Autotrophic and heterotrophic soil respiration responds asymmetrically to drought in a subtropical forest in the Southeast China

Both increasing frequency of drought and drought duration are expected for many terrestrial ecosystems under on-going climate change. However, our understanding of the drought effect on soil respiration (Rs), which comprises the second largest carbon (C) flux of the global C cycle, remains limited. To explore the effects of reduced precipitation on Rs and its components, we conducted an experiment of throughfall rainfall exclusion during two consecutive growing seasons in a subtropical forest in the Southeast China. Following throughfall exclusion Rs declined rapidly, and did not recover until three to four months following rewetting, in both 2014 and 2015. During the experiment, throughfall exclusion significantly reduced autotrophic soil respiration (Ra); however, heterotrophic soil respiration (Rh) was unaffected, resulting in a reduced contribution (Ra/Rs) from 33 ± 1% for the control to 16 ± 3% under throughfall exclusion. Experimental drought significantly reduced soil microbial C and fine root biomass, and subsequent to rewetting, soil microbial C recovered quickly, but fine root biomass relapsed slowly. Our results suggested that prolonged drought decreases Rs through modifications in soil microbial activities and fine root metabolic capacity, which are induced by reduced soil water availability. Moreover, our results imply that drought-induced reductions in Rs originate primarily from Ra. Our results highlight the need to account for asymmetric responses to drought between Ra and Rh when predicting the reaction of the ecosystem C balance in response to future drought events.