Effects And Driving Mechanisms Of Combined Application Of Organic Fertilizers On Soil Environmental Quality Of Paddy Field

Combined application of organic fertilizers was significant for improving soil physical properties,increasing soil nutrients and biological activities.However,the irregular utilization of organic fertilizers could induce agricultural non-point source pollution.Therefore,it was of great significance to evaluate the comprehensive effects quantitatively and driving mechanisms of combined application of organic fertilizers on soil environmental quality of paddy field.Firstly,soil environmental quality evaluation index system for paddy field with combined application of organic fertilizers was constructed based on the coupling analysis including meta-analysis,variation analysis,correlation analysis,factor analysis,and random forest mode based on the data collected from the 137 peer-reviewed articles in the present study.Secondly,soil environmental quality of paddy field with long-term(8 years)combined application of organic fertilizers including five treatments: chemical fertilizers applied only(T1),pig manure substituted 20% chemical nitrogen fertilizer(T2),biogas residue substituted 20% chemical nitrogen fertilizer(T3),compost from pig manure substituted 20% chemical nitrogen fertilizer(T4),green manure in early rice and rice straw in late rice substituted 20% chemical nitrogen fertilizer(T5)was determined by factor analysis method,fuzzy comprehensive evaluation method,and entropy method respectively.The effects of long-term combined application of organic fertilizers on soil bacteria communities,fungi communities and carbon cycle genes of paddy field were further analyzed through high-throughput sequencing technology.The relational models between bacteria communities,fungi communities,carbon cycle genes and soil environmental quality were explored,and the driving mechanisms of combined application of organic fertilizers in the paddy field on the soil environmental quality from the micro-level was clarified.The main results obtained in the present study were as follows:(1)Soil environmental quality evaluation index system with 10 indicators including soil pH,bulk density,electrical conductivity,organic carbon,total nitrogen,available potassium,available phosphorus,microbial biomass carbon,cadmium and copper for the combined application of organic fertilizers in paddy fields was constructed.(2)The ranges of soil environmental quality comprehensive scores according to factor analysis method,fuzzy comprehensive evaluation method,and entropy method for the five treatments were-1.32～7.31,0.20～0.83,and 0.37～1.36,respectively.The trend of the comprehensive scores of the five treatments following the three evaluation methods was consistent.Moreover,the range of the soil environmental quality comprehensive score based on factor analysis method was widest among the three methods which indicated that factor analysis method was most suitable for assessing soil environmental quality.The soil environmental quality of the five treatments was divided into three levels based on cluster analysis: I level(T2,T4),II level(T3,T5),and III level(T1),which indicated that combined application of organic fertilizers could improve soil environmental quality of paddy field significantly.Moreover,the effects of different organic fertilizers types on soil environmental quality were not consistent.(3)The results of bacteria communities sequencing indicated that the diversity(Shannon index and Simpson index)and richness(Observed OUT and Chao1 index)of soil bacteria communities for the treatments of T2,T3,T4,and T5 were significantly improved compared with the treatment of T1.Regression analysis revealed that soil environmental quality of paddy field with combined application of organic fertilizers was significantly positively correlated with the diversity and richness of bacteria communities.Compared with the treatment of T1,the relative abundance of Proteobacteria under the treatments of T2,T3 and T4 were significantly increased by 27.30%,25.22%,and 41.99% respectively.The network analysis indicated that nine genera among the Proteobacteria affected soil environmental quality of paddy field with combined application of organic fertilizers significantly,and they played a positive role on soil environmental quality.(4)As for fungal communities,the treatments of T2 and T4 significantly reduced the relative abundance of Ascomycota with 9.02% and 11.70% receptively compared with the treatment of T1.While,the treatments of T2,T3,and T4 significantly increased the relative abundance of Basidiomycete with 9.11%,11.08%,and 20.15% receptively.Moreover,the treatment of T4 significantly improved soil the relative abundance of Zygomycota with 30.87%.Network analysis demonstrated that the fungal genera of Drechslerella 、 Coniosporium 、 Eutypa 、 Fonsecaea 、 Rasamsonia 、 Thielavia and Cladophialophora in Ascomycota,Moniliophthora in Basidiomycota and Mortierella in Zygomycota affected soil environmental quality significantly.(5)There were 10 genes according to CAZy(Carbohydrate-Active enzymes)database which significantly influenced soil environmental quality.Among the 10 genes,the genes of AA1 、AA5 、GH103、CE4、CE14、CBM48 and GT51 had significant positive effects on soil environmental quality.On the contrary,the genes of GH63,GH1 and GH2 played negative roles on soil environmental quality.