BG3.4

Current land surface models hold large uncertainties in the predictions of how key biosphere processes such as photosynthesis, respiration and transpiration will respond to the combined effects of rising atmospheric CO₂, nutrient enrichment and changes in water availability. Recent developments in optimality theory provide new approaches to explicitly predict coordinated changes in leaf photosynthetic traits, specifically stomatal conductance (g_s) and the maximum capacities of carboxylation (V_cmax) and electron transport (J_max) (e.g. Prentice et al., 2014; Smith et al., 2019; Harrison et al., 2021). These novel formulations show promising results when tested with meta-analyses and global data sets. However, support from manipulative experiments that include changes in CO₂-growth conditions remains scarce. Here we summarize the results from two manipulative experiments using walk-in growth chambers in which a variety of species were exposed to sub-ambient, ambient and elevated growth CO₂ in combination with either a Phosphorous (P) treatment or a drought treatment, and compare the experimental results with predictions from optimality theory. The P treatment exposed plants to either severe P limitation at an N:P ratio of 45:1 or severe Nitrogen (N) limitation at an N:P ratio of 1:1, with a similar supply rate of N. The drought treatment consisted of a continuous dry-down after an initial period of unstressed establishment and growth. Results of the combined CO₂-nutrient treatment showed significant effects of growth CO₂ and P supply on V_cmax and J_max, as well as the whole-plant biomass at the point of harvest. Interaction effects between growth CO₂ and P supply were observed for g_s, the light-saturated photosynthesis rate, leaf P content, and the N:P ratio of the leaf. Results of the combined CO₂-drought experiment showed that g_s, V_cmax and J_max decreased significantly under rising CO₂ treatments, whereas whole-plant biomass at the point of harvest increased significantly. When scaled with non-stressed conditions, g_s and light-saturated photosynthesis declined consistently across CO₂ treatments. These experimental results align with quantitative predictions of g_s, V_cmax and J_max based on optimality theory. However, additional formulations are required to predict whole-plants growth responses as well as changes in plant nutrient-stoichiometry.

References

Harrison SP, Cramer W, Franklin O, Prentice IC, Wang H, Brännström Å, de Boer H, Dieckmann U, Joshi J, Keenan TF, et al. 2021. Eco-evolutionary optimality as a means to improve vegetation and land-surface models. New Phytologist 231: 2125–2141.

Prentice IC, Dong N, Gleason SM, Maire V, Wright IJ. 2014. Balancing the costs of carbon gain and water transport: testing a new theoretical framework for plant functional ecology. Ecology Letters 17: 82–91.

Smith NG, Keenan TF, Prentice IC, Wang H, Wright IJ, Niinemets Ü, Crous KY, Domingues TF, Guerrieri R, Ishida FY, et al. 2019. Global photosynthetic capacity is optimized to the environment. Ecology Letters 22: 506–517.

How to cite: de Boer, H., van Dijk, J., van der Ploeg, M., and Wagner-Cremer, F.: Experimental support for optimization of photosynthetic biochemistry and leaf gas exchange in response to combinations of rising CO2, drought stress and phosphorous deficit., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10521, https://doi.org/10.5194/egusphere-egu22-10521, 2022.

Nutrient cycles are tightly linked to the carbon cycle in tropical forests, controlling its responses to environmental change such as elevated CO₂ concentrations (eCO₂). In tropical wet forests of the Amazon, plants tend to grow slower in low fertility soils, while their relative investments in nutrient acquisition are likely higher due to costly mechanisms of nutrient mobilization. In low fertility soils where weatherable minerals have been depleted, decomposition and mineralization of soil organic matter and plant litter by free-living microorganisms may represent the dominant nutrient source for plants. The availability of soil nutrients for plants is a key constraint on tropical forest growth under future climate change. However, soil microbial communities not only drive the mineralization, but require nutrients for growth and biomass production themselves, and therefore from a plant’s perspective, soil microbial communities may act as a source or sink of nutrients under eCO₂.

Here, we employ the process-based terrestrial biosphere model QUINCY (Thum et al., 2019), coupled to the microbial-explicit soil model JSM (Yu et al., 2020) to model shifts in coupled nutrient and carbon cycling rates at field sites with differing soil fertility in the Amazon forest sites, and to confront the modeled ecosystems with elevated CO₂. QUINCY-JSM reflects our current understanding of governing carbon and nutrient cycling feedbacks, allowing for dynamic plant carbon investment in growth and nutrient acquisition, and microbial-explicit growth, turnover, and nutrient cycling. We compiled a unique dataset of forest growth, soil and litter chemistry, as well as microbial growth and stoichiometry from a set of Amazon forest plots that cover a large gradient in soil fertility. Microbial stoichiometry and soil texture data are used to calibrate QUINCY-JSM, and data on forest aboveground growth, microbial growth, litter chemistry, and soil carbon and nutrients are used for model evaluation. We test the hypothesis that the contribution of microbial-driven nutrient mineralization to the nutrient supply of plants increases with lowering soil fertility and explore the soil microbial-induced nutrient feedback to eCO₂-induced carbon sequestration in wet lowland Amazon forest sites. We examine the consequences of model assumptions on the conditions in space and time under which soil microorganisms alleviate or enforce the plants’ nutrient limitation under eCO₂. Directly testable hypotheses for old-growth wet forests’ response to elevated CO₂ in ecosystem-scale experiments like AmazonFACE are formulated.

Thum, T., Caldararu, S., Engel, J., Kern, M., Pallandt, M., Schnur, R., et al. (2019). A new model of the coupled carbon, nitrogen, and phosphorus cycles in the terrestrial biosphere (QUINCY v1.0; revision 1996). Geoscientific Model Development, 12(11), 4781–4802. https://doi.org/10.5194/gmd-12-4781-2019

Yu, L., Ahrens, B., Wutzler, T., Schrumpf, M., & Zaehle, S. (2020). Jena Soil Model (JSM v1.0; Revision 1934): A microbial soil organic carbon model integrated with nitrogen and phosphorus processes. Geoscientific Model Development, 13(2), 783–803. https://doi.org/10.5194/gmd-13-783-2020

How to cite: Fleischer, K., Yu, L., Fuchslueger, L., Reichert, T., Caldararu, S., Quesada, B., and Zaehle, S.: Modeling soil-microbial nutrient cycling feedbacks to elevated CO2 concentrations in old-growth tropical forest sites, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11276, https://doi.org/10.5194/egusphere-egu22-11276, 2022.

The carbon fertilization effect under increasing atmospheric carbon dioxide (CO₂) may contribute to removing 30% of anthropogenic CO₂, with mature forests central to this. However, the ability of mature forests to continue to act as a long-term sink of carbon (C) is dependent on the availability of essential nutrients; nitrogen, and phosphorus. It has been suggested root exudates may increase under elevated CO₂ (eCO₂) as a mechanism to acquire these nutrients from soil, via priming of the soil microbial community to increase nutrient turnover, or abiotic release. However, this is yet to be tested in a mature forest. Furthermore, it is unknown if root exudate composition also changes in response to eCO₂, as has been observed for drought. Given the role of root exudates in nutrient acquisition, their response to elevated CO₂ in a mature temperate forest may be a key mechanism for nutrient acquisition, supporting their ability to act as a long-term sink of CO₂.

We used the unique Birmingham Institute of Forest Research (BIFoR) free air carbon enrichment experiment (FACE), where a mature temperate deciduous forest dominated by English Oaks (Q. robur) is fumigated with eCO₂ at +150 ppm above the ambient atmospheric CO₂ concentration during the growing season, since 2017. Root exudates were collected quarterly from summer 2020 to summer 2021 from in-situ fine (<2 mm) oak roots in the O horizon, accessed via root boxes, into a soil-free bead-filled static cuvette system over 24 hours. Root exudates were analyzed for total dissolved carbon and nitrogen content, and roots and exudates from Summer 2020 underwent metabolomic analysis to investigate changes in composition. Root exudation rates were normalized to root surface area.

Carbon exuded by fine roots was 40% higher under elevated CO₂ across the year, with a clear seasonal trend whereas nitrogen exudation rate did not significantly differ between elevated CO₂ and control plots with no seasonal trend. Enhancement of C exudation resulted in a trend of a relatively larger C:N ratio, indicating a compositional change under eCO₂, despite no differences in root C:N. Untargeted metabolomic analysis of root exudates collected in Summer 2020 confirmed significant changes in composition of root exudates. Compounds associated with the metabolism of amino acids, carbohydrates and cofactors and vitamins, and biosynthesis of secondary metabolites were upregulated under eCO₂, and this was also reflected in the metabolome of the roots.

The increased carbon exudation rates reflected higher photosynthetic rates observed in oaks leaves under eCO₂, and compositional changes indicated by lower nitrogen exudation rates, relative to carbon. Furthermore, compositional changes investigated via metabolomics revealed significant changes in the metabolome, pointing to potential eCO₂ cascading impacts on nutrient acquisition strategies of mature oaks. These must be accounted for to be able to fully account for nutrient constraints of C uptake by forests under future climates, including within CNP-coupled and ESM models. 

How to cite: Reay, M., Pastor, V., Kourmouli, A., Hamilton, L., Sayer, E., Hartley, I., and Ullah, S.: Root exudation rate increases, and composition changes in a mature temperate forest under elevated carbon dioxide, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5026, https://doi.org/10.5194/egusphere-egu22-5026, 2022.

This research investigates the cascading effects of elevated carbon dioxide (eCO₂) fumigation of a mature temperature forest, with a particular focus on the fluxes of greenhouse gases (GHG) nitrous oxide (N₂O), methane (CH₄) and carbon dioxide (CO₂). A field experiment was performed at the Birmingham Institute of Forest Research Free Air Carbon dioxide Enrichment facility (BIFoR FACE), where an oak dominated mixed mature woodland has been under eCO₂ since 2017. Fluxes were quantified in situ using the Licor 8100A – an infrared gas analyser measuring total soil respiration (R_s) as CO₂, and a Picarro greenhouse gas analyser (G2508), measuringN₂O and CH₄. Preliminary data from 2019 – 2021 have been analysed and are built on an earlier dataset from 2017-2018, and the role of soil temperature and soil moisture is considered. With more carbon allocation belowground, we expect an increase in microbial activity and consequently larger R_s. Overall, R_s was higher under eCO₂ in 2017-2018; however, in years 2019 to 2021, the absolute difference in respiration between eCO₂ and control plots gradually decreased and even switched in 2021, with a slight increase in R_s for control plots compared to eCO₂ plots. Moreover, annual fluxes of N₂O and CH₄ were detectable and in general we observed N₂O emission and CH₄ consumption. My presentation will discuss R_s and N₂O and CH₄ fluxes and highlight the role of eCO₂ as well as environmental and soil conditions that regulate the GHG fluxes, allowing us to compute the net global warming potential of forests under future climates.

How to cite: Douwes Dekker, N., Barba, J., Kourmouli, A., Mackenzie, R., Gauci, V., Hamilton, L., Pendall, E., Yamulki, S., and Ullah, S.: Soil – atmosphere exchange of greenhouse gases under future climates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10237, https://doi.org/10.5194/egusphere-egu22-10237, 2022.

The effects of climate change on ecosystem productivity and water fluxes have been studied in various types of experiments, but it is still largely unknown whether and how the experimental approach itself affects the results of such studies. We use data from high precision weighable lysimeter from two contrasting experimental approaches to determine and compare the responses of water fluxes and aboveground biomass to climate change in low mountain range permanent grasslands. The first approach is based on a controlled increase in atmospheric CO₂ concentration and surface temperature (type manipulative). Space-for-time substitution along a gradient of climate conditions was used in a second approach (type: observational). The Budyko framework was used here to determine if the soil ecosystem is energy or water limited.

Under energy-limited conditions and elevated temperature, actual evapotranspiration increased, while seepage, dew, and aboveground biomass decreased. Elevated CO₂ mitigated the effects on actual evapotranspiration. Under water-limited conditions, increased temperature decreased actual evapotranspiration, and aboveground biomass correlated negatively with increased drought.

Our results reveals that the responses of soil water fluxes and biomass production of both experimental approaches depend mainly on the status of ecosystems in terms of energy or water limitation. To better understand ecosystem responses to climate change and identify potential tipping points, climate change experiments must include sufficiently extreme boundary conditions so that responses to single and multiple forcing factors can be comprehensively studied. Manipulative and observational climate change experiments complement each other well in this regard, and thus the approaches should be combined in future research on climate change impacts on grasslands.

How to cite: Groh, J., Forstner, V., Vremec, M., Herndl, M., Vereecken, H., Gerke, H. H., Birk, S., and Pütz, T.: Response of water fluxes and biomass production to climate change in permanent grassland soil ecosystems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5950, https://doi.org/10.5194/egusphere-egu22-5950, 2022.

How to cite: Moustakis, Y., Fatichi, S., Onof, C., and Paschalis, A.: Assessing ecosystem responses to different drivers of climate change, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5399, https://doi.org/10.5194/egusphere-egu22-5399, 2022.

To prevent excessive global warming, we have faced a situation to reduce the net carbon dioxide (CO₂) emission. However, how the Earth’s terrestrial biosphere behaves under negative emission is highly uncertain. Here we show that there is a strong hysteresis in terrestrial carbon cycle in response to CO₂ ramp-up and -down forcing. Due to this strong hysteresis lag, terrestrial biosphere stores more carbon at the end of simulation than its initial state, lessening the burden on the net negative emission. This hysteresis is latitudinally-dependent, showing a longer timescale of reversibility in high-latitudes and particularly carbon in boreal forests can be stored for a long time. However, the hysteresis of the carbon cycle in the pan-Arctic region strongly depends on the presence of permafrost processes. That is, an unexpected irreversible carbon emission might occur in permafrost even after achieving net-zero emission, which implies the importance of the permafrost processes, highly uncertain in our current knowledge.

How to cite: Park, S. and Kug, J.-S.: Hysteresis of terrestrial carbon cycle to CO2 ramp-up and -down forcing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10878, https://doi.org/10.5194/egusphere-egu22-10878, 2022.

In recent years, perturbations of precipitation regime have intensified due to climate change and it has led to frequent droughts and waterlogged periods. Under these conditions, plants show specific responses and these events often lead to yield losses. In order to have a better understanding of the impact of these perturbations on carbon and nitrogen dynamic transfers within plants as well as between plants, soil and atmosphere, a modelling approach was combined with measurements. The 1D/2D DAISY model was used as it has already been evaluated and compared to others frequently used models (Palosuo et al., 2011; Kollas et al., 2015) and as it meets most of our criteria. Indeed, this open-source model simulates plant production and keeps track of water, nitrogen and carbon content in soil and within plants with an hourly time resolution. Moreover, a SVAT component along with Farquhar formalism for photosynthesis allows DAISY to simulate energy, CO₂, N₂O and H₂O exchanges (Hansen et al., 2012).

Our objective is to apply this model to a 4-year crop rotation (winter wheat, potato, winter wheat, sugar beet) in Lonzée that is representative of Belgian agricultural system. As this site is part of the ICOS network, fluxes (CO₂, N₂O and H₂O) are measured along with continuous meteorological and edaphic conditions data as well as a regular follow-up of organs biomass and LAI. Soil texture has been assessed, identifying Lonzée soil as a silty loam (USDA).

As a first step in this modelling procedure, a global sensitivity analysis (GSA) was performed according to the Morris method. To our knowledge, no GSA had been carried out on DAISY before and, moreover, this step is often overlooked in modelling even though it can give useful information. Sensitivity indices were computed at each time step, providing information on which parameter are influential in general but also at specific phenological stages and under specific conditions such as droughts or waterlogged periods. Furthermore, GSA identified which parameters need thorough measurement or estimation and detected interaction effects between parameters.

The followed methodology and the obtained results will be presented as well as an analysis over the agricultural cycle (from sowing to harvest), leading to propositions to improve the ICOS experimental set-up.

References

Hansen, S., Abrahamsen, P., Petersen, C. T., & Styczen, M. (2012). DAISY: Model Use, Calibration and Validation. 55(4), 1315–1333.

Kollas, C., Kersebaum, K. C., Nendel, C., Manevski, K., Müller, C., Palosuo, T., Armas-Herrera, C. M., Beaudoin, N., Bindi, M., Charfeddine, M., Conradt, T., Constantin, J., Eitzinger, J., Ewert, F., Ferrise, R., Gaiser, T., Cortazar-Atauri, I. G. de, Giglio, L., Hlavinka, P., … Wu, L. (2015). Crop rotation modelling-A European model intercomparison. 70, 98–111. https://doi.org/10.1016/j.eja.2015.06.007

Palosuo, T., Kersebaum, K. C., Angulo, C., Hlavinka, P., Moriondo, M., Olesen, J. E., Patil, R. H., Ruget, F., Rumbaur, C., Takáč, J., Trnka, M., Bindi, M., Çaldaĝ, B., Ewert, F., Ferrise, R., Mirschel, W., Şaylan, L., Šiška, B., & Rötter, R. (2011). Simulation of winter wheat yield and its variability in different climates of Europe: A comparison of eight crop growth models. 35(3), 103–114. https://doi.org/10.1016/j.eja.2011.05.001

How to cite: Delhez, L. and Longdoz, B.: Retrieving useful information from global sensitivity analysis performed on soil-plant-atmosphere model DAISY, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8481, https://doi.org/10.5194/egusphere-egu22-8481, 2022.

Reclamation of lignite mines with the establishment of tree plantations is very important for the post-mining land restoration, especially during the ongoing transition to the post-lignite era. Black locust (Robinia pseudoacacia L.) is a species that have been extensively used worldwide in relevant projects, however its ecophysiological responses and its contribution to carbon cycle has not been extensively studied yet.

In this study, we provide a 9-year estimation (2013-2021) of carbon dynamics, in terms of GPP fluctuation, for a 20-year old black locust plantation, located in the restored areas of the Lignite Center of Western Macedonia, Greece. GPP estimation was performed with the use of a satellite LUE model. The model was evaluated with a combination of LandSat 8 and Sentinel 2 products and validated with eddy covariance measurements performed in a two-year period. The novelty of the model was the combined use of two water-stress indices, one for the ecosystem water deficit effects, expressed through the Land Surface Water Index and one for the atmospheric drought-liked effects, expressed through VPD.

Our results highlight the seasonal pattern of GPP fluctuation of the site on both annual and interannual time-scale. According to our findings, the fast-growing plantation has reached its peak development very early, as for the period 2013-2021, no significant trend in both GPP and vegetation indices during the summer period was observed. On the other hand, a significant increase of the growing period was observed, that was mainly referred to a constant increase in October GPP during the 9-year period. GPP during leafless period was found to have a significant contribution to annual GPP, mainly due the activity of the well-developed grass understory vegetation. From the studied environmental parameters, VPD and summer precipitation was found to be more strongly correlated to summer GPP and air temperature to springtime GPP of the leafless period.

How to cite: Markos, N. and Radoglou, K.: Long-term monitoring of CO2 fluxes and development of a forest plantation in a post-mining reclamation site with the use of eddy covariance measurements and satellite imagery, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11597, https://doi.org/10.5194/egusphere-egu22-11597, 2022.

The past decade with the drought years of 2015, 2018 and 2019 in Central Europe revealed strikingly the devastating consequences of severe and repeated drought/heat events on forest ecosystems. Nevertheless, responses of the water balance of trees and forest stands to such repeated drought events and subsequent recovery are poorly understood. The estimation of the water consumption of trees and forest ecosystems is a crucial part of many process-based models, especially under severe drought events. One major factor for the calculation of the water consumption per tree by xylem sap flow measurements is the radial profile of the xylem sap flow density decreasing towards the inner part of the sapwood. However such profiles are very scarce, especially under repeated and severe drought events.

Here, we present the changes of the profile of xylem sap flow density of European beech and Norway spruce within a five-year throughfall-exclusion (TE) experiment and a subsequent recovery. Two different methods were used to measure the xylem sap flow density up to 8cm sapwood depth, i.e. the heat dissipation and heat field deformation method. In beech, there was no difference in the linear radial profile between the TE and the CO (control) trees. However, under drought, for spruce the xylem sap flow density was strongly reduced along the profile by about 48 ± 16 % and the profile changed to an exponential decrease compared to the linear decrease for CO trees. Even two years upon drought release, the profile has not recovered. The reduction in the xylem sap flow density profile of drought stressed spruce was accompanied by heavy loss of fine roots and a reduction of the leaf area.

The use of standardized xylem sap flow profiles without the consideration of drought induced changes would lead to an overestimation of the water consumption of more than 30%. These results stress the importance of the radial profile measurements for the calculation of water balance of trees and thus forest ecosystems.

How to cite: Gebhardt, T., Hesse, B., Hikino, K., Grams, T., and Häberle, K.-H.: Xylem sap flow density along the radial profile is strongly reduced by repeated drought in mature spruce but not in beech, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11059, https://doi.org/10.5194/egusphere-egu22-11059, 2022.

Plants are highly sensitive to the water status of the soil in which they grow, with too little soil moisture causing drought stress whereas too much soil moisture causing flooding stress. This stress in response to opposing water conditions can be understood from the fact that plant growth demands both sufficient water uptake from the soil and rapid gas exchange with the environment. Given that drought and flooding events can occur in the same system and even consecutively, a single model simulating plant responses to a continuum of soil water conditions from drought to flooding would be attractive. However, as far as we know, such a model with sufficient mechanistic biological details currently does not exist. In this study we propose a theoretical framework of an integrated mechanistic model that is capable of describing plant responses to both flooding and drought, building on the biophysics of plant water transport and gas exchange and its dependence on environmental conditions. Since the restricted root water uptake and stomatal activity that limits gas exchange through photosynthesis are essential processes in both scenarios, we propose using a combined SPAC-Farquhar model that describes these processes as a “backbone” of the envisioned model framework. Further we propose to add processes related to oxygen dynamics and hormonal signaling, as oxygen deficit serves as the main driver of flooding stress to which hormonal signaling plays an essential role in plant response. The model aims to mimic various responsive strategies by different plant species. These strategies include isohydric and anisohydric strategies for drought response, and “escape” and “quiescence” strategies for flooding response. Stomatal activities of isohydric plants are reported to be more sensitive to leaf water potential but less sensitive to abscisic acid compared to those of anisohydric plants. This can be achieved by assigning different sensitivity coefficients. Plants that are tolerant and adaptive under flooding stress can generally switch between the “escape” and “quiescence” strategy, depending on shoot ethylene concentration. This integrated model framework is envisioned to mimic the complex behavior of plant responses to consecutive drought and flooding events, as the physiological processes involved occur on various time scales, ranging from sub-hours to weeks. Moreover, hormones and certain irreversible morphological changes can serve as “memory factors”, leading to the history-dependent nature of plant responsive behavior. We hope that the proposed theoretical model framework will serve as a basis for model research on resilience to combined drought and flooding in both agriculture and natural vegetation systems.

How to cite: Chen, S., Sasidharan, R., Dekker, S. C., ten Tusscher, K. H. W. J., and de Boer, H. J.: Parallels between drought and flooding: an integrated framework for plant eco-physiological responses to water stress, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11625, https://doi.org/10.5194/egusphere-egu22-11625, 2022.

Cold and high-latitude terrestrial ecosystems, such as the arctic and the subarctic, store large amounts of carbon (C) in the soil. These ecosystems are already experiencing rapid rates of temperature increase compared to other regions of the Earth. It is expected that warmer conditions will increase soil CO₂ efflux by enhancing soil microbial and root respiration. However, our understanding of warming effects on soil C cycling is limited to short-term observations (1-5 y of warming) and under few discrete warming levels.

This study is embedded within the FutureArctic project, and we take advantage of geothermally heated subarctic grasslands in the ForHot research site in Iceland, with 13 years of a stable and constant soil warming gradient. The main objective of this research is to get deeper insights into how rising temperatures will affect soil carbon fluxes in subarctic grasslands ecosystems.

Using automated long-term soil chambers, we are continuously measuring soil CO₂ efflux rates along a soil warming gradient ranging from +0°C to ca. +14°C above ambient soil temperature. Furthermore, complementary manual soil CO₂ efflux measurements allow us to cover higher spatial variability and to assess how soil warming affects the main soil CO₂ source components (i.e., autotrophic, and heterotrophic respiration) via the trenching approach.

Here, we present preliminary results of the soil CO₂ efflux along a soil warming gradient in Iceland, including time series of the first year of the study. Overall, soil CO₂ efflux increased along the soil warming gradient. We found that heterotrophic respiration is the main source component of total soil CO₂ efflux. Both autotrophic and heterotrophic respiration increased with warming, however, the relative contribution of each source component was unresponsive to warming. Ongoing analysis of isotopic soil CO₂ in the automated measurements will allow the partition between biogenic and geogenic sources of soil CO₂ in the studied geothermal system and accurately describe the soil respiration response to warming.

How to cite: Protti Sanchez, F., Janssens, I., Sigurdsson, B. D., Sigurdsson, P., and Bahn, M.: Soil CO2 efflux along a soil warming gradient in subarctic grasslands in Iceland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11425, https://doi.org/10.5194/egusphere-egu22-11425, 2022.

Nitrogen enrichment due to atmospheric nitrogen deposition has affected plant growth and microbial activity globally, leading to an increase in soil organic carbon in many ecosystems. Drylands cover ~45% of the global land area and constitute ~32% of the global carbon stocks, but the response of dryland carbon storage to atmospheric nitrogen deposition remains unclear and understudied relative to mesic systems. Observations from mesic systems suggest that nitrogen enrichment can increase the efficiency by which microbes incorporate carbon into mineral-associated forms if pH stays constant. Under acidification, a common response to nitrogen deposition, microbial biomass and enzymatic organic matter decay often decrease, leading to a build-up in plant-derived particulate organic carbon. However, in drylands, where organic carbon often associates with mineral surfaces via Ca-bridging, acidification may also abiotically decrease mineral-associated organic carbon if Ca is leached. In this study we tested how experimental nitrogen deposition affects different soil organic carbon fractions in drylands through microbial and abiotic effects.

We used four long-running nitrogen deposition experiments in Mediterranean shrub- and grassland ecosystems in Southern California, where two of the sites showed strong nitrogen-induced acidification (pH drop by ~1.5 units). We studied changes in soil organic carbon fractions, soil extracellular enzyme activity, microbial carbon stabilization efficiency and exchangeable Ca. Experimental nitrogen deposition had relatively small effects on soil organic carbon storage, which appeared to be mostly driven by soil physicochemical changes. Particulate organic carbon did not increase despite previously reported increases in plant biomass and decreases in microbial biomass and extracellular enzyme activity in acidified sites. Furthermore, microbial carbon stabilization efficiency was unaffected by N fertilization in non-acidified sites and decreased in short-term but not long-term incubations in acidified sites. Importantly, mineral-associated organic carbon decreased significantly by 20% in response to N fertilization at one of the acidified sites, likely as result of pH-induced loss of Ca, which dropped by 48%. Our measurements suggest that long-term effects of nitrogen fertilization on dryland carbon storage might be primarily abiotic in nature, such that drylands, which may undergo acidification and where Ca-stabilization of soil organic carbon is prevalent, may be most at risk for loss of mineral-associated organic carbon.

How to cite: Püspök, J., Aronson, E. L., Hanan, E. J., Schimel, J. P., Allison, S. D., Vourlitis, G. L., and Homyak, P. M.: Microbial and Abiotic Effects of Experimental Nitrogen Deposition on Dryland Soil Organic Carbon Storage, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10552, https://doi.org/10.5194/egusphere-egu22-10552, 2022.

Agricultural used wetlands with high SOC stocks cover large parts of northeast (NE) Germany. Drainage and modification of groundwater levels by agricultural water management during the last century not only lead to a change in their hydrological processes but also reversed many biogeochemical processes like soil C dynamics and GHG emissions. In addition, climate projections indicate that climate change will substantially alter seasonal precipitation and temperature regimes in NE Germany with an increasing risk of severe summer droughts such as in 2018. Both might have the potential to significantly increase SOC stock losses and GHG emissions. Hence, there is an emergent importance to investigate the interconnectivity between water level, soil C dynamics, and GHG emissions.

To better understand this interconnectivity, we investigated the influence of a different GWL on dynamics of GHG emissions and the net ecosystem C balance (NECB) as a proxy for SOC stock changes. Therefore, GHG emission measurements and estimates of NECB were performed for four weighable lysimeters containing soil monoliths, which were established during 2009 in an agricultural used wetland area (Spreewald region, 51^◦52’N, 14^◦02’E). The study site represents an agricultural used (pasture) grassland typical for the Spreewald region. Weighable lysimeters were used to simulate two different GWL regimes: growing season dropdown of GWL due to e.g. summer drought vs. no growing season GWL dropdown. GHG emission measurements (CO₂ (R_eco and NEE)_, CH₄ and N₂O) were conducted campaign wise every 2 to 4 weeks from 2021 onwards, using a manual (N)FT-NSS closed chamber system (Livingston and Hutchinson 1995). In addition, environmental conditions, aboveground biomass development (e.g. plant height, RVI, NDVI) and in situ water parameters (e.g., oxygen, pH, hydrogen carbonate, el. conductivity, temperature, redox potential) were obtained.

Here we present GHG emission measurements and NECB estimates for the first study year of 2021. Higher GWL generally resulted in a lower biomass production. Consequently, clear differences between the two different GWL´s were also obtained in case of derived CO₂ flux components R_eco and GPP as well as to a lower extend for overall NEE, with higher GWL showing an only slightly higher overall net CO₂ exchange. Thus, higher NECB values were detected for lower GWL. In contrast, overall GHG emissions (incl. CO₂, CH₄ and N₂O) were lower for lower compared to higher GWL.

How to cite: Antonijevic, D., Dietrich, O., Monzon, O., Niedermayr, B., Vaidya, S., Macagga, R., Augustin, J., and Hoffmann, M.: The influence of different groundwater levels (GWL) on C and GHG dynamics of an agricultural used wetland area, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4811, https://doi.org/10.5194/egusphere-egu22-4811, 2022.