Abstract:

Ammonia (NH₃) emissions are a major pathway for nitrogen loss in agricultural fields. NH₃ entering the atmosphere can return to the surface through dry and wet deposition, thereby becoming an important source of external nutrients in lakes. For agricultural production areas near lakes, systematically quantifying the volatilization of NH₃ from cropland and atmospheric nitrogen deposition is crucial for optimizing croplands nitrogen management and preventing non-point source pollution. Using a near-lake agricultural production area (i.e., the Gusheng area) as a case study, this study estimated NH₃ emissions from cropland during the main crop-growing season of 2022 (May to October) using data from a farmer survey. We also assessed the dry and wet deposition fluxes of atmospheric NH₃ and other major reactive nitrogen species (such as nitric acid, nitrogen dioxide, particulate ammonium nitrogen, and particulate nitrate nitrogen in the air, and ammonium nitrogen and nitrate nitrogen in precipitation) based on field measurements. In addition, this study elucidates the relationship between NH₃ emissions from croplands and nitrogen deposition, clarifying the contribution of NH₃ deposition to the total atmospheric nitrogen deposition, and quantifying the amount of atmospheric nitrogen deposition to the near-source surface of Erhai Lake. The results indicated that during the main growing season of 2022, the total NH₃ emissions from croplands in the Gusheng area were 11.3 tons, with the largest contribution from corn production (approximately 41.7%), followed by tobacco and rice (34.0% and 24.3%, respectively). During the same period, the monthly flux of total inorganic nitrogen deposition (the sum of dry and wet deposition) at the near-lake monitoring site ranged from 0.66 to 1.57 kg·hm⁻² (in N, same below), and summed 6.51 kg N·hm⁻². Owing to the influence of fertilization activities in the field, gaseous NH₃ is the main contributor to deposition. The dry deposition fluxes of NH₃ accounted for 42%−70% of the total inorganic nitrogen deposition in different months. Moreover, the total NH₃ dry deposition flux represented 10.7% of the NH₃ emitted from the farmlands throughout the major growing season. The atmospheric inorganic nitrogen input to the surface of Erhai Lake near the monitoring site was estimated to be 35.9 t. Our findings suggest that reducing agricultural NH₃ emissions is a crucial measure for controlling both nitrogen deposition and the nitrogen load entering Lake Erhai.

Key words: reactive nitrogen, ammonia emission, atmospheric deposition, water pollution, Lake Erhai basin

摘要：

氨（NH₃）挥发是农田氮素损失的主要途径，进入大气的 NH₃会通过干、湿沉降返回地表，成为湖泊外源营养盐输入的重要贡献源。针对近湖的农业生产区，系统定量农田NH₃挥发与大气氮沉降对优化农田氮素管理、防治水体面源污染等至关重要。以洱海流域典型近湖的古生片区农田为例，结合农户生产调研数据和大气氮沉降的连续监测数据，估算了2022年主要作物生长季（5-10月）农田NH₃挥发和大气NH₃及其他主要活性氮（如空气中硝酸、二氧化氮、颗粒态铵态氮和硝态氮；降水中铵态氮和硝态氮）的干、湿沉降通量，阐明农田NH₃挥发与NH₃干沉降之间的关系，明确大气NH₃干沉降在大气氮素总沉降中的贡献及近源的洱海湖区大气氮沉降量。结果表明，2022年主要作物生长季古生片区农田NH₃挥发总量为11.3 t，其中玉米生产的贡献最大，为4.7 t（约占挥发总量的41.7%），其次为烤烟和水稻（占比分别为34.0%和24.3%）。同一时期内，近湖监测点的大气无机氮沉降总量（干沉降与湿沉降之和）的月通量范围为0.66-1.57 kg·hm⁻²（以N计，下同），总计为6.51 kg·hm⁻²。不同月份大气NH₃干沉降量占相应月份大气无机氮沉降总量的42%-70%，说明气态NH₃是片区大气氮沉降的主要活性氮组分。此外，主要作物生长季NH₃干沉降总量占到农田NH₃排放的10.7%。据估算，在研究时期内，大气沉降监测点周边洱海水面的大气无机氮沉降量为35.9 t。这一结果表明大气氮沉降是洱海水体氮素的重要输入源。综上，减少农业NH₃排放将是降低大气氮沉降及洱海氮素入湖负荷的重要途径。

关键词: 活性氮, 氨排放, 大气沉降, 水体污染, 洱海流域