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.

Shandong Province is the main wheat and maize producing province in North China. Clarifying the spatial distribution characteristics of wheat and maize straw resources and nutrient substitution potential of straw return of this province could provide the decision support for comprehensive utilization of crop straw and fertilizer reduction.

In this research, the wheat and maize straw yield and substitution potential of nutrient by straw return in different areas of Shandong Province were estimated by straw-to-grain ratio method, based on statistic yearbook data and published literature review.

The annual outputs of wheat and maize straw were 29.28 and 22.52 million tons, respectively, in Shandong Province during 2019-2021. The proportions of wheat and maize straw resources in different regions were ranked as follows: North Shandong (34.9% and 34.7%)>Southwest Shandong (27.6% and 24.9%)>Central Shandong (20.2% and 21.4%)>East Shandong (9.1% and 11.3%)>Southeast Shandong (8.2% and 7.8%). The order of collectable wheat straw per unit area was North Shandong (5 573 kg·hm^-2)>Southwest Shandong (5 380 kg·hm^-2)>Central Shandong (5 361 kg·hm^-2)>Southeast Shandong (5 088 kg·hm^-2)>East Shandong (4 968 kg·hm^-2). The order of collectable maize straw per unit area was Southwest Shandong (5 121 kg·hm^-2)>North Shandong (5 034 kg·hm^-2)>Southeast Shandong (4 807 kg·hm^-2)>Central Shandong (4 790 kg·hm^-2)>East Shandong (4 709 kg·hm^-2). At the municipal level, the straw resources were mainly distributed in Heze, Dezhou, Liaocheng, Jining and Weifang. The wheat and maize straw yields of these cities accounted for 57.5% and 55.5%, respectively. However, the proportions of wheat and maize straw outputs from Dongying, Zibo, Rizhao and Weihai were only 7.2% and 8.0%, respectively. In Shandong Province, the amounts of nutrient from wheat straw were 0.1394 million tons of N, 0.0513 million tons of P₂O₅, and 0.5090 million tons of K₂O. For maize straw, the outputs of nutrient were 0.1665 million tons of N, 0.0658 million tons of P₂O₅, and 0.3660 million tons of K₂O. In the regional scale, the substituting potentials of N, P₂O₅ and K₂O through wheat straw return were 16.7-18.7, 7.8-8.7 and 110.4-123.8 kg·hm^-2, respectively. The maize straw incorporation could substitute chemical fertilizers of N 25.4-27.6, P₂O₅ 11.8-12.8 and K₂O 83.2-90.5 kg·hm^-2 theoretically.

In summary, there were some differences in total straw yield and collectable straw per unit area among different wheat-maize rotation areas of Shandong Province. Therefore, these appropriate policies should be formulated based on local conditions to promote efficient utilization of wheat and maize straw resources. The fertilizer replacement potential of returned straw should be considered for nutrient management in wheat-maize rotation system, which was conducive to chemical fertilizer reduction and green agricultural development.