Effects of Agroforestry Intercropping on Tea Yield and Soil Biochemical Functions in the Red Soil Region of Southern China

Agroforestry intercropping is increasingly recognized for improving soil quality and crop productivity, yet its effects on soil nutrient dynamics, enzyme activities across soil profiles, and tea yield remain insufficiently understood. Here, we assessed how four systems—monoculture tea (CK), Osmanthus–tea (OT), Michelia–tea (MT), and Osmanthus–Michelia–tea (OMT)—influence soil properties and spring tea yield in hilly plantations of southern China. Across systems, the OMT configuration produced the highest spring tea yield, representing a 39.5% increase relative to CK, accompanied by a 19.0% increase in tea bud density. In the 0–20 cm soil layer, OMT markedly enhanced soil organic matter by 48.4%, total nitrogen by 25.8%, and available nitrogen and phosphorus by 24.9% and significant margins, respectively, while also stimulating enzyme activities—urease (+34.1%), sucrase (+17.2%), dehydrogenase (+43.9%), amylase (+17.2%), and cellulase (+60.7%). In the 20–40 cm layer, OMT increased soil organic matter (+48.4%), total nitrogen (+25.8%), and available nitrogen, and elevated key enzyme activities, including sucrase (+46.5%), acid phosphatase (+16.3%), and polyphenol oxidase (+20.1%). Correlation and principal component analyses further revealed strong positive associations among nutrient enrichment, enzyme activation, and tea yield. These findings demonstrate that the OMT agroforestry configuration enhances nutrient availability and enzymatic function throughout the soil profile, thereby promoting higher tea yield. Overall, OMT substantially improved spring-season soil fertility and productivity, highlighting its potential for sustainable tea plantation management.