Concentration change and migration characteristics of nitrogen in the paddy field of Ningxia Yellow River Irrigation Area

Excessive use of nitrogen and irrational irrigation are the primary drivers of aggravated non-point farmland pollution. It is therefore urgent to develop effective coupling modes of water and nitrogen use in agricultural production in Ningxia Yellow River Irrigation Area that reduces non-point farmland pollution of the Yellow River. To that end, an experiment was conducted in typical paddy fields of the area to study the effect of irrigation and nitrogen use on non-point pollution. Based on the results, high concentrations of NH₄⁺-N and NO₃^--N were noted in surface water of paddy fields after fertilization. Compared with NH₄⁺-N, the peak concentration of NO₃^--N was a little delayed and built up more slowly. During the periods of topdressing and the entire experimentation, NH₄⁺-N average concentration in field surface waters was obviously related with nitrogen input and irrigation amount. The average concentration of NO₃^--N in field surface water was significantly related with nitrogen input, and not irrigation amount. The concentrations of NH₄⁺-N in seepage waters at depths of 30 cm and 60 cm were strongly influenced by fertilizer dose. The dynamics of NH₄⁺-N concentration in the seepage waters were similar to that of NH₄⁺-N in field surface water. There were larger lags in the peak concentration of NH₄⁺-N at 90 cm soil depth, and which was much lower than that in the upper soil layers. The concentration of NH₄⁺-N at 120 cm soil depth steadily increased. The average concentration of NH₄⁺-N in the different layers over the entire period of experimentation was significantly related with nitrogen input, and not irrigation amount except at the depth of 30 cm. The concentrations of NO₃^--N in seepage water at 30 cm and 60 cm soil depths dropped sharply after the first irrigation, but gradually increased after each fertilization. The concentrations of NO₃^--N at 90 cm and 120 cm soil depths slowly dropped. A significant correlation between NO₃^--N concentration and nitrogen input was only noted in seepage water at 30 cm soil depth. The overall result suggested that deficit irrigation limited nitrogen leaching.