Ditch‐buried straw return (DB‐SR) is a novel soil tillage and fertility building practice that is effective in regulating soil carbon and nitrogen dynamics and hydrothermal processes in rice–wheat rotation systems. However, the effects of DB‐SR on soil bacterial community are still largely unclear. We deciphered soil bacterial community with high‐throughput sequencing under various returning approaches, burial depths, and straw amounts after 6.5 years of DB‐SR application. Our results showed that DB‐SR structured distinctive soil bacterial community with rotary tillage straw return (RT‐SR; one‐way analysis of similarities [ANOSIM]: P < .01). RT‐SR significantly reduced soil bacterial diversity by 3.87%, but DB‐SR could maintain it (P > .05). These variations were mainly caused by water content‐driven changes in soil organic carbon. Also, bacterial community composition was distinctive among burial depth treatment (one‐way ANOSIM: P < .05), and deeper burial reduced species richness and diversity (P < .05). Variation in C/N ratio could mostly explain the alterations in soil bacterial community structure under different burial depths. Moreover, the amount of straw buried had no significant effect on soil bacterial species richness or diversity (P > .05), but bacterial community composition was more dissimilar with increasing straw amount (one‐way ANOSIM: P < .01). Our results suggest that long‐term DB‐SR can maintain the bacterial community structure in the surface soil layers when compared with conventional RT‐SR, but taking the current production level into consideration, the burial depth should not be greater than 20 cm for incorporating the full amounts of straws.

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The yield of direct-seeded rice has been shown to decrease after straw amendment. However, the reasons for the yield decrease, and any measures to alleviate it, are currently unknown. We hypothesised that straw return exerts negative effects on soil fertility and on root growth of direct-seeded rice, which subsequently reduces the remobilisation of reserves to grains under continuous flooding (CF); and that alternate wetting and drying (AWD) irrigation can alleviate these negative impacts. Field and greenhouse experiments were conducted to test the hypotheses, by comparing CF and AWD in combination with two wheat-straw treatments (incorporation and mulching). Under CF, wheat-straw incorporation decreased soil available phosphorus by 23–79%, root biomass by 10%, leaf biomass by 13%, and leaf area by 15% compared with the control with no straw incorporation; negative effects on these characteristics were lessened if the straw was mulched. The AWD treatment alleviated the negative effects of straw incorporation compared with CF, and straw mulching with AWD had no negative effects or resulted in positive effects. The results suggest that CF along with straw incorporation limits soil phosphorus availability, root growth and grain yield by affecting photosynthate accumulation and remobilisation. AWD irrigation mitigates these undesirable effects by decreasing soil total reductants, which subsequently increases soil pH and plant-available phosphorus. The proposed AWD treatment could be a promising strategy for the sustainable production of direct-seeded rice.

Summary Research on aggregate‐associated organic carbon (AOC) stability has increased, but its response to nitrogen (N) and straw (S) application after nitrogen reduction in alkaline sandy loam soil remains unclear. A 2‐year field study and a short‐term incubation experiment were performed to investigate the combined effect of the N + S application on aggregate distribution and stability, and AOC content and mineralization in eastern China. The study involved three N amounts (75, 150 and 300 kg N ha−1) with or without straw amendments under continuous cotton–barley rotation. The N + S application promoted the formation of larger macroaggregates (8–2 mm), with larger mean weight diameter and geometric mean diameter than with N application alone. Aggregate stability increased significantly with increasing rate of N application. The amount of N had less effect on AOC content in the 0–20‐cm layer and cumulative mineralization of AOC in both layers compared with the N + S application. The addition of straw in promoting AOC, however, was more evident in the 20–40‐cm layer. Compared with N fertilizer alone, applying N + S increased the cumulative mineralization of AOC by 27.7% in the 0–20‐cm layer and by 80.9% in the 20–40‐cm layer for different rates of N and aggregate sizes. In addition, at 0–20‐cm depth, there was less C mineralization in macroaggregates than in microaggregates, indicating that macroaggregates showed better protection of AOC in that layer. Our study suggests that N application alone had a large effect on aggregate stability, whereas combined N + S application had a large effect on AOC content and its mineralization in the soil studied. Highlights We studied aggregate and aggregate‐associated organic carbon stability under N and straw application in alkaline sandy loam soil. Nitrogen and straw treatment carried out with prior application of a gradient of excessively large N inputs. Macroaggregates showed better protection of AOC in the 0–20‐cm soil layer. N and N + S applications had large effects on aggregate stability, AOC content and its mineralization.

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