Straw returning mediates soil microbial biomass carbon and phosphorus turnover to enhance soil phosphorus availability in a rice-oilseed rape rotation with different soil phosphorus levels

The alleviation of phosphorus (P) limitation by increasing carbon (C) input is an effective strategy for improving crop production and P uptake efficiency. However, 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, increased the MBC:MBP ratio in Po-low soil, and increased the MBP turnover rates and fluxes in both soils. 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 observed 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 is beneficial for improving P utilization.