Study On The Atmospheric Nitrogen Depositionin Xichuan Area Of Danjiangkou Reservoir

Atmospheric deposition is an important way for nitrogen input to the biosphere.In recent decades,the emission of nitrogen compounds from fossil fuel combustion and fertilizer application has increased dramatically.The ecological problems affected by nitrogen deposition have increased prominently.Nitrogen deposition has been one of the hotspots in the research area of biogeochemical cycle.In contrast to other ecosystems,the research on nitrogen deposition under the special surface condition of reservoir is rarely reported in the literature.Danjiangkou Reservoir is the water source of the Middle Line of the South-to-North Water Transfer Project.TDN concentration of the reservoir water was over class V of water quality.Under the condition that the non-point source pollution around the reservoir has been controlled,atmospheric nitrogen deposition may be the important way for nitrogen input to the reservoir.To test this conjecture,we conducted an experiment of nitrogen deposition on Danjiangkou Reservoir.The study sites was located in Xichuan part of Danjiangkou Reservoir.Sampling sites were set around the Xiaotaipingyang waters which is the core area of Xichuan part.Field observation,field investigation and indoor analysis were used to systematically study the dry,wet and total nitrogen deposition flux in Danjiangkou Reservoir.The results revealed the contribution of nitrogen deposition to the exogenous nitrogen input to the reservoir,clarifying the composition characters of nitrogen deposition,and preliminarily demonstrated the origin and variation laws of nitrogen deposition.On the basis,the measures for controlling nitrogen pollution of the reservoir were explored.The results showed that:The flux of total dissolved nitrogen?TDN?in Danjiangkou Reservoir was 39.55kg/hm²·a which means TDN was an important external nitrogen input source of the reservoir.The wet deposition flux of TDN was 18.06 kg/hm²·a,and the dry deposition flux was 21.49 kg/hm²·a.In the study period,the concentration of TDN wet deposition began to rise in September which reached its peak in January or March,and then declined.The dry deposition flux of TDN increased from December to July,and then declined.Land use types had a significant impact on TDN wet deposition concentration and TDN dry deposition flux in Danjiangkou Reservoir.Ammonia nitrogen?NH₄⁺-N?,nitrate nitrogen?NO₃^--N?and soluble organic nitrogen?DON?were important nitrogen deposition forms in Danjiangkou Reservoir.Their deposition flux was 17.62 kg/hm²·a,13.56 kg/hm²·a and 8.44 kg/hm²·a,respectively.The dry deposition flux of NH₄⁺-N was 8.04 kg/hm²·a,the wet deposition flux was 10.58 kg/hm²·a,of which the highest wet deposition flux happed from April to September,accounting for 71.5%of the annual input.The dry deposition flux of NO₃^--N was 8.71 kg/hm²·a,the wet deposition flux was 4.85 kg/hm²·a,of which the highest wet deposition flux happed from April to July,accounting for 54.75%of the annual input.The dry deposition flux of DON was 4.74 kg/hm²·a and the wet deposition flux was 3.64 kg/hm²·a,of which the highest wet deposition flux happed from April to September,accounting for 82.54%of the annual input.Land use types had a significant impact on the wet deposition concentration of NH₄⁺-N and the dry deposition flux of NO₃^--N in Danjiangkou Reservoir area.TDN deposition flux accounted for 0.24 times of the total nitrogen input to the reservoir and the potential threat to reservoir water quality cannot be ignored.The annual total deposition flux of NH₄⁺-N was 1.47 times greater than the riverine input to the reservoir.NH₄⁺-N/NO₃^--N in dry and wet deposition was in the same trend with agricultural fertilization activities around the reservoir.The analysis of NH₄⁺-N/NO₃^--N ratio shows that NH₄⁺-N was the most important form of nitrogen deposition in Danjiangkou Reservoir from 2015 to 2016.NH₄⁺-N and NO₃^--N were both important of nitrogen deposition in Danjiangkou Reservoir from 2016 to 2017.The analysis of?¹⁵N indicated that the main source of NH₄⁺-N deposition to the reservoir is agricultural fertilization,the main source of NO₃^--N deposition was the emissions of thermal power plants and automobiles.Under the condition of non-point source pollution being controlled,nitrogen deposition flux contributes greatly to the input of exogenous nitrogen in Danjiangkou Reservoir which is a problem that must be solved for water quality protection.The main compounds of nitrogen deposition in Danjiangkou Reservoir are NH₄⁺-N and NO₃^--N.The main sources of NH₄⁺-N deposition are automobile exhaust and coal-fired emission from thermal power plants.The results can provide an empirical basis for exploring the nitrogen deposition flux and characteristics under the unique surface conditions of reservoirs,provide data supporting for evaluating the contribution of deposition to reservoirs'external nitrogen input,and provide a scientific basis for the selection of water quality management measures of reservoirs in China.