Atmospheric Nitrogen Deposition Characteristics Of Typical Vegetation Types In Sejila Mountains

Nitrogen is one of the important elements to maintain the structure and function of ecosystems,and the nitrogen cycle is mainly introduced into the surface ecosystems through atmospheric nitrogen deposition.With the increase of human activities,the large combustion of fossil fuels,the large use of nitrogen fertilizers and the increase in transportation emissions have led to the emission of large amounts of reactive nitrogen into the atmospheric environment,thus affecting the nitrogen cycle.Through the quantitative analysis of atmospheric nitrogen deposition in the monitoring points of typical vegetation type areas of typical vegetation type areas of warm temperate coniferous and broad-wide mixed forests in urban area of Nyingchi City and Sejila Mountains,cold and wet dark coniferous forests in subalpine cold temperate and subalpine cold and wet shrubland meadows,the variation characteristics of atmospheric nitrogen deposition in typical vegetation type areas of Sejila Mountains were discussed.Provide data and theoretical support for the sustainable development of the Sejila Mountains ecosystem.The concentrations of NH₄⁺-N,NO₃^--N,DON and TN were mainly measured in atmospheric nitrogen wet deposition,and the sedimentation flux was calculated by combining precipitation,and the concentrations of atmospheric active nitrogen gas NH₃and NO₂were mainly determined.The main conclusions are as follows:（1）The wet deposition fluxes of inorganic nitrogen（DIN）in precipitation ranged from 4.63 to 9.95 kg N ha^-1,and the DIN deposition fluxes generally showed a trend of warm temperate moist mixed coniferous forests in mountains(9.95 kg N ha^-1)>subalpine-alpine cold wet scrub meadows(7.41 kg N ha^-1)>subalpine cold temperate cold wet dark coniferous forests(4.81 kg N ha^-1)>urban area of Nyingchi City(4.63 kg N ha^-1).Atmospheric nitrogen deposition fluxes were strongly related to precipitation,and NH₄⁺-N and NO₃^--N deposition fluxes in inorganic nitrogen deposition at each monitoring site were significantly and positively correlated with precipitation（p<0.05）,and inorganic nitrogen deposition fluxes were mainly concentrated in the rainy season.（2）The organic nitrogen（DON）deposition flux in precipitation was at a high level at all monitoring sites,ranging from 13.39 to 25.41 kg N ha^-1,accounting for 66%-75%of the wet deposition TN,showing a trend of warm temperate wet coniferous mixed forests in mountains(25.41 kg N ha^-1)>subalpine-alpine cold wet scrub meadows(14.43 kg N ha^-1)>urban areas of Nyingchi City(13.98 kg N ha^-1)>subalpine cold-temperate cold-wet dark coniferous forest(13.39 kg N ha^-1).The linear fit between precipitation and DON wet deposition flux was consistent with a positive correlation,the organic N wet deposition flux increased with precipitation.However,there are relatively few studies on organic nitrogen deposition,and the impact of organic nitrogen deposition on the ecosystem is not yet clear.（3）The whole-season TN wet deposition fluxes at the monitoring sites in the urban area of Nyingchi City and the typical vegetation type area of the Sejila Mountains were as follows:warm-temperate moist mixed coniferous forest in the mountains(35.36 kg N ha^-1)>subalpine-alpine cold-wet scrub meadow(21.83 kg N ha^-1)>urban area of Nyingchi City(18.61 kg N ha^-1)>subalpine cold-temperate cold-wet dark coniferous forest(18.20 kg N ha^-1).The TN wet deposition at each monitoring site was concentrated in the rainy season,and the correlation coefficients between deposition flux and precipitation were all large and influenced by precipitation.（4）In the seasonal variation of atmospheric N dry deposition concentration,the seasonal mean NH₃concentration in the typical vegetation type area of the Sejila Mountains showed a trend of montane warm temperate moist mixed coniferous forest(6.58μg N m^-3)>subalpine cold temperate cold wet dark coniferous forest(0.90μg N m^-3)>subalpine-alpine cold wet scrub meadow(0.47μg N m^-3).This may be related to the greater influence of NH₃by temperature,with the average temperature of mixed coniferous forests being greater than that of dark coniferous forests and scrub meadows,and the average temperature of dark coniferous forests being greater than that of scrub meadows.The seasonal mean NO₂concentrations in the typical vegetation type area of the Sejila Mountains showed a trend of montane warm-temperate moist mixed coniferous forests(1.97μg N m^-3)>subalpine cold-temperate cold-wet dark coniferous forests(1.18μg N m^-3)>subalpine-alpine cold-wet scrub meadows(0.23μg N m^-3).The NO₂concentrations were relatively low in the subalpine-alpine cold-wet scrub meadows due to their higher elevation and the absence of significant anthropogenic emission sources nearby.The mountainous warm-temperate moist mixed coniferous forests and subalpine cold-temperate cold-wet dark coniferous forests are close to major traffic arteries and are influenced by traffic exhausts resulting in relatively high NO₂concentrations.（5）In terms of source analysis,the approximate source is simply judged according to the ammonium-to-nitrogen ratio,NH₄⁺-N represents the agricultural source,and NO₃^--N represents the industrial source.The main sources of contribution of the urban area of Nyingchi City in summer and autumn are agricultural sources,and the contribution of agricultural sources and industrial sources in spring and winter is comparable.The mixed coniferous and broad-coniferous forests in the temperate zone of the mountains are mainly agricultural sources in summer,the agricultural sources and industrial sources in autumn and spring are equally contributed,and the industrial sources are mainly used in winter.Subalpine cold temperate cold and wet dark coniferous forests and subalpine-alpine cold and wet shrubland meadow vegetation type areas are mainly composed of agricultural sources throughout the season.In summer,it is in the growing season of plants and the temperature is high,which is conducive to the volatilization of NH₃,resulting in a generally high ammonium to nitrogen ratio in summer.In the urban area of Nyingchi City and the humid coniferous mixed forest in the temperate zone of mountain warmth,it was greatly affected by human activities,resulting in low ammonium nitrogen.