Methodology for studying nitrogen loss from paddy fields under alternate wetting and drying irrigation in the lower reaches of the Yangtze River in China

Nitrogen (N) loss from paddy fields is a major cause of water eutrophication. Alternate wetting and drying (AWD) irrigation is a promising measure for controlling nutrient loss. Nevertheless, quantifying N loss from paddy fields under AWD to water bodies is difficult because of 1) the sharp fluctuations in soil moisture and 2) the variation in runoff N concentrations caused by the probable first flush effect. Consequently, studies on N loss are usually confined to plot experiments. From June to November 2020, a 2.5 ha field-scale experiment was conducted in Shanghai during the rice growth stage. The proposed methodology included recording the water levels in a paddy field, quantification of runoff, and implementation of automatic sampling procedures. The electrical conductivity (EC) of runoff was applied to study the loss characteristics of nutrients. The antecedent field water level (AFWL) in paddies was used to evaluate the effect of soil moisture on the field water capacity, which was indicated by the runoff curve number (CN). It was found that AWD significantly altered the CN. When the AFWL dropped to 100 mm beneath the soil surface, the CN dropped to 40. The EC was closely related to the N concentration (adjusted R² = 0.76, n = 176) when water samples with N concentrations ranging from 0.28 to 109.32 mg/L were analyzed. A hydrograph of EC versus runoff velocity during rainfall events indicated the significant effects of dilution caused by rainfall; therefore, the estimate of N loss should be based on high-frequency sampling. There was a more significant first flush effect on N loss during rainfall events than with artificial drainage. However, due to the high frequency of artificial drainage, N loss through artificial drainage should also be fully taken into account. In conclusion, the quantity and quality of N loss were significantly affected by wide fluctuations in soil moisture under AWD and the first flush effect of rainfall.