Effects of Reduction in Nitrogen Fertilizer in Combination with Iron Powder on Reactive Nitrogen Loss in Rice-wheat Rotation Farmland

Rice-wheat rotation cropland is an important source of ammonia volatilization, nitrous oxide (N₂O) emissions and nitrate leaching in China's Yangtze River Base. Although reducing the amount of nitrogen fertilizer can reduce the adverse environmental consequences brought on by irrational nitrogen fertilization, it may result in the risk of crop yield reduction. Application of iron powder can stimulate the growth of iron-reducing bacteria in flooded paddy soil and enhance their nitrogen-fixing activity. In this study, the conventional nitrogen fertilization rate (100%N), 80% of the conventional nitrogen fertilization rate (80%N), 80%N in combination with iron powder (80%N + Fe), 60% of the conventional nitrogen fertilization rate (60%N), 60%N in combination with iron powder (60%N + Fe), no nitrogen application (0%N), 0%N in combination with iron powder (0%N + Fe), yielding a total of 7 treatments. Each treatment has four replicates (plots). The effects of reduction in nitrogen fertilizer in combination with iron powder on crop yields, ammonia volatilization, N₂O emission and mineral nitrogen leaching were investigated by in situ field experiments. The results show that the wheat yield was increased by 9.7% in 80%N + Fe treatment compared with 80%N in 2021. The rice yields were comparative between 80%N + Fe treatment and the conventional nitrogen fertilization rate. The 80%N + Fe treatment could ensure both wheat and rice yields relative to the conventional N fertilization rate. Ammonia volatilization in paddy fields decreased significantly with the reduction in nitrogen fertilization rate (P < 0.05); 80%N + Fe treatment further decreased the cumulative ammonia volatilization amount and ammonia volatilization intensity by 50.99% and 47.41%, respectively (P < 0.05). The cumulative N₂O emission in the 80%N + Fe treatment decreased by 78.16% in the wheat season of 2021 compared with the 80%N treatment (P < 0.05). It decreased by 18.27% compared with the conventional nitrogen fertilization rate in the rice season. Meanwhile, 80%N + Fe treatment decreased the leaching of nitrate nitrogen in deep soil leachate. Therefore, 80%N in combination iron powder could maintain stable crop yields while reducing the nitrogen losses in the rice-wheat rotation cropland, probably because iron powder enhances the nitrogen-fixing activity of iron-reducing bacteria in flooded paddy fields. The 20% reduction in nitrogen fertilizer input mainly resulted in lower nitrogen losses including volatilization, N₂O emissions and nitrate leaching. Yet the corresponding soil microbial mechanism deserves further exploration.