Nitrogen flux and its manipulation in the cropland ecosystem of the North China Plain

The North China Plain (NCP) is one of the areas of intensive cereal production in China, producing large winter wheat and summer maize. A critical challenge facing the agro-production sector of the NCP, however, includes over-application of chemical fertilizers, excessive soil nutrient accumulation and declining nutrient use efficiency. This study investigated relationship between nitrogen (N) flux and crop nutrient uptake/loss processes under conventional fertilization at 400 kg(N)·hm^-2·a^-1 in the winter wheat/summer maize crop rotation system in the piedmont region of NCP. The management practices of the fields were adjusted on the basis of the characteristics of N output fluxes. The results showed that in the region, a total N input of 561~580 kg·hm^-2 and output of 468~494 kg·hm^-2 resulted in a N surplus of 86~93 kg·hm^-2 per year. Organic N load in the study area was 24~36 kg·hm^-2. N loss via ammonia volatilization and NO₃^--N leaching was 60 kg·hm^-2 and 47~84 kg·hm^-2 per year, respectively, collectively accounting for 30% of applied N fertilizer in the region. N loss via nitrification-denitrification was only 5.0~8.7 kg·hm^-2 per year, accounting for 1%~2% of applied N fertilizer. Ammonia volatilization and NO₃^--N leaching were the main modes of N loss, and therefore the main drivers of lower N fertilizer utilization rate in the study area. Appropriate management practices such as adjustments of the rates and time of N fertilization and irrigation were important for decreasing ammonia volatilization, NO₃^--N accumulation in deep soil profile and therefore the mitigation of NO₃^--N loss in the region. Amendments with wheat/corn straw changed the rates of N transformation and migration in the soil, which also resulted in low NH₃ loss and NO₃^--N leaching. Analysis of precision fertilization via NO₃^--N or hyper-spectra information feature also showed that the main indices of N loss were the amount of soil available nutrient and crop nutrient content. This laid the basis for efficient fertilizer application in the study area. Control-release fertilizer and precision fertilization techniques improved crop nutrient uptake, which in turn increased N use efficiency. An integrated fertilizer utilization technique for sustainable environmental and agricultural development in the piedmont region of NCP was possible.