Displays

Warming, drying, and intensified water stress is expected in many ecosystems over the next century. In dry environments, evaporative cooling becomes increasingly limited and must be replaced with alternative means of heat dissipation if canopy and leaf temperature are to be maintained within the physiological range and mortality avoided. We have shown that in dry environments when latent heat flux is minimal, net radiation is high, and thermal radiation emission is suppressed, pine forest canopies can efficiently cool through a massive sensible heat flux, facilitated by the low aerodynamic resistance of the open canopy (a so-called ‘Convector Effect’). Using novel methodology, we also show that this phenomenon may originate at the leaf-scale, associated with needle properties, changes in heat transport characteristics across the canopy profile, and propagating across scales can ultimately influence the boundary layer, the local atmospheric dynamics, and potentially regional climate.

How to cite: Yakir, D., Muller, J., Tatatrinov, F., Mauder, M., and Rotenberg, E.: Non-radiative heat dissipation across scales in a water stressed pine forest: from the leaf to the planetary boundary layer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11276, https://doi.org/10.5194/egusphere-egu2020-11276, 2020.

Greenhouse gases such as CO₂ and CH₄ are emitted by various sources. Among the natural ecosystems, forests and wetlands are believed to emit sizeable amount of these gases by means of autotrophic, heterotrophic respirations and bacterial activities. Additionally, a relatively new source has been detected; the emission of CH₄ by trees and plants. A growing evidence suggests that a significant amount of CH₄ is generated especially by the trees in forested ecosystems. Eddy-covariance (EC) based technique is widely used to estimate the GHGs and energy fluxes in natural ecosystems. The net ecosystem exchange (NEE), measured by an EC system, typically represents the net CO₂ fluxes arising due to the biosphere's photosynthetic and respirative processes. The net flux derived by this system, is subsequently partitioned into two components, the respired carbon and the assimilated carbon. However, the partitioning processes may have their own shortcomings which introduce significant errors. To reduce the uncertainty, the NEE needs to be constrained by some additional measurement. We have used a real time GHG analyzer in association with an existing EC system in a tropical forest of north-east India, the Kaziranga National Park, to better constrain the above two carbon fluxes. The GHG analyzer provided CO₂ and CH₄ concentrations as well as their carbon isotopic ratios. The isotopic data were used to partition the EC derived NEE records, which showed a good agreement with the EC measurements within the limits of experimental uncertainty. However, long-term observation is required to establish the potential of this relatively new method in this endeavour. Additionally, the isotopic data provided a strong evidence of plant generated CH₄ , which was apparently not possible to identify by the conventional means. This work will present the three years (2016-2018) of NEE data to demonstrate the unique characteristics of the carbon transfer processes of one of the major forested regions of north-east India, with a special reference to its partitioning by means of the isotopic analysis carried out on a campaign mode observation in Feb 2019.

How to cite: Chakraborty, S., Metya, A., Datye, A., K. Deb Burman, P., Sarma, D., Gogoi, N., and Bora, A.: Eddy covariance and CRDS based techniques of GHGs measurements provide additional constraint in partitioning the net ecosystem exchange, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-901, https://doi.org/10.5194/egusphere-egu2020-901, 2020.

Terrestrial vegetation growth is hypothesised to increase under elevated atmospheric CO₂, a process known as the CO₂ fertilisation effect. However, the magnitude of this effect and its long-term sustainability remains uncertain. One of the main limitations to the CO2  fertilisation effect is nutrient limitation to plant growth, in particular nitrogen (N) in temperate and boreal ecosystems. Recent studies have suggested that decreases in observed foliar N content (N%) and δ¹⁵N indicate widespread nitrogen limitation with increasing CO₂  concentrations. However, the factors driving these two variables, and especially the foliar δ¹⁵N values, are complex and can be caused by a number of processes. On one hand, if the observed trends reflect nutrient limitation, this limitation can be caused by either CO₂ or warming driven growth. On the other hand, it is possible that nutrient limitation does not occur to its full extent due to plant plastic responses to alleviate nutrient limitation, causing a decrease in N%, but changes in the anthropogenic N deposition 15N signal cause the observed δ¹⁵N trend. In reality, it is likely that all these factors contribute to the observed trends. To understand ecosystem dynamics it is important to disentangle the processes behind these signals which is very difficult based on observational datasets only.

We use a novel land surface model to explore the causes behind the observed trends in foliar N% and δ¹⁵N. The QUINCY (QUantifying Interactions between terrestrial Nutrient CYcles and the climate system) model  has the unique capacity to track ecologically relevant isotopic composition, including ¹⁵N in plant and soil pools. The model also includes a realistic representation of plant plastic acclimation processes, specifically a representation of nitrogen allocation to and inside the canopy in response to nitrogen availability, so implicitly to changes in CO₂  concentrations. We test the different hypotheses above behind the observed changes in N% and δ¹⁵N separately and quantify the contribution of each of the factors towards the observed trend. We then test the different hypotheses against existing observations of N% and δ¹⁵N from the ICP Forests database and other published datasets such as the global dataset of Craine et al. 2018.

Our study showcases the use of an isotope-enabled land surface model in conjunction with long-term observations to strengthen our understanding of the ecosystem processes behind the observed trends.

How to cite: Caldararu, S., Thum, T., Nair, R., and Zaehle, S.: Disentangling the long-term foliar 15N signal using a land surface model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9702, https://doi.org/10.5194/egusphere-egu2020-9702, 2020.

High impact invasive plant species, such as the N-fixing and water-spending tree Acacia longifolia, are a major threat to ecosystem functioning worldwide. For example, Acacia's impact on nutrient and water-cycling in Mediterranean dune ecosystems is well understood. However, early detection of such impacts remains challenging. Therefore, novel approaches are required to map functional indicators of high invader impact. Here, we tested in a real world context if the stable isotopes δ¹³C and δ¹⁵N could be such mappable indicators. First, we show that A. longifolia differs regarding its biochemical leaf traits from the native species of the same growth form particularly regarding leaf N content as well as δ¹³C and δ¹⁵N. This may indicate a high impact on N and water cycling, and can be retrieved from hyperspectral data. Second, the impact of the invader on N cycling was mapped joining the spatial distribution of δ¹⁵N with airborne laserscanning data. Foliar δ¹⁵N of a non-fixing, native species increased in vicinity of invasive stands indicating an uptake of N previously fixed by the invader. Finally, those impacts possibly result in an increase of productivity of the whole dune ecosystem even when invader cover is low. This increase can be mapped integrating hyperspectral imagery with LiDAR data. Thus, there is potential to retrieve functional indicators of high impact including stable isotopes using remote sensing.

How to cite: Große-Stoltenberg, A., Hellmann, C., Thiele, J., Oldeland, J., and Werner, C.: Stable isotopes as early indicators of high impact after plant invasion: A remote sensing perspective, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11472, https://doi.org/10.5194/egusphere-egu2020-11472, 2020.

Arbuscular mycorrhizal fungi (AMF) are important partners in plant nutrition, as they increase the range to scavenge for nutrients and can access resources otherwise occlude for plants. Under water shortage, when mobility of nutrients in soil is limited, AMF are especially important to acquire resources and can modulate plant drought resistance. Strategies of plants to cope with water and nutrient restrictions are shaped by the intensity of aridity. To investigate the effect of aridity on plant-AMF associations regarding drought resistance and plant nutrient acquisition, a ¹³CO₂ pulse labeling was conducted across an aridity gradient. In a semiarid shrubland (66 mm a^-1), a Mediterranean woodland (367 mm a^-1), and a humid temperate forest (1500 mm a^-1), root and soil samples were taken from 0-10 cm and 20-30 cm soil depth before labeling and at 1 day, 3 days, and 14 days after labeling. Carbon (C), nitrogen (N), and phosphorus (P) stocks as well as AMF root colonization, extraradical AMF biomass (phospho- and neutral lipid fatty acids (PLFA and NLFA) 16:1w5c), specific root length (SRL), and root tissue density (RTD) were measured. Plant C investment into AMF and roots was determined by the ¹³C incorporation in 16:1w5c (PLFA and NLFA) and root tissue, respectively. Soil C:N:P stoichiometry indicated a N and P limitation under humid conditions and a P limitation in the topsoil under Mediterranean conditions. N stocks were highest in the Mediterranean woodland. A strong correlation of the AMF storage compound NLFA 16:1w5c to C:P ratio under semiarid conditions pointed to a P limitation of AMF, likely resulting from low P mobility in dry and alkaline soils. With increasing aridity, the AMF abundance in root (and soil) decreased from 45% to 20% root area. ¹³C incorporation in PLFA 16:1w5c was similar across sites, while relative AMF abundance in topsoil (PLFA 16:1w5c:SOC) was slightly higher under semiarid and humid than under Mediterranean conditions, pointing to the importance of AMF for plant nutrition under nutrient limitation. Additionally, PLFA 16:1w5c contents in soil were higher with lower P availability in each site, underlining the role of AMF to supply P for plants under P deficiency. Under humid conditions (with strong N and P limitation) and semiarid conditions (with strong water limitation), root AMF colonization increased with lower N availability, displaying the role of AMF for plant N nutrition under nutrient and/or water shortage. Under humid and Mediterranean conditions, SRL decreased (0.5 and 0.3 times, respectively) and RTD increased (1.9 and 1.7 times, respectively) with depth, indicating a drought tolerance strategy of plants to sustain water shortage. Under semiarid conditions, SRL increased with depth (2.3 times), while RTD was consistently high, suggesting an increasing proportion of long-living fine roots with depth as scavenging agents for water. These relations point to a drought avoidance strategy of plants as adaptation to long-term water limitation. Under strong nutrient limitation, as under humid and semiarid conditions, AMF are crucial to sustain plant nutrition and to enhance plant resistance to water shortage.

How to cite: Stock, S., Köster, M., Boy, J., Godoy, R., Nájera, F., Matus, F., Merino, C., Abdallah, K., Leuschner, C., Spielvogel, S., Gorbushina, A., Dippold, M., and Kuzyakov, Y.: N and P limitation shapes plant-AMF interactions across an aridity gradient, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12829, https://doi.org/10.5194/egusphere-egu2020-12829, 2020.

Nutrient imbalances induced by anthropogenic N deposition may fundamentally alter plant activity and consequently, their role in biogeochemical cycling. Mechanistic understanding of N cycle processes is commonly informed by ¹⁵N tracers in fertilizer applications, but over the long term most N is obtained by plants is from litter mineralization rather than ‘new’ deposition N or mineral fertilizer applications. In many ecosystems this litter pool is dominated by root remains.

Here, we will compare results between two experiments: a ¹⁵N-labelled root litter experiment, and an associated conventional ¹⁵N-mineral fertilizer experiment, both located in a typical, spatially heterogeneous, seasonally-arid Spanish dehesa amended with N and NP fertilizers to induce nutrient imbalances. We show that recovery of the litter tracer in soil and plants was substantially higher than the fertilizer N, especially in microsites under trees, which are rich in organic inputs. In contrast, recovery of mineral tracers was strongest in more resource-poor grassland areas.  Plant acquisition of N from the organic source was also affected by the concurrent P addition treatment while we found little evidence for a similar effect in mineral additions.

Our results imply that scaling nitrogen cycle processes informed by isotope tracers from experiments to ecosystems depends heavily on appropriateness of methodology, especially in interpreting short-term traces applied as fertilizer N.

How to cite: Nair, R., Morris, K., Moreno, G., Migliavacca, M., and Schrumpf, M.: Comparing the fate of N from fertilizer treatments and root litter turnover in a Mediterranean Savanna, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6849, https://doi.org/10.5194/egusphere-egu2020-6849, 2020.

Among the Mediterranean vegetation, olive and cedar trees are significant symbols, with the former considered among the oldest trees in the Mediterranean basin. In Lebanon, those trees survive at diverse altitudes, standing as a testimony to their long history and socio-economic role.

The Mediterranean basin is classified as an area vulnerable to climate change. Its species persisted in this area due to the low amplitude of temperature change between the last glacial period and the Holocene. The Middle East and North Africa region is a major contributor worldwide to global health and climate change emissions over the past three decades.

Understanding how these trees have and will survive the different cultural, climatic and environmental shocks, and how will they continue to persist among upcoming changes, is a scientific challenge.

Trees are considered a good archive for environmental and climatic data. Using stable isotope (C,N,S,O,H) to study tree response to climatic and environmental factors are now widely used. They can act as important tracers of how plants today and in the past, have interacted and responded to their abiotic and biotic environments. The O and C isotopic of bulk wood or purified cellulose from tree rings, has offered good record of the ecophysiology of the plants, resources they use, and environments they inhabit, now and in the past.

Due to the development of MCICPMS technique, Hg, Pb contents and isotopes can be analyzed to help reveal the problem between climate and anthropogenic contamination pollution effect. We can track the source of pollution and measure concentration through the content and isotopes within different tree tissues (leave, stem, wood). Thus, pollution and climatic records can be obtained on tree archives over various time scales through metal isotopes (Pb, Hg) and stable isotopes (CNHOS).

This study aims to examine the present and past conditions of monumental olive and cedar trees, through studying and comparing the present and past isotopic and radiogenic variation; and create a dataset to help anticipate and predict climatic discrepancies using interdisciplinary approaches.

Two ancient olive groves were selected, Bchaaleh (1300m-North), Kawkaba (672m-South), and one cedar tree site, Maasser El Chouf (1700m-Mount Lebanon).

 Leaves, stems and rainwater samples are collected on monthly basis, and soil sediment and litter collected on quarterly basis from the olive sites. For cedar, seasonal collection is conducted to achieve a multi isotopic study for the present. To create data for the past, 212 wood cores were collected from 32 centennial olive trees and 21 cores were extracted of 8 cedar trees.

We expect to establish a database of stable and radiogenic isotopic signatures of recent and past olive and cedar elements. In addition to having a comprehensive interpretation of stable and radiogenic isotopic variations at seasonal scale through the applied time series, and calibrating between the isotopes of the tree and current climate. The study of trace elements contents, Pb and Hg isotopic ratio, will allow the reconstruction of anthropogenic pollution evolution of trees, tracing the sources of pollution.

Tree rings will provide information on paleoclimate and dating it back from the beginning of the industrial period.

How to cite: Tabaja, N., Chalak, L., Amouroux, D., Tessier, E., Bosch, D., Angeli, N., Fourel, F., Jomaa, I., El Haj Hassan, F., and Bentaleb, I.: Climatic, environmental and pollution traceability of the monumental Olive and Cedar trees of Lebanon: Lessons from the past to the present , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20787, https://doi.org/10.5194/egusphere-egu2020-20787, 2020.

Microorganisms influence the composition of the atmosphere, the cycling of elements within and through ecosystems, the functioning of agricultural ecosystems on which humans depend, and human health. Microorganisms are also the most metabolically flexible and the most taxonomically and evolutionarily diverse organisms on Earth. Yet deciphering how that diversity imprints on the processes they influence at larger scales has proven challenging, because of the overwhelming complexity of microbial communities, and because of the difficulty of quantifying how microbial taxa assimilate and transform elements in the environment. New approaches that blend traditions from microbial ecology and ecosystem science make it possible to explore how the diversity and physiology of microorganisms shape ecosystem biogeochemistry and how it responds to global environmental change. Quantitative stable isotope probing has revealed cases where ‘omics data scales to quantitative ecology and biogeochemistry. Lab and field warming experiments in tropical, temperate, boreal, and arctic ecosystems point to generalized relationships between microbial growth rates (measured using isotope-enabled omics) and carbon release from soil through respiration (measured at the whole-system scale using classic techniques). Phylogenetic signals describe these relationships, indicating the potential for grounding biogeochemistry within the evolutionary histories of the organisms involved. In a meta-analysis across 27 independent experiments, quantitative stable isotope probing also indicates the general importance of predatory bacterial taxa in microbiomes, a role that increases in response to resource pulses, and which may provide a trophic theoretical underpinning to processes like the soil priming effect. More generally, such approaches hold potential for linking microbial diversity to carbon and element cycling in ecosystems. Historically, the diversity, complexity, and intractability of microbial ecosystems has relegated their study to either a reductionist descriptive tradition in microbial ecology or to a simplistically quantitative one in ecosystem science. Yet, new ideas and tools are poised to push microbial ecology forward to a point where it can more meaningfully integrate with ecological fields at larger scales, from populations to ecosystems to the globe.

How to cite: Hungate, B.: Isotope tools scaling soil microbial ecology to biogeochemistry , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4348, https://doi.org/10.5194/egusphere-egu2020-4348, 2020.

Tropical rainforests play a major role in the terrestrial carbon (C) cycle. However, to date little is known about the mechanisms and processes controlling C fluxes in tropical forests. Within the C cycle of a forest, trees allocate a substantial amount of photoassimilates belowground, and fuel respiration by stems, roots and microorganisms. This link between assimilation and respiration represents a significant pathway by which assimilated C is quickly returned to the atmosphere. However, the nature of this coupling, including the speed of above- to below-ground C allocation and the proportion of rapidly metabolized assimilates is yet unknown for mature tropical rainforest systems. Furthermore, the role of tree species and size and the relative roles of canopy versus understory plants are still unresolved.

Drought spells can exert a major control on the C balance of tropical forest ecosystems by altering C uptake, the partitioning of C and the dynamics of C allocation and belowground utilization. As such responses are difficult to measure in tropical rainforest, the consequences of drought for the dynamics of recent C in stem and soil respiration in this biome remain unclear.

To assess and quantify these processes, we made use of the Tropical Rain Forest at the Biosphere 2 research complex in Arizona, US. This infrastructure provides unique opportunities to study drought effects on the C dynamics in a controlled environment. We simulated a drought spell for eight weeks and continuously measured stem and soil CO₂ fluxes using isotope laser spectroscopy before and during the drought as well as during the subsequent rewetting period. Our study is part of a large-scale experiment that aims to disentangle C- and water-cycle processes underpinning ecosystem responses to drought from a molecular to an ecosystem-scale level, with particular focus on plant-soil and plant-atmosphere interfaces.

We performed two canopy-scale ¹³CO₂ pulse labeling campaigns under ambient environmental conditions and towards the end of the experimental drought. We traced the allocation dynamics of recently assimilated C to soil respiration and to stem respiration of dominant tree species. First results show that the allocation of assimilates from the canopy to soil-respired CO₂ took several days and was affected by tree size and species identity. Under drought, tracer efflux from stems and soils was  slowed down, with strong species-specific differences. Our results will allow novel insights into the combined effects of tree size, species identity and drought on the allocation dynamics and respiratory utilization of photoassimilates in tropical rainforest.

How to cite: Ingrisch, J., Meeran, K., Kübert, A., Ladd, N., van Haren, J., Werner, C., Meredith, L., and Bahn, M.: Tracing recent carbon from photosynthesis to stem and soil respiration in an experimental tropical rainforest in response to drought, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-868, https://doi.org/10.5194/egusphere-egu2020-868, 2020.

The direct measurement of soil gases provides insight into the biogeochemical processes responsible for micro- and macro-nutrient cycling, respiration, signaling, and environmental responses.  The concentrations and isotopic signatures of soil gases are effective messengers of the microbial pathways active in the soil.  We have developed and deployed a high frequency sensor consisting of new diffusive soil probes coupled with a Tunable Infrared Laser Direct Absorption Spectrometer (TILDAS) to monitor a range of soil gas species to investigate biogeochemical soil processes.

An array of soil probes was deployed at the Tropical Rainforest at Biosphere 2 in Arizona as part of the Water Atmosphere and Life Dynamics (WALD) experiment in 2019-2020.  Probes were located in a root zone and nearby control area, and at several depths via a soil pit.  These probes were coupled with a TILDAS to monitor isotopologues of nitrous oxide (N₂O) including ¹⁴N¹⁵NO, ¹⁵N¹⁴NO, N₂¹⁸O, and methane (¹³CH₄ and ¹²CH₄), as well as CO₂. During the WALD experiment, the probe-TILDAS system followed the impact on the soil biome of a 2 month induced drought in the rainforest and the subsequent return of rain.  The high temporal resolution of the system allowed us to monitor each probe every 2 hours and thus observe changes in the composition of soil gases that reflect biogeochemical processes and pathways.  CO₂ and thus respiration decreased significantly during the drought and was slow to recover.  Differences in N₂O mixing ratios and isotopic signatures (both site preference and bulk 15N) in the root zone versus a controlled soil region were observed during both drought and rewetting periods.  Changes in nitrogen and carbon cycles and the microbial pathways during the induced drought and rewetting as reflected in these observations will be discussed.

How to cite: Shorter, J., Roscioli, J., Meredith, L., and Gil-Loaiza, J.: Soil Biogeochemical Response to Drought Conditions in the Biosphere 2 Rainforest, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11636, https://doi.org/10.5194/egusphere-egu2020-11636, 2020.

Tracers provide a way to determine, follow and quantify biochemical cycles and energy fluxes within the soil-plant-atmosphere continuum (SPAC). Thereby, different tracers, such as dyes, carbonyl sulfite or stable isotopes are employed. One major disadvantage of many tracers is, that very often, plants have to be destructively harvested to analyze the tracer concentration, making it difficult to measure continuous fluxes. Additionally, for stable isotope studies, fractionation or exchange effects can make interpretation and quantification of biogeochemical fluxes difficult. Novel tracers that are already frequently used in animal systems, are fluorescent C-dots. These nanoparticles (5-50 nm diameter) provide a non-destructive imaging option using “in vivo imaging systems” (IVIS). We examined a first approach to apply and measure C-dots as possible tracers in tree saplings of three species (Picea glauca, Pinus strobus, Tsuga canadensis). Roots were excavated from soils and exposed to 20 µmol/l liquid silica-based nanoparticles (diameter of 5.1 nm) labeled with a near-infrared fluorophore, cyanine 5.5 (excitation maximum 646 nm, emission maximum 662 nm). Subsequent continuous IVIS imaging showed real-time uptake and transport of the C-dots by the trees. Respective fluorescence intensity revealed the concentration of C-dots in each of the tissues at measured time steps. Subsequent cross-sectioning of roots, stems and leaves elucidated the internal transport pathway of the C-dots inside the saplings’ tissues. Finally, measurements of stomatal conductance, photosynthesis, transpiration rate, stem hydraulic conductivity and pre-dawn leaf water potentials in comparison to control saplings revealed no phytotoxic effect by the C-dots on plant functioning. In conclusion, C-dots provide a non-toxic new technique for measuring biochemical cycles within the SPAC. Future applications include high resolution tracing of water flow cycles and turnover times within the SPAC, compound specific analyses of root exudation or determining mechanisms of pest influences on plant metabolism.

How to cite: Hafner, B. D., Singh, K., and Bauerle, T. L.: C-dots as non-toxic, non-destructive novel tracers to measure biochemical cycles in the soil-plant-atmosphere continuum, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12426, https://doi.org/10.5194/egusphere-egu2020-12426, 2020.

The interplay of the processes controlling carbon partitioning into plant primary and secondary metabolisms, such as respiratory CO₂ release and Volatile Organic Compound (VOC) biosynthesis, is still not fully understood. Pyruvate is a key metabolite which is formed in primary metabolism and acting as substrate in numerous secondary pathways forming many BVOCs, such as isoprene, volatile terpenoids, oxygenated compounds (acetaldehyde, acetic acid), benzenoids and fatty acid oxidation products (several wound VOCs), which can be emitted by plants. Within the ERC project VOCO, we established an innovative analytical setup enabling us to simultaneously measure stable carbon isotope composition of leaf exchanged CO₂ (Infrared Isotope spectroscopy IRIS) together with VOC release (PTR-TOF-MS and GC-MS-C-IRMS). Position specific ¹³C-labeled pyruvate and ¹³CO₂ were applied in tracer experiments to elucidate carbon partitioning at metabolic branching points into VOCs vs. CO₂ in drought stressed Scots pine trees.

We observed treatment specific patterns of VOC emission including volatile terpenoids, oxygenated BVOCs and green leaf volatiles. Tracing ¹³C, we elucidated if compounds were de novo synthesized from ¹³C-labeled pyruvate or ¹³CO₂. Position-specific labeling with [1-13C]-pyruvate and [2-13C]-pyruvate suggested that most VOCs were synthesized from the C2-C3 moiety of pyruvate whereas the C1 position was decarboxylated resulting in ¹³CO₂ release by dark respiration in the light. We observed drought stress related shifts in ¹³CO₂ release and VOC emissions. Our observations suggest that VOC emissions are associated with significant pyruvate C1 decarboxylation which is released to the atmosphere. This novel approach contributes to a better understanding of the metabolic links of processes of primary and secondary plant metabolisms which is relevant for the vegetation-atmosphere exchange of CO₂ and VOCs. 

How to cite: Kreuzwieser, J., Grün, M., Eiblmeier, M., Bamberger, I., Yanez-Serrano, A. M., Fasbender, L., and Werner, C.: Drought stress affects carbon partitioning between plant primary and secondary metabolism in Scots pine trees, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20441, https://doi.org/10.5194/egusphere-egu2020-20441, 2020.

Pine Mistletoe (Viscum album ssp. austriacum) is a hemi-parasite shrub species, whose survival and development rely on water and mineral resources obtained through the xylem sap from the host tree. Mistletoe can produce photosynthates in its green organs on its own. On the other side, as they are connected to the phloem of the host tree, they may also be able to retrieve  carbon assimilates from their host and/or vice versa. However, the dynamics and the steering factors of this relationship remains unclear. We conducted ¹³C- labelling experiments with mature Scots pine (Pinus sylvestris) infected by mistletoes in a long-term (15 years) irrigation experiment in Switzerland (Pfynwald, Valais) to investigate the transport of carbon assimilates and nutrients between the host and the parasite under different soil moisture (600 mm vs 1200 mm of precipitation per year). Three irrigated and three control (natural xeric) pine trees infected by mistletoes were ¹³C labelled for 4 hours. During the ¹³C labelling of the trees, the following two experiments were carried out. 
    1) Wrapping experiment: 3-4 clusters of mistletoes on the labelled trees were wrapped with aluminium foil and enclosed in plastic bags to prevent mistletoe photosynthesis using ¹³C-enriched CO₂ and to investigate a potential host-mistletoe carbon transfer.
    2) Girdling and removal experiment: The phloem of 12 host tree branches (6 control & 6 irrigated) infected by mistletoes was manually removed (ca. 2 cm in width, basipetally of the mistletoes) to stop the downward transport of photosynthates from the girdled branches. Additionally, host needles or mistletoes were removed from the girdled branches to investigate the respective contribution of photosynthesis by the host needles vs. mistletoes to the host branch carbon balance.
     In the wrapping experiment, wrapped and unwrapped mistletoe leaves and stems were harvested at 10 times points over 6 months. In the girdling and removal experiment, needles, twigs (i.e. xylem and phloem) and mistletoe leaves were harvested at 7 time points over 14 days. In both experiment, bulk organic matter of each tissue was analysed to trace the ¹³C signal.
     We found that there was no ¹³C signal in the wrapped mistletoe leaves, indicating that there was no C-transfer from the host to the mistletoes via the phloem sap. Mistletoe removal from girdled branches decreased ¹³C-labelled carbon assimilates in host xylem and phloem. Meanwhile, when needles were completely removed from the girdled branches, host xylem and phloem were still able to acquire ¹³C from the mistletoes. These results suggest that mistletoes can support the host with carbon resources, which might be especially important when the host is C resource limited.
Our study provides new insights into the relationship between the hemi-parasite and its host tree. Mistletoes may play a role as C source providers to support the host and maintain a symbiotic relationship to survive. 

How to cite: Wang, A., Rigling, A., Lehmann, M., Saurer, M., Gessler, A., Du, Z., and Li, M.: Active support of mistletoe to the host tree with carbon assimilation under source-limitation – results from a 13C labelling experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6960, https://doi.org/10.5194/egusphere-egu2020-6960, 2020.

Analysing stable isotope composition of biologic components can be a powerful tool to reconstruct past environmental conditions, physiological responses, and to trace metabolic pathways. The analysis of the carbon-bound non-exchangeable hydrogen isotope ratios (δ²H_NE) in carbohydrates can be challenging, partly due to the exchangeability of oxygen-bound hydrogen in the same molecule with those in water or vapour. To eliminate such sample alterations, carbohydrates have been nitrated to substitute exchangeable hydrogen with nitrate ester. However, the nitration of carbohydrates is time consuming, needs high sample amount, has several safety issues, and the nitrated products of short-chained carbohydrates are instable. δ²H_NE of sugars derived from living organisms or directly from the environment are thus still limited and not widespread available. Here we optimized recent δ²H_NE methods, with the focus on plant-derived non-structural carbohydrates such as starch, sugars, and sugar alcohols. The exchangeable hydrogen is replaced via equilibration with water vapour of a known isotopic composition to calculate δ²H_NE. In this presentation, we will explain the new δ²H_NE method, discuss precision, accuracy, as well as referencing strategies, and give a first outlook for future applications in plant and environmental sciences.

How to cite: Schuler, P., Joseph, J., Cormier, M.-A., Werner, R. A., Saurer, M., and Lehmann, M. M.: New method for hydrogen isotope analysis of non-structural carbohydrates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20785, https://doi.org/10.5194/egusphere-egu2020-20785, 2020.

Sagebrush (Artemisia tridentata) ecosystems span a wide range of environmental conditions in the Western US, where they are the most extensive semiarid vegetation type. Water availability is recognized as the major driver of the structure of sagebrush communities, however less is known about the associated biogeochemical processes. By characterizing large-scale biogeographical patterns of carbon (C), nitrogen (N), phosphorus (P), and sulfur (S) elemental and isotopic compositions in soil and vegetation, we aimed to: (1) detect potential nutrient limitations, (2) identify element sources (weathering, decomposition, or atmospheric deposition), and (3) identify the nature and rates of biogeochemical processes. We sampled sagebrush leaves together with intra- and inter-canopy soil at 50 sites across Wyoming, where sagebrush extends across strong climate and soil type gradients. We expect the nature, rates, and coupling of biogeochemical processes to correlate with patterns of water availability since it is a key control of microbial activity, and the diffusion of enzymes and substrates. Nutrient availability for plants may also follow the same pattern. For example, in moist sites, increased N and P availability may result from higher organic matter decomposition rates than in dry sites, potentially alleviating N limitation to plant growth. However, an excess of P relative to N may occur at high decomposition rates and where the soil parent material is P-rich, leaving N as the limiting nutrient. Conversely, warm and dry sites may have a greater proportion of N being lost through fractionating pathways and a more open N cycle, resulting in high soil and foliar d15N values. We expect leaf d34S to reflect contrasting sources, notably helping to decipher the relative importance of the inputs from atmospheric deposition and weathering (i.e. sedimentary material deposited under anoxic conditions). By improving our understanding of the biogeochemical processes associated with vegetation productivity patterns along a macro-climatic gradient, our data could provide insights into future ecosystem status and help designing disturbance recovery strategies.

How to cite: Brédoire, F., Ayayee, P. A., Ben Tekaya, S., van Diepen, L. T. A., and Williams, D. G.: Soil and leaf CNP&S stoichiometry and isotopic composition in sagebrush ecosystems of Wyoming, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4784, https://doi.org/10.5194/egusphere-egu2020-4784, 2020.

Abstract: A canopy chamber system is useful to measure gas exchanges in the plant ecosystem. A transparent chamber has been generally used to measure canopy fluxes in the field, such that the light source for photosynthesis depends on the solar radiation. However, it is challenging to measure stable canopy fluxes in the field due to changeable solar radiation conditions in nature. In this study, we constructed a new chamber system to measure canopy fluxes using a Light Emitting Diode (LED) as a light source. Upon the construction, we aim to measure quantitative gas exchanges to estimate the amount of photosynthesis and evapotranspiration using the constructed chamber system in crop fields. While diverse chamber systems have been developed to measure canopy gas exchanges so far, no attempt has been made to create such an LED chamber system according to our best knowledge. The new chamber system was composed of a chamber, LEDs with a maximum power capacity of 1,800W, a water pump for cooling, and a gas analyzer (LI-850, LI-COR, USA). This LED chamber system can estimate the amount of photosynthesis and evapotranspiration rate of plants by measuring both CO₂ and H₂O fluxes in the ecosystem. These measurements can contribute to the practical assessment of crop productivity as well as scientific advancement in plant ecophysiology.

Keywords: Crop, evapotranspiration, gas exchange, LED, chamber, photosynthesis

Acknowledgement: This research was supported by "Cooperative Research Program for Agriculture Science & Technology Development (Project No. PJ013841022018)" from Rural Development Administration, Republic of Korea.

How to cite: Shin, T., Jeong, S., and Ko, J.: Construction of a Light Emitting Diode Chamber for Measurement of Gas Exchange in the Plant Ecosystem, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5590, https://doi.org/10.5194/egusphere-egu2020-5590, 2020.

Synchrotron X-ray computed tomography (SRXCT) imaging is a technique now commonly deployed for non-destructive 3D visualisation of root morphology in soil environments. However, visualising the internal anatomy of roots in soil using SRXCT can be difficult since the energy required for sufficient X-ray transmission through soil often results in poor contrast between root tissues. This reduces the amount of obtainable information about root anatomy and the effects of the soil environment on plant root internal structure. Contrast media is often used in SRXCT imaging to increase the contrast between tissues, enabling greater ease of both visualisation and image processing for internal structures of biological material.

In this work, we demonstrate that by introducing root material exposed to iodinated contrast media we can overcome these limitations and visualise internal root anatomy of in vivo roots intact within soil. To achieve this, we undertook time-resolved SRXCT imaging of juvenile maize plants growing in a specially designed growth system over a period of 24 hours. This system was designed such that only the base of the primary root would be suspended into iodinated contrast media whilst the rest of the root system remained in soil partially saturated with water, and the plant remained intact and alive. This enabled the basal section of primary root to take up iodinated contrast media without dispersal of the contrast media into the soil. Following the time-resolved imaging of the root system, leaf and stem material were then imaged using SRXCT and mapped using synchrotron X-ray florescence (SRXRF). Using this system, we were able to visualise and segment anatomical root features that are otherwise difficult to capture in vivo in soil using non-destructive 3D imaging such as vascular bundles (including phloem, xylem and proto-xylem) and structures within the cortex. We also gained inferences into fluid flow and transport within in vivo roots in soil based on this technique. The SRXCT imaging as well as the SRXRF mapping of stem and leaf material confirmed transport of the iodinated contrast media through plant vasculature and the distribution into leaf venation. This investigation demonstrates the quantity of data on internal root anatomy and fluid transport for in-vivo roots in soil that could be yielded from SRXCT and SRXRF in future.

How to cite: Scotson, C., Williams, K., McKay Fletcher, D., Koebernick, N., van Veelen, A., and Roose, T.: Improving Visualisation of In-Vivo Root Anatomy and Fluid Transport in Soil Using Iodinated Contrast in Synchrotron XCT and XRF, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8718, https://doi.org/10.5194/egusphere-egu2020-8718, 2020.

Climate change exerts increasing pressure on tropical rainforests enhancing their susceptibility to environmental stress. Plants' abilities to rapidly adjust their metabolism are critical for reducing the stress effects caused by extreme external conditions. Plants produce a wide spectrum of volatile organic compounds (VOCs) to cope with oxidative and thermal stress. The distribution and amount of VOC production thereby vary greatly not only among species but also organs, such as leaves and roots. Within the framework of our large-scale ecosystem manipulation experiment, Biosphere 2 Water, Atmosphere, and Life Dynamics (B2-WALD), we aimed to produce deeper insights into carbon partitioning between primary and secondary metabolism under drought stress, notably into CO₂ and VOCs.

In particular, we investigated how drought stress influences organ-specific carbon allocation between processes of primary and secondary metabolisms and to what extent allocation into secondary metabolism protects plants from drought. The tropical rainforest mesocosm in Biosphere 2, University of Arizona, provides a unique system for ecosystem manipulation studies. We implemented a drought stress experiment, excluding rainfall for two months. To investigate changes in carbon allocation, we performed labelling experiments with position-specific ¹³C-labelled pyruvate on leaves and roots of several tropical tree and shrub species before and during the drought period. We used ¹³CO₂ laser spectroscopy and high-sensitivity proton-transfer-reaction time-of-flight mass spectrometry to enable real-time analysis of metabolic pathways and carbon turnover, using leaf- and root-chambers to quantify fluxes.

Considering our preliminary results, net CO₂ assimilation strongly declined under rain exclusion, due to stomatal closure. Consequently, respiration rates declined strongly in leaves as well as in roots. The response of VOC emissions, however, varied among organs. In leaves, we found that the emission of some VOCs declined under drought stress (acetone, monoterpenes), while other fluxes increased or stayed the same (isoprene). We will present detailed data on [1-13C]- and [2-13C]-pyruvate allocation within primary and secondary metabolism, such as decarboxylation processes and VOC-production. To our knowledge, this is the first time that real-time measurements of ¹³C-labelled root VOC-emissions were conducted, enabling this comparative analysis of drought induced stress effects on leaf- and root-emissions.

How to cite: Daber, L. E., Bamberger, I., Ladd, S. N., Kreuzwieser, J., Fudyma, J., Loaiza, J. G., De Leeuw, J., Shi, L., Bai, X., Purser, G., Krechmer, J. E., Meredith, L., and Werner, C.: Understanding drought induced responses in leaf and root CO2 and VOC fluxes through position specific isotope labelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9343, https://doi.org/10.5194/egusphere-egu2020-9343, 2020.

Trees contribute substantially to the carbon cycling between the biosphere and atmosphere. Tropical ecosystems in particular are known to exchange not only CO₂ with the atmosphere, but also a wide variety of volatile organic compounds (VOCs). With their high reactivity and short life time, VOCs are known to play not only a crucial role in atmospheric chemistry but also in plant signaling and interactions. Due to climate change periods of sustained drought are thought to increase in future and have the potential to alter the carbon balance of tropical ecosystems drastically. However, combined VOC and CO₂ flux measurements are rare and thus a quantitative understanding of carbon exchange fluxes in rainforest species during and after drought periods has not yet been reached.

Thus, we used the unique opportunity to study changes of VOC and CO₂ flux patterns of the rainforest mesocosm of Biosphere 2 (University of Arizona) in response to an experimentally induced drought period and during the recovery (Biosphere 2 Water, Atmosphere, and Life Dynamics experiment; B2-WALD). This provides us novel information about stress responses of a rainforest ecosystem and its ability to recover, specifically to drought stress. Real-time fluxes of CO₂ and VOC exchange were measured by means of ¹³CO₂ laser spectroscopy and proton-transfer-reaction time-of-flight mass-spectrometry (PTR-TOF-MS) using leaf chambers on five different tree and understory species.

While photosynthesis decreased during the drought, changes in VOC flux patterns were more diverse. For example, isoprene emissions increased with dry conditions, whereas fluxes of acetone declined. Here we will present and discuss our first results on leaf gas exchange measurements of different VOCs and CO₂ and their response to drought and recovery.

How to cite: Bamberger, I., Daber, L. E., Gil Loaiza, J., Purser, G., De Leeuw, J., Ladd, S. N., Meredith, L., Kreuzwieser, J., and Werner, C.: Drought effects on the carbon balance and VOC emissions of a tropical rainforest ecosystem, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9460, https://doi.org/10.5194/egusphere-egu2020-9460, 2020.

Monoterpenes are used by plants as antioxidants in the defense against reactive oxygen species and are also contributors to the formation of secondary organic aerosol and cloud condensation nuclei. Understanding how the emissions of monoterpenes from biogenic sources change due to different stresses such as drought is of importance as more frequent drought events are expected to occur in the future due to climate change. Monoterpenes such as alpha pinene and limonene exist as optical isomers in mirror image forms, (+) and (-). Studies on the effect of different stresses on plant emissions commonly measure the sum of enantiomers rather than conducting separate measurements for the individual enantiomers [1]. Recent measurements of chiral monoterpenes have highlighted the importance of independently measuring the individual enantiomers of a chiral pair, due to differences such as environmental drivers [2] and local measurement source [3]. Despite the enantiomers of the same monoterpene having the same chemical properties, they can interact differently with biologically active chiral molecules such as those that exist as olfactory receptors within insect antennae [4].

The atmospheric dynamics of chiral monoterpenes from beneath the canopy of the tropical rainforest biome at Biosphere 2, Arizona, USA, were measured during pre-drought, drought and rewetting using an online GC-MS during the B2 Water, Atmosphere and Life Dynamics campaign (B2WALD). Furthermore, sorbent tube samples were obtained from different forest compartments, to investigate the compartment specific chiral VOC emission. Drought was found to be a driver of a change in the enantiomeric excess of specific monoterpenes. (-) alpha pinene was the dominant monoterpene present in agreement with results from the Amazonian rainforest despite there being no atmospheric chemistry in the Biosphere greenhouse. Interestingly, during the pre-drought phase, due to the conditions in the greenhouse, (-) alpha pinene showed an average daily maximum at 11:00 while (+) alpha pinene peaked at 15:00, coincident with peak light and temperature respectively. By the rewet phase, the average daily maximum for (-) alpha pinene shifted to 13:00, coincident with peak Isoprene, whilst it remained at 15:00 for (+) alpha pinene. The average maximum daily mixing ratios of (+) and (-) alpha pinene, during the drought phase, increased by a factor of 4 and 2 respectively, when compared to the pre-drought values. These results reveal distinct source mechanisms for individual enantiomers and the differing impact drought has on the individual enantiomers in a rainforest ecosystem.

How to cite: Byron, J., Werner, C., Ladd, N., Meredith, L., Purser, G., and Williams, J.: Changes in chiral monoterpenes during drought in a rainforest reveal distinct source mechanisms, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21427, https://doi.org/10.5194/egusphere-egu2020-21427, 2020.

As climate change brings warmer temperatures and reduced precipitation to forests globally, it is vital to understand how plants adapt to drought and temperature stress. Plant activity emits biogenic volatile organic compounds (BVOC), and stress-induced changes in BVOC concentrations have important implications on secondary VOC and aerosols formation due to atmospheric reactions with ozone and other oxidants. Measurements of BVOC emissions were made at the Biosphere 2 rainforest facility near Oracle, AZ, USA from September to December 2019. Time resolved BVOC vertical concentration gradients were measured continuously using  a proton transfer reaction time-of-flight mass spectrometer (PTR-QiToF-MS, Ionicon) sampling sequentially from five levels of a vertical tower positioned at 1 m, 3 m, 7 m, 14 m and 20 m above the forest floor. Emissions of a full range of primary BVOCs are estimated from concentrations and air exchange rates. The changes in ecosystem BVOC emission are evaluated under normal precipitation conditions, then throughout a controlled 2-month drought period, and finally through a re-wet period where rain was re-introduced to the rainforest. Analysis aims to show the vertical gradient of BVOC emissions from the forest plants, as well as how BVOC concentrations changed throughout the different stress periods. BVOCs that are important for plant physiology and atmospheric science, such as isoprene and higher terpenoids, as well as other compound classes such as volatile short chain and medium chain fatty acids, will be investigated in detail. These results will give insight into how plant emissions are affected under stress, either as protective mechanisms or due to desiccation-induced responses.

How to cite: Blomdahl, D., Meredith, L., Werner, C., Ladd, N., Langford, B., Nemitz, E., van Haren, J., Bamburger, I., Purser, G., Byron, J., and Misztal, P.: Biogenic VOC emissions under drought and temperature stress, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10910, https://doi.org/10.5194/egusphere-egu2020-10910, 2020.

Microbial metabolic functions and biogeochemical pathways of the complex rhizosphere-soil-microbe interactions change with aboveground vegetation and the ecosystem response to environmental changes. Soil trace gases and current genomic approaches have been valuable to characterize in-situ microbial activity. However, there is a lack of understanding of the complexity of the belowground processes, the time frame of microbial community responses to environmental changes and the degree to which microbial activity can be inferred current -omics approaches. In the nitrogen cycling at a field scale, microbial diversity or gene abundance sometimes does not explain N₂O emissions or even gene expression, there some bacteria that cannot be cultivated, and in general –omics involve destructive soil sampling that is prone to changes of the in-situ soil conditions. Additionally, field soil sampling may not capture the heterogeneity of the soil or specific area of study.

Volatile Organic Compounds (VOCs) produced in the rhizosphere play an important role in microbial nutrient cycling. VOCs are produced by plants and microorganisms as a response to biotic or biotic stressors or the type of carbon sources available.

Here, we present how subsurface soil gas measurements in an enclosed ecosystem during the Water, Atmosphere, and Life Dynamics experiment (B2-WALD) at the Tropical Rainforest biome of Biosphere 2 (Arizona, USA) during an induced controlled drought. We present initial results of a unique non-destructive approach that simultaneously couples a) new hydrophobic-porous subsurface soil probes, b) high-resolution Tunable Infrared Laser Direct Absorption Spectrometers (TILDAS) to analyze in situ trace gas isotopomers, and c) a proton transfer reaction mass spectrometer (VOCUS, high resolution volatile organic compound gas analyzer) for VOC quantification. We measured soil gas isotopic composition of N₂O and VOCs-- comparing rhizosphere and control areas before and during the drought. We will focus our discussion on VOCs and its potential as makers of microbial interactions and signaling as a response to an environmental stressor like drought.

In this project, we demonstrate the feasibility of online coupling of soil probes with high-resolution instrumentation to measure products from nitrogen cycling and nonmethane VOC production in soils as a response to soil-plant microbe interactions. In addition, this approach could be a potential tool to constraint inferences derived from different –omics approaches.

How to cite: Gil Loaiza, J., Meredith, L., Krechmer, J., Claflin, M., Roscioli, R., and Shorter, J.: Rhizosphere Volatile Organic Compounds: A real-time approach using diffusive soil probes on a controlled Tropical Rainforest, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12684, https://doi.org/10.5194/egusphere-egu2020-12684, 2020.

Ecosystem processes present a complex interplay between different components, such as vegetation, soil, and the rhizosphere. All these different components can emit (or even uptake) a plethora of volatile organic compound (BVOC) with highly dynamic response to environmental changes. However, processes controlling carbon allocation into primary and secondary metabolism such as VOC synthesis or respiratory CO₂ emission remain unclear. De novo synthesis of BVOC depends on the availability of carbon, as well as energy provided by primary metabolism. Thus, carbon allocation may compete between primary and secondary metabolism, which are linked via a number of interfaces including the central metabolite pyruvate. It is the main substrate fulling respiration, but also a substrate for a large array of secondary pathways leading to the biosynthesis of many volatile organic compounds, such as volatile isoprenoids, oxygenated VOCs. Within the European Research Council (ERC) Project VOCO we developed a novel technological basis to couple CO₂ fluxes with VOC emissions based on simultaneous real-time measurements of stable carbon isotope composition of branch, root, and soil respired CO₂ and VOC fluxes (Fasbender et al. 2018). Position specific ¹³C-labeled pyruvate feeding experiments are used to trace partitioning within the metabolic branching points into VOCs versus CO₂ emissions, bridging scales from sub-molecular to whole-plant and ecosystem processes. Positional 13C-labelling will trace real-time sub-molecular carbon investment into VOCs and CO₂, enabling mechanistic descriptions of the underlying biochemical pathways coupling anabolic and catabolic processes.

To trace ecosystem scale interactions, we implemented a whole-ecosystem labelling approach in the world’s largest controlled growth facility: the Biosphere 2 Tropical Rainforest. In the Biosphere 2 Water, Atmosphere, and Life Dynamics (B2-WALD) experiment, we applied an ecosystem scale drought and tracing carbon allocation and dynamics of VOC, CO₂ and H₂O fluxes from leaf, root, soil and atmospheric scales. The overarching goal of B2-WALD is to track, biological mechanisms controlling the fate of CO₂, VOC and water cycling in an ecosystem under change in an interdisciplinary approach. This comprehensive data set will be used for carbon and water partitioning from the metabolic to ecosystem scale

Fasbender L., et al. (2018). A novel approach combining PTR-TOF-MS, ¹³CO₂ laser spectroscopy and ¹³C-metabolite labelling to trace real-time carbon allocation into BVOCs and respiratory CO₂. PLOS One,13: e0204398

How to cite: Werner, C., Ladd, N. S., and Meredith, L. and the B2 WALD: Tracing ecosystem scale interactions of volatile organic compound (VOC) and CO2 emissions by position-specific and whole ecosystem isotope labelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11264, https://doi.org/10.5194/egusphere-egu2020-11264, 2020.

Tropical rain forests are greatly dependent on water supply and are highly efficient in water cycling. Soil infiltration rates as well as tree transpiration rates are high in these often seasonally dry ecosystems. Both deforestation and climate change have been shown to cause drought stress in tropical forests, the former through the increase of runoff and reduction in evapotranspiration, the latter mainly through the reduction in precipitation and transpiration.

Although great efforts have been made to determine the ecosystem and species responses to variable water supply, many processes determining how tree species in tropical ecosystems impact and are impacted by the water cycle (water uptake and redistribution, and stem storage) remain poorly understood. Water movement through trees, as measured by a D₂O pulse label in the rainwater, was found to be high variable and species dependent in a previous experiment in the Biosphere 2 tropical rainforest (Evaristo et al. 2019). We hypothesized that differential rooting depth and/or stem water storage could be the main causes for the difference in water label transport through the trees.

Our study is part of a large-scale experiment in the Biosphere 2 tropical forest that uses isotope labeling (¹³C and D) to trace C- and water-cycle processes underpinning ecosystem responses to drought from a molecular to an ecosystem-scale level. Here, we focus on the water cycling of this ecosystem and how it is impacted by controlled drought and rewetting conditions. Detailed continuous measurements of both the water pools (soil and stem) and movement (stems, atmospheric fluxes) will be used to determine individual tree (including different species) and whole ecosystem responses to drought. These data will be presented in light of their implications for tropical forest water movement and drought vulnerability.

Reference

Evaristo J, Kim M, van Haren J, Pangle LA, Harman CJ, Troch PA, McDonnell JJ, (2019) Characterizing the fluxes and age distribution of soil water, plant water and deep percolation in a model tropical ecosystem. Water Resources Research, 55(4), 3307-3327.

How to cite: van Haren, J., Kuhnhammer, K., Kuebert, A., Beyer, M., Tuller, M., Babaeian, E., Hu, J., Dubbert, M., Meredith, L., and Werner, C.: Water cycling (pools and movement) through an enclosed tropical forest in response to drought., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9712, https://doi.org/10.5194/egusphere-egu2020-9712, 2020.

Functional group-specific water use strategies are vital in understanding plant performance under current and future global climate change related drought scenarios. Different functional groups have different strategies to regulate their above ground water use and loss in order to respond to drought stress. Here, we studied the ecohydrological response of a controlled rain forest system to a 10-week lasting experimental drought (Biosphere 2 Water, Atmosphere, and Life Dynamics, B2 WALD project). Using gas exchange chambers, we specifically investigated the response of the two main rain forest functional groups - three canopy tree species and two understory species - in their above ground water use efficiency. Rates and isotopic fluxes of transpiration, assimilation and night respiration were monitored in high temporal resolution. In combination with plant physiological information (i.e., leaf water potential) a complete picture of their above ground water use could be gained. We expect that the deep rooting canopy tree species will be able to keep their above ground water use constant while the shallow rooting understory species will have to adapt their water use efficiency to budget their water reserves and resources.

How to cite: Kübert, A., Kühnhammer, K., Bamberger, I., Daber, E., De Leeuw, J., Bailey, K., Hu, J., Ladd, S. N., Meredith, L., van Haren, J., Beyer, M., Dubbert, M., and Werner, C.: Above ground response of rainforest functional groups to experimental drought , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9868, https://doi.org/10.5194/egusphere-egu2020-9868, 2020.

Online (or: in situ) methods for measuring soil and plant water isotopes have been identified as an innovative and crucial step to address recently identified issues in studying water uptake using stable isotope techniques.

During a controlled three month drought and rewetting experiment at the Biosphere 2 (B2) enclosed rainforest, a recently developed online method for measuring stem water isotopes (Marshall et al., 2019), namely ‘stem borehole equilibration’, was combined with online monitoring of soil water isotopes and transpired water isotopes as well as sap flow and stem water storage. This enabled us to study root water uptake depths of different tree species and dynamic changes during the dry down and rewetting. After two months of drought, the system was supplied with isotopically labelled water (deuterated water) from down below via a pipe system spanning across the complete B2 rainforest in order to identify deep water uptake of the rainforest trees and hydraulic redistribution.

Results show that – as expected – all monitored trees responded to the drought by changing their root water uptake towards deeper soil depths while sap flow rates of most trees decreased. When rewetting the system, deep water uptake from the base of B2 (between 2.5m and 4m soil depth) was identified in all large, mature trees (Clitoria faichildiana, Hibiscus tilliaceus, Hura crepitans, Pachira aquatica). No deep water uptake was found in the smaller trees (mainly Pachira aquatica). Furthermore, stem water storage was notably different between species and affected their adaptation to drought and response to rewetting. The labelled water was also identified in the transpired water more than one month after re-starting rainfall at B2.  However, no hydraulic redistribution was identified.

The holistic approach for monitoring the interactions of soils and plants provides inevitable insights into the adaptation of (enclosed) rainforests under drought and might have implications for natural rainforests. In particular, the capability of large trees to develop deep roots and the role of stem water storage are important elements for adaptation to climatic changes and need to be studied further under ‘real’ conditions.

References

Marshall, J.D., Cuntz, M., Beyer, M., Dubbert, M., Kühnhammer, K., 2019. Borehole equilibration: testing a new method to monitor the isotopic composition of tree xylem water in situ. Front. Plant Sci.

How to cite: Kuehnhammer, K., van Haren, J., Kuebert, A., Dubbert, M., Ladd, N., Meredith, L., Werner, C., and Beyer, M.: Tracing dynamic water uptake and transport from root to canopy by online monitoring of water isotopes in an enclosed tropical forest in response to drought, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18324, https://doi.org/10.5194/egusphere-egu2020-18324, 2020.

As global average temperature continues to increase and precipitation events become less predictable, understanding the long-term effects of drought on ecosystems is of increasing importance. However, it is difficult to study phenomena such as drought due to their unpredictable nature and the fact that it is difficult to tag and track the movement of water and carbon through an entire ecosystem. Within the framework of the controlled ecosystem manipulation experiment (WALD- Water, Atmosphere and Life Dynamics) at Biosphere 2, a deliberate drought in the enclosed tropical rainforest biome presented a unique opportunity to study responses in carbon and water cycling due to water stress. Within the scope of this study, the goal of this project was to examine the effect of prolonged water stress on different species within the rainforest and understand how the plants coped with the stress on an ecosystem level. This was accomplished by weekly plant water potential measurements (WP) before, during, and after the drought, as well as leaf sampling for relative leaf water content (RWC) and xylem sampling for water isotope measurements. For both predawn and midday WP, we found significantly different species responses; for Ceiba pentandra and Pachira aquatica, WP did not decrease during the drought, while for Hibiscus tiliaceus and Hibiscus rosa sinensis, WP decreased dramatically during the drought. After the additional of moisture from deeper depths, both C. pentandra and Hura crepitans (largest trees) responded the fastest by increasing in WP, while H. tiliaceus and H. rosa sinensis had the slowest recovery in WP, and only after rewetting from above had occurred. RWC also revealed different responses by different plant species, with Phytolacca dioica and H. rosa sinensis showing the highest RWC values throughout the experiment. The relationship between RWC and WP was also not consistent among species, with half of the species exhibiting a positive relationship, while the other half exhibiting a negative relationship. Other factors such as trunk capacitance and or leaf shedding during the drought might explain some of these contrasting relationships. Establishing such associations could lead to the development of tools that remotely assess average leaf water content of an area of forest via spectral reflectance and use those data to approximate the water stress of plants in that area, a very valuable asset when dealing with such geographically extensive phenomena as drought. 

How to cite: Kerman, S., Bailey, K., van Haren, J., Kübert, A., Kühnhammer, K., Werner, C., Ladd, N., Meredith, L., and Hu, J.: Plant Water Relation and Drought: Relationship Between Plant Water Potential and Relative Leaf Water Content in Different Tropical Plants, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12797, https://doi.org/10.5194/egusphere-egu2020-12797, 2020.

Wood cellulose records environmental conditions via its isotopic composition, which can be used to reconstruct different environmental events or patterns. However, it has been suggested that there can be a decoupling of the δ¹⁸O of cellulose and environmental conditions due to a lag from post-carboxylation processes. Thus, studying the dynamics of intra-seasonal tree growth provides a unique way to examine how the δ¹⁸O of cellulose responds to environmental and ecophysiological processes. There are two main factors that contribute to the δ¹⁸O signature of cellulose: the isotopic content of the source water and the leaf evaporative enrichment effect, both of which can vary under natural settings. Thus, separating the source water signal from the atmospheric humidity signal in the δ¹⁸O of cellulose can be difficult. In this study, we took advantage of a highly controlled ecosystem scale study at the University of Arizona Biosphere 2 tropical forest biome, where a drought treatment was implemented with a deep re-wetting component followed by a shallow re-wetting component. Continuous measurements of δ¹⁸O of atmospheric water vapor, soil water and xylem water as well as targeted gas exchange measurements of stomatal conductance and transpiration were made throughout the study. We also collected the δ¹⁸O of phloem sugars and cellulose to address how well the Roden et. al. (2001) cellulose model estimated observed δ¹⁸O values. One main objective was to examine how the fraction of carbonyl oxygen atoms that exchange at the cambium during cellulose biosynthesis, or P_ex, is altered, since recent studies suggest that P_ex can very among species, across aridity gradients, and throughout the growing season. Thus, this highly instrumented experiment allows us to look at variations in P_ex at a high temporal scale. By examining potential shifts in P_ex throughout the formation of a tree ring, we can increase the robustness of reconstructions by targeting specific woody anatomy to capitalize on the different signals of source water and the evaporative effect laid down in wood cellulose.

How to cite: Bailey, K., Hu, J., Warner, C., Ladd, N., Meredith, L., van Haren, J., Beyer, M., Lehman, M., and Prohaska, N.: How does drought affect the δ18O cellulose record? A Biosphere 2 experiment., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11282, https://doi.org/10.5194/egusphere-egu2020-11282, 2020.

A promising tracer for partitioning the global balance of CO₂ is carbonyl sulfide (COS or OCS), a trace gas with leaf-level mechanisms shared with carbon dioxide (CO₂) and water (H₂O). COS is therefore used to derive insights into photosynthesis and transpiration at ecosystem to global scales. However, it remains unclear whether COS reflects photosynthesis or stomatal conductance most strongly, as its leaf biochemical and physical processes are not perfectly analogous to either CO₂ or H₂O. There is therefore a need to evaluate the models that encapsulate our current understanding of leaf and soil COS fluxes and predictions of carbon and water cycling against independent constraints in tractable experimental systems.

In this study, we measured ecosystem, leaf, and soil fluxes of COS in the model Biosphere 2 (B2) Tropical Rainforest across a controlled whole ecosystem drought manipulation. We simultaneously, measured the stable isotopes of CO₂, H₂O, and their isotopes (¹³C-CO₂, ¹⁸O-CO₂, ²H-H₂O, ¹⁸O-H₂O) on atmosphere, leaf, and soil measurement streams connected to atmospheric towers, leaf chambers, and soil flux chambers. During the B2 Water, Atmosphere, and Life Dynamics (B2 WALD) campaign, the enclosed ecosystem received no rain for 66 days and was first rewet at depth (2-4 m) at 54 days. Here, we compare COS fluxes to simultaneous and independent measurements of GPP and transpiration from the leaf to ecosystem scales across ecosystem control, drought, and recovery. We further integrate COS measurements with the aforementioned isotopic tracers of carbon and water cycling into the Simple Biosphere Model (SiB3). Our goal is to explore the strengths and limitations of COS as a tracer of ecosystem processes dynamically responding to severe and controlled ecosystem drought.

How to cite: Meredith, L., Commane, R., Baker, I., Gil-Loaiza, J., van Haren, J., Ladd, N., and Werner, C.: Carbonyl sulfide reflections of leaf and ecosystem processes in a tropical rainforest under controlled drought, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12392, https://doi.org/10.5194/egusphere-egu2020-12392, 2020.

Forest cover exerts a significant control on the partitioning of precipitation between evapotranspiration and surface runoff. Thus, understanding how plants take up and transpire water in forested catchments is essential to predict flooding potential and hydrologic cycling. A growing literature underscores the importance of integrating whole-plant hydraulics, including such processes as the spatial variability of root distribution and the temporally dynamic nature of root water uptake by depth in understanding the relationship between changes in vegetation and hydrology. The analysis of stable isotopes of water (¹⁸O and ²H) sourced from soils and plant tissue has enabled the estimation of tree root water uptake depths and water use strategies. Despite the general acceptance of stable water isotopic data to estimate plant hydraulic dynamics, this methodology imposes assumptions that may produce spurious results. For example, end member mixing analysis neglects time-delays during tree-water storage. Also, it is likely that hydraulic redistribution processes of plants, which transport water across soil depths and both into and out of plant tissue, modify δ¹⁸O and δ²H; the isotopic signature of a collected sample may thus reflect a history of transport and exposure to fractionating processes not accounted for in analysis. We tested the feasibility of C-dots, core-shell silica polyethylene-glycol coated fluorescent nano-particles (5.1 nm diameter) in 20 µmol/l solution with H₂O labeled with a near-infrared fluorophore, cyanine 5.5 (excitation maximum of 646 nm, emission maximum of 662 nm), as an alternative to stable water isotopes in the investigation of plant hydraulics. We examined the absorption and transport of C-dots through soil, as well as roots and aerial structures of Eastern hemlock (Tsuga canadensis), Eastern white pine (Pinus strobus), and white spruce (Picea glauca) saplings (n = 12 each) via an IVIS-200 luminescence in-situ imaging system. We compared the fluid mechanics, residence times and mixing schemes of C-dots with ²H-labeled water during transport within these plant species to establish the nanoparticles as a viable alternative through a split-root hydraulic redistribution experiment under moderate and severe drought conditions. We present a residence-time distribution to elucidate the mixing scheme of C-dot solution and calibration curves to aid future studies. This research is the premier assessment of this nanoparticle as an alternative tracer to stable water isotopes, and as such may yield insights for broader applications.

How to cite: Singh, K., Hafner, B., Knighton, J., Walter, M. T., and Bauerle, T.: C-dots as a novel silica-based fluorescent nanoparticle tracer to investigate plant hydraulics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12504, https://doi.org/10.5194/egusphere-egu2020-12504, 2020.

Hydrogen (H) stable isotope analysis of specific plant organic compounds has become of interest as a tool for ecological, environmental and palaeoclimatological studies. Aside from the influence of leaf water evaporative enrichment on the δ²H composition of organic compounds, hydrogen isotope fractionation occurs during carbon metabolism in the plant (ε_bio). To get a better understanding of the metabolic signal recorded in ε_bio, we explored the variation of δ²H in cellulose and n-alkanes, and its relationship with phylogeny and other plant traits. Leaf material of a large set of species in the eudicot clade was collected in the botanical garden at the University of Basel, cellulose and n-alkanes were extracted, δ²H in both compounds and δ¹⁸O in cellulose were analysed. It was found that modelled leaf water differences only explain part of the observed variation of δ²H in organic compounds. δ²H appears to be related to phylogeny and a wider assessment of trait data is currently being undertaken to test for signal associations with physiological traits. This study helps address at which taxonomic level the variation of δ²H is found; illuminate plant physiological traits that can be responsible for shaping species specific δ²H values in organic compounds; as well as, provide novel insights into the δ²H covariation between cellulose and n-alkanes.

How to cite: Baan, J., Holloway-Phillips, M., and Kahmen, A.: Variation in hydrogen stable isotopes in cellulose and n-alkanes: phylogenetic signal and related traits, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16450, https://doi.org/10.5194/egusphere-egu2020-16450, 2020.