Study On Agricultural Ammonia-nitrogen Release And Its Contribution To Nitrogen Deposition In The Xichuan Reservoir Area

Nitrogen deposition is an important way for nitrogen to enter the biosphere.Excessive nitrogen inputs have caused negative effects such as soil nutrient loss,water eutrophication,biodiversity loss and nitrogen saturation,which in turn affects the health and service functions of terrestrial and aquatic ecosystems.Ammonia nitrogen occupies a dominant position in global inorganic nitrogen deposition.Therefore,it is of great significance to clarify the pollution level and source of regional NH₃.This paper takes the first-level protected small Pacific waters in the Xichuan reservoir area of the Danjiangkou Reservoir as the research object.Through the combination of remote sensing interpretation,field positionion observation,field in-situ simulation experiment,laboratory detection and analysis,and isotope traceability methods,we analyzed the distribution characteristics of ammonia sources around the reservoir,clarified the release characteristics of agricultural ammonia sources and the isotope characteristics of ammonia nitrogen deposited in the reservoir area,and estimated the contribution of the main ammonia sources.Research indicated:（1）Within the distribution range of ammonia sources around the reservoir area,croplands land was the most important land use type in the study area.（2）Agricultural activities changed the release amount of soil ammonia in different land use types,which had the greatest impact on the release of soil ammonia in dryland.The response of soil ammonia release to fertilization differed with the application of different fertilizers.Ammonia release from soils applied with urea was more sensitive to fertilization.The release rate of ammonia in the soil applied with urea was the highest and the release time was the longest,followed by compound fertilizer,and the release rate of ammonia in the soil applied with organic fertilizer was the lowest and the release time was the shortest.There were obvious seasonal differences in soil ammonia emission.The release rate of soil ammonia in summer was higher than that in autumn,and the difference of release amount was small.（3）The concentration of gaseous NH₃ in the study area ranged from 20.56 to 110.44μg·m^-3,and theδ¹⁵N value ranged from-30.0 to-7.2‰.The concentration of NH₄⁺in particulate matter ranged from 0.04 to 8μg·m^-3,and theδ¹⁵N values ranged from+2.3 to+38.4‰.The concentration of NH₄⁺in dry deposition ranged from 0.24 to 1.97 mg·L^-1,and the value ofδ¹⁵N-NH₄⁺ranged from-26.5 to+6.8‰.The concentration of NH₄⁺in wet deposition ranged from 0.16 to 5.26 mg·L^-1,and the value ofδ¹⁵N-NH₄⁺ranged from-14.6 to+6.3‰.The ammonia nitrogen concentration and itsδ¹⁵N value were significant different in time（P<0.01）,but not in space.The concentration of gaseous NH₃,particulate matter and theδ¹⁵N-NH₄⁺in dry deposition were significant correlated with the average temperature and precipitation（P<0.01）.The concentration of NH₄⁺in the particulate matter and theδ¹⁵N value of gaseous NH₃ were significantly negatively correlated with average temperature and precipitation（P<0.01）.（4）The f values（f=NH₄⁺/（NH₃+NH₄⁺））of the conversion of gaseous NH₃ to particulate NH₄⁺in the atmospheric particulates with a particle size D≤2.5μm and D>2.5μm in the study area of the reservoir were 0.9 to 22.8%,0.1 to 2.6%,respectively.There was a significant difference in f values in time（P<0.01）.（5）The gaseous NH₃,dry and wet deposition of atmospheric ammonia nitrogen in the study area were mainly derived from agricultural sources,with contribution rates of66.1±4.4%,57.7±2.0%,62.1±3.7%,respectively,and all had spatial-tempoeal changes.The contribution of gaseous NH₃ and dry deposition of africultural ammonia sources were the largest in summer and the smallest in winter.The contribution of wet deposition of agricultural ammonia sources were the largest in summer and the smallest in spring.Spatially,the maximum and minimum contribution rates of gaseous NH₃ to agricultural ammonia sources ere in the scenic spot（Heijizui）and the water intake（Taocha）,respectively.The maimum and minimum contribution rates of agricultural ammonia sources to dry deposition were in orchard（Dangzikou）and water intake（Taocha）,respectiverly.The maximum and minimum contribution rates of agricultural ammonia sources in wet deposition were in agricultural area（Tumen）and wharf（Songgang）.