In order to provide the theoretical support for soil structure improvement and phosphorus activation of Shajiang black soil, the regulatory effects of wheat and maize straw return on aggregate size distribution and phosphorus fractions of soil aggregates were investigated.

This study was based on a long-term straw return experiment (since 2008) conducted in Mengcheng County, Anhui Province. The experimental design included four treatments: conventional fertilization (F), conventional fertilization plus wheat straw return (FWS), conventional fertilization plus maize straw return (FMS), and conventional fertilization plus wheat and maize straw return (FWMS). Soil samples of cultivated layers (0-20 cm) were collected at wheat maturity stage in 2023 to determine the composition of soil aggregates and the content of phosphorus fractions in different particle-size aggregates. The effects of wheat and maize straw return on the distribution of phosphorus fractions in soil aggregates, the phosphorus activation rates of different size aggregates and their contribution rates to soil available phosphorus were analyzed.

Compared with the F treatment, the proportions of >2 mm aggregates in Shajiang black soil under FWS, FMS and FWMS treatments were significantly increased by 35.9%, 30.6% and 39.1%, respectively, while the proportions of 0.25-2 mm aggregates and microaggregates (0.053-0.25 mm) were significantly decreased. Wheat and maize straw incorporation significantly increased the mean weight diameter (MWD) of water-stable aggregates and decreased the percentage of aggregate disruption (PAD). Under the condition of straw return, the contents of soil total phosphorus and available phosphorus were significantly increased by 6.5%-26.0% and 21.1%-37.6%, respectively. Wheat and maize straw return could significantly elevate the contents of labile phosphorus fraction NaHCO₃-Pi (17.1%-51.3%) and moderate labile phosphorus fraction NaOH-Po (19.5%-46.2%) in Shajiang black soil. Compared with the F treatment, the soil available phosphorus contents and phosphorus activation rates of 0.25-2 mm aggregates under the straw return treatments were significantly increased by 39.3%-63.9% and 37.5%-51.7%, respectively. The relative contribution rates of >2 mm aggregates to soil available phosphorus under FWS, FMS and FWMS treatments were significantly increased by 19.0%, 17.3% and 22.3%, respectively, compared with the F treatment. However, straw incorporation significantly reduced the relative contribution rates of microaggregates (0.053-0.25 mm) and silt and clay particles (＜0.053 mm) to soil available phosphorus. The contents of labile phosphorus (H₂O-P, NaHCO₃-Po) and moderate labile phosphorus (NaOH-Po) in 0.25-2 mm aggregates were increased under straw return.

Wheat and maize straw return was an effective measure to improve the composition and stability of soil aggregates in Shajiang black soil, and could enhance the soil phosphorus supply capacity.