Spatial And Temporal Characteristics Of Nitrogen Deposition And Its Effects On Plant Diversity And Element Stoichiometry In Shaanxi Province

Natural ecosystems are mostly nitrogen-deficient systems.In recent years,human life,agriculture and industrial activities have emitted a large amount of active nitrogen into the atmosphere.Reactive nitrogen often returns to the surface in the form of nitrogen dry and wet deposition.The impact of long-term nitrogen deposition on plant diversity and elemental stoichiometry in terrestrial ecosystems is one of the important issues of ecological and environmental disciplines.The climate,vegetation and agricultural production conditions in Shaanxi vary greatly from south to north.In recent years,the economic crops represented by fruit trees have developed rapidly.In the Zhouzhi and Meixian areas of the northern foot of the Qinling Mountains,the kiwifruit producing areas with the largest concentrated area in the world have been formed.The Loess Plateau in northern Shaanxi is a concentrated apple producing area in China.Compared with food crops,the amount of fertilizer applied to economic crops is high.There are also large differences in industrial development and population density from south to north in Shaanxi.These factors will undoubtedly affect the nitrogen deposition characteristics in different regions of Shaanxi.However,it is not clear about the spatial and temporal variation characteristics of nitrogen deposition in different regions of Shaanxi,especially in orchard concentrated production areas,as well as the effects on plant element stoichiometry and plant diversity.Based on the monitoring experiments of nitrogen deposition in different regions of Shaanxi,this paper analyzes the spatial and temporal characteristics of nitrogen deposition in Shaanxi from the northern foot of Qinling Mountains to northern Shaanxi.The stoichiometric differences of the same plant elements in different regions were analyzed by combining the plant leaves,roots and soil collected near the nitrogen deposition monitoring points.The effects of simulated nitrogen deposition on plant diversity and plant stoichiometry were studied based on nutrient addition experiments.The main conclusions obtained include:Draw the following main conclusions:（1）During the monitoring period from July 2020 to November 2022,the annual total nitrogen deposition flux in each region was 5.88-24.59 kg N hm^-2.Compared with ten years ago,nitrogen deposition has a decreasing trend,but it is still at a high level.Each monitoring point showed that wet deposition contributed greatly to total nitrogen deposition flux,and dry deposition contributed less to total nitrogen deposition.Except for Huoditang,all areas had the risk of nitrogen saturation.Except for Huoditang,NH₄⁺-N was the main contributor to nitrogen deposition at all monitoring points,accounting for 47.59%-54.39%of total nitrogen deposition.Large-scale centralized agricultural activities may be the main reason for high-level nitrogen deposition at each monitoring point.The temporal variation and regional difference of atmospheric nitrogen deposition and its components are related to the annual precipitation and its seasonal distribution,temperature,agricultural activity intensity,industrial and fossil fuel combustion.Temperature and precipitation had significant effects on the distribution of nitrogen deposition（p<0.01）.（2）There were differences in plant element content and element stoichiometry in different regions.The contents of soil total nitrogen,mineral nitrogen and plant nitrogen in Meixian County,Zhouzhi Jiufeng Town,Zhouzhi Minzhu Village and Luochuan Jiuxian Town were relatively high.This is consistent with the monitoring data of nitrogen deposition and the flux of nitrogen deposition in different regions in the literature.Long-term nitrogen deposition makes the average N:P of plants in Meixian County greater than 14,and the N:P of plants such as Robinia pseudoacacia is greater than 16,making the plants have changed from nitrogen limitation to phosphorus limitation.Considering the long-term effects of nitrogen deposition caused by human agricultural activities and industrial activities on plants,its impact on the stoichiometry of plant elements and its impact on natural ecosystems cannot be ignored.（3）Nutrient addition experiments showed that fertilization reduced plant diversity and plant C:N increased plant C:P and N:P.Low nitrogen addition can alleviate the effect of high nitrogen addition on organisms.In summary,with the large-scale development of agriculture and industry,the nitrogen deposition flux in many areas of Shaanxi has exceeded the ecosystem load.Long-term nitrogen deposition will reduce plant diversity,increase plant nitrogen content,reduce plant C:N,and increase plant N:P.Therefore,attention should be paid to the effects of nitrogen deposition on plant diversity and plant ecological stoichiometry.