Apparent variations in nitrogen runoff and its uptake in paddy rice under straw incorporation

Straw incorporation to soils is a widespread practice to promote the sustainability of agriculture. However, the potential effects of straw incorporation with the prolonged time on nitrogen (N) runoff loss from paddy fields are not well studied. The current study aims to assess the effects of straw incorporation on the processes influencing N runoff patterns, and its impacts on crop yield, N uptake, soil TN, and soil organic matter (SOM). To address the knowledge gap, field experiments with rice (Oryza sativa L.)-wheat (Triticum aestivum L. cv.) rotation, rice-tobacco (Nicotiana tabacum L.) rotation, and double-rice cropping were conducted in subtropical China from 2008 to 2012. Each rotation had three N treatments: zero N fertilization (CK), chemical N fertilization (CF) and chemical N fertilization combined with straw incorporation (CFS). The treatment effects were assessed on total N (TN) runoff loss, crop yield, N uptake, soil TN stock, and SOM. Results showed that TN runoff was reduced by substituting part of the chemical N fertilizer with straw N in the double rice rotation, while crop N uptake was significantly (P<0.05) decreased due to the lower bioavailability of straw N. In both rice-wheat and rice-tobacco rotations, in contrast, TN runoff in CFS was increased by 0.9–20.2% in the short term, when straw N was applied in addition to chemical N, as compared to CF. However, TN runoff was reduced by 2.3–19.3% after three years of straw incorporation, suggesting the long-term benefits of straw incorporation on TN loss reduction. Meanwhile, crop N uptake was increased by 0.8–37.3% in the CFS of both rotations. This study demonstrates the challenges in reducing N runoff loss while improving soil fertility by straw incorporation over the short term but highlights the potential of long-term straw incorporation to reduce N loss and improve soil productivity.