Global Evidence of the Unimodal Response of Ecosystem Respiration to Soil Moisture.

Ecosystem respiration (ER) is the largest flux of carbon from land to the atmosphere and is strongly influenced by soil moisture. Nevertheless, the response of ER to soil moisture remains poorly understood. Here, the ER-soil moisture response curves at 135 sites from the global FLUXNET are analyzed. In contrast to the typically assumed monotonic increase in respiratory carbon emissions with increasing soil moisture derived from land surface models, the study shows that 106 sites exhibit a unimodal soil moisture response with a peak in ER at an apparent optimum soil moisture (SMER opt), implying a prevalent inhibitory effect of soil moisture excess on ER. Among the 12 biotic and abiotic variables examined, the SMER opt is mostly related to local water availability, with drier sites showing lower SMER opt than wetter sites, possibly reflecting water adaptation of ER. This adaptation is further supported by a field experiment that exclusively manipulates water and holds other factors constant, showing a downward shift in SMER opt after long-term water deficit. These results reveal the large variation of SMER opt and the water adaptation of ER, providing novel insights for understanding and forecasting carbon-climate feedbacks.