Straw Returning Mediates Soil Microbial Biomass Carbon and Phosphorus Turnover to Enhance Soil Phosphorus Availability in Rice-Oilseed Rape Rotation

The alleviation of phosphorus (P) limitation by increasing carbon (C) input was an effective strategy in crop production. Yet, the effects of straw returning on soil microbial biomass P (MBP) turnover and P fractions in paddy-upland rotation remain poorly understood. Soil MBP turnover involves the mineralization and immobilization of organic P (Po), potentially altering P fractions and reducing the risk of P loss. The effects of straw returning on soil MBP turnover, P fractions and crop P utilization during the rice-oilseed rape rotation were evaluated in two field experiments with different soil Po levels. The treatments included no P fertilizer (-P), P fertilizer (+P) and P fertilizer plus straw returning (+P+S). The crop P uptake and cumulative P utilization efficiency were increased by straw returning. Straw addition promoted microbial biomass, decreased the MBC: MBP ratio in Po-high soil and increased the MBC: MBP ratio in Po-low soil, and increased MBP turnover rates and fluxes. According to structural equation modeling (SEM), soil MBC and MBP had significant effects on crop P uptake directly or via P fractions. In Po-high soil, straw addition increased inorganic P (Pi) (NaHCO3 and NaOH) and decreased Po content by increasing the MBP turnover, while the Po-low soil mainly increased Po content. Among multiple variables, MBP was the most important driving factor controlling P uptake in Po-high soil, while MBC and NaOH-Pi were the best driving factors in Po-low soil. Additionally, the soil MBC, MBP and MBC: MBP ratios were higher in the oilseed rape season than in the rice season. Consequently, straw returning improves soil P availability by increasing MBP turnover, which was beneficial for improving P utilization.