Grasslands are often water-limited ecosystems with high belowground carbon allocation. Their root systems and soil microbial communities play an important role in regulating the soil carbon pool, and properly managed, grasslands may contribute to climate change mitigation via sequestration of carbon (C) in soils. However, it is still uncertain how roots and microbial communities are affected by drought and changes in precipitation patterns in combination with management for soil C sequestration. Expected longer dry periods and more intense precipitation events will evoke soil microbial responses that may feed-back on soil carbon storage.

We set up an experiment in southern Sweden in 2019 to investigate the response of belowground biodiversity to chronic drought (via partial rainfall exclusion) and carbon amendments in the form of a compost addition. We sampled belowground plant biomass, root traits and soil microbial communities from two grasslands with different land use history and over several depths. We extracted and sequenced fungal and bacterial DNA using metabarcoding, and carried out vegetation inventories at the sites - including plant biomass and relative abundance of plant functional types. At the same time, we monitored changes in soil moisture and soil organic matter from topsoil to deep soil, to assess the effect of the treatments throughout the soil profile.

After three years of treatment, we expected to observe changes in the root systems and soil microbiota in response to decreased precipitation, as well as interactions between soil moisture and the organic matter added through the compost amendment. In addition, we expected to see shifts in the composition of fungal functional groups involved in organic matter decomposition and mycorrhizal symbionts limited to the rooting zone. The analyses suggest that changes in soil C and soil moisture affect only the topsoil. While overall root biomass did not change significantly in the treatment plots over the course of the experiment, we observed a slight increase in rooting depth and root mass density and a decrease in fine root length under drought. The results of this study will contribute to assess ecosystem responses to drought, and to evaluate the potential for soil carbon sequestration in grasslands and its possible impacts on belowground biodiversity.

How to cite: Guasconi, D., Cousins, S., Fransson, P., Manzoni, S., and Hugelius, G.: Effects of soil carbon management and drought on grassland root systems and soil microbial communities, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7749, https://doi.org/10.5194/egusphere-egu23-7749, 2023.

The rise in atmospheric CO₂ concentrations, and the associated increase in global warming and likelihood of severe droughts, is altering terrestrial carbon (C) and nutrient cycling, with potential feedback to climate change. Soil microbial communities and their functioning represent a major research area in this context. Microbes regulate important biogeochemical functions, including C fluxes between the biosphere and atmosphere and the availability of essential nutrients for plant growth, such as nitrogen (N) and phosphorous (P). Thus, improving our ability to quantify microbial responses to climate change is of utmost importance. While each climate change factor has been widely studied individually, it was shown that their combined effect is difficult to predict from the simple knowledge of each single factor.

In 2013, a climate change experiment (“ClimGrass”) was set up on a montane grassland in Austria, with the aim to assess the potential interaction of multiple climate change factors (warming, elevated CO₂ and drought) on the functioning of managed grasslands. The experimental design followed a response surface model approach for warming and elevated CO₂, with each factor having two levels of increase above ambient (+1.5 and +3°C for warming and +150 and +300ppm for elevated CO₂). Drought was nested on this experimental design within a subset of treatments and implemented in multiple years. This design, combined with multiple harvests across seasons and years, allowed us to test the potential for interactive, non-linear and seasonal effects of multiple climate change factors.

Across multiple years and seasons, we analyzed parameters related to soil microbial communities and their functions in relation to the biogeochemical cycles of C, N and P. By using a large range of approaches, from in situ stable isotope labelling to the analyses of functional genes, we covered different aspects related to the cycling and stability of C in soil and to major processes involved in nutrient cycling.

In this talk, I will provide an overview of the multiple experiments carried out in ClimGrass. I will show that combined elevated CO₂ and warming can have minor but important interactive and non-linear responses that cannot be predicted by studying each factor individually. Seasonality represents a major mediator of climate change effects on important microbial functions, an aspect that is often overlooked. I will also focus on the response of soil microbial communities to drought and the implications of combined warming and elevated CO₂ treatments.

How to cite: Canarini, A. and the ClimGrass: Effects of multiple climate change factors and their seasonal variation on the soil microbial community and its functions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13067, https://doi.org/10.5194/egusphere-egu23-13067, 2023.

Mountain ecosystems contribute substantially to global carbon and nitrogen biogeochemical cycles. Although soil respiration, and microbial biomass, activities and diversity have been extensively studied at different altitudes worldwide, little is known on causal link between environmental drivers, microbial functions and emissions of greenhouse gases (GHGs) in soils of different elevation. Here, by measuring in-situ GHG fluxes, soil properties, organic matter (OM) quality, microbial enzyme activities, biomass and gene abundances, we investigate factors that control long-term GHG fluxes (carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O)) in natural soils with an elevational gradient of ~2400 m across Switzerland with different vegetation covers. Results showed that CO₂ and N₂O fluxes increased significantly with elevation from top to the treeline, but slightly decreased from the treeline to bottom. Contrastingly, no significantly patterns of CH₄ fluxes across the whole elevation were observed. Spearman correlations revealed that the increased CO₂ and N₂O fluxes were highly correlated to the significant increases in soil temperature, moisture, organic matter (OM) quantity and quality (increases in the relative contribution of humic-like vs. fresh-like OM), bacterial and fungal biomass and gene abundances. Structural equation model, hierarchical partitioning and random forest regression further confirmed that, in addition to soil temperature and moisture, SOM quantity and quality are the most driving factors of microbial activity and respiration. Our study highlights the importance of OM quality as a driving factor of soil microbial metabolic activities in Alpine soils across the elevation, and predicts a potential increase in GHG emissions in high-altitudinal soils with the expected upwarding-shifting treeline under climate warming.

How to cite: Han, X., Pascual, A. D., Casas-Ruiz, J. P., Donhauser, J., Jordaan, K., Ramond, J.-B., Priemé, A., Romaní, A. M., and Frossard, A.: Drivers of soil microbial activities and greenhouse gas emissions along an elevational gradient, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9252, https://doi.org/10.5194/egusphere-egu23-9252, 2023.

Changes in temperature and water availability under global warming will alter soil bacterial and fungal community structures and thus ecosystem functioning across the globe. We sampled large-scale temperature and aridity gradients across Greenland, Europe, Spain, the Swiss Alps and South Africa to understand microbial long-term adaptation to climatic conditions in soils and to predict microbial responses to climate change. We found that bacterial communities from South African soils were distinct from those in European and Greenlandic soils, largely explained by high relative abundances of Firmicutes. Conversely, fungal communities additionally differed between European and Greenlandic soils and thus seem to be more affected by oceans acting as geographical barrier compared to bacteria. Interestingly, bacterial communities in hyperarid soils from Northern Greenland clustered with hyperarid soils from Southern Spain and South Africa indicating that these communities share taxa adapted to low water availability despite their distinct geographical origin and temperature regimes. Within regional gradients in Europe and Greenland microbial community structures sequentially shifted along the gradients of temperature and aridity, whereas in the South African gradient soil physicochemical properties such as pH and texture that were not related with aridity were important drivers of microbial community structures. Shifts in fungal and bacterial community structures along climatic gradients occurred in parallel with changes in microbial functions, such as extracellular enzyme activities, greenhouse gas fluxes as well as abundances of functional genes involved in soil carbon and nitrogen cycling. Collectively, our results suggest that alterations in microbial community structures along climatic gradients, which serve as a proxy for climate change over time, translate into an alteration in ecosystem services provided by the community members. Moreover, at the global scale our study indicates that bacterial communities are mainly controlled by environmental conditions whereas fungal communities are more influenced by geographic barriers.

How to cite: Donhauser, J., Jordaan, K., Han, X., Doménech Pascual, A., Casas-Ruiz, J. P., Romaní, A. M., Frossard, A., Ramond, J.-B., and Priemé, A.: Bacterial and fungal communities along temperature and aridity gradients are linked with soil functions across global biomes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17332, https://doi.org/10.5194/egusphere-egu23-17332, 2023.

The terrestrial soil organic matter (SOM) pool size depends on the balance between SOM formation and stabilization of decomposing plant litter relative to mineralization as CO₂.  Decomposition and mineralization processes are to large extents mediated by microbial decomposer communities. In addition, labile fractions released by decomposing litter can be stabilized in mineral associations (MAOM). High latitude ecosystems are particularly affected by global warming. Increasing temperatures can stimulate litter decomposition and increase nutrient mineralization, thereby increasing nutrient (e.g. nitrogen) availability for plants allowing higher productivity and subsequent plant organic matter inputs. On the other hand, if warming accelerates SOM decomposition stronger than formation processes it can cause large carbon and nutrient losses.

Tracing ¹³C/¹⁵N labelled above-ground litter we aimed to disentangle if soil warming changes the balance between litter derived SOM formation through microbial communities, over particulate organic matter (POM) and MAOM stabilization across a decadal geothermal soil warming gradient (from ambient up to +5 °C) in a grassland in Iceland. In addition, we added three levels of inorganic N to disentangle potential direct warming from indirect (over plant feedbacks) effects of increased warming on SOM dynamics.  We found that over the course of two years warming accelerated litter decomposition rates and litter-derived carbon turnover by the microbial community,  and we could recover more litter-derived carbon recovered in particulate organic matter (POM) nor MAOM. Nitrogen additions triggered a faster decomposition of structural above-ground litter compounds, but did not influence carbon turnover in different soil fractions. On the other, hand we found that overall the absolute amount of soil carbon decreased in response to warming. We therefore conclude that direct warming not only increased SOM formation, but also decomposition and mineralization processes, and – at least in the studied grassland – plant inputs may not have counterbalanced warming induced losses.   

How to cite: Fuchslueger, L., Verbrigghe, N., Soong, J. L., Meeran, K., Vicca, S., Cotrufo, F. M., Sigurdsson, B. D., Bahn, M., and Janssens, I.: Soil warming accelerates above-ground litter decomposition and soil organic carbon turnover, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13536, https://doi.org/10.5194/egusphere-egu23-13536, 2023.

Alpine and sub-alpine areas react very sensitive to global climate change and carbon cycling therein has been understudied, so far. A major component of plant litter that is commonly regarded as hardly decomposable is lignin. Consequently, the improved knowledge on degradation of lignin and soil organic carbon in alpine areas is of great importance to better understand their response to climate change. Therefore, we conducted a closed-jar incubation experiment under controlled conditions. ¹³C labelled plant litter (above ground litter from Lolium perenne) was added to two different soils from a sub-alpine area, one pasture soil and one forest soil originating from Jaun, Switzerland. To investigate the effect of increasing temperatures, the incubation was conducted under three different temperature regimes (average growing season temperature of 12.5°C, +4°C (16.5°C) and +8°C (20.5°C)) for the period of one year with five consecutive destructive samplings.

Lignin phenols were extracted using the CuO oxidation method, subsequent sample clean-up and quantification by GC-FID. Compound-specific stable carbon (δ¹³C) isotope composition of the lignin phenols was measured by GC-IRMS.

For all treatment groups, lignin concentrations decreased over the period of one year. The average decrease across all treatment groups was -22.7%. The decrease was slightly higher for the forest soil (-24.9%) than for the pasture site (-20.5%). No significant difference was observed between the control soil with and without added labelled litter. Average lignin decrease for the pasture soil was highest for the lowest temperature (-27.1%). For the two higher temperature treatments the decreases were identical with -17.1% and -17.3%. For the forest soil, the decrease was highest for a temperature of 16.5 °C (26.9%) and slightly lower for 12.5°C (25.7%). Surprisingly, the lowest decrease was observed for 20.5°C (22.1%).

The evolution of the ¹³C labelled litter signal enables the assessment of the degradation of fresh litter in the soils. For all different soils and incubation temperatures, the amount of litter-derived lignin phenols decreased by more than 50% already within two weeks after litter addition. In the further course of time, the ¹³C signal decreased much more slowly but remained considerably different from control soils. A possible explanation for this is a high availability of easily degradable carbon within the litter, providing enough energy to produce enzymes for lignin degradation.

Over the course of a year, also older lignin in the control samples degraded in a similar range as in the samples with litter addition, with a strong decrease in the initial phase and a slower decomposition in the later phase. This can be explained by the better availability of carbon at the beginning of the experiment and missing fresh litter during the later course.

Contrary to expectations, the degradation of lignin did not increase with rising temperature. This could be due to a lower temperature optimum of the current microbial community which is adapted to the current sub-alpine temperature regime. A complementary field incubation will show whether and how the laboratory results can be transferred to field conditions.

How to cite: Püntener, D., Speckert, T. C., and Wiesenberg, G. L. B.: Rapid Lignin Degradation in a Laboratory Incubation Experiment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6390, https://doi.org/10.5194/egusphere-egu23-6390, 2023.

Wildfires occur regularly in boreal forests of Northern Canada and are increasing in frequency and intensity due to the impacts of projected global climate change. A by-product of these forest fires is pyrogenic carbon (PyC) as a residue of incomplete combustion. The short-and long-term dynamics of this important soil organic carbon (SOC) pool in permafrost-affected mineral soils, however, is largely unknown. Here we studied eleven boreal forest soils at distinct landscape positions under continuous (northern sites) and discontinuous (southern sites) permafrost. In these we assessed the short-term fate of ¹³C-labeled PyC and its precursor grass organic matter over two year in-situ soil core incubations. Further, we isolated PyC by hydrogen pyrolysis (PyC_HyPy) for quantification and radiocarbon measurements to investigate long-term pools across the landscape.

Losses of PyC after two years were dominated by decomposition with up to three times more PyC losses at northern sites (36%) compared to the southern sites (11%). The losses of the grass organic matter were substantial (69-84%) but losses were larger in southern soils. The PyC persistence depended on site and soil specific properties and not solely on its chemical resistance. Fresh PyC was increasingly decomposition in nutrient limited mineral soils under continuous permafrost, indicating that polyaromatic compounds can act as a nutrient source. Mineral interactions were important and contributed to the stabilization of ~40% of recovered PyC. Mineral-associated PyC mainly remained in particulate forms as identified on the microscale using NanoSIMS. Beside large grass organic matter losses, remaining fractions were recovered predominantly as particles in northern soils but highly dispersed on mineral surfaces in the southern soils on the microscale. Our results highlight that permafrost-affected boreal forest soils are sensitive to fresh PyC and organic matter inputs with substantial losses by decomposition even under continuous permafrost conditions with unique stabilisation mechanisms at the organo-mineral interface.

On the long-term, we identified that the native PyC_HyPy represents a millennial age carbon pool with significantly higher ages in continuous (5.5-7.8 cal. ka BP; F¹⁴C=0.44-0.54) than in discontinuous (1.2-2.2 cal. ka BP; F¹⁴C=0.76-0.88) permafrost soils in 0-15 cm soil depth. The PyC_HyPy was markedly older than the bulk SOC (modern with F¹⁴C=0.65-1.11). With soil depths, PyC_HyPy ages increased to >18 cal. ka BP (F¹⁴C<0.10) in cryoturbated soils. In accordance with the age, the PyC_HyPy stocks were larger at northern (3.4 ± 0.3 Mg PyC_HyPy ha^-1) compared to southern (1.4 ± 0.1 Mg PyC_HyPy ha^-1) sites. The PyC_HyPy stocks were found to be independent of permafrost intensity and landscape position within the regions and did not reflect observed SOC variability. By considering the results of the two-year incubation with the long-term observations, we identified that the processes on different temporal scales are not directly linked. A better understanding of PyC dynamics along temporal and spatial scales is required to evaluate soil carbon feedbacks of high-latitude soils with global warming and associated permafrost thaw and shifts in vegetation and wildfire regimes.

How to cite: Schiedung, M., Ascough, P., Bellè, S.-L., Hilton, R. G., Hoeschen, C., Schweizer, S. A., and Abiven, S.: Dynamics of pyrogenic carbon in permafrost-affected soils on short- and long-timescales, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2845, https://doi.org/10.5194/egusphere-egu23-2845, 2023.

Permafrost thaw can stimulate microbial decomposition and induce soil carbon (C) loss, potentially triggering a positive C-climate feedback. However, earlier observations have concentrated on bulk soil C dynamics upon permafrost thaw, with limited evidence involving soil C fractions. Here, we explore how the functionally distinct fractions, including particulate and mineral-associated organic C (POC and MAOC) as well as iron-bound organic C (OC-Fe), respond to permafrost thaw using systematic measurements derived from one permafrost thaw sequence and five additional thermokarst-impacted sites on the Tibetan Plateau. We find that topsoil POC content substantially decreases, while MAOC content remains stable and OC-Fe accumulates due to the enriched Fe oxides after permafrost thaw. Moreover, the proportion of MAOC and OC-Fe increases along the thaw sequence and at most of the thermokarst-impacted sites. The relatively enriched stable soil C fractions would alleviate microbial decomposition and weaken its feedback to climate warming over long-term thermokarst development.

How to cite: Yang, Y. and Liu, F.: Divergent changes in particulate and mineral-associated organic carbon upon permafrost thaw, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1443, https://doi.org/10.5194/egusphere-egu23-1443, 2023.

Ombrotrophic bogs are rainwater-fed, water-logged, anoxic, and carbon-rich ecosystems with low concentrations of dissolved inorganic terminal electron acceptors (TEAs), such as nitrate and sulfate. Consequently, methanogenesis is expected to dominate carbon turnover in many of these systems and to result in an approximatively equimolar formation of CO₂ and CH₄. Yet, numerous studies have reported elevated molar CO₂:CH₄ formation ratios in peat bog soil incubations, indicating that anaerobic respiration prevails over methanogenesis despite the apparent scarcity of inorganic TEAs. To explain anaerobic respiration, particulate organic matter (POM) was proposed to act as previously unrecognized TEA. Here, we present results from combined in situ field studies and laboratory peat soil incubations to assess electron transfer to oxidized POM (POM_ox) and its effects on CO₂ and CH₄ formation. In situ studies consisted of deploying litter mesh bags containing POM_ox in the anoxic, water saturated subsurface of three ombrotrophic bogs – Lungsmossen (LM), Storhultsmossen (SM), and Björsmossen (BM) – for one year. The electron accepting capacity (EAC) of the retrieved POM decreased by 0.16±0.02 mmol e^-/g dry POM in LM, 0.15±0.02 in SM, and by 0.17±0.01 mmol e^-/g dry POM in BM, as compared with the buried POM_ox, demonstrating extensive electron transfer to the buried POM over the course of one year. Extents of POM_ox reduction were similar for different depths, as tested in BM bog. Exposure of the reduced POM to air (i.e. O₂)resulted in an increase in its EAC, supporting that POM acts as a reversible TEA at the oxic-anoxic interface of peat soils. We complemented these in situ field studies with laboratory incubations of reduced POM collected from the same three bogs. Methanogenic conditions were observed in BM peat soil incubations, which were used for further studies. Amending BM soils with POM_ox and glucose resulted in increases in CO₂:CH₄ formation ratios of several orders of magnitude. These findings pointed towards anaerobic respiration using POM_ox as TEA, thereby suppressing methanogenesis. Taken together, our work provides evidence for POM_ox reduction in situ and substantiates the important role of POM as TEA in controlling CH₄ formation in ombrotrophic bogs.

How to cite: Obradović, N., Läubli, S., Schmitz, R., Schroth, M., and Sander, M.: Electron transfer to peat particulate organic matter in ombrotrophic bogs and implications for methane formation: a combined field and laboratory study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12388, https://doi.org/10.5194/egusphere-egu23-12388, 2023.

Global warming effects on soil organic carbon (SOC) stocks are expected to be site specific but current process-based models still struggle to forecast spatially explicit long-term trends properly. This study estimated such long-term effects of global warming on European SOC stocks using a novel data-driven space-for-time approach. In principle, this approach estimated site-specific SOC stocks under future climate from SOC stocks in comparable soils but in regions that are already exposed to such climate today. About 20k observations of the LUCAS soil dataset were used to train a machine learning model that predicted SOC stocks from current climate as well as static environmental properties (e.g. geology, soil type, soil texture). Then, this SOC model was used to forecast future SOC stocks in Europe under various CMIP6 climate scenarios. Preliminary results suggest Europe’s top 20 cm of mineral soil to loose on average 2 to 6 Mg SOC ha^-1 by the end of this century. But global warming-induced changes in SOC showed pronounced regional differences. SOC was anticipated to even rise under global warming in some areas, particularly in Northern European forest ecosystems. In vast parts of southern Europe, unprecedented future climate limited the applicability of the data-driven SOC model. This was the case for up to 49% of all sites in the most extreme climate scenario. In contrast, for the remaining 51% of sites in all climatic scenarios, equivalent "soil-climate twins" could be successfully located elsewhere in contemporary Europe. It is proposed that outcomes from data-driven space-for-time models could complement and act as cross-checks for process-based modelling outputs to gain confidence in long-term projections of SOC stocks under global warming.

How to cite: Schneider, F.: How does global warming affect European SOC stocks? A data-driven space-for-time approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12926, https://doi.org/10.5194/egusphere-egu23-12926, 2023.

Radiocarbon measurements provide a powerful tool to assess the persistence of soil organic carbon (SOC). While soil depth is generally one of the most important predictors of soil radiocarbon age, it remains unclear whether this relationship is due to an overall decrease in C input with depth or to changing importance of climatic or mineralogical constraints on SOC decomposition. Due to this lack of mechanistic understanding, we argue that the relationship between soil radiocarbon age and SOC abundance may be a better proxy than depth to investigate SOC persistence. To test this hypothesis, we use globally collected soil radiocarbon data from the International Soil Radiocarbon Database (ISRaD) to examine the influence of climate and soil mineralogy on the relationship between SOC concentration and radiocarbon age at the profile level. Our analysis includes about 600 soil profiles covering all major climatic zones and soil types (except aridisols). We show that extreme climatic and mineralogical constraints can lead to a similar accumulation of old SOC throughout the soil profile but for very different reasons. Climatic extremes include soils from tundra/polar regions where C input and decomposition are constrained by low temperatures and water availability. Mineralogical extremes include volcanic soils (andisols) that are dominated by highly reactive amorphous minerals that limit SOC accessibility. Across all climate zones and for a given SOC concentration, arid soils tend to have younger radiocarbon ages compared to temperate soils. Tropical soils have the youngest SOC at the surface due to high C input and show a relatively uniform distribution of SOC and radiocarbon ages globally. In terms of mineralogy, soils dominated by low-activity clays (oxisols and ultisols) show younger radiocarbon ages than soils dominated by high-activity clays (all other soil types except andisols) for a given SOC concentration. These first results have far-reaching implications for better understanding SOC vulnerability and benchmarking soil carbon models for representing SOC turnover and persistence across climate zones and soil types.

How to cite: von Fromm, S. F., Hoyt, A. M., Doetterl, S., and Trumbore, S. E.: Climate and mineral controls on global soil radiocarbon profiles, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5070, https://doi.org/10.5194/egusphere-egu23-5070, 2023.

As the largest terrestrial carbon (C) pool, the feedbacks of soil C to global environmental change have significant implications for our future climate. It is increasingly recognized that studying solely bulk soil C and nitrogen (N) responses to global change is not sufficient. Because different fractions of soil organic matter (SOM) have distinct controls, they likely respond differently to global changes. To investigate the responses of SOM fractions to global change we must combine data synthesis with mechanistic experiments. We investigated the responses of particulate and mineral-associated organic matter (POM and MAOM) to global change through a global meta-analysis and an increased precipitation experiment. In our meta-analysis we found that POM was more strongly influenced by global change than MAOM and that these fractions responded uniquely to global changes. In particular, increased precipitation caused opposing, but non-significant, responses of POM and MAOM C (decrease and increase, respectively) when investigated with meta-analysis. In investigating POM and MAOM responses to an increased precipitation experiment, we find greater support for changes in plant biomass and diversity driving changes in SOM fractions than changes in microbial necromass. Unique, statistically stronger, and plant- and microbially-informed responses of SOM fractions as compared to bulk SOM suggest these fractions are useful for understanding SOM responses to global change. Altogether, our work provides strong evidence that fractionating SOM into POM and MAOM will help determine whether soil C will feed back positively or negatively to climate change.

How to cite: Rocci, K. S. and Cotrufo, M. F.: Soil carbon and nitrogen responses to global change are informed by soil organic matter fractions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1496, https://doi.org/10.5194/egusphere-egu23-1496, 2023.

The response of soil carbon represents one of the key uncertainties in future climate change due to competing soil carbon driven feedbacks. The ability of Earth System Models (ESMs) to simulate both present day and future soil carbon is therefore vital for reliably estimating global carbon budgets required for Paris agreement targets. In this presentation, the simulation of both present day and future soil carbon is investigated within CMIP6 ESMs.

The ability of CMIP6 ESMs to simulate present day soil carbon is evaluated against empirical datasets, where a lack of consistency in modelled soil carbon remains from the previous generation of models (CMIP5). This underestimation is particularly dominant in the northern high latitude soil carbon stocks. The results suggest much of the uncertainty associated with modelled soil carbon stocks can be attributed to the simulation of below ground processes, and greater emphasis is required on improving the representation of below ground soil processes in future developments of models.

Projections of soil carbon during the 21^st century are also evaluated to quantify future soil carbon changes in CMIP6 ESMs and to assess the uncertainty of the soil carbon induced feedback to climate change. The response of soil carbon is broken down into changes due to increases in Net Primary Productivity (NPP) and reductions in soil carbon turnover time (τ_s), with the aim of isolating the differing responses which influence changes in future soil carbon storage. A reduction in the spread of soil carbon projections is identified in CMIP6 compared to CMIP5. However, similar reductions are not seen in the components due respectively to changes in NPP and τ_s. The relationship between the induced soil carbon changes due to NPP and τ_s is investigated and their overall effect on the future soil carbon response is presented.

How to cite: Varney, R., Chadburn, S., Burke, E., and Cox, P.: Soil carbon in CMIP6 Earth System Models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13638, https://doi.org/10.5194/egusphere-egu23-13638, 2023.

Floodplain soils within mountainous watersheds are dynamic reservoirs of carbon (C), and experience seasonal flooding due to snowmelt and drainage. Climate change is shifting snowpack levels, making these ecosystems vulnerable to more frequent extreme flood and drought years. Here we show how extreme flooding or drought events, and associated variations in redox conditions, impact the dominant controls on microbial C cycling within and export from floodplain soils. Employing in-field monitoring with advanced analytical and molecular tools in the subalpine East River watershed (Gothic, Colorado) we compared seasonal flooding impacts in extremely low and high river discharge years (2018 and 2019, respectively), foreshadowing climate change projections. Our results show that reduced conditions during flooded periods caused reductive dissolution of Fe oxide minerals, mobilizing previously mineral-bound organic C and enhancing export of dissolved organic carbon (DOC). At the same time, flooding decreased CO₂ production and selectively preserved chemically reduced DOC, likely due to metabolic constraints on microbial respiration. Upon drainage and re-oxygenation of floodplain soils, however, CO₂ production increased, but was limited by the concurrant entrapment of DOC by newly precipitated Fe oxides within the soils. Compared to the low discharge year, extreme flooding during high dicharge years underminded mineral protection and heightened mineral constraints, suppressing CO₂ production and enhanced DOC export from floodplain soils. We conclude that seasonal flooding events shift the relative and interactive impacts of mineral and metabolic constraints on microbial C cycling in floodplains, altering the balance between CO₂ and DOC export. Our results suggest that extreme hydrological events expected with climate change will shift the control on and pathways of C loss from floodplains.

How to cite: Keiluweit, M. and Anderson, C.: Hydrological extremes shift controls on and pathways of carbon loss from mountainous watersheds, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13944, https://doi.org/10.5194/egusphere-egu23-13944, 2023.

Mediterranean forests are considered to be highly vulnerable ecosystems to climate change because they are water- and nutrient-limited. However, experimental evidence of the combined effects of increasing drought and warming on biogeochemical cycles in these ecosystems is still extremely scarce. To fill this gap, we analyzed during four consecutive years the impacts of rainfall reduction (RE; ambient vs. ~30% reduction in rainfall), soil warming (W; ambient vs. ~ 1 °C increase) and their interaction on biogeochemical cycling of C, N and P in Mediterranean forests. Rainfall exclusion translated into quick significant reductions of soil organic matter (SOM), enzymatic activity (β-glucosidase, urease and acid phosphatase activities) and nutrient availability (ammonium, nitrate and phosphorus) one year after the application of the treatments. These effects were consistent over time. Warming acted synergistically with rainfall reduction to further decrease C-related variables (SOM and β-glucosidase). SOM reduction in forest soils might be the result of delayed leaf senescence as a drought tolerant trait in the forest trees. Warming also had direct positive effects on N- and P- related variables that partially counteracted the negative effects of rainfall reduction on these variables. Overall, our results showed that the different components of climate change (drought and warming) have complex direct and interactive effects on biogeochemical cycles of Mediterranean forests that differ among soil nutrients (C, N, P). Consequently, drought and warming might cause an unbalance in natural biogeochemical cycles of Mediterranean forests, with important consequences for ecosystem functioning.

How to cite: Villa-Sanabria, E., Gallardo, A., Gutiérrez, E., Serrano, M. S., Homet, P., and Gómez-Aparicio, L.: Climate change unbalances biogeochemical cycles of C, N and P in Mediterranean forests, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16164, https://doi.org/10.5194/egusphere-egu23-16164, 2023.

Forest soils are crucial for many ecosystem services that rely on soil organic matter (SOM) stability. Carbon allocated to roots and released as exudates to the rhizosphere plays a key role in SOM stabilization. Under periodic drought, elevated root exudation and SOM accumulation have been reported. Yet, whether root exudates control SOM formation and stability in mature forests once the drought ends is largely unknown. We examined whether root exudates from P. abies and F. sylvatica trees relate to SOM formation and stability in soil depth profiles one year following five years of experimental drought (Kroof experiment, Germany). We collected root exudates throughout the rooting zone and combined the data with thermogravimetric analysis of SOM in the rhizosphere and non-rooted soil. We found that the rhizosphere of both species was characterized by stable SOM fractions that did not decrease post-drought, suggesting potential protection of SOM due to rhizodeposition and root exudates. In contrast, stable SOM fractions decreased relative to controls in non-rooted topsoil below P. abies, indicating a loss of stabilized SOM from drought-affected and re-wetted soil. Our measurements provide valuable insights into post-drought SOM formation and mechanisms of SOM stabilization in forest ecosystems under climate change.

How to cite: Brunn, M., Hafner, B. D., Bölscher, T., Hikino, K., Jungkunst, H. F., Kučerík, J., Neff, J., Pritsch, K., Sayer, E. J., Weikl, F., Zwetsloot, M. J., and Bauerle, T. L.: Post-drought root exudation defines soil organic matter stability in a temperate mature forest, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11241, https://doi.org/10.5194/egusphere-egu23-11241, 2023.

The increasingly warmer and drier climate will change the geographic distribution of tree species. Here, apart from direct effects of changes in temperature and soil moisture on tree physiology, indirect effects via interactions between soil environmental conditions and the bioavailability of nutrients may play an additional role. Specifically, this comprises effects of desiccation and rewetting on soil properties, and of drought and drought release on root-soil interactions in the rhizosphere. Such effects may be particularly strong for phosphorus, considering its low solubility and mobility in soils.

For this study, we considered data from 44 forest sites across Switzerland representing soil moisture gradients for each of the four major tree species beech (Fagus sylvatica L.), oak (Quercus sp.), pine (Pinus sylvestris L.) and spruce (Picea abies Karst). First, we explored the relations between the nutrient status of the soil (total N, hydrogencarbonate extractable inorganic and organic P, microbial P, exchangeable K, Mg, Ca) and soil environmental conditions during four years before soil sampling (average water potential and temperature, number of dry days and drying-rewetting [DRW] cycles as defined by a given water potential threshold). Second, we performed the same analysis using the nutrient status of mature trees (as indicated by nutrient concentrations in bark) instead of the soil nutrient status.

Results indicate a strong influence of DRW on the soil’s P status, whereas other nutrients in the soil are only little affected. On beech and oak sites, the N and P status of the trees increased with increasing moisture and decreasing temperature, and the P status was in addition negatively affected by the length and number of DRW cycles during which high drought stress levels were reached. By contrast, the Ca status of the trees increased with temperature and soil dryness.

Considering observations by others regarding positive effects of high plant nutrient status on a plant’s resilience to drought, our results call for more in-depth studies on the feedback-loops between soil water supply, soil nutrient availability, plant-physiology and root-soil interactions.

How to cite: Luster, J., Ulmann, A., Herzig, A., and Walthert, L.: Soil and plant nutrient status in temperate forests as affected by long-term drying-rewetting conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16676, https://doi.org/10.5194/egusphere-egu23-16676, 2023.

Soil carbon turnover time (t_SOC), the ratio between soil organic carbon stocks (SOC), and soil heterotrophic respiration (R_h), is a critical factor in determining soil carbon storage and a key parameter in terrestrial carbon models. While t_SOC is generally expected to increase with drying conditions, its interactions with the carbon fluxes and soil moisture are still poorly constrained. Our study centered on a five-year manipulation experiment in the Yatir semi-arid pine forest in Israel, where supplement irrigation eliminated the summer drought. Soil CO₂ fluxes (F_s) and soil organic carbon (SOC) stocks were measured under trees and in open areas in a "control" forest plot (CTRL) and an 0.1 ha “irrigated” plot (IRRI). During the dry period (May-November), daily average F_s in the open areas was near zero in the CTRL but significant in the IRRI plots (0.06 and 2.02 µmol CO₂ m^-2 s^-1 respectively, with a similar trend under the trees). Annual-scale fluxes in the open areas were 82 and 321 g C m^-2 yr^-1 in the CTRL and IRRI plots, respectively (with similar trends under trees). Using published results from the same site enabled us to partition F_s and estimate R_h, which indicated that under the drought conditions (CTRL) t_SOC was x5 longer in the open area (and x2 longer under trees) compared with the non-droughted (IRRI) plot. However, no significant changes in the SOC stock down to 40 cm (the typical soil depth at this site) were observed.  Furthermore, there were no differences between treatments in regard to the ratio between the stable mineral-associated organic carbon fraction and the particulate organic carbon fraction. The stability of SOC stocks, despite the large changes in t_SOC suggests that carbon inputs must have increased proportionally to match the changes in carbon outputs and provided the main source for the increased F_s. The results indicate that changes in the intensity of the seasonal drought can result in large changes in fluxes and t_SOC values with little impact on soil carbon storage and its stability.

How to cite: Yalin, D., Qubaja, R., Tatarinov, F., Rotenberg, E., and Yakir, D.: Soil moisture manipulation in a semi-arid pine forest demonstrates large changes in carbon turnover time with no change in soil carbon stock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11793, https://doi.org/10.5194/egusphere-egu23-11793, 2023.

Soil respiration is a measure of the flux of greenhouse gases and accounts for the release and uptake of CO₂, CH₄ and N₂O. It is a function of heterotrophic microbial activity through the mineralisation and immobilisation of organic matter and the symbiotic relations formed in the rhizosphere, contributing to the autotrophic component. It is a primary process within woodlands that contribute to the efflux of CO₂, the sink capacity of CH₄ and the variable flux of N₂O. Understanding how woodland soils will react to rising atmospheric CO₂ levels is critical for budgeting, modelling, and providing insightful management strategies for global forests. The Birmingham Institute of Forest Research is a Free Air Carbon Enrichment facility (BIFoR-FACE), whereby the seasonal and diurnal fumigation of CO₂ is closely controlled. Providing localised and enriched atmospheric CO₂ levels across the canopy of a mature temperate Oak dominant woodland that is representative of 2050 levels.

The flux of CO₂ from the soil has been continuously measured within fumigated treatment (eCO₂) and ambient control (aCO₂) arrays since 2017, with capabilities to additionally measure CH₄ and N₂O being added in 2020. Across all arrays, CO₂ fluxes showed significant negative correlations with soil moisture but significant positive correlations with soil temperature. Initial trends from 2017 - 2020 indicated that eCO₂ arrays had a higher efflux of CO₂ relative to paired aCO₂ arrays, with this pattern switching in 2021. During the 2021 and 2022 fumigation seasons, eCO₂ arrays have seen a decline in the efflux of CO₂, to levels lower than aCO₂ plots. Mean values during 2022 for the efflux of CO₂ within eCO₂ arrays were 2.63 μmol m^-2 s^-1 (n = 18762) versus 3.62 μmol m^-2 s^-1 (n = 22856) for aCO₂ arrays. Uptake of CH₄ and efflux of N₂O were not significantly different between arrays, although eCO₂ arrays had lower CH₄ uptake and N₂O efflux. Indicating a potential decline in the efflux of CO₂ but a reduced uptake of CH₄ from temperate woodlands under future atmospheric conditions. Further investigation will now look to understand the mechanistic drivers behind these changes, focusing on the microbial heterotrophic contribution as a potential mediator of these noted flux rates changes under eCO₂

How to cite: Armstrong, A.: How will forest soils ‘breathe’ in 2050? Soil respiration of CO2, CH4 and N2O under elevated atmospheric CO2., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8621, https://doi.org/10.5194/egusphere-egu23-8621, 2023.

Estimating the effect of past century global warming on agricultural soil carbon stocks

Christopher Poeplau, Rene Dechow

Climate change is likely to affect soil organic carbon (SOC) stocks across the globe. Therefore, modelling efforts are undertaken to estimate the effect of future climate change on SOC stocks at different spatial scales and for various climate change scenarios. However, global average air temperature already increased by more than 1°C, which most likely already affected and affects global SOC stocks. Agricultural soils were observed to lose SOC in many parts of the world, which is partly interpreted as climate-driven. For deconfounding management and climate change effects, the latter needs to be estimated comprehensively. In this study, an established FAO framework, including the global SOC map as well as the RothC and MIAMI models, was used to model global agricultural topsoil SOC stock dynamics from 1919 to 2018 as attributable to climate change.

On average, global agricultural topsoils lost 2.5±2.3 Mg C ha^-1 with constant net primary production (NPP) or 1.6±3.4 Mg C ha^-1 when NPP was modified by temperature and precipitation. These loss rates per °C were comparable to those observed in long-term geothermal warming experiments, which are also presented as a source of validation. Regional variability in SOC stock changes was explained by the complex patterns of alterations in temperature and moisture, as well as initial SOC stocks as major drivers of mineralisation and partly also C inputs in the models. On average, SOC losses have been a persistent feature of climate change in all climatic zones during the past century. This needs to be taken into consideration in reporting or accounting frameworks and halted in order to mitigate climate change and secure soil health. At the same time, the estimated climate-driven loss rates were partly much smaller than observed SOC losses from agricultural soils, indicating that other management-related drivers have been more important. 

How to cite: Poeplau, C. and Dechow, R.: Estimating the effect of past century global warming on agricultural topsoil carbon stocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3380, https://doi.org/10.5194/egusphere-egu23-3380, 2023.

Forests play a crucial role in the carbon biogeochemical cycle. A significant amount of carbon is lost every year through soil respiration (SR). SR is known to respond exponentially to soil temperature (ST), but it is still unclear how SR responded to the long-term increases in temperature. Hence, we currently cannot predict if global warming would result in an overall increase or decrease of CO₂ release from forest soils. 
In this context, our study aims to understand how SR has changed over time in a mixed deciduous forest in Switzerland. The study site is located in the Lägeren (CH-LAE) forest at 689 m a.sl., which is mainly composed by European beech trees (Fagus sylvatica) and Norway spruce (Picea abies). SR was measured with a closed chamber through survey campaigns in 2006, 2007, 2021 and 2022. In addition, continuous measurements of forest floor Net Ecosystem Exchange (NEE_ff) with a below-canopy Eddy covariance system have been running since 2014. We then partitioned these fluxes to obtain forest floor respiration (R_ff). A random forest analysis was performed to investigate SR and R_ff drivers; SR responses to ST were analysed with the Lloyd-Taylor (1994) equation.  
The aims of this research are (1) to compare the magnitude of SR and R_ff, (2) to evaluate the drivers of SR and R_ff, and finally (3) to investigate the change of ST, SR and R_ff over time. We expect that ST is the main driver, and that the magnitude of SR and R_ff is comparable. Moreover, we hypothesize a long-term acclimation of SR and R_ff to the increasing air and soil temperatures recorded at the study site. We indeed found ST driving SR and R_ff, except for drought conditions when soil moisture becomes a limiting factor. We also observed that the sensitivity of SR to ST has increased over time, suggesting higher CO₂ fluxes from the forest soil with increasing temperature due to climate change.

How to cite: Scapucci, L., Krebs, L., Burri, S., and Buchmann, N.: Long-term responses of soil and forest floor respiration to increasing temperature in a mixed deciduous forest, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17160, https://doi.org/10.5194/egusphere-egu23-17160, 2023.

Earthworm species are grouped into three ecological categories, endogeic, epigeic and anecic from their nutritional behaviors, their strategies of morpho-functional adaptations and localization in the soil profile. These earthworms play a major role in the biogeochemical cycling of carbon and nitrogen through their bioturbation activity and cast production that have specific physical, chemical and biological characteristics. They affect the sequestration of soil C through their influence on soil organic matter (SOM) protection processes. However, CO₂ and N₂O emissions from cast depend on earthworm species with different physical, chemical and microbiological properties even when these species belong to the same ecological category. Hence the importance of investigating their functional traits to see how they affect greenhouse gas emissions. To do this, we measured the emissions of N₂O and CO₂ in earthworm cast, then we characterized the physical-chemical properties from elemental and spectroscopic analyses. Finally, these results were related to different morphological (pigmentation, color, size), anatomical (gizzard, pharynx, morren's gland size) and physiological traits. Our hypothesis is that the casts of pigmented and colored earthworms, with a large pharynx and gizzard, morren's gland, secreting a lot of mucus and water produce more CO₂, N₂O.

How to cite: Zi, Y., Dignac, M.-F., Bottinelli, N., Capowiez, Y., Florio, A., and Rumpel1, C.: Effects of earthworm functional traits on CO2 and N2O emissions from casts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15615, https://doi.org/10.5194/egusphere-egu23-15615, 2023.

Extreme weather events such as prolonged flooding and extended drought are predicted to increase in frequency and intensity due to climate change. Drying and rewetting influence soil nutrient cycling and greenhouse gas emissions, particularly where nutrient inputs are high such as in agricultural systems. Flooding and drought events therefore directly influence climate change, nutrient fate and nutrient use efficiency. Soil wetting events can stimulate nitrous oxide (N₂O) hot moments (disproportionately high emission rates over a short temporal period). Antecedent soil moisture conditions influence these hot moments, however this relationship and the mechanisms underlying it are not yet fully understood.

Characterisation of N₂O hot moments in response to current and future climatic conditions is essential to inform land management practices and nutrient application regimes. This work explores the relationship between hydrological events and resultant hot moment dynamics, and aims to elucidate the mechanisms fundamental to these processes.

In this study, soil samples were subjected to four treatment conditions (n=5) for a 14-day dry period: 5%, 20%, 35% and 50% water filled pore space (WFPS). After this period, all soils were fertilised (100 kg N ha^-1 ammonium nitrate) and simultaneously wetted to 90% WFPS for a further 14 days, to stimulate an N₂O hot moment. Gas emissions (N₂O, CO₂, CH₄) and soil chemistry (NO₃^-, NH₄⁺, dissolved organic carbon) were analysed throughout the 28-day incubation, and untargeted metabolomics analysis was conducted on day 14 of the dry period.

Our results showed hot moments to intensify under pre-drought conditions, with 5% and 20% WFPS considered a drought, versus 35% and 50% WFPS considered moist. For the first time, we showed extreme drought (5% WFPS) to significantly influence hot moment dynamics compared with moderate drought and moist conditions, with emissions occurring more abruptly and to a greater intensity over a 3-day, versus > 14-day, timeframe. Possible explanations for this shift include microbial osmolyte accumulation during drought and secretion upon rewetting, resulting in a labile C pool (immediate C availability); microbial cell death during drought or rewetting (immediate C availability via necromass); or shifts in microbial community structure, or gene expression rate, following rewetting. Untargeted metabolomics analysis is being conducted to determine the extent of osmolyte accumulation between treatments, including the nature of said osmolytes for indication of species likely involved in accumulation, and to probe any disparities in active microbial metabolic pathways, and therefore function, between treatments.

In summary, our results indicate antecedent conditions to significantly influence N₂O hot moments following wetting, with extreme droughts appearing to shift biogeochemical process dynamics compared with dry-to-moist conditions. Microbial activity, function and substrate availability may play explanatory roles in this shift, with untargeted metabolomics promising a powerful tool to probe underlying functional mechanisms.

How to cite: Withers, E., Cardenas, L., Jones, D. L., and Chadwick, D. R.: Extreme drought influences N2O hot moment intensity and duration, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-473, https://doi.org/10.5194/egusphere-egu23-473, 2023.

A diverse and broad range of sizes and materials – the total mobile inventory (TMI) – is mobile in soil (Lehmann et al., 2021). Constituting matter exchange between surface and subsurface ecosystems, it is still almost unknown how dissolved, colloidal to particulate fractions (>0.45 µm) fluctuate in seepage of undisturbed soil in both the short (intra-event scale) and long terms (seasons and multiannual periods). In the topographic groundwater recharge area of the Hainich Critical Zone Exploratory (NW-Thuringia), we monitor TMI dynamics using tension-controlled lysimeters in topsoil and subsoil under forest, pasture, and cropland. Soil seepage and precipitation were sampled on a regular (biweekly) and event-scale cycle and analyzed by physico-/hydrochemical, spectroscopic, and molecular biological methods. Within >6.5 years, fluctuations in the TMI were mainly driven by atmospheric conditions (precipitation, temperature), showing pronounced seasonality in the signature of dissolved components (e.g., sulphate) and the seepage pH. In hydrological winter, the total export of bacteria from soil was 1.5-fold higher compared to summer. However, episodic, and strong infiltration events, for instance after snowmelts or heavy rain storms, resulted in increased mobilization of particles. Noteworthy, the export of particulate organic carbon (POC) during winter infiltration events accounted for ~80% of the total annual translocation. Taking >20% of the total mobile OC on average, mobile POC, thus, must be taken into account in carbon balances and for ecosystem interactions. Against the background of increasing climate variability and extreme conditions due to climate change, we furthermore investigated intra-event dynamics of the TMI by increased temporal resolution to improve the understanding of the factors influencing the translocation. During a snow melt event (February 2021) and a simulated heavy rain event (artificial irrigation, August 2021), strong intra-event fluctuations of the TMI (e.g., nitrate, DOM, microorganisms) were also caused by effects of preferential flow. Besides improving our understanding of the soil carbon balance, our long-term monitoring identifies possible controlling factors for the coupling and supply of subsurface ecosystems (aeration zone, groundwater) and for their chemical, biological and functional fluctuations.

Lehmann, K., Lehmann, R., & Totsche, K. U. (2021). Event-driven dynamics of the total mobile inventory in undisturbed soil account for significant fluxes of particulate organic carbon. Science of The Total Environment, 143774.

How to cite: Lehmann, K., Lehmann, R., Herrmann, M., Schroeter, S., and Totsche, K. U.: (Intra-event) dynamics of the total mobile inventory in soil - importance for carbon balances and coupling of subsurface ecosystems, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9883, https://doi.org/10.5194/egusphere-egu23-9883, 2023.

Patterns of nitrous oxide (N₂O) emissions related to water-filled pore space and gas diffusivity have been described as evidence of denitrification activity. However, the complexity of factor combinations in managed agroecosystems calls for comprehensive studies relating to N₂O emissions and water content dynamics, both spatially and temporally. This study examined the impact of two water table levels and free drainage (60 cm, 120 cm and free drainage, WT60, WT120 and FD) in combination with land management (conservation and conventional agriculture) on the seasonal variation of surface N₂O flux and aeration/water stress on the soil profile. N₂O emissions and volumetric water content dynamics were measured on a lysimeter experiment over three years (2018-2020). Preliminary results show that N₂O emissions were driven by fertilization over time irrespective of water table level and land management. In the topsoil, WT60 and WT120 promoted longer periods of aeration stress under conservation agriculture compared to conventional, whereas FD increased water stress days under conventional agriculture. At 30 and 60 cm depth, water content under FD remained mostly within the range of nonlimiting for plant growth, and under WT60 and WT120 was generally above the aeration limit in-season and over time. Correspondently, calculated relative gas diffusivity was limiting for conservation agriculture in the topsoil compared to conventional agriculture, and below the anoxia threshold for both land management at 30 and 60 cm depth. This suggests that the subsoil could become a potential hotspot for N₂O production under shallow water levels. Cumulative surface N₂O appears to be related to the cumulative number of aeration stress days derived from the nonlimiting water range, with variation across years and land management.

How to cite: Politi Moncada, M., Longo, M., Dal Ferro, N., and Morari, F.: Seasonal variation of nitrous oxide flux and aeration/water stress at different water table levels, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2619, https://doi.org/10.5194/egusphere-egu23-2619, 2023.

Tidal marsh is a large reservoir of soil organic carbon (SOC), considered one of the most efficient natural carbon sinks. However, the future carbon pool of this ecosystem has large variability and uncertainty due to climate change such as temperature rise and elevated atmospheric CO₂ concentration. Microbial-mediated SOC decomposition is a key process that regulates the carbon cycle of tidal marsh. This process is largely temperature-dependent, thus understanding the response of temperature sensitivity (Q10) of SOC decomposition to climate change is critical to improving our prediction capability of the tidal marsh carbon cycle and the climate feedback. However, the response of Q10 of SOC decomposition on climate change in tidal marsh ecosystem is yet to be revealed, hampering our prediction capability of the future tidal marsh carbon cycle. Here, we elucidate the effect of warming and elevated CO₂ concentration on the Q10 of SOC decomposition at a Salt Marsh Accretion Response to Temperature eXperiment (SMARTX). Surface sediments were collected in 2022 (6 years after manipulation started) from ambient, +5.1℃ (W), elevated CO₂ (750ppm, eCO₂), and +5.1℃+elevated CO₂ (W+eCO₂) plots and the Q10 of SOC decomposition was determined. W significantly decreased aerobic SOC decomposition rate most likely due to the labile carbon depletion and thermal adaptation, whereas eCO₂ and W+eCO₂ increased the aerobic decomposition rate at high temperatures (25 and 30℃). Anaerobic SOC decomposition rate was not affected by W whereas eCO₂ and W+eCO₂ significantly increased anaerobic decomposition rate at all temperatures. Q10 of aerobic SOC decomposition was not affected by W whereas eCO₂ and W+eCO₂ significantly increased. Q10 of anaerobic SOC decomposition was not affected by climate change. Overall, our finding demonstrates that elevated CO₂ concentration increases the vulnerability of soil carbon stock to warming in tidal marshes, with implications for modeling the future carbon cycle of the ecosystem.

How to cite: Lee, J., Yang, Y., Kang, H., Noyce, G., and Megonigal, P.: Temperature sensitivity of soil organic carbon decomposition responses to warming and elevated CO2 in the tidal marsh ecosystem, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4782, https://doi.org/10.5194/egusphere-egu23-4782, 2023.

Tidal marshes are an important transition zone connecting marine, freshwater, and terrestrial ecosystems. Tidal marshes are known to be sensitive to global climate change such as elevated CO₂ and warming. In particular, it is yet to be revealed whether the such change may increase or decrease carbon stock in tidal marshes. Although soil microorganisms play an important role in carbon storage in tidal marshes by determining carbon mineralization, little is known about the interactive effects of elevated CO₂ and warming on soil microbial communities in field conditions. Here, we elucidate the effects of 6-year experiment of climate manipulation on soil microbial communities in a tidal marsh. The manipulation experiment included 4 treatments of 1) elevated atmospheric CO₂ concentrations (750 ppm) only, 2) warming (+5.1 °C) only, 3) both elevated CO₂ and warming, and 4) ambient conditions. Elevated CO2 significantly changed the structure of the RNA-derived (active) and DNA-derived (total) soil microbial community, but warming did not affect either. The relative abundances of Acidobacteria, Actinobacteria, Chloroflexi, and Planctomycetes were higher in the DNA-derived soil microbial communities than in the RNA-derived soil microbial communities, whereas those of Campilobacterota, Desulfobacteria, Gammaproteobacteria, Myxococcota, and Spirochaetota were higher in the RNA-derived soil microbial communities. In addition, elevated CO₂ changed the microbial communities from r- to K-strategists in both RNA and DNA-derived communities, suggesting that this may offset additional C input by roots in a future elevated atmospheric CO₂ environment. This study provides a better understanding of microbial response to the combined effects of elevated atmospheric CO₂ concentrations and global warming in the tidal marsh ecosystems.

How to cite: Yang, Y., Lee, J., Noyce, G., Megonigal, P., and Kang, H.: The long-term effects of elevated atmospheric CO2 and warming on soil microbial communities in a tidal marsh ecosystem, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4785, https://doi.org/10.5194/egusphere-egu23-4785, 2023.