The Study On The Spatial And Temporal Variation Of The Impact Of Nitrogen Deposition On Vegetation Cover In China From 2001-2017

In the last decade,rapid agricultural intensification,industrial and urban development,and increased fuel use in transportation and energy production have led to a dramatic increase in anthropogenic reactive nitrogen emissions,which are deposited in the atmosphere through dry and wet pathways involving oxidized nitrate NO_x and reduced ammonium NH_x species.It has been shown that increased nitrogen deposition has far-reaching effects on both natural and anthropogenic ecosystems.China is one of the three regions with the highest nitrogen deposition in the world,but we do not know the mechanisms of nitrogen deposition on vegetation growth in China.Given the rapid socioeconomic changes and the parallel development of industry,combustion technology,and agricultural intensification in China,understanding the patterns of nitrogen deposition and the impact of nitrogen deposition on vegetation is critical for environmental policy decisions in China,the largest developing country.Therefore,we chose China as the study area and explored how the effects of nitrogen deposition on vegetation growth varied using three types of vegetation data,SIF,NIR_V,and NDVI,as well as a variety of climatic data,through trend analysis,correlation analysis,multiple linear regression analysis,contribution calculation,and structural equation modeling,and validated the results using site data.The main conclusions drawn are as follows:（1）During the study period,the Chinese annual means of the three vegetation indices showed fluctuating increasing trends at the rates of 0.014·a^-1,0.001·a^-1 and0.002·a^-1 on the time scale.the Chinese annual means of NO_x and CO₂ data showed fluctuating increasing trends at the rates of 11.36346 t·0.1°^-2·a^-1 and 1.91498 ppm·a^-1.The Chinese annual average of NH_x data showed a fluctuating decreasing trend at-0.89815 t·0.1°^-2·a^-1.The Chinese annual averages of precipitation and temperature data show fluctuating increasing trends at 2.895 mm·a^-1 and 0.00739℃·a^-1,and the Chinese annual averages of solar radiation data show fluctuating decreasing trends at-0.51725MJ·a^-1.This reflects that the vegetation ecology of China is good and the climate ecology is generally stable.（2）The median correlation coefficients of SIF,NIR_V and NDVI data with nitrate-N data in China were positive,with an overall positive correlation.The median correlation coefficients of SIF,NIR_V and NDVI data with ammonium nitrogen data in China were negative,with an overall negative correlation.The median correlation coefficients of SIF,NIR_V and NDVI with precipitation,air temperature and CO₂ data in China were positive,and the overall correlation was positive.The median correlation coefficients of SIF,NIR_V,and NDVI with solar radiation data in China are negative,and the overall correlation is negative.The effect of nitrate nitrogen on vegetation was overall facilitative,while the effect of ammonium nitrogen on vegetation was overall inhibitory.（3）The median regression coefficients of SIF,NIR_V and NDVI data with nitrate-nitrogen data in China were positive,and the overall effect of nitrate-nitrogen on vegetation was facilitative.This is due to the fertilization characteristics after nitrate-nitrogen deposition.The median regression coefficients of SIF,NIR_V and NDVI data and ammonium nitrogen data in China were negative,and the overall effect of ammonium nitrogen on vegetation was inhibitory.This is due to the uptake characteristics of the two nitrogen fertilizers by different vegetation types after the deposition of nitrate and ammonium nitrogen.The median contribution of SIF,NIR_Vand NDVI data to the nitrate-nitrogen data in China was positive,and the overall effect of nitrate-nitrogen on vegetation was facilitative.The median contribution of SIF,NIR_Vand NDVI data to ammonium nitrogen data in China was negative,and the overall effect of ammonium nitrogen on vegetation was suppressed.（4）From the results of the site data,the effect of site N deposition data on vegetation is promoted from the overall point of view,which is due to the fact that plant roots are competing for N in areas where N sources are not sufficient.However,when broken down,the effect of nitrate nitrogen on vegetation is mainly facilitative,while the effect of ammonium nitrogen on vegetation is somewhat inhibitory.The impact of both nitrate and ammonium nitrogen deposition on vegetation in China showed a fluctuating upward trend from 2001 to 2017,but at the same time,there were two significant differences,i.e.,the impact of nitrate nitrogen deposition on vegetation was promoted and gradually increased in 17 years,while the impact of ammonium nitrogen deposition on vegetation was inhibited and gradually decreased in 17 years.This is mainly due to the different application characteristics of the two nitrogen fertilizers,i.e.,the application effect of excess ammonium nitrogen deposition can instead have a suppressive effect on vegetation,while nitrate nitrogen is not bothered by the excess.