Soil Respiration Components and their Temperature Sensitivity Under Chemical Fertilizer and Compost Application: The Role of Nitrogen Supply and Compost Substrate Quality

Understanding autotrophic (Ra) and heterotrophic (Rh) components of soil respiration (Rs) and their temperature sensitivity (Q(10)) is critical for predictingsoil carbon (C) cycle and its feedback to climate change. In agricultural systems, these processes can be considerably altered by chemical fertilizer and compost application due to changes in nitrogen (N) supply and substrate quality (decomposability). We conducted a field experiment including control, urea, and four compost treatments. Ra and Rh were separated using the root exclusion method. Composts were characterized by chemical analyses, C-13 solid-state nuclear magnetic resonance, and lignin monomers. Annual cumulative Ra, along with root biomass, increased with soil mineral N, while Rh was suppressed by excessive N supply. Thus, Ra was stimulated but Rh was decreased by urea alone application. Annual Rh was increased by application of compost, especially that containing most lignin vanillyl and syringyl units, O-alkyl C, di-O-alkyl C, and manganese. However, during the initial period, Rh was most effectively stimulated by the compost containing most carbohydrates, lignin cinnamyl units, phenolic C, and calcium. Ra was mediated by N release from compost decomposition and thus exhibited similar responses to compost quality as Rh. The Rh Q(10) was reduced, while Ra Q(10)was increased by chemical fertilizer and compost application. Moreover, the Rh Q(10) negatively related to soil mineral N supply and compost indicators referring to high substrate quality. Overall, our results suggest that N supply and substrate quality played an important role in regulating soil C flux and its response to climate warming.