Differential responses of auto- and heterotrophic soil respiration to water and nitrogen addition in a semiarid temperate steppe

Evaluating how autotrophic (SRA), heterotrophic (SRH) and total soil respiration (SRTOT) respond differently to changes of environmental factors is critical to get an understanding of ecosystem carbon (C) cycling and its feedback processes to climate change. A field experiment was conducted to examine the responses of SRA and SRH to water and nitrogen (N) addition in a temperate steppe in northern China during two hydrologically contrasting growing seasons. Water addition stimulated SRA and SRH in both years, and their increases were significantly greater in a dry year (2007) than in a wet year (2006). N addition increased SRA in 2006 but not in 2007, while it decreased SRH in both years, leading to a positive response of SRTOT in 2006 but a negative one in 2007. The different responses of SRA and SRH indicate that it will be uncertain to predict soil C storage if SRTOT is used instead of SRH to estimate variations in soil C storage. Overall, N addition is likely to enhance soil C storage, while the impacts of water addition are determined by its relative effects on carbon input (plant growth) and SRH. Antecedent water conditions played an important role in controlling responses of SRA, SRH and the consequent SRTOT to water and N addition. Our findings highlight the predominance of hydrological conditions in regulating the responses of C cycling to global change in the semiarid temperate steppe of northern China.