Intercropping Green Manure Species with Tea Plants Enhances Soil Fertility and Enzyme Activity and Improves Microbial Community Structure and Diversity in Tea Plantations

To investigate the effects of intercropping green manure on the tea plantation ecosystem, this study was conducted using 40-year-old Camellia sinensis cv. “Fuding Dabai” tea plants at the Tea Experimental Base of the Jiangxi Institute of Cash Crops. Four treatments were established: clean tillage (CK), tea intercropped with ryegrass (Lolium perenne, TRG), tea intercropped with rapeseed (Brassica napus, TRP), and tea intercropped with alfalfa (Medicago sativa, TAL). The study systematically evaluated the effects of green manure on tea yield, soil nutrient content, enzyme activity, and microbial community structure. The results showed that intercropping with green manure significantly increased the bud density, hundred-bud weight, and yield of tea in spring, summer, and autumn, with the TAL treatment showing the best overall performance. In terms of soil physicochemical properties, green manure treatments significantly improved soil organic matter, total nitrogen, available nitrogen, available phosphorus, and available potassium contents, with TRP and TAL showing the most pronounced improvements. Enzyme activity analysis indicated that the TRP treatment significantly enhanced the activities of amylase, urease, and invertase. High-throughput sequencing results revealed that green manure treatments significantly increased both the number of bacterial and fungal OTUs (Operational Taxonomic Units) and alpha diversity indices. The TAL and TRP treatments showed superior performance in terms of Shannon, Chao, and ACE indices compared to CK. Principal coordinate analysis (PCoA) indicated that green manure had a greater influence on fungal community structure than on bacterial structure. Correlation analysis demonstrated that dominant microbial taxa were significantly associated with soil nitrogen, phosphorus, and potassium levels, suggesting that green manure modulates microbial community composition by improving soil nutrient status. Intercropping green manure significantly increased tea yield and soil quality compared with clean tillage. Alfalfa intercropping (TAL) increased tea yield by 49.61%, 40.88%, and 43.79% in spring, summer, and autumn, respectively, compared with the control. Soil organic matter and total nitrogen under TAL were 29.02% and 15.67% higher than the control, while rapeseed intercropping (TRP) increased available phosphorus by 186%. TAL and TRP also enhanced microbial diversity, with bacterial Shannon index values 14.11% and 11.25% higher than the control. These results indicate that alfalfa intercropping is the most effective green manure practice for improving tea plantation productivity and soil ecology.