Nitrogen and phosphorus leaching characteristics and temporal and spatial distribution patterns in northern China farmlands

The main agricultural production areas in northern China are the black soil area, fluvo-aquic soil area, and cinnamon soil area. In the area N and P leaching is a common cause of groundwater pollution, but the leaching characteristics and distribution patterns (temporal and spatial) are unclear. The in situ monitoring of field leakage ponds and literature data analysis were used to analyze N and P leaching characteristics at 285 monitoring sites using the four main planting patterns (spring maize, winter wheat-summer maize rotation, open-field vegetables, and greenhouse vegetables). The results showed that the average N and P leaching rates were 30.7 kg(N)·hm^-2 and 0.09 kg(P)·hm^-2 for spring maize, 49.9 kg(N)·hm^-2 and 0.07 kg(P)·hm^-2 for winter wheat–summer maize rotation, 51.7 kg(N)·hm^-2 and 0.10 kg(P)·hm^-2 for open-field vegetables, and 117.5 kg(N)·hm^-2 and 0.74 kg(P)·hm^-2 for greenhouse vegetables. Fertilizer application and irrigation, often determined by the planting pattern, were positively correlated with N leaching. Therefore, high fertilizer and water amounts used in vegetable fields resulted in more N and P leaching than observed in grain fields. In fields using the same planting pattern, the fluvo-aquic soil area had the greatest total N loss intensity, followed by the cinnamon soil area; the black soil area had the least intensity. Different soil textures resulted in different leached N amounts when the same fertilization and irrigation practices were used. Fields using the same planting pattern also had different leaching amounts because of regional differences in fertilization and irrigation practices. Annual N and P leaching was mainly affected by rainfall intensity, and total N leaching was positively correlated with rainfall intensity. If no leaching events occurred in the previous year, a sharp increase in leaching was observed in the following year. Spatially, the cinnamon and fluvo-aquic soil areas were the primary N leaching risk areas, especially, some provinces with large vegetable planting areas (particularly those with large greenhouse areas)showed high N and P leaching risks. Northern Chinese agricultural areas are primarily at risk for N leaching, but also P leaching; cinnamon and fluvo-aquic soils are the highest risk areas. Regionally, N and P leach mainly from grain fields, but as vegetable field size increases, so does the risk of N and P leaching.