Accumulation and leaching of nitrate in soils in wheat-maize production in China

Application rates of fertilizers in China often exceed crop requirements, resulting in high accumulation of nitrate (NO3) in the soil. Nitrate that has accumulated in soils is highly prone to leaching, directly threatening the quality of groundwater. A study was conducted to assess the magnitude of NO3 accumulation and leaching in China, to identify factors controlling NO3 accumulation and leaching, and to develop strategies that can be used to minimize NO3 leaching. Data were compiled from 212 studies conducted in China, amounting to 1077 observations of the NO3 content of the 0-100 cm soil profile in wheat and maize fields after harvest. Leaching of NO3 was significantly correlated with NO3 accumulation in the soil. NO3 leaching increased with 0.058 and 0.34 kg NO3-N ha(-1) per season for wheat and maize, respectively, for every 1 kg ha(-1) increase in NO3-N accumulation in 0-100 cm. This mainly related to lower precipitation during the wheat season and intensive rainfall in the maize season. Accumulation of NO3 in maize systems was 50% lower than for wheat when fertilized at the same rate, due to differences in rainfall between seasons. Soil NO3 accumulation was higher in heavy textured soils than in freely draining lighter textured soils, as most of NO3 leached out of 0-100 cm soil in lighter textured soils. Compared to flood irrigation, sprinkler irrigation increased NO3 accumulation by 17% and 152% for wheat and maize, respectively, due to lower irrigation and leaching rate. The level of nitrate accumulation in Chinese arable soils has become a significant hazard to drinking water, so good agricultural management is essential. Soil NO3 accumulation and leaching in China can be reduced by source and process control, such as reducing fertilizer application, using slow or controlled release forms of fertilizers, and regulating irrigation.