Mechanism Of Straw Returning To Improve Soil Microbial Phosphorus Cycle And Phosphorus Utilization In Rice-Oilseed Rape Rotation

Rice-oilseed rape rotation is the dominant cropping system in the middle region of the Yangtze River in China,and is an effective way to ensure food and oil security with the amount of agricultural land existing.The problems of heavy soil phosphorus（P）accumulation,low P availability and regional distribution differences in agricultural production,and the large number of straw resources from multi-season crop cultivation are prominent.In the rice-oilseed rape rotation,seasonal wet and dry cycles cause complex changes in soil P forms.Straw returning is an effective measure to promote soil P availability to improve crop P utilization.However,the microbial regulation mechanism of straw returning to rice-oilseed rape rotation soil P mobilization and the substitution effect on P fertilizer need to be further clarified.Therefore,it is important to clarify the effects of P fertilizer and straw returning on soil P availability and utilization in rice-oilseed rape rotation,the regulatory mechanism of straw carbon（C）input on microbial P turnover and soil P mobilization,which is important for promoting straw resource management and improving P fertilizer utilization in rice-oilseed rape rotation areas.In this study,we investigated the effects of straw returning on soil P utilization and microbial P mobilization in rice-oilseed rape rotation soils by combining field and culture experiments.The main results were as follows:（1）In WX,SY and WH sites,compared with no P application,P application（60kg P₂O₅/hm²）increased rice yield by 6.7%,24.4%and 42.9%and oilseed rape yield by25.5%,229.0%and 501.9%,respectively,and there was a consistent trend of increase in crop P accumulation.Compared with no straw returning,the yield of rice and oilseed rape increased by 4.4%-10.6%and 6.6%-18.8%,respectively,and P accumulation increased by 10.3%-13.3%and 5.8-23.0%,respectively,after straw returning（6 t/hm²）.The increase effect of straw returning on the cumulative P utilization in WX site with higher soil active P（6.0 percentage points）was higher than that in SY and WH sites with lower soil active P（1.1-2.5 percentage points）.The effect of P application and straw returning on Olsen-P enhancement was higher in WX site（10.5 and 2.6 mg/kg）than in SY and WH sites（4.6-4.8 and 0.6-0.7 mg/kg）.In addition,soil Olsen-P was increased by 3.8,2.6 and 1.5 mg/kg per 100 kg/hm of P surplus in WX,SY and WH sites,and by 3.8,2.8 and 2.0 mg/kg after straw returning,respectively.（2）Straw addition increased microbial biomass C and P（MBC and MBP）,decreased the MBC/MBP in WX site（7.6%）,increased the MBC/MBP in SY and WH sites（3.5%-7.1%）,and increased the MBP turnover rates and fluxes in all soils.In WX site with higher soil active P,straw addition increased inorganic P（Pi）（NaHCO₃ and NaOH）content（10.8%）and decreased organic P（Po）content（7.2%）by increasing the MBP turnover,while the SY and WH sites with lower soil active P mainly increased Po（NaHCO₃ and NaOH）content（18.6%-24.2%）.Among multiple observed variables,MBP was the most important driving factor controlling P uptake in WX site,while MBC and NaOH-Pi were the best driving factors in SY and WH sites.Additionally,the soil MBC,MBP and MBC/MBP were higher in the oilseed rape season than in the rice season.（3）Straw addition promoted Po mineralization in medium P soil（Pi/Po increased12.7%-17.3%）and promoted Po accumulation in low P and high P soils（Pi/Po decreased 12.7%-18.0%and 6.7%-9.8%）.Low P and high P soils had the highest microbial P and C limitations,respectively.In medium P soil,the P and C limitations were between those of low P and high P soils.Straw addition reduced microbial P limitation in low P and MP soils.In addition,species number,diversity and synergy of microbial communities were high in medium P soil.Straw addition modified microbial community composition and resulted in a greater increase in the relative abundance of keystone P-solubilizing microorganisms,such as Rhizobiales and Bradyrhizobium,in medium P soil.We concluded that straw addition promoted Po mineralization when the soil Olsen-P content was between 8.1-18.9 mg/kg and the corresponding MBC/MBP ratio was between 25.6-36.9.（4）In the rice-oilseed rape rotation,straw returning increased soil phoD and pqqC gene abundance and improved their community composition,accelerating microbial P mobilization.The increase in phoD and pqqC gene abundance and labile Po by straw returning was higher in the oilseed rape season（37.8%,32.0%and 17.4%）than in the rice season（12.0%,15.6%and 6.2%）.High P input decreased phoD gene abundance and increased pqqC gene abundance.The abundance of phoD and pqqC genes and alkaline phosphatase activity were higher in the rice season than in the oilseed rape season.In addition,the number of significant negative and positive correlations between soil phoD and pqqC gene species abundance concerning P forms was higher in the rice season than in the oilseed rape season.The interaction of soil phoD and pqqC genes explained more active P in the oilseed rape season than in the rice season.（5）Straw return could achieve P fertilizer substitution,and the P fertilizer substitution rate was higher in the oilseed rape season（31.8%-42.7%）than in the rice season（14.0%-24.9%）.The sensitivity of P accumulation to soil Olsen-P,microbial biomass P and C（MBP and MBC）was higher in oilseed rape than that in rice.The effect of MBP and MBC on Olsen-P was higher in the rice season than in the rape season,while MBP showed a higher direct role in oilseed rape P accumulation.P fertilizer and stubble C input were the key factors for the increase in crop yield,P accumulation,soil MBP and Olsen-P.Straw return increased MBC through C input,and MBC served in P availability.In addition,the response of soil MBP,MBC and Olsen-P to C and P inputs was higher in the oilseed rape season than in the rice season.In summary,straw returning regulates microbial nutrient limitation and community composition through C input,improves soil MBP and turnover,promotes P fraction mobilization,and increases soil P supply capacity,thus improving P utilization and achieving the P fertilizer substitution effect in the rice-oilseed rape rotation system.