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In a future world, ecosystems will be affected by a concomitant increase in atmospheric CO2 concentrations, temperature and drought events. While the individual effects of elevated CO2, warming and drought on plant and ecosystem productivity are comparatively well understood, there is a major lack of experimental studies testing for their interactive effects. In a multifactor experiment (ClimGrass) established in 2013 on a managed montane grassland in Central Austria we tested how elevated CO2 (eCO2), warming (eT) and drought individually and interactively affect productivity and tissue stoichiometry.Treatment effects varied and amplified across the eight treatment years, partly related to shifts in species composition. Above-ground net primary productivity (ANPP) was generally increased when eT and eCO2 were combined, while it was not consistently affected by the individual treatments. Drought and drought recovery effects on ANPP, gross primary productivity (GPP) and belowground carbon allocation were amplified when drought was combined with eT and eCO2. Both under current and future (eT, eCO2) scenarios drought altered tissue stoichiometry by decreasing phosphorus concentrations during drought and increasing nitrogen and potassium concentrations post-drought. Overall, our study suggests that in the temperate grassland studied drought had an overriding effect on productivity and tissue stoichiometry, which was amplified by warming, but only weakly altered by elevated CO2.

Photosynthesis and soil respiration represent the two largest fluxes of CO2 in terrestrial ecosystems and are tightly linked through belowground carbon (C) allocation. Drought has been suggested to impact the allocation of recently assimilated C to soil respiration; however, it is largely unknown how drought effects are altered by a future warmer climate under elevated atmospheric CO2 (eT_eCO2 ). In a multifactor experiment on managed C3 grassland, we studied the individual and interactive effects of drought and eT_eCO2 (drought, eT_eCO2 , drought × eT_eCO2 ) on ecosystem C dynamics. We performed two in situ 13 CO2 pulse-labeling campaigns to trace the fate of recent C during peak drought and recovery. eT_eCO2 increased soil respiration and the fraction of recently assimilated C in soil respiration. During drought, plant C uptake was reduced by c. 50% in both ambient and eT_eCO2 conditions. Soil respiration and the amount and proportion of 13 C respired from soil were reduced (by 32%, 70% and 30%, respectively), the effect being more pronounced under eT_eCO2 (50%, 84%, 70%). Under drought, the diel coupling of photosynthesis and SR persisted only in the eT_eCO2 scenario, likely caused by dynamic shifts in the use of freshly assimilated C between storage and respiration. Drought did not affect the fraction of recent C remaining in plant biomass under ambient and eT_eCO2 , but reduced the small fraction remaining in soil under eT_eCO2 . After rewetting, C uptake and the proportion of recent C in soil respiration recovered more rapidly under eT_eCO2 compared to ambient conditions. Overall, our findings suggest that in a warmer climate under elevated CO2 drought effects on the fate of recent C will be amplified and the coupling of photosynthesis and soil respiration will be sustained. To predict the future dynamics of terrestrial C cycling, such interactive effects of multiple global change factors should be considered.

Photosynthesis and soil respiration represent the two largest fluxes of CO

Plants can modulate the source and magnitude of water uptake under environmental stresses, ultimately constraining water and energy fluxes across Earth’s surface. These alterations are scarcely quantified for future climatic scenarios such as warming, elevated atmospheric CO2 (eCO2), and droughts—all projected by the end of this century. Here we use diurnal soil moisture dynamics throughout the 2019 growing season to quantify the impacts of these three global change factors on root water uptake in a managed C3 mountain grassland in Austria; a key agricultural landscape within central Europe. To determine whether plants alter water uptake via root trait adjustments, we then compared water uptake to root morphological traits. We expected that 1) drought and eCO2 (+300 ppm) would reduce root water uptake relative to ambient conditions due to supply limitation and a lower stomatal conductance, whereas 2) greater vapor pressure gradients in warmed systems would elevate transpiration rates, increasing root water uptake. Plants reduced water uptake in droughted plots by ~35% regardless of other factors applied, due to decreased water extraction from the soil surface during the peak drought. Warmed plots had unexpectedly lower water uptake by 17-25% relative to control plots. Finally, vegetation in eCO2 plots displayed similar water uptake to plots under ambient conditions; however, eCO2 effects did buffer warming effects, such that plots with eCO2 and warming extracted less water than those subjected to warming alone. Root morphological traits showed strong linear correlations (R > 0.7, or R < -0.7) to root water uptake in ambient, drought, and eCO2 plots, yet no significant relationship was found for plots under warming or multifactor treatments. Relationships were strongest and most abundant following a drought. This suggests that—though plants may optimize root structure for drought recovery—plants may alter their root systems to account for resource limitations other than water in a warming climate. Altogether, we show that warming, eCO2, and droughts may significantly alter the root water extraction in managed C3 mountain grasslands, but changes in water availability alone may not fully explain plant water uptake responses.

Climate change is expected to lead to an increase in frequency and severity of extreme climatic events like summer drought. Drought and rewetting have strong impacts on soil respiration, which constitutes the largest flux of CO2 from terrestrial ecosystems to the atmosphere. However, little is known about the role of biotic and abiotic factors in driving CO2 production and transport across the soil profile and how these processes are affected by repeated drought events. Soil CO2 transport can be assessed using the flux-gradient approach, a method which assumes that diffusion is the only transport mechanism for CO2 through soil, with diffusion rates primarily dependent on air-filled pore space. It is therefore generally assumed that the calculated soil CO2 concentration gradient translates directly into soil CO2 efflux, however, a discrepancy between measured soil CO2 efflux and modeled soil CO2 concentration gradients can indicate presence of non-diffusive transport mechanisms.In a multiyear drought and rewetting experiment at a mountain meadow in the Austrian Alps, we compared soil CO2 production, transport and efflux for plots which were exposed to two and twelve subsequent years of experimental summer drought, respectively, versus plots with ambient precipitation and soil moisture. We measured soil respiration using automated chambers and assessed the production and transport of CO2 using the flux-gradient approach on data obtained with solid-state sensors in three soil depths through the soil profile. We tested the hypothesis that drought-driven reduction in soil respiration will be more intense for the 12-year drought treatment, but the CO2 pulse induced by rewetting will be higher. We furthermore expected that non-diffusive transport mechanisms would play a crucial role during drought and would be more pronounced in the 12-year drought treatment compared to the 2-year drought treatment. Data analysis is currently in progress, the findings will be presented at the conference.

Biogeochemical processes in soils largely control the atmospheric mixing ratio of nitrous oxide (N2O). The growing use of nitrogen (N) fertilizer in agriculture drives anthropogenic N2O emissions, which currently surpass projections with some of the highest emissions. In order to adapt mitigation strategies and to model the future N cycle it is crucial to fully understand N2O emission pathways in a changing climate. The underlying processes, attributed to microbial transformation of N, primarily occur via the oxic nitrification and anoxic denitrification pathways. These processes depend greatly on soil, plant and ecosystem properties, which in turn rely on meteorological drivers (e.g. air temperature and precipitation). This means that the many environmental factors that drive microbial activity and N2O emissions in soils are vulnerable to climate change, including extreme events such as droughts. Consequently, the rates of nitrification and denitrification are expected to be strongly impacted by changing climatic conditions, which could also alter the N2O production and consumption dynamics across the soil profile. This study aims to understand how N2O production and consumption pathways respond to the individual and combined effects of warming, elevated atmospheric CO2 concentration, and drought-rewetting events in managed mountain grassland. For the first time, we use online, in-situ stable isotopic measurements of both surface N2O emissions and of N2O across the soil profile to distinguish pathways for N2O production and consumption. Different modeling approaches will be used to reconstruct production and consumption dynamics from soil gas isotopic measurements, and to upscale results to examine global relevance.

Assessing the future of water resources in terrestrial biomes is contingent on observations from climate-manipulation experiments. Global change in the Anthropocene could produce various permutations of warming, atmospheric carbon levels, and moisture availability; however the impact on ecosystem hydrology is largely studied individually (e.g., elevated CO2 or temperature) rather than interactively. We sought to specify how various combinations of drought, elevated CO2 (+150 ppm, +300 ppm) and warming (+1.5°C and + 3°C) may alter the partitioning of soil moisture in the root zone of mountain grassland. Using spectral techniques, we transformed these high resolution data (i.e., 4 soil depths and every 15 min) into the frequency domain to study the interactive effects of climate change on sub-hourly to seasonal soil moisture signals. Diurnal moisture signals in heated plots (+3°C in air temperature) were up to 3x stronger (in amplitude) during summer drawdown compared to plots receiving heat and elevated CO2 (+300 ppm). This preliminary analysis suggests that elevated atmospheric carbon may buffer heat-driven soil moisture losses in grassland root zones by reducing transpiration fluxes during seasonal dry periods.

Microbial growth and carbon use efficiency (CUE) are central to the global carbon cycle, as microbial remains form soil organic matter. We investigated how future global changes may affect soil microbial growth, respiration, and CUE. We aimed to elucidate the soil microbial response to multiple climate change drivers across the growing season and whether effects of multiple global change drivers on soil microbial physiology are additive or interactive. We measured soil microbial growth, CUE, and respiration at three time points in a field experiment combining three levels of temperature and atmospheric CO2, and a summer drought. Here we show that climate change-driven effects on soil microbial physiology are interactive and season-specific, while the coupled response of growth and respiration lead to stable microbial CUE (average CUE = 0.39). These results suggest that future research should focus on microbial growth across different seasons to understand and predict effects of global changes on soil carbon dynamics., Eva Simon et al. show non-linear responses and non-additive interactive effects of climate change on microbial physiology, but not on microbial carbon use efficiency (CUE). These findings suggest that future research should focus on microbial growth instead of CUE alone to understand and predict effects of global changes on soil carbon dynamics.

As the climate warms, drought events are expected to increase in intensity and frequency, with consequences for the carbon cycle. Soil respiration (Rs) accounts for the largest flux of CO2 from terrestrial ecosystems to the atmosphere. While the drought responses of Rs have been well studied, it is uncertain how they will be modified in a future world, when higher temperatures will occur in combination with elevated atmospheric CO2 concentrations. In a global change experiment in a managed temperate grassland, we studied drought and post-drought responses of Rs dynamics under current versus likely future conditions (+3°, +300 ppm CO2). Furthermore, to understand the soil CO2 production (Ps) and transport dynamics underlying Rs fluxes we continuously monitored in-situ soil CO2 concentrations across the soil profile. Our results show that Rs was higher and that drought-induced reductions in Rs were delayed under future compared to current conditions. Peak drought reductions and post-drought pulses of Rs were more pronounced in the future scenario. Annual Rs was reduced by drought only under current but not under future conditions. An in-depth analysis of soil CO2 gradients and fluxes across the soil profile showed that elevated CO2 stimulated Ps primarily in the main rooting horizon and that warming affected Ps also in deeper soil layers. We found that both in the current and the future scenario drought led to the strongest reductions of Ps in the most productive soil layers, which also exhibited the largest depletion of soil moisture. We conclude that a future warmer climate under elevated CO2 amplifies soil CO2 production and efflux and their peak drought and post-drought responses, but delays the onset of the drought responses and thereby eliminates the overall drought effect on annual soil CO2 emissions.

Field measurements of aboveground net primary productivity (ANPP) in temperate grasslands suggest that both positive and negative asymmetric responses to changes in precipitation (P) may occur. Under normal range of precipitation variability, wet years typically result in ANPP gains being larger than ANPP declines in dry years (positive asymmetry), whereas increases in ANPP are lower in magnitude in extreme wet years compared to reductions during extreme drought (negative asymmetry). Whether the current generation of ecosystem models with a coupled carbon–water system in grasslands are capable of simulating these asymmetric ANPP responses is an unresolved question. In this study, we evaluated the simulated responses of temperate grassland primary productivity to scenarios of altered precipitation with 14 ecosystem models at three sites: Shortgrass steppe (SGS), Konza Prairie (KNZ) and Stubai Valley meadow (STU), spanning a rainfall gradient from dry to moist. We found that (1) the spatial slopes derived from modeled primary productivity and precipitation across sites were steeper than the temporal slopes obtained from inter-annual variations, which was consistent with empirical data; (2) the asymmetry of the responses of modeled primary productivity under normal inter-annual precipitation variability differed among models, and the mean of the model ensemble suggested a negative asymmetry across the three sites, which was contrary to empirical evidence based on filed observations; (3) the mean sensitivity of modeled productivity to rainfall suggested greater negative response with reduced precipitation than positive response to an increased precipitation under extreme conditions at the three sites; and (4) gross primary productivity (GPP), net primary productivity (NPP), aboveground NPP (ANPP) and belowground NPP (BNPP) all showed concave-down nonlinear responses to altered precipitation in all the models, but with different curvatures and mean values. Our results indicated that most models overestimate the negative drought effects and/or underestimate the positive effects of increased precipitation on primary productivity under normal climate conditions, highlighting the need for improving eco-hydrological processes in those models in the future.