Effects of Woodland Conversion to Tea Plantations and Tea Planting Age on Soil Organic Carbon Accrual in Subtropical China

Tea (Camellia sinensis (L.) Kuntze) plantations have been rapidly expanding in recent years in developing countries, but there is a lack of knowledge about the effects of woodland conversion to tea plantations and tea plantation aging on soil organic carbon (SOC) accumulation in subtropical regions, which may be a critical issue for accurately estimating the regional C balance in tea planting areas. Thus, in this study, we selected four tea plantations with ages ranging from 5 to 23 years, along with an adjacent mature pine forest (PF, more than 60 years of age), to investigate the effects of woodland conversion to tea plantations and stand age on SOC. Lignin phenols and amino sugars were used to distinguish the contributions of plant-derived C and microbial-derived C to SOC. The results showed that when PF is converted to a tea plantation, 54.12% of the SOC content in the topsoil is lost, with reductions of 84.77% in plant-derived C and 10.23% in microbial-derived C; however, there is a slight increase in microbial-derived C in the deep-layer soil. The tea planting age only has a negative effect on microbial-derived C in the topsoil. Additionally, the plant aboveground biomass, ratio of carbon to nitrogen, total nitrogen concentration, and clay–silt content are key environmental variables influencing SOC accrual, explaining 59.8% of the total variance. SOC and plant-derived C are thus implicated in the quick response to decreasing plant inputs with land conversion and do not accumulate with increasing tea plantation age under the current tea plantation management practices. Generally, more attention should be focused on SOC loss with woodland conversion to tea plantations at the regional scale, and more effective practices can be applied to enhance SOC accrual in subtropical tea plantations.