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 the risk of water pollution, especially the excessive content of nitrogen and phosphorus in groundwater. Clarifying the characteristics of nitrogen and phosphorus leaching is a prerequisite for formulating targeted measures to control farmland nutrient loss. In this paper, by analyzing the relevant literature in this field, the research hotspots of nitrogen and phosphorus leaching in farmland at home and abroad in the past two decades were clarified. The main monitoring methods of leaching and their advantages and disadvantages were discussed, and the key factors affecting leaching and its environmental effects were summarized. Finally, the existing problems and future research directions of 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 mainly concerned at home and abroad. Long−term excessive input of fertilizer is the main reason for the accumulation and leaching of nitrogen and phosphorus in soil. Long−term excessive input of fertilizer leads to surplus. Some nitrogen is discharged into the atmosphere with ammonia, nitrous oxide and other gases, and some nitrogen and phosphorus enter rivers and groundwater with runoff and leaching. NO₃⁻-N was the main form of nitrogen leaching. Phosphorus is mostly immobilized in the soil in the form of insoluble particles, and the vertical migration is less. However, with the increase of phosphorus fertilizer, leaching has gradually become an important way of phosphorus loss. The current quantitative methods of nitrogen and phosphorus leaching mainly include in−situ field monitoring and artificial simulation of rainfall remodeling leaching process. Each method has its advantages and disadvantages. In the research process, monitoring methods should be selected according to local conditions such as soil, crop and purpose. Affected by factors such as fertilization, rainfall irrigation, soil type and land use, the risk of nitrogen and phosphorus leaching in farmland varies from place to place. The amount of fertilizer input in the main agricultural areas is large, the surplus of soil nitrogen and phosphorus and the pollution of groundwater are more serious than other areas, and the environmental risk is still very serious. In the future, in the face of extreme climate change, it is necessary to strengthen the research on the microbial process and driving mechanism of deep soil in dryland while assessing the degree of nitrate pollution in groundwater, accurately identify the main pollutant types for prevention and control, continue to develop monitoring methods suitable for long−term, continuous and similar natural conditions, and 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, and carry out classification, zoning and grading management.