Objectives The study was to investigate the effects of green manure incorporation combined with nitrogen reduction on maize yield, soil nutrients, and microbial communities in the capital region, aiming to propose an optimal combination of hairy vetch incorporation amount and nitrogen fertilizer rate for the hairy vetch-maize rotation system around Beijing, China.

Methods A field experiment under hairy vetch (Vicia villosa Roth.)－maize rotation system was conducted in the experimental base of Tianjing Agricultural Academy across 2021−2022. The six treatments were composed of green manure planting during winter time (GM) and the incorporation rate as well as the nitrogen fertilizer rate in maize season, including: neither GM nor nitrogen application (CK); no GM + conventional N rate (WF100N); low GM (15 t/hm²)+ 15% N rate reduction (GML85N); low GM+ 30% N rate reduction (GML70N); high GM (30 t/hm²)+15% N rate reduction (GMH85N); and high GM+30% N rate reduction (GMH70N). At maize harvesting stage, the maize grain yield and nutrient accumulation, the top soil physicochemical properties and the microbial community composition were analyzed. The correlation among the various indices were calculated.

Results Compared with the WF100N treatment, GML70N maintained a stable maize grain yield, while GML85N and high-amount green manure incorporation treatments (GMH70N and GMH85N) resulted in significant yield increases (P<0.05). The maize grain yield under the GML85N treatment was significantly higher than that under GML70N in 2021, with no significant increase observed in 2022. For the GMH85N treatment, the maize yield was consistently higher than that under GMH70N over the two years, with increases of 5.8% and 2.6%, respectively. Under the same N reduction level, the GMH70N treatment only significantly increased yield by 5.2% compared to GML70N in 2021, with no significant difference observed between GMH85N and GML85N. Compared with the CK and WF100N treatments, the green manure incorporation with nitrogen reduction treatments significantly increased the nitrogen, phosphorus, and potassium contents and their accumulation in maize grains (P<0.05). Compared with WF100N, GMH70N and GMH85N treatments significantly increased soil organic matter by 15.3% to 26.6%. All green manure incorporation with nitrogen reduction treatments elevated microbial biomass carbon, nitrogen, and phosphorus contents (MBC, MBN, MBP) than WF100N, while reducing soil moisture content and nitrate nitrogen content; and, except for GML70N, also increased soil total nitrogen, total phosphorus, and available phosphorus contents (P<0.05). In the green manure incorporation with nitrogen reduction treatments, the proportion of beneficial bacteria such as Actinobacteria and Proteobacteria in the bacterial community increased, and the proportion of Basidiomycota in the fungal community also rose. Mantel analysis revealed that the bacterial community was significantly correlated with soil nitrate nitrogen content, moisture content, and indicators such as MBC, MBN, and MBP, while the fungal community showed a weaker correlation with soil physiochemical properties. Partial least squares path modeling (PLS-PM) indicated that nitrogen application rate could directly influence maize grain yield and nitrogen accumulation, as well as indirectly affect them by altering soil physicochemical properties. In contrast, green manure incorporation primarily influenced yield and nitrogen accumulation by improving soil properties.

Conclusions Green manure incorporation combined with reducing nitrogen fertilizer application can synergistically achieve stable maize yields and soil fertilization enhancement in the capital region. Green manure improves soil health by increasing soil organic matter, stimulating microbial proliferation, and optimizing microbial community composition. The nutrients provided by both low- and high-amount green manure incorporation can partially replace chemical fertilizers, ensuring adequate nutrient supply for maize. Under the tested conditions, incorporating a high amount of green manure (30 t/hm²) combined with a 30% reduction in nitrogen fertilizer application demonstrated significant advantages and is recommended as a model for green production of winter green manure-maize systems in the capital region.