Soil nitrogen storage and recovery efficiency in double paddy fields under reduced nitrogen dose and increased crop density

Rice production technology is transforming to natural resources-saving and environment-friendly techniques, such as straw incorporation, reduced nitrogen (N) application. Simultaneously, machine transplanted rice with high plant density has been rapidly developed with continuous reduction in rural labor and rising labor costs. Therefore, it is important to investigate N sink and use efficiency under straw incorporation, reduced N application and increased plant density for natural resources-saving and environment-friendly rice production. Five cultivation modes of machine-transplanted double-cropping rice were set up under straw incorporation in this study. The cultivation modes included zero-N and conventional density (T1), conventional N dose and density (T2), conventional N dose and increased density (T3), reduced N dose and conventional density (T4) and reduced N dose and increased density (T5). Soil N storage capacity and recovery efficiency in double cropping rice fields were analyzed under five treatments. The results showed that compared with T2 treatment, the amount of basic fertilizers of T5 treatment dropped by 20% in total N and density increased by 27.3% for both early and late rice. Total N content and storage capacity of the 0-10 cm and 10-20 cm layers of T5 were not significantly different from those of T2, but available N content of T5 decreased by 15.6% in the 0-10 cm soil layer and by 8.8% in the 10-20 cm soil layer, compared with T2. Total storage of available N was decreased by 10.7% in the 0-20 cm soil layer. Compared with T2 treatment, T5 treatment significantly reduced surplus soil N, with loss amount and loss rate of N fertilizer significantly dropping respectively by 51.7% and 15.5%. Agronomic efficiency of N in early and late rice under T5 treatment increased respectively by 33.6% and 23.0%, compared with T2 treatment. Uptake efficiency of N increased respectively by 5.9% and 6.3% and physiological efficiency of N increased by 16.3% and 3.7%, compared with T2 treatment. The results indicated that total N storage capacity of soils with reduced N and increased density under rice straw return did not significantly reduce, but alkali N storage capacity reduced significantly in the short-term. However, it had the potential to significantly reduce N loss and increase N utilization in double cropping paddy fields.