Soil Nitrogen Accumulation And Water-nitrogen Regulation In Cotton Field On The Upper Reaches Region Of Tarim River Catchment

Nitrogen fertilizer is the most widely used chemical fertilizer in agricultural production.Its fate after application may include crop absorption,soil residue and loss.The losses may include ammonia volatilization,nitrous oxide（N₂O）emission,and leaching.Ammonia volatilization and N₂O emission directly cause pollution of the atmospheric environment and various environmental issues such as acid rain.Leaching leads to eutrophication of the water body and excessive nitrate content in drinking water.The accumulation of nitrogen in the soil profile is a potential basis for leaching,and the greater the accumulation,the higher the risk of leaching contamination.Finding out the adverse effects of the long-term application of nitrogen fertilizers in intensive agricultural regions on soil nitrogen accumulation and loss,as well as the whereabouts of fertilizer nitrogen,has vital theoretical and practical significance for reducing fertilizer application amount,increasing efficiency,and lowering environmental pollution risks.The unique climatic conditions in Xinjiang,including favorable light,precipitation,and temperature,create excellent natural conditions for cotton planting.As a result,Xinjiang has become the primary cotton producing region in China.In 2020,the cotton cultivation area in Xinjiang reached 2.502 million hectares,securing the top spot for both cotton acreage and yield in China.Southern Xinjiang,which is the main cotton cultivation area in Xinjiang,occupies a crucial position in Xinjiang’s cotton production.Despite this,there is a lack of research on accumulation of soil nitrogen,the distribution and environmental impacts of nitrogen fertilizer application in the primary cotton farming regions of southern Xinjiang.To address this gap,this study investigated the nitrogen accumulation characteristics and driving factors in the cotton fields in the upper reaches of the Tarim River on a regional scale.This study also conducted in-situ soil column experiments and field plot experiments to examine the leaching,gaseous losses,accumulation,migration,and regulation mechanism of nitrogen fertilizer in the cotton fields under different irrigation modes.The study aims to provide a theoretical basis for improving the utilization rate of nitrogen fertilizer in cotton fields in southern Xinjiang and reducing the pollution of fertilizer nitrogen to the environment.The following main research results were obtained.（1）The soil mineral nitrogen accumulation characteristics and influencing factors in the cotton area of the upper reaches of the Tarim River were clarified.Soil samples of 200 cm soil layer were collected from 108 sites across an 8000 km²area,spanning 160 km from east to west and 50 km from north to south,in the cotton cultivation region of the upper Tarim River.It was found that the mineral nitrogen accumulation in the 0-200 cm soil in the cotton cultivation area was dominated by nitrate nitrogen.The soil nitrate nitrogen accumulation for the three different land uses followed the order of jujube orchards>cotton fields>wasteland,with an amount of respective 614kg·hm^-2,465 kg·hm^-2,and 39 kg·hm^-2.The nitrate nitrogen was predominantly accumulated in the 0-100 cm soil layer,with the 0-100 cm soil nitrate nitrogen accumulation in jujube orchards,cotton fields,and wasteland accounting for 62%,68%,and 88%of the 0-200 cm nitrate nitrogen accumulation,respectively.Despite the increase in soil nitrogen due to fertilization,long-term cultivation and irrigation practices reduced the accumulation of nitrate nitrogen in the soil.The spatial distribution of soil nitrate nitrogen accumulation followed a pattern of higher accumulation in the west and lower accumulation in the east in the cotton cultivation area of the upper reaches of the Tarim River.The farmland surrounded by the three major plain reservoirs,farmland around the source of the Tarim River,and the surroundings of the main urban area had relatively higher soil nitrogen accumulation.The amount of nitrogen fertilizer,average annual irrigation and precipitation,and agricultural land area were found to be the main driving factors influencing the spatial distribution of nitrate nitrogen in the region.（2）The characteristics of mineral nitrogen content in the surface water and groundwater of the cotton cultivation area in the upper reaches of the Tarim River were analyzed.The water samples were collected randomly from the cotton cultivation area spanning approximately 8000 km² in the upper reaches of the Tarim River.The collection included a total of 100 shallow water wells with a depth range of 15-20 m,approximately 100farmland alkali discharge canals,about 20 primary farmland irrigation canals,and various segments of the upper reaches of the Tarim River.The samples were analyzed to determine their ammonium and nitrate nitrogen contents.The results revealed that the average ammonium nitrogen in surface water was 3.25 mg·L^-1,while the average ammonium nitrogen in groundwater was 1.05 mg·L^-1.Approximately 42%of the surface water samples exceeded the V standard（GB3838-2002）,while 76%of the groundwater samples exceeded the V standard（GB/T 14848-2017）.The average nitrate nitrogen in surface water was 0.88mg·L^-1,and the average nitrate nitrogen in groundwater was 3.29 mg·L^-1.Approximately13%of surface water samples and 2%of groundwater samples had nitrate nitrogen contents that exceeded the standard.Interestingly,the levels of ammonium nitrogen and nitrate nitrogen in the water body were not significantly correlated with the levels of ammonium nitrogen or nitrate nitrogen in the soil.Additionally,the study found that in the common system of salt discharge and domestic sewage discharge,the alkali discharge canals had higher ammonium nitrogen content compared to nitrate nitrogen content.This phenomenon was attributed to the water-saving drip irrigation planting mode and multi-source co-drainage system adopted in this region.（3）The mechanism of the effects of irrigation and nitrogen fertilizer application on cotton growth and nitrogen leaching was revealed.Two typical soil textures,sandy loam and silty loam,were selected,and 60 cm of undisturbed soil was collected in the field using PVC pipes to make undisturbed soil columns.The cotton plant nitrogen uptake,soil nitrogen accumulation,leaching loss,and leaching rate were examined under various conditions.These included two different soil textures,two irrigation modes,three levels of irrigation amount,and three levels of nitrogen fertilizer application amounts.Results showed that when the nitrogen application rate was increased from 270 kg·hm^-2 to 360 kg·hm^-2,the cotton biomass and nitrogen accumulation were significantly increased.However,when the nitrogen application rate increased to 450 kg·hm^-2,the cotton biomass and nitrogen accumulation did not increase further.Cotton plants absorbed the most nitrogen with a drip irrigation water volume of7500 m³·hm^-2or flood irrigation water volume of 9000 m³·hm^-2.Additionally,sandy loam soil resulted a significantly higher cotton biomass and nitrogen accumulation compared to silty loam soil.The amount of water and nitrogen input,input method,soil texture type,and crop nitrogen uptake worked together affecting the nitrogen residue in the soil.In silty loam soil,the treatment of flood irrigation with medium water volume(9000 m³·hm^-2)and high nitrogen application rate(450 kg·hm^-2)resulted a highest nitrate-nitrogen residue in the 0-60 cm soil layer.In sandy loam,the drip irrigation with high nitrogen treatment had higher nitrate-nitrogen residue in the 0-20 cm soil layer,while flood irrigation and high nitrogen treatment had higher nitrate-nitrogen residue in the 40-60 cm soil layer.Under the condition of excessive water input,long-term high-frequency irrigation with excessive fertilization were the main reasons for the increased nitrate-nitrogen residue.From the results of the soil column test,it can be seen that the amount of water and nitrogen input and the irrigation mode were the primary factors affecting the leaching of soil nitrogen.The total leaching loss and loss rate were the highest under high water and high nitrogen regardless of drip irrigation or flood irrigation.Therefore,to mitigate nitrogen leaching loss in cotton production,it is important to regulate not only the amount of irrigation water and nitrogen application but also the frequency of irrigation.（4）The regulation mechanism of nitrogen fertilizer in terms of cotton growth and nitrogen distribution was elucidated.This study employed field trials that used water meters to precisely control water and fertilizer.Six different nitrogen fertilizers were applied under various irrigation methods to investigate the effects on cotton growth,soil and plant nitrogen accumulation,and ammonia volatilization.Nitrogen balance was also analyzed under different treatments.Results showed that compared with the normal water and nitrogen input,nitrogen fertilizer control measures such as nitrogen reduction+nitrification inhibitor,nitrogen reduction+urease inhibitor,nitrogen reduction+urease inhibitor+nitrification inhibitor+straw returning under drip irrigation and furrow irrigation conditions were effective in promoting cotton growth by increasing plant height,number of true leaves,and leaf SPAD.Drip irrigation was found to be more effective than furrow irrigation.However,reducing nitrogen by 40%under furrow irrigation had the potential risk of reducing cotton production.The study also demonstrated that ammonia volatilization in the cotton field soil was relatively high,ranging from 7.95 to 21.97 kg·hm^-2,which accounted for 3.2 to 8.8%of the total nitrogen application.Drip irrigation had a significantly higher ammonia volatilization compared to furrow irrigation due to the increased number of soil surface applications.Compared with normal nitrogen fertilization,under furrow irrigation,nitrogen reduction+nitrification inhibitor increased ammonia volatilization,while nitrogen reduction+urease inhibitor,nitrogen reduction+straw returning,and nitrogen reduction+urease inhibitor+nitrification inhibitor+straw returning measures reduced soil ammonia volatilization risk.After cotton harvesting,soil nitrate nitrogen was mainly found in the0-40 cm soil layer,and all control measures had a positive effect on reducing soil nitrate nitrogen accumulation compared to normal water and nitrogen input.In summary,the long-term improper use of nitrogen fertilizers and irrigation practices in the upper Tarim River region resulted in a significant accumulation of nitrate-nitrogen in the farmland soil profile compared with natural ecosystems.Specifically,the nitrate-nitrogen accumulation in the 0-200 cm soil profile of the cotton fields was 2616kg·hm^-2.Approximately 5-10%of the nitrogen applied to the cotton field was lost through leaching.Nitrate leaching was higher under drip irrigation than flood irrigation.Additionally,prolonged and high-frequency irrigation increased the loss of nitrogen through leaching.Based on a comprehensive analysis of many aspects,including cotton yield,soil nitrate-nitrogen residue,and ammonia volatilization,it is recommended that the most suitable nitrogen fertilization strategy for cotton cultivation in this region,when utilizing drip irrigation,is the combination of nitrogen reduction and the use of a urease inhibitor.This method is recommended for optimal results.