Recovery of [sup.15]N-labeled urea and soil nitrogen dynamics as affected by irrigation management and nitrogen application rate in a double rice cropping system

A sophisticated soil microcosm system and [sup.15]N-labeled urea were used to investigate nitrogen (N) use efficiency and soil N dynamics in a rice monoculture system in two successive seasons. Topsoil (0 cm-20 cm) and subsoil (20 cm-50 cm) samples were collected from a traditional double rice cropping field in the Jiangxi Province, China, and these soil samples were derived from Quaternary red clay. Treatments were randomly assigned with two irrigation regimes and three N application rates (no application control, 80% traditional rate and 100% traditional rate noted as N0, N1 and N2, respectively). The levels of [sup.15]N recovery of plants, [sup.15]N and N remaining in soil were determined. Moreover, the N dynamic of soil solution from different layers of the soil profile was surveyed. The results showed that the effects of irrigation management and N application rate varied in different rice growing seasons. Irrigation regimes had remarkable effects on grain yield and chemical [sup.15]N fertilizers (CF-[sup.15]N) uptake. When compared to flood irrigation (FI), the shallow water depth with wetting and drying (WD) increased grain yield up to 5.7%-20%. Although the highest grain yield was obtained with reduced N application level, both N apparent recovery (NAR) and [sup.15]N use efficiency (the percentage of plant N uptake derived from applied N, % Ndfan) significantly decreased with increasing N inputs. However, the interaction between irrigation management and N application rate on grain yield and N use efficiency (NUE) of CF-[sup.15]N were not significant. A survey of soil solutions every 5 days indicated that N[H.sub.4.sup.+]-N was the main residual form of N, and high N[H.sub.4.sup.+]-N leaching was observed. When compared to FI, WD decreased vertical N[H.sub.4.sup.+]-N and TN leaching, especially at 10-50 cm depths of soil profile in the second season. N[H.sub.4.sup.+]-N was the main N residual form in the soil profile. Therefore, in this study, the WD irrigation regime and reduced rate (N1) was the optimal irrigation and fertilizer management strategy to increase the NUE of CF-N, increase the after effects of CF-[sup.15]N, decrease leaching loss of CF-[sup.15]N and minimize the shallow groundwater pollution risk, which were all beneficial for the ecological environment. Keywords Double rice cropping system * [sup.15]N-labeled urea * Irrigation * Leaching * N Use Efficiency (NUE). Soil solution