Effect Of Organic Alternative On The Soil Nitrogen Leachate In Double-season Paddy Field And Its Mechanism

As an important nutrient source in the process of reducing the amount of chemical fertilizer,using organic fertilizer to replace chemical fertilizer can effectively reduce the environmental problems caused by excessive application of chemical fertilizer.In recent years,it was found that the application of manure could increase the nutrient loss in farmland.However,the effects of replacing chemical fertilizer with organic fertilizer on soil nutrient utilization and loss in double-cropping rice fields in Dongting Lake area are not clear.This paper focuses on the direction of nitrogen in the sustainable development of paddy fields and the process of its migration and transformation in the soil.From the perspective of optimizing the replacement of organic fertilizer,field positioning experiments were carried out.Picked up from the soil solution device,Biolog functional microbial analysis,high-throughput sequencing technologies and structural equation model to study the organic alternative rice nitrogen whereabouts and leaching in the process of change,paddy soil and soil solution morphological changes and migration patterns,soil nitrogen in surface electrochemical properties,functional microbial components and microbial community structure change and its influence factors From macro to micro,the effects and mechanisms of soil nitrogen leaching,inorganic nitrogen adsorption and fixation led by electrochemical properties on soil surface and N cycling microorganism led by N form change during the replacement of organic fertilizer were systematically studied.The theoretical basis was provided for the application of organic fertilizer replacement in double cropping rice field.The main conclusions obtained are summarized as follows:(1)The change of dissolved inorganic N(DIN)leaching loss in late rice season and its influencing factors under different fertilization treatments(no fertilizer,chemical fertilizer,chemical fertilizer plus hog manure,hog manure,and chemical fertilizer plus oil cake)for five consecutive years were investigated from a macroscopic point of view.The results shown that,the leaching loss of DIN from paddy soil decreased with the application ratio of manure application.Furthermore,compared with CF,the soil properties and surface electrochemical properties significantly changed with manure application.In all,manure application significantly decreased bulk density(9.84-20.49%,P<0.05)and specific surface area(26.38-44.88%,P<0.05).And manure application also significantly increased the amount and density of soil surface charge(7.97.55-98.44%,P<0.05)and soil water-holding capacity(19.05-42.86%,P<0.05).In addition,NH4~+-N was the main form of DIN in leachate during rice growth,and the risk of NH4~+-N leaching was decreased with manure application.As NH4~+could easily adsorbed by soil colloid,and the adsorption capacity of NH4~+was increased with manure application due to the increase of soil surface electrochemical properties in manure treatments(β=-0.88,P<0.05).Thus,manure application increased the capacity of NH4~+adsorption by soil colloid,in which surface charge increased with organic matter application.(2)Electrochemical properties of soil are the main cause of leaching risk of DIN After application of organic fertilizer,the increase of organic matter in soil changes the charge distribution on the surface of particles,and affects the fractions and fate of N in soil.The change of N fractions in soil after organic substitution and the effect of organic substitution on the fate of N in paddy field during rice growth period were further studied by continuous extraction method.The results were that,organic fertilization significantly increased apparent nitrogen loss(20.79%-45.57%,P<0.01).At the same time,compared with CF,manure fertilization significantly increased the content of soil Stable Nitrogen(41.42%-72.19%,P<0.05),Microbial Nitrogen(2.77-7.92 times,P<0.05)and Chronic nitrogen(39%-66.78%,P<0.05).In addition,the activities of nitrogen cycling-related enzymes(LAP and NAG)were significantly increased with organic fertilized.The results of structural equation model showed that the increase of apparent nitrogen loss in paddy field was mainly due to the accumulation of organic nitrogen in soil with manure application(P<0.05)and the change of the activities of enzymes involved in nitrogen cycle(P<0.001).These results indicated that the factors affecting soil nitrogen conversion were the important reasons for changing nitrogen use and loss from paddy fields.(3)Combined with the culture test and Biolog ECO method,the main factors affecting the change of soil N fractions after organic substitution were explored.The results shown that,the forms and contents of Fe and Mn oxides in soil were not significantly changed with manure application.However,the content of soil organic carbon in manure fertilizer treatments were significantly higher than that in CF(23.16%-78.40%,P<0.05).At the same time,organic fertilization significantly increased the diversity(1.37-2.11 times,P<0.05)and evenness(1.37-2.11 times,P<0.05)of functional microorganisms in soil.And compared with CF,manure treatments significantly increased the metabolic capacity of microorganisms to amino nitrogen,esters and acids(P<0.05).The results of redundancy analysis and structural equation model showed that the change of soil microbial community was the main factor for the change of nitrogen fractions and content in paddy field with fertilization.The results of NH4~+assimilation and abiotic fixation experiments under different fertilization treatments also proved that the biological assimilation was the key mechanism of soil nitrogen conversion under organic fertilizer treatments.(4)Dissolved organic nitrogen was the main form of nitrogen in leachate of paddy field.Manure fertilization significantly increased the risk of dissolved organic nitrogen(DON)leaching from paddy field(P<0.05).At the same time,the diversity of soil microbial community significantly increased with manure application,which increased about 4.05%at the order level and 32.38%at the genus level(P<0.05).The changes of soil microbial communities significantly affected the changes of DON content in surface water and leachate(P<0.05).All results indicated that the change of microbial community was the dominant factor affected the difference of DON leaching within fertilization.The results also shown that,the microorganisms involved in the metabolism of glutamate were the main community composition of the nitrogen cycling microorganisms in paddy soil with fertilization.Meanwhile,organic fertilization significantly increased the abundance of denitrifying bacteria gene.In addition,Glutamate-metabolizing microorganisms and denitrification bacteria increased the DON content by affecting the transformation of nitrogen in soil,and then affected the runoff and leaching loss of DON(P<0.05).In conclusion,organic fertilization significantly increased the apparent nitrogen loss in the double-cropping rice field system in Dongting Lake region,especially of the DON leaching.