Linking soil profile NO concentration with surface flux in a cotton field under drip fertigation.

It remains unclear how the source and rate of nitrogen (N) fertilizers affect NO concentration and effluxes along the soil profile under the drip-fertigated agricultural system. A plot-based field study was performed in 2017 and 2018 in a cotton field in arid northwestern China, with an objective to elucidate the impact of the applications of conventional urea (Urea), polymer-coated urea (ESN) and stabilized urea (SuperU) at rates of 120 and 240 kg N ha on concentration and efflux of NO in the soil profile and its relationship with NO surface emissions. The in-situ NO concentrations at soil depths of 5, 15, 30 and 60 cm were measured and used to estimate soil profile NO effluxes. Estimates of surface NO flux using the concentration gradient-based (GM) were compared with those measured using the chamber-based (CM) method. In both years, soil NO concentrations at all depths increased in response to basal N application at planting or in-season fertigation events. However, N rate or source did not affect soil NO concentrations or effluxes at each depth. Surface emissions of NO were mostly associated with that presented in the top layer of 0-15 cm. Surface NO efflux determined by GM was poorly or not associated with those of chamber measurements, which was attributed to the low NO production restricted by soil moisture condition under the drip-fertigated condition. These results highlight the challenge of applying the enhanced efficiency N fertilizer products in the drip-fertigated agricultural system.