BG3.22

Forests under elevated atmospheric CO₂ concentration as a result of climate change are expected to require more available nitrogen (N) to sustain the enhanced CO₂ uptake for photosynthesis and C storage. Therefore, it is essential to evaluate how CO₂ fumigation of forests will affect availability of N to trees. Main pathways to sustain the high N demand are increasing biological N fixation (BNF), increasing N turn-over and reducing N losses. The purpose of this research is to explore the effects of elevated CO₂ on soil N cycling in a temperate forest under the Birmingham Institute of Forest Research (BIFoR) Free Air Carbon Dioxide Enrichment facility. We hypothesize that under CO₂ fertilization, trees will allocate more carbon belowground to enhance microbial activity for increasing N mineralization as well as enhancing BNF to meet N demands. We also hypothesize that the subsequent microbial activity will up-regulate N₂O and N₂ emissions due to denitrification. BNF by free-living organisms is investigated using the ¹⁵N assimilation method. Mineralization and N gas production rates is determined using the ¹⁵N pool dilution and ¹⁵N-Gas flux method at the same time. Early results are showing trends toward an enhancement of N mineralization and microbial N immobilization rates. However, BNF in the forest floor is hardly detectable more likely because of the high N deposition in the area; therefore, it doesn’t appear to be responsive to CO₂ fumigation. This research is expected to help us improve our understanding of the changes and magnitude of nutrient availability and gaseous losses under future climates.

How to cite: Rumeau, M., Ullah, S., Mackenzie, R., Carillo, Y., Reay, M., and Sgouridis, F.: Nitrogen cycling in forest soils under elevated CO2: response of a key soil nutrient to climate change, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-178, https://doi.org/10.5194/egusphere-egu22-178, 2022.

Soil microorganisms are frontline managers of the terrestrial carbon cycle. To better understand and model their effects under a changing climate, it is critical to determine which microbial ecophysiological traits are associated with soil organic matter formation – particularly mineral-associated organic matter (MAOM). Yet major uncertainty surrounds the traits that regulate this process, and how environmental context (e.g. spatial habitat, moisture conditions) shapes the manifestation of these traits. Microbial carbon-use efficiency (CUE) is posited to be a particularly key microbial trait, yet direct evidence for this relationship is sparse, and few other microbial traits have been directly tested as predictors of MAOM formation.

To investigate the relationship between different microbial traits and MAOM, we conducted a 12-week ¹³C tracer study to track the movement of rhizodeposits and root detritus into microbial communities and SOM pools under moisture replete (15 ± 4.2 %) or droughted (8 ± 2%) conditions. Using a continuous ¹³CO₂-labeling growth chamber system, we grew the annual grass Avena barbata for 12 weeks and measured formation of ¹³C-MAOM from either ¹³C-enriched rhizodeposition or decomposing ¹³C-enriched root detritus. We also measured active microbial community composition (via ¹³C-quantiative stable isotope probing; qSIP) and a suite of microbial traits that may be important in soil carbon formation, including community-level carbon-use efficiency, growth rate, and turnover (via the ¹⁸O-H₂O method), extracellular enzyme activity, bulk ¹³C-extracellular polymeric substances (EPS), and total microbial biomass carbon (¹³C-MBC).

We found that different microbial traits were associated with MAOM formation in the rhizosphere versus the detritusphere, and their effect was influenced by soil moisture. In the rhizosphere, fast growth and turnover were positively associated with MAOM, as were total ¹³C-MBC and ¹³C-EPS production. In contrast, growth rate was negatively associated with MAOM formation in the detritusphere, as were CUE, ¹³C-MBC, and ¹³C-EPS. However, extracellular enzyme activity was positively associated with MAOM in the detritusphere. These results, paired with data on the chemical composition of MAOM (via STXM-NEXAFS) suggest that traits associated with fast growth and death rates, as well as high necromass yield, generate microbial-derived MAOM in the rhizosphere, whereas traits associated with resource acquisition generate plant-derived MAOM in the detritusphere. We also present ¹³C-qSIP data demonstrating that fungal taxa are more active in the detritusphere, whereas certain bacterial phyla (e.g., Firmicutes) are more active in the rhizosphere. Together, our results show that distinct traits, communities, and pathways of MAOM formation predominate in the rhizosphere versus the detritusphere. New research should focus on a broader suite of microbial traits – including but not limited to CUE – to model the role of microbes in MAOM formation in distinct habitats and moisture conditions of the soil.  

How to cite: Sokol, N., Foley, M., Blazewicz, S., Estera-Molina, K., Greenlon, A., Firestone, M., Hungate, B., Slessarev, E., Liquet, J., and Pett-Ridge, J.: Soil habitat and drought shape microbial traits associated with mineral-associated soil carbon formation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-409, https://doi.org/10.5194/egusphere-egu22-409, 2022.

Soils are the dominant global source of the important greenhouse gas nitrous oxide (N₂O). The anthropogenic input of nitrogen (N) into soil ecosystems increases the rate of soil N cycling, and thus enhances soil N₂O emissions. N₂O is produced during microbial N transformation processes, mainly via oxic nitrification and anoxic denitrification processes. These predominant pathways depend heavily on soil environmental conditions, such as soil moisture, aeration and substrate availability, which are modulated by weather and climate conditions, atmospheric composition and land use. Consequently, N₂O emission rates and pathways are likely to be affected by future global changes in climate and atmospheric composition. However, the combined effects of elevated carbon dioxide (eCO₂) and elevated air temperature on both N₂O emission rates and pathways are unclear, as the effects can be synergistic, antagonistic or additive, and they can be further influenced by additional interacting disturbances (e.g. summer drought).

Here we test how soil N₂O fluxes and emission pathways respond to environmental changes in a multifactorial climate manipulation experiment, combining warming and eCO₂, as well as precipitation manipulation to simulate an extreme drought during the growing season in a managed montane grassland. For the first time, we combine in-situ surface N₂O flux measurements with online high-time resolution isotopic measurements, soil N₂O isotope depth profiles, molecular microbial ecology, and complementary soil and microclimate measurements. Under future global change conditions, we expect increasing N₂O emission rates, as well as an increasing importance of denitrification, due to the effect of large emission pulses following rewetting. In addition, we hypothesize that drought effects overrule other environmental change factors. Our results will provide an unprecedented insight into the effects of global changes on soil N dynamics and soil N₂O emissions in managed montane grasslands. Furthermore, these findings will help to improve the modelling of N dynamics at the atmosphere-biosphere interface, which will be used to derive soil N₂O production and consumption pathways, based on soil N₂O isotope measurements, and to upscale the results to examine their potential global relevance.

How to cite: Stoll, E., Diaz-Pines, E., Reinthaler, D., Radolinski, J., Schloter, M., Schulz, S., Duffner, C., Tian, Y., Wanek, W., Pötsch, E., Glatzel, S., Zechmeister-Boltenstern, S., Bahn, M., and Harris, E.: Understanding soil N2O emissions and production pathways in a changing climate by coupling automated chambers with isotope measurements, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-500, https://doi.org/10.5194/egusphere-egu22-500, 2022.

Strong recent challenges of the long-standing paradigm that macroclimate predominantly controls decomposition questions the accuracy of climate change predictions. With a novel approach combining three experiments at continental scale, using 104 litter types in 194 plots in six major European forests, we show that the confusion around the macroclimate control on decomposition is mostly an experimental artefact. The relative role of decomposition drivers was incorrect when disrupting the natural context of locally-produced litter decomposing locally, with either a focus on litter characteristics neglecting microenvironmental context, or on environmental drivers neglecting local litter characteristics. Our data reaffirm macroclimate and its interaction with litter characteristics as predominant decomposition drivers. Conversely, standard litter types overrated microenvironmental control and failed to predict local decomposition of plot-specific litter. Our findings provide support for a strong macroclimate component in predictive decomposition models and call for cautious interpretation of data from experiments using standard litter types.

How to cite: Joly, F.-X., Scherer-Lorenzen, M., and Hättenschwiler, S.: Cutting the Gordian knot of climate control on decomposition , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1192, https://doi.org/10.5194/egusphere-egu22-1192, 2022.

IPCC climate models (RCP8.5) suggest 4°C warming until 2100, which could potentially accelerate soil carbon loss, greenhouse gas release, and thus promote global warming. Despite low carbon concentrations, subsoils (> 30 cm) store more than half of the total global soil carbon stocks. Retaining this is crucial to mitigate soil carbon greenhouse gas release. However, how deep soil carbon will respond to warming and how increased root-derived carbon could contribute to carbon stabilization in subsoils is under-studied and largely unknown. We aim to i) quantify the decomposition rate of root-litter at different depths with a +4°C warming field experiment, ii) assess whether various plant polymers will degrade differently in heated and control plots, iii) identify decomposition products of plant biomass remaining after three years of incubation.

In a field experiment in a temperate forest, ¹³C labelled root-litter was added at different soil depths (10-14, 45-49, 85-89 cm) in 2016 and retrieved half of the cores in 2017 and the remaining half in 2019. So far, we measured bulk soil carbon concentrations and d¹³C-composition of individual microbial biomarkers (PLFA).

Results confirm that bulk carbon concentrations and d¹³C values follow typical depth trends, except for the three horizons containing ¹³C labelled root-litter incubations. Next, we will quantify above- and below-ground biomarkers (cutin and suberin, respectively) and determine compound-specific ¹³C-composition in each molecular fraction in heated and control plots. We suspect that the presumably difficult to degrade compounds (cutin, suberin, and lignin polymers) will degrade slower than bulk organic matter over the observation period, and likely faster in heated than control plots. However, early results from warming experiments provide circumstantial evidence that also these compounds might degrade in synchrony with the bulk organic matter.

How to cite: Sun, B., Zosso, C., Wiesenberg, G., Schmidt, M., and Torn, M.: How will belowground plant biomass in deep soil respond to warming in temperate forests?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2433, https://doi.org/10.5194/egusphere-egu22-2433, 2022.

Storms represent a major disturbance factor in forest ecosystems, but the effects of windthrows on soil organic carbon (SOC) stocks are quantitatively poorly known. Here we present a comprehensive analysis of windthrow-induced changes in SOC stocks in Swiss forests by combining field-based measurements and modelling simulations. We measured the SOC stocks of 19 windthrown forests across Switzerland, about 10 and 20 years after they were disturbed by the storms ‘Lothar’ and ‘Vivian’ and compared them to the stocks of adjacent intact forests. We also calibrated the process-based model Yasso07 for additional 77 windthrown forests. Our results show that the effect of windthrow on SOC is strongly related to the size of the initial SOC stocks in the organic layer. In absolute and relative terms, the largest SOC losses occurred in high-elevation forests with thick organic layers, where initial SOC stocks decreased by up to 90% (or 30 t C ha^-1). In contrast, SOC stocks of low-elevation forests with thin organic layers were hardly affected. The likely reason for this pattern is the high stocks of easily mineralizable organic matter in thick organic layers of mountain forests, while at low elevations a greater SOC fraction is stabilized by mineral interactions. Modelling simulations further show longer-lasting SOC losses and a slower recovery of SOC stocks after windthrow at high-elevations compared to low-elevations, due to a slower regeneration of mountain forests and associated lower C inputs into soils. We also upscaled the SOC changes after windthrow to the whole forested area of Switzerland and estimated a total SOC loss of ~0.3 Mt C after the storms ‘Lothar’ and ‘Vivian’. Our results provide strong empirical evidence that windthrows can reduce the SOC stocks of forest ecosystems, with mountain forests being hotspots for SOC losses.

How to cite: Mayer, M., Rusch, S., Didion, M., Baltensweiler, A., Walthert, L., Zimmermann, S., and Hagedorn, F.: High soil organic carbon losses in response to forest windthrow, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3521, https://doi.org/10.5194/egusphere-egu22-3521, 2022.

Earth system model are designed to capture our present knowledge of soil-carbon-climate interactions. However, uncertainties remain high because mechanistic insights are available at fine scales for which we can never achieve unbiased resolution for global modeling. Consequently, the key challenge gaining global or regional overviews of soil carbon-climate feedbacks is to identify the scale that best reflects the underlying soil processes without getting lost in details. According to latest findings, the dominant control of soil carbon persistence varies with climate, which suggests that overarching proxies at a critical mesoscale combine climatic and soil factors and could enable regionally tailored approaches. Here, the Holdridge Life Zone (HLZ) classification proved to be more than a descriptive tool to guide our understanding of soil carbon-climate interaction allowing for linking top-down (from global to local) and bottom-up (from local to global) approaches. In the talk we will present the results for the indiviaul 38 HLZ and present possibilities to add soil internal controls. Regionally tailored solutions can lead to better management of soil carbon. Improving ‘translations’ from the scales relevant for process understanding to the scales of decision-making is key to sustainable soil management and to improve predictions of the fate of our largest terrestrial carbon reservoir during climate change.

How to cite: Jungkunst, H., Brunn, M., Goepel, J., Ott, S., and Horvath, T.: Finding the sweet scale to understand processes and climate control over soil carbon stocks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3647, https://doi.org/10.5194/egusphere-egu22-3647, 2022.

Methane (CH₄) is over 25 times stronger greenhouse gas than CO₂ on a molar basis, and its concentration increases continuously. Its global budget, however, is yet to be constrained, and the greatest uncertainty is associated with natural sources and sinks. Terrestrial ecosystems are known to uptake atmospheric CH₄ by a group of microbes known as methanotrophs. Previous studies have noted that the controlling variables for methane oxidation in forest soils are moisture content, temperature and nutrient availability. However, previous estimation on CH₄ oxidation in terrestrial soils exhibit great discrepancy between model estimates and field observations. In this study, we measured CH₄ uptake rates monthly in temperate pine forests and seasonally in subtropical forests in Korea for 2 years. In addition, soil chemical properties and microbial composition were monitored to reveal the key controlling variable for the uptake rates. Deciduous forests showed the highest CH₄ uptake rate followed by mixed forests and the lowest was observed in coniferous forests. Air-filled porosity and the abundance of methanotrophs were correlated with CH₄ uptake rate. Our results as well as meta-analysis of 207 measurements from 84 literature showed that soil organic matter (SOM) content significantly correlated with soil CH₄ uptake rate at both regional and global scales, indicating that SOM can be a robust controlling factor for CH₄ oxidation. We speculate that SOM content affects soil CH₄ oxidation via alters air-filled porosity and available carbon source for facultative methanotrophs. The amount of CH₄ oxidation in global forests estimated by a model based on SOM content is 22.22 Tg, which far exceeds previous estimation of 17.46 Tg.

How to cite: Kang, H., Lee, J., and Oh, Y.: Soil organic matter as a key controlling variable for methane oxidation in forest soils – microbial analysis and global estimation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3970, https://doi.org/10.5194/egusphere-egu22-3970, 2022.

Carbon cycling in alpine soils is prone to changes with temperature increase, for instance because of reduced frost periods (Zierl & Bugmann, 2007). Afforestation throughout the last decades and in future with warming climate and land-use change will influence carbon dynamics. To investigate the climate-driven response of carbon cycling in alpine soils, we conducted a jar incubation experiment under controlled conditions using ¹³C-labelled plant material and traced the decomposition of the organic material under different increasing temperature regimes.

Approximately 20 kg of soil samples were collected from the uppermost 10 cm of a 130-year old tree stand and a pasture site from a sub-alpine afforestation sequence in Jaun, Switzerland. The samples were sieved to 2 mm, roots and stones were removed. 50 g of the soil material was incubated in 2 l glass jars.

To investigate the degradation of the organic material, dried and cut shoots of ¹³C labelled plant material (Lolium perenne L.) were added to the soil samples. Additionally, samples without added plant material were incubated as a control group. The incubation was conducted at three different temperature regimes: 12.5°C (average growing season temperature, weather station by WSL-SLF, 2021), 16.5°C (+ 4°C) and 20.5°C (+ 8°C). Destructive sampling was conducted after 0, 2, 4, 8, and 26 weeks. NaOH traps were exchanged every 3-4 days in the beginning and every 3 weeks during later stages of the experiment to trace the respiration of CO₂ and the ¹³C label.

The measured basal respiration shows a temperature dependence. The values are highest at 20.5°C and subsequently decreased to 16.5°C and 12.5°C with the lowest basal respiration. Surprisingly, the basal respiration of the forest soil is always higher than that of the pasture soil of the same incubation temperature. This partially contradicts previous findings (Nazaries et al., 2015) and might be related to the more resilient microbial community in the forest compared to the pasture soil.

Litter-induced respiration increased sharply after litter application and then decreased again. The pasture soil shows higher cumulative respiration for each temperature compared to the forest soil incubated at the respective temperature. After the highest litter-induced respiration of the pasture soil at 20.5°C at the beginning, this is surpassed by that of the pasture soil with 16.5°C from about 40 days after the beginning of incubation. This could indicate a temperature optimum of the current soil microbial community closer to 16.5°C rather than to 20.5°C. These initial results indicate a different sensitivity of the soil microbial community and consequently also carbon cycling in alpine soils to future rising temperature depending on vegetation cover.

Nazaries, L., Tottey, W., Robinson, L., Khachane, A., Al-Soud, W. A., Sørenson, S., & Singh, B. K. (2015). Shifts in the microbial community structure explain the response of soil respiration to land-use change but not to climate warming. Soil Biology and Biochemistry, 89, 123–134.

WSL-SLF (2021), IMIS Weather Station Fochsen-Jaun, WSL, Davos/Switzerland.

Zierl, B., & Bugmann, H. (2007). Sensitivity of carbon cycling in the European Alps to changes of climate and land cover. Climatic Change, 85(1–2), 195–212.

How to cite: Püntener, D., Speckert, T. C., Thomas, C. L., and Wiesenberg, G. L. B.: Non-uniform Soil Respiration of Soils from an Afforestation Sequence in a Laboratory Incubation Experiment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4506, https://doi.org/10.5194/egusphere-egu22-4506, 2022.

Trees invest up to one third of the carbon (C) fixed by photosynthesis into belowground allocation, including fine root exudation into the rhizosphere. Rising soil temperatures in a warmer world could modify the allocation of labile C below ground, thereby affecting biogeochemical cycling in forest soils. Up to date our understanding of how fine root exudation of labile carbon compounds responds to warming is yet emerging and in-situ analyses under warming conditions are scarce, especially in mature forests. Using a C-free cuvette incubation method, we investigated in situ rates of root exudation from fine roots in a mature spruce forest across three seasons after 14 and 15 years of warming in the Achenkirch soil warming (+4°C) experiment. In addition, we run a complementary short-term experiment on root exudation, during which we increased soil temperatures on warmed plots stepwise up to a difference of 12°C between treatments within a few days. We found no effect of long-term soil warming on in situ root exudation rates (n = 120 roots sampled). Mean exudation rates per biomass were 16.23 ± 4.03 and 17.94 ± 2.94 µg g^-1 h^-1 on control and warmed plots respectively, with highest rates found in the late growing season in both treatments. Exudation rates were positively related to the specific root length and were negatively related to soil moisture, but unrelated to soil inorganic N availability and in situ soil temperature. However, the short-term temperature manipulation resulted in an exponential increase of estimated root exudation rates with soil temperature. Our results therefore indicate that fine root exudation from mature trees in the studied ecosystem is inherently controlled by soil temperature, but an interplay with other parameters such as nutrient availability, root morphology and/or soil moisture are the dominant controlling mechanisms across the seasons in the long run. Our observations further indicate that the long-term soil warming by 4°C caused only a subtle increase in root exudation per fine root surface area or per fine root biomass.

How to cite: Heinzle, J., Liu, X., Tian, Y., Kwatcho-Kengdo, S., Borken, W., Inselsbacher, E., Wanek, W., and Schindlbacher, A.: Effects of soil warming on in situ fine root exudation rates in a temperate forest soil, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4539, https://doi.org/10.5194/egusphere-egu22-4539, 2022.

The capacity of forest soils and trees to sequester C is closely linked to soil nitrogen (N) bioavailability, a major control of microbial and plant growth and functioning. Recent meta-analyses indicated that both, soil organic C and N cycling, intensify with climate/soil warming, though little studies investigated this in long-term (decadal) warming experiments. Changes in N cycling processes have been addressed by measuring total and labile N pools, net and gross N process rates, and changes in extracellular enzyme activities. An alternative approach, integrating over longer time intervals, is to study the natural ¹⁵N abundances of different soil and plant N pools. In this study we quantified the natural ¹⁵N abundances (d¹⁵N values) of coarse and fine litter, fine roots, soil organic N, extractable organic N, microbial biomass N, ammonium and nitrate at the long-term soil warming experimental site in Achenkirch (Tyrol, Austria). This site is one of the few climate manipulation experiments in forests operating for more than 14 years and has provided unique insights into the effects of global warming on forest ecosystem processes. We analyzed ecosystem compartments across three seasons (May, August, October 2019), to investigate the consistency of warming effects on soil N cycle processes. Moreover, we developed an isotope fractionation model to decipher the isotope fractionations of the studied soil N processes and the fractions transformed by them, i.e. for depolymerization, microbial uptake, N mineralization, nitrification and soil N losses. Overall, the consistent increase in fine root δ¹⁵N in warmed soils indicated a general opening of the soil N cycle (greater N losses), which was mirrored in increased ammonium d¹⁵N values, the latter implying increased fractions of ammonium being oxidized to nitrate. Higher fractions of ammonium being nitrified makes labile N more amenable to N losses, either by leaching of nitrate or by denitrification losses. Since nitrification and denitrification exhibit strong isotope fractionation effects against ¹⁵N, the lost N is concomitantly ¹⁵N depleted, while residual substrates remaining in the ecosystem become ¹⁵N enriched, thereby explaining the ¹⁵N enrichment with increasing N cycling and N loss rates in warmed soils. 

How to cite: Wanek, W., Bachmann, M., Inselsbacher, E., Heinzle, J., Tian, Y., Kwatcho-Kengdo, S., Shi, C., Borken, W., and Schindlbacher, A.: Effects of long-term soil warming on soil organic and inorganic nitrogen cycling in a temperate forest soil as assessed by measurements of natural 15N abundances of soil N pools, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4938, https://doi.org/10.5194/egusphere-egu22-4938, 2022.

Climate change is expected to alter plant growth and traits, and in turn alter the quantity and quality of plant-derived carbon inputs to soils. Even though root-derived carbon inputs are more likely to be stabilized as soil organic matter (SOM) than aboveground inputs, our models of how climate change influences plant traits and SOM often focus on aboveground plant dynamics. This is in part because our knowledge of root trait linkages to SOM is limited. Recent efforts synthesizing root trait and soil data make it possible to harmonize these data towards an improved representation of roots in terrestrial biosphere models but a conceptual framework to do this is missing. To this end, we review processes that bridge root traits to SOM formation and stabilization and suggest future model improvements. We estimated that 80% of total global soil carbon is in the rooting zone. We then determined that root traits relevant for SOM can be broadly divided into those pertaining to either living or dead roots, within which, the amount, characteristics, and lifespan of living roots, and the decomposability of dead roots and root fragments are particularly important for SOM. Model recommendations included improved allocation regimes, representation of root interactions with microbes and minerals, and incorporation of root-trait variation across the heterogeneous soil matrix. Our review provides a framework necessary for data syntheses and modelling of root trait-SOM linkages to understand future changes in SOM driven by changing plant inputs in a warmer and elevated atmospheric CO₂ world. 

How to cite: Malhotra, A. and the Root trait-soil carbon working group: Linking root traits to soil carbon: model and data gaps, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5625, https://doi.org/10.5194/egusphere-egu22-5625, 2022.

Climate models predict an increase in the average temperature, an increase in heat and drought periods in summer and heavier rainfall events for Austria (APCC, 2014; IPCC, 2021). Eventually, advancing droughts can lead to substantial yield losses, which will be more pronounced on soils with low water storage capacity (Eitzinger et al., 2013). Especially in dry regions like the Marchfeld (east of Vienna), Austria’s most productive region for grain and vegetables, this may have severe consequences for food security.

In our study, we investigated the combined effects of different soil types and altered precipitation on the soil-plant-nexus in a lysimeter facility from 2017-2019. This facility consists of 18 gravitation lysimeters representing the three main soil types of the Marchfeld, namely calcaric Phaeozem (Ps), calcic Chernozem (Ch) and gleyic Phaeozem (Pg). Half of the lysimeters were irrigated according to current precipitation patterns and half according to the precipitation pattern predicted for the period 2071-2100 in the Marchfeld region, simulating drought periods and heavy rain events. Spring wheat, spring barley and winter wheat were cultivated in all lysimeters in 2017, 2018 and 2019, respectively. Mustard was cultivated as a cover crop after spring wheat and incorporated as mulch. The following spring barley had substantially higher yields than spring wheat. This might be due to the improved water infiltration and organic matter input provided by the cover crop (Kirchman, 2011) and/or the larger crop damage by animals in 2017. 

Drought events resulted in an average decline of grain yield by 66% (p<0.05) in spring wheat, 40% (p=0.13) in spring barley and 39% (p<0.05) in winter wheat. In all soil types, the yield of winter wheat was higher than of the other crops, which could indicate its ability to make better use of water resources than the spring crops. This underlines the importance of optimizing sowing dates as an adaptation strategy to climate change. The increase of the δ¹³C value, an indicator for the stomata conductance, in the “predicted” scenario confirmed that drought stress was limiting plant growth.  δ¹³C values were also higher in the Phaeozem than in the Chernozem, with the first having a lower soil water holding capacity.

In contrast to biomass, nitrogen content of grain did not change between current and predicted precipitation patterns, indicating no impairment of grain quality. Furthermore, the nitrogen use efficiency tended to be higher in the current scenario than in the predicted scenario and was highest for winter wheat. Overall, plant biomass, plant nitrogen content and plant δ¹⁵N values were differently affected by soil type, however as there were no significant interaction effects with precipitation, plants responded identically to the precipitation pattern on all soil types, i.e. significant decline in crop production under drought stress.

Our results exemplify the pressing need to develop and implement adaptation strategies in agriculture, taking into consideration local pedoclimatic characteristics. Introducing more resilient crop species, diversifying the crop rotation and increasing the system’s water use efficiency are promising measures that should be investigated further.

How to cite: Miloczki, J., Prommer, J., Wawra, A., Berthold, H., Hösch, J., Formayer, H., Kisielinska, W., Spiridon, A., Hood-Nowotny, R., Spiegel, H., Baumgarten, A., and Watzinger, A.: Climate change induced drought inhibits plant growth in agricultural systems – A lysimeter study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6123, https://doi.org/10.5194/egusphere-egu22-6123, 2022.

Soils contain the largest actively-cycling terrestrial carbon pool, which is itself composed of chemically heterogeneous and measurable pools that vary in their persistence. Fundamental uncertainties in terrestrial carbon-climate feedbacks still depend on the timing, sign, and magnitude of the response of soil carbon, and its underlying pools, to environmental change. However, model comparisons typically focus on benchmarking only bulk soil carbon stocks and climatological temperature sensitivities. Underlying microbial and mineral-associated pools, and their response to global change, have received increasing attention among empirical studies, yet data limitations still hinder benchmarking of these pools and processes in models at ecosystem- to global-scales. Here we examined the distribution of carbon within particulate and mineral-associated fractions across an ensemble of global soil biogeochemical models, and compared model estimates to a global database of soil fractions. We found that, while bulk soil carbon stocks were seemingly comparable in magnitude and geographic distribution across the models and observations, the spread in underlying pools was much more pronounced. Indeed, the ensemble of models varied nearly 6-fold in the proportion of carbon in mineral-associated fractions, and the majority of models greatly underestimated mineral-associated carbon stocks compared to the observations. Latitudinal differences between the models resulted in divergent pool-specific climatological temperature sensitivities, with implications on projections to global change scenarios. Our study elucidates key structural and theoretical differences between models that drive divergent soil carbon projections, and clearly highlights the need to benchmark underlying carbon pools, in addition to bulk soil carbon stocks.

How to cite: Georgiou, K., Wieder, W. R., Abramoff, R. Z., Koven, C. D., Riley, W. J., Ahlström, A., Bouskill, N. J., Hartman, M., Pellegrini, A., Pierson, D., Sulman, B., Slessarev, E., Zhu, Q., Pett-Ridge, J., and Jackson, R. B.: Global distribution and climatological temperature sensitivity of soil organic matter fractions differ between observations and models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6813, https://doi.org/10.5194/egusphere-egu22-6813, 2022.

Mountain floodplains are characterized by spatiotemporal variations in soil redox conditions that arise due to dynamic hydrological and resulting biogeochemical states. Under oxygen-depleted conditions, solid phase Fe(III) can serve as terminal electron acceptor (TEA) in anaerobic microbial respiration. It remains unclear, however, to what degree the redox properties of Fe(III) phases limit rates of anaerobic respiration and hence organic matter degradation. Here, we assess such limitations in iron-rich soils collected across a gradient in native redox states from the Slate River floodplain (Colorado, U.S.A.). We incubated soils under anoxic conditions and quantified electron transfer to TEAs, TEA reactivity toward electrochemical reduction, and CO₂ production. Fe(III) reduction occurred together with CO₂ production in native oxic soils; no Fe(II) nor CO₂ production was observed in native anoxic soils. Initial CO₂ production rates increased as the reactivity of TEAs toward electrochemical reduction increased across all soil depths and, thus, native soil redox states. The low redox reactivity of TEAs was likely caused by higher acid-extractable Fe(II) concentrations rather than higher crystallinity of Fe(IIII) mineral phases based on analysis of Fe(III) mineral identity and crystallinity using Mössbauer spectroscopy. Our findings indicate that the low redox reactivity of TEAs limited microbial respiration rates in our incubation experiments. This work advances our understanding of controls on anaerobic microbial respiration and can help anticipate organic matter degradation under future hydrological conditions.

How to cite: Aeppli, M., Thompson, A., Dewey, C., and Fendorf, S.: Redox properties of particulate electron acceptors affect anaerobic microbial respiration under oxygen-limited conditions in floodplain soils, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7946, https://doi.org/10.5194/egusphere-egu22-7946, 2022.

Climate change is disproportionately impacting mountain ecosystems, leading to large reductions in winter snow cover, earlier spring snowmelt and widespread shrub expansion into alpine grasslands. Yet, the combined effects of shrub expansion and changing snow conditions on abiotic and biotic soil properties remains poorly understood. We used complementary field experiments to show that reduced snow cover and earlier snowmelt have effects on soil microbial communities and functioning that persist into summer. However, ericaceous shrub expansion modulates a number of these impacts and has stronger belowground effects than changing snow conditions. Ericaceous shrub expansion did not alter snow depth or snowmelt timing, but did increase the abundance of ericoid mycorrhizal fungi and oligotrophic bacteria, which was linked to decreased soil respiration and nitrogen availability. Moreover, by combining molecular sequencing, enzyme assays, greenhouse gas flux measurements, soil biogeochemical analyses, and ¹⁵N labelling, we show that reduced winter snow cover and shrub expansion alter the seasonal dynamics of plant growth (i.e., net ecosystem exchange and plant N-uptake), with important consequences for the seasonal dynamics of soil microbial communities, their functioning, and alpine biogeochemical cycles. In conclusion, our findings suggest that changing winter snow conditions have cross-seasonal impacts on biotic and abiotic soil properties, but shifts in vegetation can modulate belowground effects of future alpine climate change.

How to cite: Broadbent, A., Bahn, M., Pritchard, W., Newbold, L., Goodall, T., Guinta, A., Snell, H., Cordero, I., Michas, A., Grant, H., Soto, D., Kaufmann, R., Schloter, M., Griffiths, R., and Bardgett, R.: Changing snow conditions and shrub expansion alter above- and belowground seasonal dynamics in alpine grasslands, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8723, https://doi.org/10.5194/egusphere-egu22-8723, 2022.

Soil carbon cycle is a large yet poorly understood component of the global carbon cycle under climate change. The transient behaviour of the soil carbon cycle is fully determined by four elements, which are carbon input, residence time,carbon pool size (or state), and the net carbon flux. Many of the past studies focused on one subset of the four elements to quantify soil carbon sequestration under climate change, often leading to contradictory conclusions. Here we assimilated data of respired soil ¹⁴C, soil ¹⁴C profile, and soil organic carbon (SOC) profile from Harvard Forest (i.e., a mid-latitude hardwood forest) into a vertically resolved process model (i.e., Community Land Model version 5, CLM5) together with estimated carbon input to fully constrain soil carbon dynamics during 1900 to 2010. Our results suggested litter pools instead of the mineral soil pools contributed to the majority of the carbon sequestration in history. Different sources of constraints effectively informed parameters of their corresponding elements in the soil system. Yet, single data constraints only provided part of the features of soil carbon cycle and cannot lead to a comprehensive interpretation of its historical dynamics. Using ¹⁴C data alone as the constraints resulted in overestimated soil carbon residence time and more sensitive responses of soil carbon sequestration to changing climate. In the future, multi-source data constraints from different global databases are essential in understanding soil carbon dynamics and accurately quantifying soil sequestration in response to the changing climate across the globe.

How to cite: Tao, F. and Luo, Y.: Quantifying soil carbon sequestration by multi-source constraints, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8949, https://doi.org/10.5194/egusphere-egu22-8949, 2022.

Drylands are unique and diverse ecosystems that occupy more than 40% of the terrestrial surface. These areas are inhabited by more than 35% of the world population. In the case of Portugal, drylands represent 37% of its territory. In many areas worldwide, climate change (CCh) is increasing the aridity leading to an expansion of drylands. However, the joint effects of different CCh drivers on the features, functions, and services of drylands remain largely unknown. Further, there is large uncertainty on how CCh-driven alterations in biotic and abiotic soil attributes will feedback CCh through greenhouse gas (GHG) fluxes.

This study aims to assess (1) the soil-atmosphere GHG exchange and soil nutrients availability and (2) their response to different CCh scenarios along an aridity gradient made up of 8 humid, arid, and semiarid-natural parks in Portugal. In winter 2019, we installed open top chambers and rainfall shelters (both separately and combined) in 24 plots to simulate the forecasted increase in temperatures (~3 °C) and reduction in precipitation (~35%), respectively. Since then, seasonal field campaigns to collect gas and soil samples as well as to measure in situ nutrients availability have been performed.

Our first data show that soil organic matter and nutrients (N and P) availability decrease along the aridity gradient whereas methanogenesis seems to be constrained along the gradient and there is not a clear response from other GHG to the aridity gradient. Soil respiration was mainly driven by the seasonal variability of soil moisture and temperature. Finally, the different CCh scenarios had their biggest effect on variables with faster turnover and the response of GHG fluxes to different CCh scenarios varied among sites, which highlights the importance of considering other site-dependent ecosystem features when trying to assess the effects of climate change on GHG fluxes.

How to cite: Serôdio, J., Férnandez-Alonso, M. J., Fangueiro, D., Freitas, H., Durán, J., and Rodríguez, A.: Greenhouse gas fluxes and nutrients availability in Portuguese drylands and their sensitivity to climate change, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10533, https://doi.org/10.5194/egusphere-egu22-10533, 2022.

Complex biogeochemical processes involving vegetation, microbial communities, symbiotic relationships and nutrient cycling determines the rates at which carbon is transferred between the atmosphere and the soils, and eventually the terrestrial carbon storage. Because of these complexities, there are still large uncertainties connected to the representation of the terrestrial carbon cycling in Earth System Models.

An emerging approach to deal with these problems is to explicitly represent microbial pools as well as physically and chemically protected soil organic matter in the model. Based on this, we have developed a soil decomposition process model designed to capture and quantify relationships between soil microorganisms and their environment, focusing on high latitude systems. Our aim with this approach is dual: 1) Testing hypotheses in the model before designing field experiments will help to set up experiments that benefits both understanding of the ecology and improving the model and, 2) Incorporating such a model into an ESM will make it possible to validate the model with observations, and to identify possible climate feedbacks related to the soil dynamics.

The model includes the decomposer activity of saprotrophic fungal and bacterial communities and the symbiotic relationship between mycorrhizal fungi and vegetation. We include separate carbon and nitrogen reservoirs for these microbes, as well as for plant litter and soil organic matter. The transfer of C and N between the reservoirs is based on rate equations using various parameterizations found in literature, and is also transported vertically following a diffusion equation.

The model is forced with litter input and climatic variables from the Community Land Model (CLM5). For calibration and validation we use subsets from a large dataset containing soil profile data for ~1000 forested sites in Norway (Strand et al. 2016). Since the sites are distributed over a large area, they cover climatic gradients in both temperature and precipitation.

Comparisons of C and N content between simulations from the new decomposition model, the standard CLM5 and the observations will be presented, as well as sensitivity tests of different parameter choices and impacts of changes in climate forcing.

How to cite: Ristorp Aas, E., Koren Berntsen, T., Eiler, A., and de Wit, H.: Representing microbial activity in a soil decomposition model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10985, https://doi.org/10.5194/egusphere-egu22-10985, 2022.

Climate predictions for arctic and subarctic regions show a higher rise in surface temperature than the global mean, which will subsequently raise the soil temperature (Ts) in those regions. We investigated the effects of soil warming duration (medium-term (11-yr) vs. long-term (>60-yr) warmed grassland) and magnitude from +0.2 to +6.2 °C on total belowground plant biomass (BPB) as well as in two functional groups: short-living fine-roots and long-living rhizomes in topsoil (0-10cm) and subsoil (10-30cm). We also analyzed the effect of plant community composition on belowground biomass and pools.

Both the duration and the magnitude of soil warming influenced the dynamic of total belowground biomass (BPB) and fine-roots and rhizomes separately. The soil warming effect varied along the soil depths. Both changes in carbon and nitrogen concentration in fine-roots and rhizomes and their corresponding biomass contributed to the significant decline in carbon and nitrogen pool in belowground plant biomass along the warming gradient. The change in the functional structure of the plant community was related to the increase in soil temperature. The proportion of forbs increased towards warmer plots and was related to the change in the BPB and soil chemistry. Our findings underline the importance of a functional approach in root research to understand better the key physiological processes like N and C cycling. We highlight the role of soil chemistry and community changes together with warming (duration and magnitude) in the fine-root and rhizome response to climate change. 

How to cite: Bhattarai, B., D. Sigurdsson, B., Sigurdsson, P., Leblans, N., Janssens, I., Truu, J., Truu, M., Kumar Devarajan, A., and Ostonen, I.: Soil warming duration and magnitude affects dynamics of fine-roots and rhizomes and C and N pools in belowground biomass in subarctic grasslands, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12663, https://doi.org/10.5194/egusphere-egu22-12663, 2022.

As semi-closed ecosystems, biotic and abiotic properties of cave environments are extensively isolated from the impacts of the surface processes, except for a few environmental parameters. Surface climatic parameters (atmospheric CO₂ ratio, temperature, and precipitation amount) and vegetation are known with their impact on the environmental parameters such as the CO₂ partial pressure, temperature, and humidity of the cave atmosphere. These properties of cave microclimate are defined over a long-term process of balancing all the input and output heat and mass fluxes under the influence of soil temperature, seepage water content and the chemical/physical properties of sinking streams as well as direct air flux from the outside into the cave. While this interaction between the surface and in-cave environmental parameters exerts a key factor on the hydro/geochemical, and microbiological properties of the cave by altering the biotic and abiotic conditions, cave environments can be considered as long-term archives of the consequences of this interaction by being highly sheltered to the surface processes. This relationship between sediment geochemistry, microbiology and environmental conditions is still not fully understood.

In this study, the relationship between bacterial diversity, sediment geochemistry, and microclimate as three major components of cave ecosystems will be examined in cave environments, the relationship between in-cave and surface atmospheric conditions as well. In order to determine the in-cave environmental conditions, micro-climatic (CO₂, temperature, humidity) and environmental (cave water pH, alkalinity) parameters were measured during the fieldwork. Sediment and water (drip water, underground river water and pond water) were sampled in two seasons (summer and winter) aseptically as triplet to determine bacterial community composition of these caves. Water, sediment, and speleothem samples from the caves were examined by Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) and Next Generation Sequencing (NGS) methods to reveal the geochemical and metagenomic features. To observe the changes in cave micro-climate for a year-long period, dataloggers were used.

How to cite: Tok, E. and Olğun Kıyak, N.: Beneath the surface: Climatic, Micro-climatic, Geochemical and Microbiological Approach to Karstic Cave Ecosystem, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12918, https://doi.org/10.5194/egusphere-egu22-12918, 2022.

Physical and chemical stabilisation mechanisms are now known to play critical roles in controlling carbon (C) storage in mineral soils. This has led to suggestions that climate warming-induced C losses may be lower than previously predicted. However, evidence has also been produced that the decomposition of older, and more protected soil organic matter (SOM) is more sensitive to temperature than unprotected and more rapidly decomposing SOM pools. Thus, the extent to which temperature controls C storage in mineral soils remains controversial, and it is not known whether the C stores in soils with large capacities for stabilising C are more, or less, vulnerable to climate warming than the C stored in soils with more limited stabilisation capacities.

By analysing data on >9,000 soil profiles from the World Soil Information Database, we found that, overall, C storage declines strongly with mean annual temperature. However, we observed very large differences in the effect of temperature on C storage in soils with different capacities for stabilising SOM, as indicated by their textural properties. In coarse-textured soils (clay contents less than 20%) with more limited stabilisation capacities, C storage declined strongly with temperature, decreasing by a factor of 1.6 to 2.0 for every 10 ^oC increase in temperature. However, in fine-textured soils (clay contents greater than 35%) with greater stabilisation capacities, the effect of temperature on C storage was more than three times smaller. This pattern was observed independently in cool and warm regions, and after accounting for potentially confounding factors including plant productivity, precipitation, aridity, cation exchange capacity and pH. The difference in the effects of temperature on C storage in soils with contrasting stabilisation capacities could not, however, be represented by an established Earth system model (ESM). To reduce uncertainties in projections of the effect of climate change on soil C losses, we suggest that ESMs could be evaluated against their ability to simulate the differences in the effects of temperature on C storage in soils with contrasting textural properties.

In conclusion, our results suggest that there are stabilised pools of SOM in fine-textured soils that may be relatively insensitive to the impacts of climate change, but that less protected pools in coarser-textured soils may be substantially more vulnerable to global warming than currently predicted. Finally, given the mismatches between data and model outputs, ESMs may not be predicting accurately the potential magnitude of soil C losses in responses to climate warming or which stocks are most vulnerable.

How to cite: Hartley, I., Hill, T., Chadburn, S., and Hugelius, G.: Temperature effects on carbon storage are controlled by soil stabilisation capacities, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13048, https://doi.org/10.5194/egusphere-egu22-13048, 2022.

Environmental changes, such as altered precipitation patterns and temperatures, but also the type of
land management, have strong impact on the vegetation structure and the associated soil carbon
storage. Vulnerable ecosystems that have always grown at the limits of system stability have small
resilience and therefore respond to the smallest changes. This is also true for the forest-steppe
ecotones at the southern border of the Mongolian taiga, with their two subtypes of light and dark
taiga. Due to drought stress, this forest only grows on the northern slopes, where the rate
evapotranspiration is smaller. Climatic change, which is very pronounced in these highly continental
areas, leads to water scarcity and thus to higher drought stress as well as an increased risk of forest
fires. In the forest-steppe ecotone in northern Mongolia, light taiga dominated by Betula is increasingly
spreading into areas previously covered by dark taiga representing coniferous forests dominated by
Pinus and Larix. Since soil organic carbon stocks are known to be related to vegetation, in this study
we aimed at assessing the spatial carbon stocks distribution of different forest-steppe ecotones
characterized by different tree compositions by using a multispectral satellite image approach. Based
on Sentinel-2 data, a supervised random forest classification was carried out using the MSAVI index
and carbon stocks from 50 soil profiles of these sites as training data. For the first time, a mean of
multi-year MSAVI was used to compensate the temporal gap between the actual image of vegetation
vitality and the comparatively inert soil organic carbon. The results were validated by ground truthing
on further 36 soil profile measurements. The validation confirmed the accuracy of the classification
and thus led to a valid area calculation. The map based on the measurement results, which was created
by the use of machine learning, illustrates that the significant differences in the spatial distribution of
the taiga subtypes and their soil organic carbon stocks balance each other out in the areas under
consideration. Since the resulting map could be validated by both soil investigations and field survey
experiences, we assume that the applied remote sensing method can be used as a basis for a realistic
area monitoring of the ecosystem under consideration to calculate the spatial change of the carbon
pool. 

How to cite: Donnerhack, O. and Guggenberger, G.: Large scale carbon mapping of forest-steppe ecotones using multispectral satellite data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13184, https://doi.org/10.5194/egusphere-egu22-13184, 2022.

Reduced carbon assimilation by plants and increased net ecosystem exchange are often considered to reduce the overall carbon sink function of drought-stressed ecosystems. However, plants and soil may respond differently under drought, leading to imprecise predictions of carbon sequestration in soil. We determined the net carbon assimilation and related it to soil organic carbon (SOC) stocks as well as to root exudate production to measure belowground carbon investment in mature trees (F. sylvatica and P. abies) exposed to experimental drought for five growing seasons. Despite more than 50 % reduction in net carbon assimilation under drought, SOC stocks increased on average by more than 30 %. The proportion of carbon allocated as root exudates increased two- to threefold under drought. Increasing amounts of carbon in organo-mineral associations suggest increased carbon stability under water-limited P. abies but not under F. sylvatica. Our data indicate that the belowground sink strength increased rapidly for the ecological and economic most relevant tree species in Europe. However, evaluating the ecosystem´s carbon sink strength by using the net ecosystem exchange alone neglects belowground SOC accumulation under drought. Although belowground-invested carbon could contribute to reducing the soil carbon-climate feedback temporarily and may support ecosystem resilience, SOC accumulated primarily in dry mineral topsoil may be vulnerable upon exposure to rewetting events.

How to cite: Brunn, M., Hafner, B., Zwetsloot, M., Sayer, E., Ruehr, N., Weikl, F., Pritsch, K., Hikino, K., Krüger, J., Lang, F., and Bauerle, T.: Experimental drought increased the forest’s belowground sink strength towards temporarily increased topsoil carbon stocks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13214, https://doi.org/10.5194/egusphere-egu22-13214, 2022.