Thresholds and interactive effects of soil moisture on the temperature response of soil respiration

Ecosystem carbon exchange is poorly understood in low-productivity, semiarid habitats. Here we studied the controls of soil temperature and moisture on soil respiration in climate change field experiment in a sandy forest-steppe. Soil CO2 efflux was measured monthly from April to November in 2003e2008 on plots receiving either rain exclusion or nocturnal warming, or serving as ambient control. Based on this dataset, we developed and compared empirical models of temperature and moisture effects on soil respiration. Results suggest that in this semiarid ecosystem the main controlling factor for soil CO2 efflux is soil temperature, while soil moisture has less, although significant effect on soil respiration. Clear thresholds for moisture effects on temperature sensitivity were identified at 0.6, 4.0 and 7.0 vol% by almost each model, which relate well to other known limits for biological activity in this sandy soil. The relationship between soil respiration and temperature was better described by the LloydeTaylor or the Gaussian functions compared to exponential function. Involving additive and interactive soil moisture effects further improved model fitting. Similarly to other low productivity semiarid ecosystems, annual soil carbon efflux values estimated by the different models were rather low (between 123.1 and 139.8 g C m2 yr1 as multi-year averages).