Quantitative study on nitrogen fate and residual effect of double cropping rice fields in Hunan

【Objective】The purpose of this paper was to study the characteristics of nitrogen absorption, utilization, loss and residual effect of double cropping rice fields under different nitrogen applications, and the fertilizer nitrogen direction and residual effect rule of double cropping rice fields in Hunan were quantified, providing theoretical basis for the establishment of scientific and reasonable nitrogen fertilizer management measures in double cropping rice fields.【Method】This study conducted a ¹⁵N micro-plot experiment in Hunan double cropping rice area in 2017-2018, set four treatments:N0(no nitrogen fertilizer), N1(with nitrogen fertilizer 90 kg/ha in both early rice and late rice), N2(120 kg/ha of nitrogen fertilizer in early rice, 135 kg/ha of nitrogen fertilizer in late rice) and N3(150 kg/ha of nitrogen fertilizer in early rice, 180 kg/ha of nitrogen fertilizer in late rice) according to the N application level(based on pure N). ¹⁵N labeling-urea was applied in 2017, the absorption and utilization of ¹⁵N, the residue of ¹⁵N in the soil and the loss rate of ¹⁵N in each treatment were studied, and the different directions and proportions of fertilizer ¹⁵N were determined., the same amount of unlabeled urea was applied in 2018, analyzing the absorption and utilization of residual ¹⁵N, the total loss rate of residual ¹⁵N in each treatment.【Result】The results of difference subtraction method showed that the nitrogen absorption use efficiency decreased significantly with the increase of nitrogen application(P< 0.05).In 2017, the nitrogen absorption use efficiencies of early and late rice were 42.14%-46.62% and 35.45%-43.08%, respectively, 37.93%-42.56% and 37.20%-44.51% in 2018.The ¹⁵N tracer method results showed that in 2017, the ¹⁵N recovery rate of early rice under different treatments was 24.49%-24.53%, and late rice was 25.32%-26.59%, late rice was slightly higher than early rice.The ¹⁵N recovery rate of all treatments was similar, with no significant difference(P> 0.05).The direction of ¹⁵N fertilizer in each treatment was basically the same, the crop absorption, soil residues and total losses accounted for about 25%, 23% and 52%, respectively.Fertilizer ¹⁵N mainly remained in the 0-20 cm soil layer, accounting for about 77% of the total residual amount, about 19% in the 20-40 cm soil layer, and about 4% in the 40-60 cm soil layer.The residual fertilizer nitrogen from the last rice season could be absorbed and utilized by the next rice season, which became the supplement of soil nitrogen pool.The residual N in the soil at 0-20 cm had the best residual effect.In 2018, the cumulative absorption rate of the residual ¹⁵N in early rice and late rice was 8.13%-9.28%, the cumulative loss rate was 38.68%-52.97%, and the final residual rate was 38.90%-52.05%.【Conclusion】The nitrogen fertilizer use efficiency of double-cropping rice field is low, and nitrogen fertilizer loss accounted for more than 50% in both early rice and late rice;nitrogen accumulation in rice mainly comes from soil, and the contribution rate of soil nitrogen is more than 71.00%.In the production of double cropping rice, the nitrogen supply capacity of the soil itself and the fertilizer residue of rice in the last season should be fully considered, and then the amount of nitrogen applied in the current season should be reduced appropriately to achieve the high utilization of nitrogen fertilizer.