Measurements Of Greenhouse Gas Emission From Double Cropping Rice Field By Reducing Nitrogen Fertilizer Application

China is the only country in the world that applies more than 200kg-N·hm-2 in nitrogen fertilizer.The heavy use of chemical fertilizers not only promotes the increase of grain yield,but also leads to serious decrease of nitrogen fertilizer utilization rate and excessive accumulation of soil nitrogen.Many studies have shown that the application of high amount of fertilizer in order to increase the yield will lead to a significant increase in the greenhouse gas emissions of farmland,especially the N2O emissions of farmland and the nitrogen application have a positive exponential relationship.Therefore,how to reduce environmental pressure while ensuring crop yield through reasonable fertilization is a subject worthy of discussion and in-depth study.In this study,the typical subtropical double-cropping rice field ecosystem in hunan province was taken as the research object.The CO2,CH4 and N2O emissions of double-cropping rice system under the condition of nitrogen fertilizer reduction were measured in situ in the field by using static opaque chamber-gas chromatograph(GC)method and in situ field observation experiment for one year.On the one hand,the greenhouse gas emission dynamics and intensity of double cropping rice under the condition of reduced nitrogen fertilizer were observed.On the other hand,relevant environmental driving factors affecting greenhouse gas emissions of double-cropping rice fields were explored to provide field in-situ observation data for assessing the total greenhouse gas emissions of double-cropping rice fields and their comprehensive emission reduction potential under different fertilizer application amounts,so as to provide reference for the compilation of greenhouse gas emission inventory of farmland in China and the formulation of emission reduction measures.The main research results are as follows:1.The CO2 emission presentsd an annual dynamic change from high in summer and autumn to low in winter and spring,and the peak emissions all occur in the middle and late stage of rice growth in the season,with no significant CO2 absorption or emissions in the fallow period.CH4 emissions were mainly concentrated about a month after transplanting,and peak CH4 emissions occurred 3-4 weeks after the application of basal fertilizer.The CH4 emission flux in late rice season was significantly higher than that in early rice season,and no significant CH4 absorption or emission occurred in fallow period.N2O emission was very low when the field is in the state of flooding,and it is mainly concentrated in the later stage of roasting field,dry and wet alternation period and falling dry period,and the peak emission generally appears in the roasting field in the current season.2.The pH of the surface water in the paddy field in the two seasons varied widely.Each time of fertilization,the pH value of the water would rise and then fall back to the initial level.The variation trends of ammonium nitrogen,nitrate nitrogen,total nitrogen,dissolved phosphorus and total phosphorus content in surface water of early and late rice season were consistent,and the peak appeared after fertilization,and the peak appeared when basal fertilizer was applied,and then dropped to the general level.The change of soil pH was significant.After fertilization in rice growing season,the soil pH decreased,but after fallow period,the soil pH increased.The change trend of soil ammonium nitrogen content in early and late rice seasons was consistent,and the peak value appeared after fertilization,and then decreased to the general level.Soil nitrate nitrogen content decreased after rice planting and irrigation,then peaked during roasting and remained low after rehydration.3.Greenhouse gas emissions of the paddy field ecosystem are affected by the complex interaction of environmental factors such as temperature,depth of field water layer,soil pH value and mineral nitrogen content.In this experiment,CO2 emission of rice farmland was significantly correlated with temperature and water depth in the field.CH4 emission of rice farmland was positively correlated with temperature,depth of water layer in the field and soil mineral nitrogen content,and negatively correlated with soil pH value.There was a significant positive correlation between N2O emission in rice farmland and soil mineral nitrogen content,and a significant correlation between N2O emission and temperature and soil pH.4.Reduced application of nitrogen fertilizer leads to differences in greenhouse gas emissions between treatments.Overall,CO2,CH4 and N2O emission fluxes under F3 treatment were higher than those under other treatments in the two rice planting seasons,and the highest emission peak was also observed under F3 treatment.In terms of cumulative emissions,CO2,CH4 and N2O emissions also decreased significantly with the reduction of nitrogen fertilizer application.However,there was no significant difference between F2 treatment and F3 treatment.In addition,GHGI under F2 treatment was significantly lower than that under F1 treatment and F3 treatment,and agronomic utilization rate of nitrogen fertilizer under F2 treatment was significantly higher than that under F1 treatment and F3 treatment.