Straw returning has gradually been adopted as an effective approach to address the serious degradation of farmland. However, the carbon/nitrogen (C/N) ratio of rice straw is generally too high for microorganisms to decompose the organic materials and release nutrients, which may minimize the benefits of straw returning to the agricultural production system. This study aimed to investigate the effects of straw returning on rice production and propose optimum nitrogen (N) management for early rice production under a straw returning system. The total N fertilizer that was evaluated was 165 kg N ha-1, urea (46% N), applied in different proportions in three stages of rice cultivation: basal, tillering, and panicle. Using no straw returning with the N fertilizer ratio of basal:tillering:panicle = 5:2:3 treatment (T1) as the control, four different N fertilizer ratios of basal:tillering:panicle, including 5:2:3 (T2), 5:2:2 (T3), 5:4:1 (T4), and 5:5:0 (T5) were set under straw returning. The return of straw decreased the available N in the soil at the tillering stage, and impeded root growth and the crop canopy from establishing, which decreased the effective panicles by 10.1% compared with that of T1, limiting the increases in rice grain yield. Increasing the N fertilizer ratio 10–20% (T3 and T4) at the tillering stage effectively increased the content of soil ammonium and nitrate nitrogen, improved the root growth, and increased the root activities by 16.0–40.5% at the tillering stage. As a result, the effective panicle number increased by 5.1–16.2%. Among these, T4 treatment maximized the benefits of straw returning the most. Additionally, increasing the N fertilizer ratio at the tillering stage increased the shoot uptake across the early rice growing season and synchronized crop N uptake with the accumulation of carbon assimilates, which enhanced the crop growth rate and increased the rice yield by 13.5–25.1%. It is concluded that increasing the N fertilizer ratio by 20% at the tillering stage is a promising strategy to increase the availability of N in the phases of high demand for this nutrient.

Long-term intensive use of mineral fertilizers in double rice-cropping systems has led to soil acidification and soil degradation. Manure fertilization was suggested as an alternative strategy to mitigate soil degradation. However, the effects of long-term mineral and manure fertilization on rice grain yield, yield stability, soil organic carbon (SOC) content, soil total nitrogen (TN) content, and the underlying mechanisms are unclear. Based on a long-term experiment established in 1981 in southern China, we compared four treatments: no fertilizer application (Control); application of nitrogen–phosphorus–potassium (NPK); NPK plus green manure in early rice (M1); and M1 plus farmyard manure in late rice and rice straw return in winter (M2). Our results showed that 37 years of NPK, M1, and M2 significantly increased rice grain yield by 54%, 46%, and 72%, and yield stability by 22%, 17%, and 9%, respectively. M1 and M2 significantly increased SOC content by 39% and 23% compared to Control, respectively, whereas there was no difference between Control and NPK. Regarding soil TN content, it was significantly increased by 8%, 46%, and 20% by NPK, M1, and M2, respectively. In addition, M2 significantly increased bacterial OTU richness by 68%, Chao1 index by 79%, and altered the bacterial community composition. Changes in soil nutrient availability and bacterial Simpson index were positively correlated with the changes in grain yield, while shifts in bacterial community were closely related to yield stability. This study provides pioneer comprehensive assessments of the simultaneous responses of grain yield, yield stability, SOC and TN content, nutrient availability, and bacterial community composition to long-term mineral and manure fertilization in a double rice-cropping system. Altogether, this study spanning nearly four decades provides new perspectives for developing sustainable yet intensive rice cultivation to meet growing global demands.