The impact of extreme summer drought on the short-term carbon coupling of photosynthesis to soil COsub2/sub efflux in a temperate grassland

摘要

Along with predicted climate change, increased risks for summer drought areprojected for Central Europe. However, large knowledge gaps exist in termsof how drought events influence the short-term ecosystem carbon cycle. Here,we present results from 13CO₂ pulse labeling experiments at anintensively managed lowland grassland in Switzerland. We investigated theeffect of extreme summer drought on the short-term coupling of freshlyassimilated photosynthates in shoots to roots as well as to soil CO₂efflux.Summer drought was simulated using rainout shelters during two field seasons(2010 and 2011). Soil CO₂ efflux and its isotopic composition weremeasured with custom-built chambers coupled to a quantum cascade laserspectrometer (QCLAS-ISO, Aerodyne Research Inc., MA, USA). During the 90 minpulse labeling experiments, we added 99.9 atom % 13CO₂ to thegrass 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 the shoots,but increased the relative allocation of fresh assimilates below groundcompared to the control grasslands. Contrary to our hypothesis, we did notfind a change of allocation speed in response to drought. Although droughtclearly reduced soil CO₂ efflux rates, about 75% of total traceruptake in control plots was lost via soil CO₂ efflux during 19 daysafter pulse labeling, compared to only about 60% under droughtconditions. Thus, the short-term coupling of above- and below-groundprocesses was reduced in response to summer drought. The occurrence of anatural spring drought in 2011 lead to comparable albeit weaker droughtresponses increasing the confidence in the generalizability of our findings.