Displays

The resistance of trees to stress events has been studied intensively, however we know little on underlying processes affecting the recovery of trees following stress release. Hence, this clearly impairs our ability to project the resilience of trees and forests to an intensification of heatwaves and drought spells.

Here we studied the legacy effects of heat and heat-drought stress on carbon (C) allocation dynamics in Scots pine. We were particularly interested in how C allocation changes post heat and heat-drought stress and how this change in allocation affects tree growth. We exposed Pinus sylvestris seedlings to increasing temperatures from 30 to 40°C within 18 days either under well-watered or drought conditions and measured stem growth, leaf water potential and above- and belowground gas exchange. Two days after stress release, we conducted a ¹³CO₂ pulse-labelling experiment in custom build single tree cuvettes (n=18) allowing us to continuously monitor ¹³CO₂ shoot and root gas exchange. We then chased the fate of the newly assimilated C from leaves to roots via soluble sugars, starch and cellulose.

Our results showed that Pinus sylvestris is able to recover gas exchange following heat release immediately in the well-watered trees, while drought-treated trees recovered slightly slower. We found indications for a stress compensatory response of the previously heat-treated trees, which tended to translocate recent assimilates faster compared to the control trees as identified in the dynamics of water-soluble carbon in the phloem and root ¹³CO₂ efflux. In addition, we found larger stem growth rates in the heat-treated trees which was also reflected by a larger investment of new assimilates to cellulose. In the trees that experienced both, heat and drought stress, C allocation differed strongly from the control trees as apparent in a half as fast C translocation from leaves to root respiration and large investments of new assimilates into starch. This delayed translocation but enhanced allocation towards C storage in needle tissues was reflected in a delayed recovery of stem growth and very low detection of the ¹³C signal in twig, root and stem cellulose. We can conclude that heatwaves of 40°C have relatively moderate responses on C allocation post-stress, whereas hot drought stress clearly affects C allocation as indicated by a delayed C transport capacity and a preferential allocation towards C storage in needle tissues. This could indicate that C allocation following hot drought stress is affected by an impaired phloem functionality, which only slowly recovers post-stress.

How to cite: Rehschuh, R., Gast, A., Jakab, A.-L., Lehmann, M., Saurer, M., Gessler, A., and Ruehr, N.: Unrevealing tree carbon allocation beyond the stress – a case study of heat and drought impacts on Pinus sylvestris , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10751, https://doi.org/10.5194/egusphere-egu2020-10751, 2020.

Rubber trees (Hevea brasiliensis) are the main source of natural rubber, extracted from latex, which exudes from the trunk after tapping. Tapped trees require large amounts of carbon (C) to regenerate the latex after its collection. Knowing the contribution of C sources involved in latex biosynthesis will help understand how rubber trees face this additional C demand. Whole crown ¹³CO₂ pulse labelling was performed on 4-year-old rubber trees in June when latex production was low and in October, when it was high. ¹³C contents were quantified in the foliage, phloem sap, wood and latex. In both labelling periods, ¹³C was recovered in latex just after labelling, indicating that part of the carbohydrates was directly allocated to latex. However, significant ¹³C amounts were still recovered in latex after 100 days and the peak was reached significantly later than in phloem sap, demonstrating the contribution of a reserve pool as a source of latex C. The contribution of new photosynthates to latex regeneration was faster and higher when latex metabolism was well established, in October than in June. An improved understanding of C dynamics and source-sink relationship in rubber tree is crucial to adapt tapping system practices and ensure sustainable latex production.

How to cite: Desalme, D., Duangngam, O., Thaler, P., Kasemsap, P., Sathornkich, J., Satakhun, D., Chayawat, C., Angeli, N., Chantuma, P., and Epron, D.: Contribution of the carbon sources involved in latex regeneration in rubber trees (Hevea brasiliensis): an in situ 13CO2 labelling experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19100, https://doi.org/10.5194/egusphere-egu2020-19100, 2020.

In trees, the use of non-structural carbon (NSC) under limiting conditions impacts the age structure of the NSC pools. We compared model predictions of NSC ages and transit times for Pinus halepensis Mill., Acer rubrum L. and Pinus taeda L., to understand differences in carbon storage dynamics in species with different leaf phenology and growth environments. We used two carbon allocation models from the literature to estimate the NSC age and transit time distributions, to simulate carbon limitation, and to evaluate the sensitivity of the mean ages to changes in allocation fluxes. Differences in allocation resulted in different NSC age and transit time distributions. The simulated starvation flattened the NSC age distribution and increased the mean NSC transit time, which can be used to estimate the age of the NSC used, the NSC remaining in the system,  and the time it would take to consume the reserves. Mean NSC ages and transit times were sensitive to carbon fluxes in roots and allocation of carbon from wood storage. Our results demonstrate how trees with different storage traits are expected to react differently to starvation. They also provide a probabilistic explanation for the “last-in, first-out” pattern of NSC mobilization from well-mixed carbon pools. In addition, they unveil determinant sink fluxes in NSC dynamics for mature trees. These findings open the possibility to better understand NSC dynamics in mature trees based on estimated NSC ages and transit times in different tree organs of species with contrasting life strategies and growth environments.    

How to cite: Herrera, D., Muhr, J., Hartman, H., Römermann, C., Trumbore, S., and Sierra, C.: Probability distributions of non-structural carbon ages and transit times provide insights in carbon allocation dynamics of mature trees , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7391, https://doi.org/10.5194/egusphere-egu2020-7391, 2020.

Large amounts of C are allocated to tree roots, but little is known about the age and dynamics of their non-structural C (NSC). We measured bomb-radiocarbon (¹⁴C) in respired CO₂, non-structural (mainly sugars), and structural (cellulose) C in roots. The steady decline of Δ¹⁴C in atmospheric CO₂ since the 1960s indicates the mean time elapsed since the C in these pools was fixed. We measured coarse (>2 mm, mean 2.91 mm) and fine (<2 mm) roots from 12 German poplar trees sampled before and after girdling of 6 of the trees. All samples were taken in 2018, an exceptionally dry summer in Europe. The mean Δ¹⁴C ±SD of root-respired CO₂ (4.1 ± 3.6 ‰) in June-July was equal to current atmospheric Δ¹⁴CO₂ (1.2 ‰), irrespective of the mean age of root cellulose. During extended incubations, the Δ¹⁴C of root-respired CO₂ increased to ~10 ‰ 8 days after harvesting and up to 42 ‰ 17 days after harvesting. The mean Δ¹⁴C of soluble sugars in the roots was ~21 ‰. In September-October, almost three months after girdling, roots from girdled trees respired CO₂ with Δ¹⁴C of 7.9 ± 6.6 ‰ vs. 2.3 ± 6.1 ‰ in the ungirdled control trees. However, in both groups the respired CO_2­-Δ¹⁴C correlated with cellulose-Δ¹⁴C (R² = 0.37, 0.26 for girdled and control trees, respectively), suggesting that roots respired more stored C in the later growing season in this drought year, independent of treatment. The Δ¹⁴C values of soluble sugars were correlated with the Δ¹⁴C values of the cellulose (R²=0.83). On average, C in sugars was fixed more recently than cellulose, suggesting mixing of young C from other parts of the tree into the roots. Stem girdling did not affect the Δ¹⁴C of soluble sugars. Average total sugar concentrations (sucrose+ glucose+ fructose) were ~42 mg g^-1 and did not vary with sampling date, root class or treatment. Starch, measured only in September-October, was higher in coarse than in fine roots (12 vs. 3.8 mg g^-1). Respiratory loss of C was higher in the fine roots (~4 mgC g^-1 day^-1) than coarse roots (~2.4 mgC g^-1 day^-1), with no effect of girdling or sampling month. When normalize (expressed per gram dry root material), the NSC reservoirs and C loss rates suggest C turnover rates are 2-fold higher in fine roots than in coarse roots. The extended incubations indicate that detached roots are able to quickly utilize stored NSC, as indicated by the sharp Δ¹⁴CO₂ increase. In comparison, stem girdling had no measurable effect on respired CO₂-Δ¹⁴C, suggesting internal re-allocation of C from the lower stem base or large roots to smaller roots, and/or lower than expected metabolic consumption of C in reaction to girdling or because of the exceptional drought.

How to cite: Hilman, B., Muhr, J., Helm, J., Kuhlmann, I., and Trumbore, S.: Respiration and C dynamics in Poplar roots, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13469, https://doi.org/10.5194/egusphere-egu2020-13469, 2020.

Mycorrhizal fungi are an important partner of almost all land plants, who trade soil nutrients, such as Phosphorus or Nitrogen, for photosynthetic Carbon (C). Moreover, mycorrhizal fungi connect multiple plants with their mycelium in so called Common Mycorrhizal Networks (CMNs). CMNs formed by ectomycorrhizal (EM) fungi are an inherent part of boreal and temperate forests, often termed the ‘wood-wide web’. However, the role of these networks for plant belowground C allocation and distribution is not well known.

Here, we examined how plant photosynthates are distributed within EM mycelium networks connecting pairs of young beech trees, addressing the following questions: (1) Is the total belowground C allocation of plant photosynthates influenced by the size of the mycorrhizal network and its access to resources? (2) Is the belowground C distribution within a CMN altered if trees have unequal access to C from photosynthesis? (3) Do CMNs amplify or alleviate competition for nutrients between connected trees?

We planted young beech trees in pots in a special two-plant box set-up which allows to control the establishment of mycorrhizal networks between them. For this, two plant pots, penetrable by fungal hyphae but not by roots, were placed inside of plastic boxes and the interstitial space was filled with quartz sand. In addition, a hyphal-exclusive N source consisting of ¹⁵N labeled peat (‘peat bag’), was buried within each plant pot. Two treatments were applied in a fully factorial design: 1) Allowing/preventing the establishment of a CMN between the pots (some pots were turned around at a regular interval to prevent the establishment of CMNs) and 2) inequality of access to photoassimilated C (in part of the boxes one of the two plants was shaded). In a ¹³C-CO₂ labeling approach, we traced ¹³C assimilated by one plant of each tree pair into belowground pools of both plants by isotope ratio mass spectrometry (EA-IRMS) and ¹³C phospholipid fatty acid (PLFAs) analysis (GC-IRMS). At the same time, we investigated plant uptake of ¹⁵N via mycorrhiza by EA-IRMS.

Our results demonstrate that plants relied mostly on their fungal partners to acquire nutrients (63% of plant N was derived from mycorrhiza-exclusive peat bags), and also directed the majority of the C allocated belowground to their mycorrhizal partners. The presence of a larger mycorrhizal network connecting to another plant and an additional N source almost doubled photosynthetic CO₂ assimilation and belowground C allocation by plants. Fungi translocated carbon via hyphal linkages preferentially into mycorrhiza-exclusive nutrient patches, even when they were located within the realm of a neighboring plant and this necessitates to cross a nutrient-poor zone of sand. Shading did not affect the belowground distribution of C.

We conclude that belowground ectomycorrhizal networks represent a significant sink strength for plant photosynthates and may thus be a major driver of C sequestration in beech forest soils. The belowground distribution of C via fungal networks is mainly related to the distribution of nutrient-rich patches in the soil and less to differences in the photosynthetic capacity of the host plants.

How to cite: Imai, B., Gorka, S., Wiesenbauer, J., Mayerhofer, W., and Kaiser, C.: Common mycorrhizal networks of European Beech trees drive belowground allocation and distribution of plant-derived C in soil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21669, https://doi.org/10.5194/egusphere-egu2020-21669, 2020.

The mycorrhizal tragedy of the commons

It is increasingly recognized that plant C allocation to mycorrhizal symbionts plays a critical role for plant nutrition and the future global CO₂ fertilization effect on plants (Terrer et al., 2019). At the same time its future impacts are hard to predict because we do not fully understand the mechanisms underlying the symbiosis. The traditional view of mycorrhizal symbiosis always helping plants has been challenged by observations of negative effects, e.g. on tree N uptake (Näsholm et al., 2013), which makes it difficult to understand why the symbiosis has evolved and why it is so widespread.

We propose, and tested, a theory explaining the contrasting findings by showing that mycorrhizal symbiosis can be both mutualistic and parasitic at the same time. Plants and fungi are connected in a mycorrhizal network where each fungus has multiple plant partners and vice versa. Each plant can gain additional N at the expense of the other plants by supplying more C to the fungi, i.e. paying a higher C price for N. At the same time the additional C supply increases N immobilization in fungal biomass, which reduces the total N export to all plants. Thus, an individual plant can gain N at the expense of its neighbors while the negative side effects are shared among all, resulting in a tragedy of the commons effect that reduces plant N uptake and drives N immobilization in the soil.

While some observations support this hypothesis, it had not yet been thoroughly tested experimentally – until now. Based on laboratory and field experiments in boreal pine forest we tested both key components of this hypothesis - individual level mutualism and the community parasitism (decline in plant N uptake). We also estimated the strength of the fungal discrimination among its plant partners, which drives the competitive C for N trading. Finally, we highlight potential consequences of these mechanisms for boreal forest C allocation and responses to rising CO₂.

References

Näsholm, T. et al., 2013. Are ectomycorrhizal fungi alleviating or aggravating nitrogen limitation of tree growth in boreal forests? New Phytologist, 198(1): 214-221.

Terrer, C. et al., 2019. Nitrogen and phosphorus constrain the CO2 fertilization of global plant biomass. Nature Climate Change, 9(9): 684-689.

How to cite: Franklin, O., Näsholm, T., and Henriksson, N.: The mycorrhizal tragedy of the commons, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12925, https://doi.org/10.5194/egusphere-egu2020-12925, 2020.

A large terrestrial carbon sink significantly influences the rate of change in atmospheric CO₂ concentrations, but uncertainties associated with its estimate are considerable. Here we combined carbon stock (CS) and eddy covariance (EC) flux measurements that were collected over a period of 15 years (2001-2016) in a 55-year-old 30 km² pine forest growing at the semi-arid timberline (with no irrigating or fertilization). The objective was to constrain estimates of the carbon (C) storage potential in forest plantations in such semi-arid lands, which cover ~18 % of the global land area. Annual integrated carbon accumulation was 145-160 g C m^-2 y^-1 over the study period based on the EC and CS approaches, with a mean value of 152.5 ± 30.1 g C m^-2 y^-1 indicating 20 % uncertainty in carbon uptake estimates. This carbon uptake reflect high carbon use efficiency NEP/GPP of 29 compared to ~21 in temperate forests, leading to the current ecosystem stocks of 7943 ± 323 g carbon m^-2 and 372 g nitrogen m^-2. In addition, carbon is mostly stored in the soil (~71 % of the current ecosystem C stock), with a long C turnover time of 59 ± 4 y (compared to mean value of 18 years in temperate forests). It is also estimated that soil carbon at the study site constitutes only ~25 % of the estimated soil saturation capacity. Irrespective of un-expected disturbances beyond those observed at the study site, the results support considerable C sink potential in semi-arid soils and forest plantations, and imply that afforestation of even 10 % of semi-arid land area under conditions similar to that of the study site, could sequester ~0.4 Pg C y^-1 over several decades.

How to cite: Qubaja, R., Grünzweig, J., Rotenberg, E., and Yakir, D.: Long turnover time and large sequestration potentials in a dry pine forest based on 15-year flux and inventory records, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21153, https://doi.org/10.5194/egusphere-egu2020-21153, 2020.

Carbon (C) assimilated in the canopy of trees is transported downwards via phloem to fuel metabolic processes, such as respiration of above- and below-ground tissues.  Part of the respired CO₂ can be dissolved into xylem water and transported along the trunk up to the canopy, causing a CO₂ efflux which is dislocated from the site of respiration. While the individual processes of C transport in the phloem and xylem in trees have been comparatively well described, little is known on the linkage of xylem–phloem pathways of C and the potential of re-assimilation of root-respired C in the canopy.

In this study, we randomly assigned a set of five-year-old oak trees (Quercus rubra) to stem infusion of ¹³CO₂ dissolved water (n=4) or ¹³CO₂ canopy labeling treatment (n=3), thereby labeling xylem C flow and phloem C flow, respectively. Using high temporal resolution isotope ratio measurement by laser spectroscopy, we monitored ¹³C in tissue and in respiratory CO₂ efflux resulting from phloem- and xylem-transported C to trace the fate of C from photosynthesis through the phloem and xylem to respiration and re-assimilation. We observed that CO₂ efflux was related to both phloem and xylem transport of ¹³C and that a quick lateral transport of sugars occurred from phloem to xylem. Furthermore, we found evidence for re-assimilation of CO₂ transported through the xylem in branches and leaf petioles. The re-assimilated ¹³C transported by the xylem was also found in stem tissues at various heights, 24 to 96 hours after labeling. Moreover, stem ¹³CO₂ efflux showed a diurnal variation, suggesting a potential incorporation of recycled C into respiratory substrate at different stem heights shortly upon re-assimilation. Our results demonstrate a phloem–xylem C linkage leading to repeated coupling of assimilation and respiration, with consequences for whole tree C dynamics.

How to cite: Yoshikawa, A. L., Bloemen, J., Ingrisch, J., Hartmann, H., and Bahn, M.: High temporal resolution 13C tracing to link xylem – phloem pathways of carbon in oak trees, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1070, https://doi.org/10.5194/egusphere-egu2020-1070, 2020.

It is predicted that climate change will cause an increase in frequency and duration of dry spells in Central Africa. This will lower yields of cassava (Manihot esculenta Crantz), a starchy root crop consumed daily by almost 800 million people in the tropics. Potassium has been considered as an important plant nutrient in mitigating the impact of drought stress because of its critical role in stomatal regulation, as an osmolyte, as well as in starch synthesis and assimilate translocation. This study aims to quantify the effects of potassium fertilizer on water use efficiency and translocation speed of new assimilates in water-stressed cassava plants at early bulking stage.

Cassava cuttings (Bailo variety), originating from the Eastern Democratic Republic of Congo, were grown in pots filled with 5 kg of calcium carbonate free sand substrate and fertilized with a complete nutrient solution either high (+K; 1.437 mM K⁺) or low (-K; 0.359 mM K⁺) in potassium. All pots were weighed every other to each day to monitor water use and were watered to field capacity. A drought treatment was imposed on half of the plants two months after planting by reducing irrigation amounts by half. Plants were put in an airtight walk-in growth chamber enriched with ¹³C-CO₂ (for 8 h) to trace the translocation of new assimilates. One, nine and twenty-four days after labelling, plants were harvested and δ¹³C values for different plant organs were analysed.

Plant water use was higher in plants under low potassium nutrition (-K) in the period prior to imposition of drought. Data on biomass production and δ¹³C and δ¹⁸O values of these plants will further help understand whether the observed difference in water use also leads to a difference in water use efficiency. Further, a ¹³C mass balance will be composed. These data, to be presented at EGU 2020, will provide information on the translocation speed of new assimilates from shoot to root and confirm whether potassium positively affects this process under dry conditions.

How to cite: Van Laere, J., Willemen, A., Ding, Y., Said, H., Resch, C., Hood-Nowotny, R., Merckx, R., and Dercon, G.: Potassium application to alleviate drought stress in cassava production: A growth chamber based carbon-13 pulse labelling experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4839, https://doi.org/10.5194/egusphere-egu2020-4839, 2020.

Carbon allocation is a crucial process in plants and ecosystems. However, does carbon allocation also impact plant phenology, in particular during autumn? Elucidation of autumn tree phenology is essential as autumn leaf senescence affects leaf nutrient resorption, next year tree growth potential and the seasonal exchange of energy and material (e.g. CO₂, H₂O, VOCs) between vegetation canopy and the atmosphere. Environmental manipulative experiments clearly show that the onset of leaf senescence in deciduous trees is not only controlled by internal cues but also that is crucially affected by environmental factors. Yet, we have not understood which are the environmental drivers of leaf senescence onset as, for example, day length, temperature, water and nutrient availability, all showed to impact autumn phenology in a given species, sites, years and experimental setting. The knowledge gap around the environmental drivers of leaf senescence might be due to the fact that, up to date, we have investigated autumn dynamics of leaves (carbon sources) but overlooked the autumn activity of branches, stem and roots (carbon sink). Are leaves in autumn needed for the tree if no sinks are active? In other words, is leaf senescence triggered by the cessation of tree growth in autumn? In more detailed, we expected that: (i) in the absence of growth-limiting environmental conditions, tree growth cessation directly controls leaf-senescence onset, and (ii) in the presence of growth-limiting conditions, photoperiod controls leaf-senescence onset – this prevents trees from starting to senesce too early. These hypotheses have been the topic of three years of monitoring and experimental campaigns in different deciduous species and European locations within the ERC project LEAF-FALL, and of ancillary data analysis work. The presentation aims to show key results and the most novel aspect of this line of research.

How to cite: Campioli, M., Mariën, B., and Dox, I.: Does carbon allocation determine the timing of autumn leaf senescence in deciduous trees? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1311, https://doi.org/10.5194/egusphere-egu2020-1311, 2020.

Carbon allocation to wood formation is the key process that drives biomass accumulation in forest ecosystems. Particularly important from a carbon balance perspective is the fraction of carbon taken up through photosynthesis (i.e. gross primary productivity) that is allocated to and sequestered in long-lasting wood tissues. This fraction is known as “biomass production efficiency” and a comprehensive understanding of its inter- and intra-annual variability in response to climatic fluctuations and ecosystem dynamics is still lacking. In this study, we assessed and reconstructed the above-ground biomass increment of three deciduous tree species, European beech (Fagus sylvatica), Sessile oak (Quercus petraea) and European hornbeam (Carpinus betulus) in Hainich National Park, Thuringia (Germany). Trees were sampled in a fixed plot design within the footprint area of a long-term eddy-covariance site (DE-Hai). We applied allometric equations to estimate tree volume and combined them with tree-ring width and wood density measurement to quantify and reconstruct the carbon stored as above-ground biomass in wood tissues. We scaled these measurements from the tree to the plot level and integrated the annual biomass increment with the carbon fluxes from the tower to quantify biomass production efficiency. Finally, we correlated species-specific growth with carbon fluxes and various climate parameters at daily, monthly, seasonal, and annual resolution to better understand, how climate variations affect carbon allocation to wood growth at this site. Our study represents a well-constrained observational framework to provide both quantitative and qualitative information on forest carbon cycling that can be used, e.g., to better parameterize tree-centered mechanistic vegetation models.

How to cite: Wei, J., Karamihalaki, M., von Arx, G., and Babst, F.: Carbon allocation to wood formation in an unmanaged deciduous forest in Thuringia (Germany)-a case study of biomass production efficiency, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1086, https://doi.org/10.5194/egusphere-egu2020-1086, 2020.

Among living tree species, Acacia raddiana (Savi) and Acacia tortilis (Forssk), species of the legume family, populate some of the hottest and driest places on earth. Our research investigates the physiological processes underlying the unique survival of these trees in their extreme environmental conditions. We measured Acacia trees in their natural habitat once a month for two years to unravel the photosynthesis dynamics and water relations. Leaf gas exchange and leaf water potential were measured, as well as atmospheric and soil parameters. Daily and annual gas-exchange curves showed higher carbon assimilation during noon and in summer, when temperature and radiation were maximal (44°C, 2000 µmol m^-2 s^-1), and the air was dry (21% RH). Additionally, we found that the maximum rate of carbon assimilation was at PAR (photosynthetic active radiation) of 3000 µmol m^-2 s^-1. Our results suggest that water did not drive net carbon assimilation but rather light and temperature, which are already close to their maximum in our hyper-arid ecosystem.

How to cite: Uni, D., Sheffer, E., Winters, G., and Klein, T.: Photosynthesis in a desert tree is driven by the highest light and temperature, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3868, https://doi.org/10.5194/egusphere-egu2020-3868, 2020.

Terrestrial Gross Primary Production (GPP), the total amount of carbon taken up by terrestrial plants, is one of the largest fluxes in the global carbon cycle – and a key process governing the capacity of terrestrial ecosystems to partly offset continuing anthropogenic CO₂ emissions. Accurate simulation of land carbon uptake and its response to environmental change is therefore essential for reliable future projections of the terrestrial carbon sink. However, there are still large uncertainties in the sensitivity of global GPP to environmental drivers. Here we use a recently developed and extensively tested generic model of GPP (the ‘P-model’), which uses satellite-derived green vegetation cover as an input, to simulate (a) trends in site-level GPP, as observed at eddy-covariance flux sites; (b) trends in global GPP, for comparison with independent geophysical estimates; and (c) quantitative spatial patterns of the sensitivity of grid-based GPP to green vegetation cover, vapour pressure deficit, temperature, solar radiation, soil moisture and atmospheric CO_2.

How to cite: Cai, W. and Prentice, I. C.: Sensitivity of global gross primary production to environmental drivers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5947, https://doi.org/10.5194/egusphere-egu2020-5947, 2020.

Hundreds of thousands of hectares have been limed in German forests in the last three decades to mitigate the effects of soil acidification. To understand the long-term impacts of liming on tree rooting behaviour and the implications for soil organic matter, we used a novel electrical approach to quantify rooting parameters of mature beech forests and compared it to the standard monolith excavation approach. At each of the three experiment sites located in northern Germany, we looked at rooting behaviour in limed plots (overall eight tons of lime per hectare applied in the 1980s and 1990s) in comparison to adjacent control plots. First, we used the standard monolith excavation approach to determine fine root biomass at the stand level. With an electrical approach called the “earth impedance method” (EIM) we subsequently estimated tree absorbing root surface area (ARSA; this is the contact area where roots take up nutrients and water). This experimental, non-destructive approach uses a low frequency alternating electric current that flows from the roots to the soil and vice versa, and the electrical impedance (resistivity) is recalculated to estimate ARSA for the sample tree. We measured the ARSA of six mature trees per plot (12 trees per site).

To summarize the results of the sampling approaches, (1) both root estimation approach measurements were positively correlated, validating the EIM; (2) the ARSA was positively correlated with tree size at each site, further substantiating the rapid and cost effective EIM; (3) however this method is vulnerable to variables that effect electrical conductivity, such as soil moisture and the thickness and makeup of the organic horizons.

Overall, no significant differences between limed and control plots were detected with either measurement approach, suggesting that despite improved soil pH conditions the tree root systems in limed plots remain relatively constant in size and capacity to take up nutrients and water.

How to cite: van Straaten, O., Čermák, J., Kulp, L., and Talkner, U.: Using a novel electrical measurement approach to measure the effects of liming on rooting parameters in German beech forests , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18385, https://doi.org/10.5194/egusphere-egu2020-18385, 2020.

Carbon storage in grassland ecosystems is realized mostly belowground. The changes in the management activities of grasslands also influence the below-ground carbon stocks. Soil carbon-dioxide efflux (Rs) takes a major part of the ecosystem’s carbon cycle. R_s includes the respiration of different components. Rs gives 60-80% of ecosystem respiration or 40-60% of gross primary production. It is known from the literature that respiration is affected by abiotic (temperature (Ts), soil water content (SWC)) and the biotic factors.

In our study we investigated the biotic one, namely the belowground carbon allocation on soil respiration. The study was performed in a semi-arid sandy grassland at Bugac (Kiskunság National Park, Hungary). The vegetation of the pasture was dominated by Festuca pseudovina, Carex stenophylla and Cynodon dactylon and the soil is a chernozem type soil with high organic carbon content.

The soil CO₂ effluxes were measured continuously by an automated soil respiration system consisted of 10 soil respiration chambers. The chambers measured 3 different experimental plots. Data was collected in every half-hour from each chamber for 6 days before the cutting event. After the cutting data was recorded from 1) non-cut, 2) half cut and 3) completely removed treatments also for 6 days. The study was repeated under laboratory conditions (constant temperature, illumination, humidity) on grass patches planted in pots. We observed that the respiration in half cut and completely removed treatments increased after they were cut off. The proportion of respiration after cutting in the completely removed treatment reduced to 85% compared to the control one. Our results highlight that the soil respiration is largely affected by belowground carbon allocation.

How to cite: Papp, M., Fóti, S., Pintér, K., Nagy, Z., and Balogh, J.: Effect of biomass cutting on soil CO2 efflux in a sandy grassland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22060, https://doi.org/10.5194/egusphere-egu2020-22060, 2020.

Rising atmospheric CO₂ concentrations may induce or aggravate nitrogen (N) limitation to plant growth. To overcome this limitation, plants may invest their newly assimilated carbon (C) into N acquiring strategies, such as root growth, root exudation or C allocation to mycorrhizal symbionts. These shifts in C allocation can increase the turnover of soil organic matter by stimulating microbial activity. As these processes are poorly quantified, their net effects on ecosystem C storage remain uncertain.

To gain a better quantitative understanding of these processes, we assessed the effect of elevated CO₂ on plant C and N allocation in a mesocosm experiment. For four months of one growing season, 64 saplings of Fagus sylvatica L. were grown in a natural beech forest topsoil. Plants were exposed to near ambient (390 ppm) or elevated (560 ppm, eCO₂) CO₂ concentrations at two levels of continuous ¹³CO₂ enrichment (δ¹³C +50 or +150‰). At the end of the experiment, we determined dry biomass, C and N concentrations and isotopic compositions for all leaves, buds, twigs, stems and fine and coarse roots for all plants. For all plants, C and N budgets and the amount of newly incorporated C were evaluated.

We found a positive effect of eCO₂ on tree growth, with the highest growth response in fine root biomass. In both CO₂ treatments, newly fixed C was preferentially allocated to roots compared to other plant compartments, but under eCO₂, we found a shift in C allocation patterns towards higher belowground C allocation. These results suggest enhanced plant investments into belowground resource acquisition. Decreased N concentrations in all plant organs of these trees under eCO₂ may indicate plant N limitation and suggest that the effect of increased belowground C allocation was insufficient to fulfil the plants N demand. Still, the observed increase in C allocation to microbial biomass in these soils may be a mechanism to enhance plant N nutrition. CO₂ concentrations also affected C allocation within the whole plant-soil-system: Under eCO₂, more C was stored in tree biomass and less C was stored in soils. Overall, there was no effect of CO₂ treatment on total mesocosm C. We will discuss these findings with regard to the N mining hypothesis.

How to cite: Eder, L. M., Weber, E., Rousk, J., Schrumpf, M., and Zaehle, S.: Elevated CO2 increases plant growth but reduces soil C storage under N limiting conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22452, https://doi.org/10.5194/egusphere-egu2020-22452, 2020.

Climate warming has been suggested to impact high latitude grasslands severely, causing considerable carbon (C) losses from soil. Warming can also stimulate nitrogen (N) turnover, but it is largely unclear whether and how altered N availability impacts soil C dynamics. Even less is known about the individual and interactive effects of warming and N availability on the fate of recently photosynthesized C in soil.  We hypothesized that warming would increase belowground C allocation, while enhanced N availability would decrease it, and that their interactive effects would be additive.

We studied a subarctic grassland located at a natural geothermal soil warming gradient close to Hveragerði, Iceland, which was established by an earthquake in 2008. We chose 14 plots along the gradient with soil warming temperatures ranging from 0 to 10°C above ambient, and fertilized a subset of plots with 50kg ha^-1 y^-1 of NH₄NO₃ twice a year prior to the study. We performed ¹³CO₂ canopy pulse labeling for an hour and tracked the ¹³C pulse through the plant-microbe-soil system and into soil respiration for ten days after labeling.

Our preliminary results show that at higher temperatures microbial activity increased, causing higher C turnover and a higher respiration of recently assimilated C from the soil. Warming significantly decreased microbial biomass, however, the recent C allocated from roots to microbes increased. This indicates a higher microbial C-limitation and a tighter root-microbe coupling under warming. Nitrogen addition increased the allocation of recent C to roots, microbial biomass, and soil respiration. The effects of N addition and warming were additive with no interaction. Our results indicate that the microbes in warmed soil may not be N limited, but could be C limited and depend more on the supply of recent C from plants. We conclude that in a future climate with warmer soils, more C may be allocated belowground, however, its residence time may decrease.

How to cite: Meeran, K., Verbrigghe, N., Fuchslueger, L., Ingrisch, J., Vicca, S., Soong, J., Müller, L., Sigurdsson, B. D., Janssens, I., and Bahn, M.: Effect of soil warming and N availability on the fate of recent carbon in subarctic grassland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18374, https://doi.org/10.5194/egusphere-egu2020-18374, 2020.