The Effects of Water-nitrogen Coupling on Uptake and Transformation of Nitrogen in Soil

【Objective】 Nitrogen (N) is the most important plant nutrient but its leaching to surface and subsurface water bodies is a great environmental concern. The dynamics of N in soil is mediated by a multitude of biotic and abiotic processes, and the aim of this project is to investigate the effect of water-nitrogen coupling on it. 【Method】 A two-year experiment involving winter wheat-summer maize rotation was conducted. It consisted of two sets of treatments. The first one kept N fertilization at 5kg with the irrigation amount varying from 10 m3 to 40 m3; the second one kept the irrigation amount at 30 m3 with N fertilization varying from 5 kg to 15 kg. Added to these are three initial soil water treatments: dry, moderate and wet. Water and nitrogen movement in each treatment was simulated using the Hydrus-1D model. 【Result】 Soil water content in the proximity of the soil surface increased drastically in the first four days after the irrigation, and spatiotemporal change in N was dominated by its movement more than by its transformation. After that, soil water in the topsoil stabilized asymptotically and the N change was dominated by transformation. N transformation in the subsoil (70~500 cm) was weak. Increasing irrigation amount or reducing initial soil water content led to a decrease in root uptake of N. The accumulated leaching of NH4+-N and NO3--N from the top 100 cm of soil increased with the irrigation amount. Increasing irrigation amount boosted mineralization, nitrification and denitrification, with their associated rates varying in the range of 1~2.5 mg/cm2, 1~4 mg/cm2 and 0~0.6 mg/cm2, respectively. Root N uptake from the top 100 cm of soil increased with N fertilization. The cumulative leaching of NH4+-N and NO3--N remained almost the same under different irrigation amounts, though increasing greatly with N fertilization. Mineralization, nitrification and denitrification in the topsoil increased with fertilization, with their rates varying in the range of 1~2.5 mg/cm2, 1.5~16 mg/cm2 and 0.3~1.2 mg/cm2, respectively. In contrast, their reactions in soil deeper than 100 cm were comparatively slow. 【Conclusion】 A rational water- nitrogen coupling can improve root N uptake and utilization. When initial soil water content was dry, coupling 525~900 m3/hm2 of irrigation with 225 kg/hm2 of N fertilization was optimal for winter wheat-summer maize rotation in the areas we studied.