Stand Age Shapes Bacterial Communities via Shifts in Soil Nutrients and Enzyme Activities in Unfertilized Rubber Plantations

Understanding how long-term monoculture shapes soil microbial communities is crucial for sustainable management of tropical rubber plantations. Here, soils from a chronosequence of rubber plantations (5, 11, 18, and 36 years) in Danzhou, Hainan, China, were analyzed to assess changes in soil nutrients, enzyme activities, and bacterial communities. Soil organic matter, total phosphorus, and available phosphorus increased with stand age, whereas alkaline-hydrolyzable nitrogen (AN) and available potassium (AK) declined, indicating a decoupling between total and labile nutrients. Enzyme activities exhibited clear age-dependent patterns, with nitrogen-acquiring enzymes peaking in older stands and carbon-cycling enzymes peaking at intermediate stages, suggesting shifts in microbial resource acquisition strategies. Bacterial community composition shifted significantly with stand age, and this variation was primarily associated with AN, AK, and pH. Bacterial α-diversity exhibited a unimodal pattern, peaking at intermediate stages (11 years) and declined slightly in older plantations, and was primarily regulated by soil nutrient pools and microbial resource acquisition capacity. Overall, stand development restructures soil nutrient availability and enzyme-mediated processes, thereby reshaping the microbial community structure in unfertilized rubber plantations. These findings underscore that maintaining balanced nutrient supply is critical for sustaining soil microbial diversity and functional stability under long-term monoculture.