Soil Nitrogen Fluxes in Wheat-Corn Double Cropping Systems in the North China Plain.

The North China Plain (NCP) is one of the areas of intensive cereal (wheat and maize) production in China. A critical challenge facing the agro-production sector of the NCP, however, is over-application of chemical fertilizers. This study investigated relationship between nitrogen (N) fluxes and crop nutrient uptake/loss processes at conventional fertilization rate of 400 kg-N/ha in the winter wheat/summer maize crop rotation system in the piedmont region of NCP. The results showed that a total N input of 561~580 kg/ha and output of 468~494 kg/ha resulted in a N surplus of 86~93 kg/ha per year in the soil. Organic N load of the above amount in the study area was 24~36 kg/ha. N loss via ammonia volatilization and NO₃^--N leaching was 60 kg/ha and 47~84 kg/ha 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/ha 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, nitrate-N accumulation in deep soil profile and therefore the mitigation of nitrate 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^--N loss and nitrate leaching. Analysis of precision fertilization via NO₃^--N or hyper-spectra information also showed that the main indices of N loss were the amount of soil available nutrient and crop nutrient content. Control-release fertilization and precision fertilization techniques improved crop nutrient uptake, which in turn increased N use efficiency.