Microbial nutrient limitations limit carbon sequestration but promote nitrogen and phosphorus cycling: A case study in an agroecosystem with long-term straw return.

Soil fertility can be increased by returning crop residues to fields due to the cooperative regulation of microbial metabolism of carbon (C) and nutrients. However, the dose-effect of straw on the soil C and nutrient retention and its underlying coupled microbial metabolic processes of C and nutrients remain poorly understood. Here, we conducted a comprehensive study on soil nutrients and stoichiometry, crop nutrient uptake and production, microbial metabolic characteristics and functional attributes using a long-term straw input field experiment. We estimated the microbial metabolic limitations and efficiency of C and nitrogen (N) use (CUE and NUE) via an enzyme-based vector-TER model, biogeochemical-equilibrium model and mass balance equation, respectively. In addition, the absolute abundances of 20 functional genes involved in the N- and P-cycles were quantified by quantitative PCR-based chip technology. As expected, straw input significantly increased C and N stocks, C: nutrients, crop nutrient uptake and growth. However, the C sequestration efficiency decreased by approximately 6.1 %, and the NO emission rate increased by 0.5-1.0 times with the increase in straw input rate. Interestingly, the microbial metabolism was more limited by P when straw input was <8 t ha but was reversed when straw input was 12 t ha. The enhanced nutrient limitation reduced both the CUE and the NUE of microbes and then upregulated genes associated with the hydrolysis of C, the mineralization of N and P, and denitrification, which consequently influenced C and N losses as well as crop growth. This study highlights that soil C and nutrient cycling are strongly regulated by microbial metabolic limitation, suggesting that adding the appropriate limiting nutrients to reduce nutrient imbalances caused by straw input is conducive to maximizing the ecological benefits of straw return.