Effects Of Atmospheric Nitrogen Deposition On Grain Yield And Methane Emission From Paddy Field Across China

Rice production is important for meeting population growth,and rice paddies are one of agricultural sources for methane?CH₄?emissions.Under the background of high nitrogen?N?deposition in China,the study on the effects of N deposition on rice yield and CH₄ emissions can not only help to ensure rice food security,but also improve the estimation accuracy of CH₄ emissions.Therefore,the effects of N deposition on rice yield and CH₄ emission in Chinese's rice fields are studied by using N deposition data retrieved from remote sensing and ecological process model DNDC.The main research conclusions are as follows:?1?The average N deposition in 2012 in China's paddy field was 21.4 kg N ha^-1 yr^-1(ranging from 5.1 to 106.7 kg N ha^-1 yr^-1),resulting in an increase in rice yield of 12.4 × 108 kg.The increased rice yield accounted for 0.6% of the total rice yield.The effect of N deposition on rice yield had a strong spatial heterogeneity.The area promoted by N deposition accounted for 15% of the total rice area,with an average increase of 270.4 kg ha^-1,mainly concentrated in Heilongjiang Province.The area inhibited by N deposition was less than 1% of the total rice area,and the average decreased yield was 24.8 kg ha^-1.The area with no effect by N deposition accounted for 85% of the total rice area.With the increase of fertilizer application,the effect of N deposition on the increase of rice yield decreased significantly.?2?N deposition had a slight inhibitory effect on CH₄ emissions,which reduced the CH₄ emissions of rice fields by 0.04 Tg?0.026-0.044 Tg?.The mechanism of N deposition to inhibit CH₄ emission has three aspects: the promotion effect of ammonium N on methane-oxidizing bacteria;the increase of soil redox potential?Eh?by nitrate N?which is not conductive to the production of CH₄?;toxic effects of nitrate and its denitrification products?nitrate?on methanogens.The average reduced CH₄ flux of single and double cropping rice by N deposition were 1.38 and 1.69 kg CH₄ ha^-1 yr^-1,respectively.Moreover,the total CH₄ emission from rice fields in 2012 in China was 8.20 Tg CH₄ yr^-1(4.80^-11.40 Tg CH₄ yr^-1,1 Tg = 109 kg).CH₄ emissions from early,late and single cropping rice were 1.12?14%?,2.86?35%?and 4.23?51%?Tg,respectively.Moreover,the spatial distribution of CH₄ emissions in paddy fields was mainly determined by flooding irrigation times of growth duration?positive correlation?,temperature of growth duration?positive correlation?,and soil clay content?negative correlation?.?3?In general,the average availability?absorbed by rice and residue in soil,converted into soil organic nitrogen and soil residue?and loss?NH3 volatilization,leaching and runoff,and nitrogen oxide emission?of N deposition were 8.1 and 6.4 kg N ha^-1,respectively,accounting for 60% and 40% of the average N deposition,respectively.The soil residue?d SIN?caused by N deposition was 7.9 kg N ha^-1,accounting for 56.9% of the average N deposition.The nitrogen oxide emission caused by N deposition was 3.7 kg N ha^-1,accounting for 24.8% of the average N deposition.The total amount of leaching and runoff caused by N deposition was 1.8 kg N ha^-1,accounting for 10.1% of the average N deposition.The NH3 volatilization caused by N deposition was 0.9 kg N ha^-1,accounting for 5.6% of the average N deposition.The average amount of N deposition absorbed by rice was 0.2 kg N ha^-1,accounting for 1.8% of the average N deposition.The average amount of N deposition converted into soil organic nitrogen?d SON?was 0.1 kg N ha^-1,accounting for 0.8% of the average N deposition.Considering both food security and environmental risks,our simulation results show that: on the promise of not reducing the rice yield,appropriately reducing the application amount of nitrogen fertilizer can maximize the utilization of N deposition,thus reducing secondary environmental pollution caused by N deposition.Under the background of high N deposition,this study estimated the CH₄ emissions from rice fields,identified the hotspots of CH₄ emissions,and illustrated the effects of N deposition on rice yield and CH₄ emissions in China.This study provides the management strategies and relate plans for China's food security departments and environmental management departments,and provides a scientific basis for international carbon emission negotiations.