Autotrophic Respiration is More Sensitive to Nitrogen and Phosphorus Supply than Heterotrophic Respiration in Semiarid Grassland

Total soil respiration and heterotrophic respiration were measured between 09:00 and 11:00 local time using an EGM-4 automated soil CO2 efflux system (PP-Systems, Amesbury, MA, USA). Autotrophic respiration was calculated as total soil respiration minus heterotrophic respiration. Soil temperature (ST) and soil volumetric water content (SWC) at 5 cm soil depth were measured adjacent to the PVC collar in parallel to soil respiration measurement. Soil temperature was measured using a digital thermometer (TP101, Hengshui Innovation Instrument Company, Hebei Province, China) and SWC using a portable soil moisture meter (FieldScout TDR 100, Spectrum Technologies, Aurora, IL, USA). Aboveground biomass (AGB) was harvested in per quadrat, and litter biomass (LB) was collected after AGB harvest. All plant aboveground and litter samples were oven-dried at 75℃ for 48 h. Soil organic carbon content (SOC), soil total N content (STN) and soil total P content (STP) were measured by the K2Cr2O7–H2SO4 oxidation method, the Kjeldahl digestion method (Kjeldahl System 2300 Distilling Unit, Foss, Sweden) and molybdenum-antimony colorimetric method (UV 2550, Shimadzu Corporation, Japan), respectively.<br>