Straw Return, Rather Than Warming, Alleviates Microbial Phosphorus Limitation in a Cultivated Mollisol

The release of carbon (C), nitrogen (N), and phosphorus (P) from soil organic matter by extracellular enzymes is essential for biogeochemical cycles and is sensitive to field management practices and climate change. However, the way in which straw return and warming shift microbial C and nutrient (N and P) limitation is not fully understood. We conducted a field experiment with straw return and warming treatments in a cultivated Mollisol on the Songnen Plain, China, and assessed their effects on extracellular enzyme activities (EEAs) for nutrient cycling and the status of microbial nutrient limitation. The length and angle of vectors defined by ratios of EEAs (C: N vs C: P) were used to indicate relative microbial investments in C-acquiring enzymes (length), and N-or P -acquiring enzymes (angle). We found that microbial biomass C and 16S gene abundances increased by 104.7 % and 18.7 % in response to straw return, but they decreased by 46.0 % and 25.5 % in response to warming, respectively. Vector analysis highlighted that microbial metabolism was co-limited by C (vector length ranging from 2.99 to 3.23) and P (vector angle ranging from 78.88 to 82.49 degrees) in Mollisols, and straw return significantly alleviated microbial P limitation, while warming did not affect microbial nutrient limitation. Moreover, struc-tural equation modeling further indicated that microbial C limitation was significantly associated with soil nutrient content, and microbial biomass was the main determinant of microbial N or P requirements. Overall, our study provides specific and useful insights into the responses of microbial nutrient limitation to straw return and warming, and improves the understanding of nutrient cycling in temperate cropland ecosystems.