Effect Of Nutrient Input On Soil Physiochemical Properties And Soil Microbial Diversity In Rice-oilseed Rape Rotation Field

Straw returning helps to improve soil structure,increase soil fertility and reduce environmental load of toxic gases emission,which can otherwise result if straw is subjected to burning.The intensive planting and production mode of rice-oil rotation in Southwest China is the main farming method for rice and oilseed production areas.The research on the effects of straw returning and chemical fertilizer application on farmland soil quality and crop yield under the rice-oil rotation double cropping system is lagging behind,especially unreasonable application will affect crop yield and soil nitrogen fixation and emission reduction capacity.In order to explore the effects of nitrogen and phosphorus fertilizer application and straw returning on the physical,chemical,biological,and microbiological properties,total crop yield,and yield parameters of rice-oil rotation field,this study set up four rice-oil double-cropping fields in Mianyang,Sichuan.4 levels of nitrogen（urea）（0,60,120,180 kg/ha）and 4levels of phosphorus（P₂O₅）fertilizers（0,60,90,120 kg/ha）were combined into 8treatments:CK（N₀P₀）,T1(N₁₂₀P₀),T2(N₁₂₀P₆₀),T3(N₁₂₀P₉₀),T4(N₁₂₀P₁₂₀),T5(N₀P₉₀),T6(N₆₀P₉₀)and T7(N₁₈₀P₉₀)and 2 rice varieties（Deyou 4727 and Jingyou781）and 2 rapeseed varieties（Mianyou 305 and Mianyou 15）completely randomized block design,studied the effects of nitrogen and phosphorus fertilizers and straw returning on the total amount of plant essential nutrients,the content of available nutrients（C,N,and P）,and the soil micro-ecosystem.The soil physical structure,lignocellulose content,enzymatic activities,and microbial diversity were also studied,which provides a theoretical basis and technical support for soil quality and crop yield under rice-oil double cropping system.The main findings of the field trial study are as follows:（1）The application of N and P fertilizers and straw returning significantly increased the yield of rice（Deyou 4727 and Jingyou 781）and oilseed rape（Mianyou305 and Mianyou 15）.Under the same straw management method,the yield of rice and rape（only Mianyou 15）increased significantly with the increase of nitrogen application rate under moderate phosphorus fertilizer（90 kg/ha）.Under the same N application level,the total yield of rice and oil rapeseed with high P fertilizer content（120 kg/ha）was significantly the highest.The yields of rice and rape were significantly highest under high fertilization（T7）as compared to CK.Under a high N application rate（≥120 kg/ha）,there was no significant difference in the total yield of Jingyou 781 and Mianyou 15 between T7 and T4.The effective panicle number（153-206）,and seed setting rate（91.68-96.49%）of the two rice varieties T7,T4,and T1 were all significantly highest.The number of grains per panicle（216 and 232 grains）of T7 treatment was significantly highest.T7 and T4showed the highest thousand-grain weight（3.18-4.89）,pod number（267-384）,grain number（4827-7047）,and thousand-grain weight（5.39-6.87）of the main branch and effective branches respectively.Combining various agronomic measures,straw returning,N,and P fertilizer management have a significant impact on rice and oil rapeseed yields.（2）The soil pH was weakly alkaline in all treatments and the EC values increased significantly in N and P fertilized treatments.Dissolved organic carbon（DOC）content was significantly high in both seasons soils with high fertilization rates（T4and T7）,and lowest values were found in moderate fertilization（T3 and T2）.Fertilization and straw returning significantly effected soil organic matter（OM）and soil enzyme activity.The T1-T3 treatments showed significantly low soil OM content in rice season（22.99-48.51%）and rapeseed season（31.50-51.57%）,respectively.Similarly,T2 and T3 treatments had the highest activity of CMCase,xylanase,and laccase activity in both seasons soils,which resulted in significantly lowest cellulose,hemicellulose,and lignin contents in the respective soil.However,alkaline phosphatase enzyme activity was significantly high in high P fertilized treatments（T6and T5）,respectively.（3）The microbial diversity indexes such as OUTs,Chao1,Shannon and Simpson index of soil bacteria and fungi were affected by N and P application in both seasons.Moderate N and P fertilization（T3 and T2 treatments）and straw returning significantly increased the OTUs and Chao1 index of bacterial and fungal communities in the rice season,while low values were found in T4 and T7 treatments.The bacterial and fungal Shannon indices in rice season were significantly high in T2and T7 treatments,respectively.In rapeseed season,under the same N application level,the bacterial OTUs and Chao1 index of 0-120 kg/ha P fertilizer treatment（T1-T4）were significantly high.However,under the same P application level（0-120kg/ha）,but high N application level（0-180 kg/ha）the bacterial OTUs and Chao1index were significantly low.The bacterial Simpson index was significantly low in T4-T7 and T1 treatments,and high values were found in T2 and T3.The fungal OTUs and Chao1 index of rapeseed T2 treatment were significantly high,and the Simpson index was significantly low.The fungal diversity indexes（OUTs,Chao1,and Simpson）of T4-T7 were opposite to those of the T2 treatment.The composition of phylum and order of dominant bacteria and fungi in rice-oilseed double-cropping soil with different N and P fertilizers were the same,but their relative abundance was different.The dominant bacterial phyla were Chloroflexi,Actinomycetes,Acidobacteria,and Proteobacteria.Micrococcales,Micromonosporales,Vicinamibacterales,Burkholderiales,and Rhizobiales were the orders with the highest relative abundance among the above bacterial phyla,respectively.The dominant fungal phyla were mainly Ascomycota,Basidiomycota,unclassified fungi,Mortierella,and Rozellomycota.Hypocreales,Agaricales,Unclassified Basidiomycetes,and Mortierellales are the main orders within the fungal phylum.In summary,straw returning and application of N and P fertilizers can significantly improve soil physical properties,soil lignocellulose degradation and biological activity of P solubilizing microorganisms,increase soil organic carbon,and total N storage.It also improves the supply capacity of essential nutrients for rice and oilseed crops,which helps to enhance the carbon fixation and N absorption capacity of crops.Among them,nitrogen application of 120 kg/ha and phosphorus application of 120 kg/ha combined with straw returning have the most significant effect to degrade straw and benefit soil microorganisms.In this study,this combination was best in terms of lignocellulose degradation,with a small compromise on yield.It also enhanced soil carbon and N fixation capacity,which has a significant effect on enriching the number of soil bacteria and microorganisms in both seasons.