ClimGrass: Grassland carbon dynamics in a changing climate

The proposed project ClimGrass: Grassland carbon dynamics in a changing climate aims to quantify and understand individual and combined effects of warming, elevated CO2 and extreme climatic events on the carbon dynamics in upland grassland, with a particular emphasis on ecosystem CO2 exchange and soil carbon (C) turnover. ClimGrass will uniquely combine multistep changes in temperature and atmospheric CO2 concentrations in a response surface approach to test for non- additive and non-linear effects and will investigate concurrent and lagged effects of extreme climatic events (drought / heatwave) in the current and in a future climate. To obtain an understanding of the processes underlying the ecosystem C balance, ClimGrass will analyse, next to productivity and litter decomposition, all CO2 flux components, including the partitioning to plant-derived (autotrophic) and soil organic matter-derived (heterotrophic) fluxes and the CO2 production across the soil profile. ClimGrass will particularly focus on belowground carbon allocation as a key process linking above- and belowground processes and will assess its implications for microbial C use efficiency and soil C turnover. To this end, ClimGrass will employ isotopic tracer approaches in nested continuous and pulse-chase labelling experiments to understand the fate of recent plant-assimilated C in the plant- soil system and its potential for altering soil C sequestration. The proposed project will provide the first comprehensive process-based assessment of effects of climate change and elevated CO2 on the productivity, CO2 fluxes and soil C turnover of a C3 grassland, a widespread and highly relevant ecosystem type in many regions of Europe. ClimGrass will provide substantially novel insights into the mechanisms governing ecosystem C dynamics under global change and will thereby contribute to understanding effects of elevated CO2 and climate change, including extreme climatic events, on a range of supporting, provisioning and regulating ecosystem services.

Climate projections suggest that in the coming decades rising atmospheric CO2 concentrations will further accelerate climate warming and will increase the occurrence of extreme climatic events such as severe droughts. To date, the interactive effects of these three major global change drivers have rarely been assessed in real-world ecosystems. The ClimGrass experiment tested for the individual and combined effects of multiple levels of warming and elevated CO2 on ecosystem carbon cycling in managed C3 grassland, which is a widespread and highly relevant ecosystem type in many regions of Europe. The ClimGrass project furthermore explored how the effects of drought on carbon dynamics in a current climate are altered under a future warmer climate with elevated CO2. Following a response surface design, the ClimGrass experiment showed that the effects of warming and elevated CO2 on plant productivity, on the plant investment in root- versus shoot production and specific root length, and on microbial carbon use for growth and respiration are highly interactive and non-linear, and change across the seasons. Based on isotopic pulse labelling experiments it was found that in a warmer, CO2-rich environment a higher proportion of the photosynthetically fixed carbon was allocated to soil bacteria and was respired in the soil. In such a future climate drought effects on carbon cycling (including photosynthesis and respiration and the accumulation of recently fixed C in roots and in soil) were more pronounced. At the same time, rewetting- and post-drought recovery effects on carbon dynamics were more rapid under future compared to current climate conditions. Furthermore, across the soil profile, the layers contributing most strongly to soil CO2 production were shifted under warming and elevated CO2 and especially under drought, when deeper soil layers remained active and while top soil layers reduced activity and non-diffusive transport decreased soil surface CO2 concentrations further. Drought altered soil nitrifier communites and nitrification rates more than the other global change factors studied, with possible consequences for plant growth recovery from drought. To be able to eventually link the outcomes of the drought experiments in ClimGrass to other studies, a conceptual framework was developed, which permits obtaining a comparable quantification of the resilience of ecosystems to extreme climatic events. This bivariate framework directly relates the resistance of ecosystem processes to and their recovery from extreme events. Overall, the findings from the ClimGrass project suggest that the effects of warming, elevated CO2 and drought on grassland carbon cycling are highly non-linear and interactive, and that, compared to current climatic conditions, in a future warmer climate under elevated CO2 carbon dynamics are more severely affected by drought, but also recover more rapidly.