The impact of extreme summer drought on the short-term carbon coupling of photosynthesis to soil CO2 efflux in a temperate grassland.

Along with predicted climate change, increased risks for summer drought are projected for Central Europe. However, large knowledge gaps exist in terms of how drought events influence the short-term ecosystem carbon cycle. Here, we present results from ¹³CO₂ pulse labeling experiments at an intensively managed lowland grassland in Switzerland. We investigated the effect of extreme summer drought on the short-term coupling of freshly assimilated photosynthates in shoots to roots as well as to soil CO₂ efflux. Summer drought was simulated using rainout shelters during two field seasons (2010 and 2011). Soil CO₂ efflux and its isotopic composition were measured with custom- built chambers coupled to a quantum cascade laser spectrometer (QCLAS-ISO, Aerodyne Research Inc., MA, USA). During the 90min pulse labeling experiments, we added 99.9atom% ¹³CO₂ to the grass sward. In addition to the isotopic analysis of soil CO₂ efflux, this label was traced over 31 days into bulk shoots, roots and soil. Drought reduced the incorporation of recently fixed carbon into shoots and increased carbon allocation below-ground relative to total tracer uptake. Contrary to our hypothesis, we did not find a change in allocation speed in response to drought, although drought clearly reduced soil CO₂ efflux rates. 19 days after pulse labeling, only about 60 % of total tracer uptake was lost via soil CO₂ efflux under drought compared to about 75 % under control conditions. Predisposition of grassland by spring drought lead to different responses to summer drought in 2011 compared to 2010, suggesting increased sensitivity of grassland to consecutive drought events as predicted under future climate change. [ABSTRACT FROM AUTHOR]