Effects of different chemical nitrogenous fertilizer application rates on soil nitrogen leaching and accumulation in deep vadose zone

In order to study the soil nitrogen migration under different application rates of chemical nitrogenous fertilizer in deep vadose zone, during the course of summer maize growing, 3 chemical N fertilizer levels (142.5, 285 and 427.5 kg/hm2) were set up, the NO3--N, NH4+-N and total nitrogen migration and accumulation in the first 500 cm depth of soil for each treatment were studied. The results showed that impacts of chemical N application rates on NO3--N, NH4+-N and total nitrogen were dramatic, the leaching and accumulation of soil NO3--N, NH4+-N and total nitrogen increased with increasing chemical N application rate, but decreased with the growth of summer maize. Soil NO3--N and total nitrogen accumulation increased with the increase of chemical N application rate in 0-200 cm soil, while the soil NH4+-N accumulation were found to be in the following descending order: the 285 kg/hm2 N application rate> the 142.5 kg/hm2 N application rate> the 427.5 kg/hm2 N application rate, but the difference was no more than 0.1 kg/hm2. After harvest the soil nitrogen accumulation decreased. The influences of different N application rates on soil NO3--N, NH4+-N and total nitrogen changes were mainly in 0-145 cm depth. The soil nitrogen was inclined to accumulate in powder sandy loam and leach into water in sandy loam. The 142.5 kg/hm2 N application rate can effectively reduce the NO3--N leaching, and reduce NH4+-N and total nitrogen content in soil, thus make the risk of groundwater pollution minimum. The groundwater in Beijing is deep, which is not polluted easily by NO3--N leaching. However, large quantity of chemical N application in long-term and the presence of soil macropores can accelerate the leaching and migration of NO3--N in the soils, thus posing a threat to groundwater quality in the Beijing area.