Changing precipitation exerts greater influence on soil heterotrophic than autotrophic respiration in a semiarid steppe

Future precipitation change is anticipated to have a profound influence on ecosystem carbon (C) cycling, especially soil respiration, the largest C flux from the terrestrial ecosystem to the atmosphere. Due to different substrate sources and biological processes, the heterotrophic (SRh) and autotrophic (SRa) components of soil respiration (SRtot) may respond to changing precipitation in different ways. Determining the differential responses of SRh and SRa to precipitation will facilitate our evaluation of soil C storage and stability under future precipitation change. Here, a 3-year precipitation manipulation experiment with 5 levels of precipitation (+/- 60%, +/- 30% and ambient growing season precipitation) was conducted in a semiarid steppe to determine the influence of precipitation on soil respiration. Results showed that SRtot, increased nonlinearly with increasing water supply, in which, SRh increased much more than SRa. Consequently, the ratio of SRh to SRtot, was enhanced significantly with increasing precipitation, i.e., from 41% to 62% as precipitation increased from 122 mm to 408 mm. Structural equation modeling analysis indicated that changes in SR h and SR C were predominated by the soil water content (SWC) and plant growth, respectively. The stronger effects exerted by changes in precipitation on SWC compared with plant growth contributed to the greater change in SR h than in SRa. Additionally, SRa exhibited a higher temperature sensitivity than SRh. Thus, the increased soil temperature in the drought treatments had greater influence on SRa than SRh, and greatly offset the influence of drought stress on SRa, leading to the smaller change in SRa compared with SRh. Our study highlights the different responses of the two soil respiration components to changing precipitation. These results indicate that future increases in precipitation in semiarid regions will increase soil heterotrophic respiration which may accelerate the turnover of soil C, and further affect the stability of soil C stock.