Mechanism Of Reducing Nitrogen Loss By Decreasing Nitrogen Application And Planting Catch Crops Of Sesbania In Rice-vegetable Rotation System In Hainan

Excessive application of nitrogen(N)fertilizer is very common to get high yield in vegetable production which causes water or air pollution.This research selected representative rice-vegetable rotation field studies the effects of different fertilization on yield and fate of nitrogen of line pepper and immobilization mechanism analysis of catch crop and the effects of catch crop returning on rice growth and N accumulation by 15N micro-plot experiment and soil column experiment.It is great significance for improving N cycle utilization in rice-vegetable rotation system,reducing environmental pollution and promoting sustainable agricultural development,main results as follow:1.Line pepper yield of optimized fertilization treatment was slightly improved but the difference was not significant compared with conventional fertilizer application treatment.The results of 15N micro-plot experiment indicated that the accumulation of nitrogen in aboveground part was almost equal during the whole growth period of line pepper in 2017;optimized fertilization treatment was significantly higher than conventional fertilizer application treatment,and the nitrogen use efficiency of optimized fertilization treatment was significantly improved in 2018.Even though more N fertilizer was applicated in conventional fertilizer application treatment which resulted in increment of the residual N in 0-40 cm soil and the loss of N.The residual N may be leached after harvest of line pepper and contributed to pollute environment.The nitrogen use efficiency of optimized fertilization treatment is significantly higher than that of conventional fertilizer treatment obtained by the percentage of fertilizer N uptake by crop to N fertilizer rate or 15N tracer technique.2.The amount of NO3--N leaching treated with different nitrogen fertilizer treatment had significant or ex-tremely significant difference,among which the leaching volume had no significant difference,and the content of NO3--N in the leaching water had significant or extremely significant difference.There was a positive correlation between the amount of nitrogen applied and the amount of leaching of nitrate nitrogen.Compared with the conventional fertilizer treatment,the optimized fertilization reduced the amount of 15N loss by 28%～60%and the amount of NO3--N leaching by 24%～27%.Therefore,the control of nitrogen fertilizer application amount can reduce the loss of nitrogen leaching and other ways at the source.Compared with the fallowing treatment,planting catch fallow crop reduiced 11%～50%15N loss,including 45%～55%NO3--N,60%～83%less NH4--N and 69%organic nitrogen leaching.There was not significant difference in the volume of leaching water and the concentration of NO3--N between the fallowing treatment and planting fallow crop treatment in the short term(within 30 to 45 days),because of the less biomass.After this term,the catch crop reduced the volume of leaching water and the concentration of NO3--N.This indicates that planting catch crop more than 1 month can reduce the leaching water volume and the NO3--N concentration in the leaching water by transpiration and absorption.In 2018,the microbial biomass carbon and nitrogen contents in rhizosphere and non-rhizosphere soils were also studied,which indicated the content of microbial biomass carbon in rhizosphere soil was significantly higher than that in non-rhizosphere soil,and the microbial biomass nitrogen in rhizosphere soil was 33%higher than that in non-rhizosphere soil,but the difference was not significant.The results showed that planting catch crop could also promote microorganisms to convert leaching NI3--N into microbial biomass nitrogen by root exudation and temporarily store it to reduce nitrogen leaching.3.Compare with fallow treatment,planting catch crop and returning the straw to the field could significantly increased the plant height,tiller number,dry matter weight,chlorophyll content and nitrogen accumulation of rice.This indicated that planting catch crop and returning the straw to the field could immobilization the nitrogen in soil and release the nitrogen through mineralization after catch crop straw returning to the field to promote the growth of rice.We analyzed the trace of 15N from the beginning of fallow period to the end of rice tillering,the results show that compared with fallow treatment,planting catch fallow crop and returning the straw to the field could significantly increase the nitrogen accumulation of rice,the residual nitrogen in soil and significantly reduce the loss of nitrogen.So it is beneficial to improve the nitrogen recycling in rice-vegetable rotation system to planting catch crop.