Mechanism of nitrogen leaching in fluvo-aquic soil and deep vadose zone
in the North China Plain

Rationally managing nitrogen (N) and water results in high crop yield and quality, maintains (or improves) soil fertility, and reduces environmental pollution. However, since the 1990s, excessive use of N fertilizer and flood irrigation has created problems in Chinese croplands, causing agricultural nonpoint source pollution and groundwater nitrate contamination. Data integration and literature review of winter-wheat summer-maize farmlands in fluvo-aquic soil in the North China Plain was used to investigate the temporal and spatial variation of N leaching, the contribution of cracks and macropores to N leaching, and N movement through soil (along the surface to groundwater continuum). The results showed that the N surplus was very high (299-358 kg·hm^-2·a^-1) when conventional management was used, resulting in high nitrate accumulation in the root and deep vadose zones. Nitrate movement in the winter wheat season was primarily caused by unsaturated flow and affected by irrigation; the nitrate movement distance was short. Water and nitrate loss from the root zone was negligible if the irrigation amount was lower than 60 mm. In the winter wheat season, tillage- and irrigation-induced cracks contributed minimally to nitrate and water movement out of the root zone. In the wet and hot summer maize season, the soil was frequently water-saturated, and small precipitation amounts lead to nitrate leaching, accounting for 81% of the annual leaching events and 80% of the annual nitrate leaching. In the summer maize season, the leached nitrate amounts were much higher than in the winter wheat season, and 71% of the total nitrate leaching was preferential flow caused by macropores. Nitrate from the root zone could be partially removed by denitrification in the deep vadose zone. In the North China Plain, avoiding high nitrate accumulation after winter wheat harvest was effective at decreasing nitrate leaching in the summer maize season. Matching the N fertilizer supply with crop demand and controlled release fertilizer in summer maize season (to avoid costly topdressing N fertilizer) may also play important roles in leaching reduction. Frequent and heavy rainfall accelerates the movement of nitrate via saturated and preferential flow to groundwater. Therefore, rational water and N management is key to reducing nitrate movement to the deep vadose zone and groundwater.