Gradient Organic Fertilizer Substitution Reshapes Soil Microecology and Microbial Carbon Utilization in Tea Plantations

Organic fertilizer substitution is widely used to improve acidified tea plantation soils, but the coupled effects of substitution gradients on soil aggregates, microbial carbon utilization, and microbial C–N–P limitation remain insufficiently quantified. This study aimed to determine how gradient organic fertilizer substitution regulates soil physicochemical properties, aggregate-size distribution, extracellular enzyme stoichiometry, and microbial carbon-source utilization in a subtropical tea plantation, and to identify a suitable substitution range under the experimental conditions. A long-term field experiment was conducted in an acidic red-soil tea plantation in Wuyishan, Fujian Province, China. Five treatments were established: pure chemical fertilizer (CF) and organic fertilizer substituting 33% (OF33), 50% (OF50), 67% (OF67), and 100% (OF100) of chemical fertilizer nitrogen. Tea growth indices, soil physicochemical properties, aggregate mechanical stability, microbial biomass C, N, and P (MBC, MBN, and MBP), extracellular enzyme activities, and BIOLOG-ECO carbon-source utilization profiles were measured. Results showed that OF33 and OF67 increased soil pH by 0.46–0.60 units relative to CF. OF33 generally improved available N and K and aggregate stability indices, whereas OF50 increased 100-bud weight and microbial carbon-source richness. OF100 increased MBC but also increased vector length and vector angle, indicating stronger relative C and P limitation. BIOLOG-ECO profiles separated treatments, and moderate substitution (OF33–OF50) broadened the carbon-source utilization niche. However, different indicators peaked under different treatments; therefore, 33–50% substitution should be regarded as a suitable range under the conditions of this experiment rather than a universally optimal rate. These findings provide a site- and cultivar-specific basis for reducing chemical fertilizer use in red-soil tea plantations.