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Green manure application is a feasible agricultural practice to effectively improve soil fertility. However, our understanding of the effects and mechanisms of different green manure residue amounts on soil fertility improvement remains insufficient. Regarding a new type of Italian ryegrass (Lolium multiflorum L.)-rice (Oryza sativa L.) rotation (IRR) system in Southern China, the effects of six Italian ryegrass residue amounts (IRR0: 15,700 kg·ha−1, IRR1: 30,700 kg·ha−1, IRR2: 35,400 kg·ha−1, IRR3: 40,100 kg·ha−1, IRR4: 44,800 kg·ha−1, IRR5: 49,500 kg·ha−1) on the soil nutrient contents, soil enzyme activities, and bacterial communities were evaluated. Compared to the CK, appropriate amounts of Italian ryegrass residue application increased total nitrogen (36.45–37.38 %), soil organic carbon (15.34–26.55 %), microbial biomass carbon (14.07–33.93 %) and microbial biomass nitrogen (30.20–47.65 %) (P < 0.05). The activities of soil polyphenol oxidase, urease, β-glucosidase, peroxidase, cellulase, and total enzyme were also improved significantly in comparison with the CK (P < 0.05). Bacterial community structures in response to IRR were different from those in the CK. The application of Italian ryegrass residue significantly enhanced the relative abundance of beneficial taxa, including Acinetobacter , Pseudomonas , and Arenimonas (P < 0.05). The 30,700–35,400 kg·ha−1 Italian ryegrass residue exhibited the most practical amounts for enhancing soil fertility, soil enzyme activities, and altering soil microbial communities, suggesting that the improvement effects of green manure application on soil were dependent on the residue returning amounts. These findings suggest that the returning of Italian ryegrass residue is an effective land management measure to realize efficient and sustainable utilization of paddy fields. • Italian ryegrass residue improved the soil properties and bacteria communities in an amount-dependent manner. • The dominant bacteria communities were constructed according to their nutrient utilization capacity. • Relative abundances of the dominant bacteria were associated with soil biochemical properties. • Application of 30,700-35,400 kg·ha-1 Italian ryegrass residue was recommended. [ABSTRACT FROM AUTHOR]

The popularized application of annual ryegrass—maize rotation (ARMR) in southern China has been proposed to fully utilize the farmlands and to increase forage yield and quality. Herein, one growth cycle of ARMR was conducted and soil bacteria were analyzed by 16S rRNA sequencing for control (CK), after the preceding crop (monoculture, or mixed sowing of annual ryegrass and oat) and the successive crop (maize). Our results indicated that the α-diversity of soil bacteria was changed in the ARMR system, which was related to the activity of urease and available phosphatase. The mixed sowing of annual ryegrass and oat in preceding crop could improve the yield and quality, while it was accompanied by unbalanced soil community. With the increased sowing proportion of oat to annual ryegrass, the soil pH increased while the soil available phosphatase decreased. The ARMR system was found to benefit the soil microenvironment by increasing the beneficial soil bacteria and enzyme activity or decreasing the harmful soil bacteria. Considering the soil bacteria α-diversity index and physicochemical properties comprehensively, the recommended sowing regime is the mixed sowing of M2 (22.5 kg·hm−2 annual ryegrass with 75 kg·hm−2 oat). [ABSTRACT FROM AUTHOR]