Research progress on the occurrence characteristics of nitrogen and phosphorus leaching in dryland farmland

Nitrogen and phosphorus leaching is the main form of non-point source pollution in dryland agriculture, which directly leads to a risk of water pollution, especially excessive nitrogen and phosphorus content in groundwater. Clarifying the characteristics of nitrogen and phosphorus leaching is a prerequisite for formulating targeted measures to control farmland nutrient loss. In this study, by analyzing the relevant literatures, the research hotspots of nitrogen and phosphorus leaching in farmlands at home and abroad over the past two decades were clarified. The main leaching monitoring methods and their advantages and disadvantages were discussed, and the key factors affecting leaching and their environmental effects were summarized. Finally, the existing problems and future research directions for nitrogen and phosphorus leaching were discussed. In the past two decades, the leaching characteristics of soil nitrogen, phosphorus, nitrate, and other solutes under different water and fertilizer conditions and management methods have been evaluated at home and abroad. Long-term excessive fertilizer input is the main reason for the accumulation and leaching of nitrogen and phosphorus in the soil. Long-term excessive fertilizer input leads to a surplus. Some nitrogen is discharged into the atmosphere along with ammonia, nitrous oxide, and other gases, whereas some nitrogen and phosphorus enter rivers and groundwater via runoff and leaching. NO₃⁻-N is the main form of nitrogen leaching. Phosphorus is mostly immobilized in the soil in the form of insoluble particles, and there is less vertical migration. However, with the increase in phosphorus fertilizer use, leaching has gradually become an important mode of phosphorus loss. Current quantitative methods for nitrogen and phosphorus leaching mainly include in situ field monitoring and artificial simulation of the rainfall remodeling leaching process. Each method has its advantages and disadvantages. Monitoring methods should be selected according to local conditions such as soil, crops, and purpose. Affected by factors such as fertilization, rainfall irrigation, soil type, and land use, the risk of nitrogen and phosphorus leaching in farmlands varies from place to place. The amount of fertilizer input in the main agricultural areas is large, with a correspondingly large surplus of soil nitrogen and phosphorus, and the pollution of groundwater is more serious than in other areas, representing a serious environmental risk. In the face of extreme climate change, it is necessary to strengthen research on the microbial processes and driving mechanisms of deep soil in drylands while assessing the degree of nitrate pollution in groundwater and accurately identifying the main pollutant types for prevention and control. Moreover, it is important to continue to develop monitoring methods suitable for long-term, continuous, and similar natural conditions and to comprehensively use in situ monitoring, remote sensing, model simulation, and other methods to quantify the nitrogen and phosphorus leaching contribution of different land use patterns in the same area. Finally, classification, zoning, and grading management should be carried out.