Changes in soil organic carbon stocks and mineralization following the replacement of secondary evergreen broadleaf forests with tea (Camellia sinensis L.) plantations

Tea plantation ecosystems have a strong potential to sequester carbon (C) and reduce CO2 emissions. However, the effects of different tea planting periods on soil organic carbon (SOC) stocks and mineralization and related mechanisms are unclear. To address this knowledge gap, we investigated the effects of replacing evergreen broadleaf forests with tea plantations on SOC stocks and mineralization rates by examining alterations in SOC pools and composition, microbial community composition, functional genes related to C-cycling and enzyme activities. The SOC content in forest, 30-, 50- and 100-year-old tea plantations were 1.91%, 2.37%, 2.87% and 3.69%, respectively, in the 0-20 cm soil depth (100-year-old > 50-year-old > 30-year-old > forest). Cumulative CO2-C emissions increased by 38.1% (114 mg C kg(-1) soil), 49.9% (157 mg C kg(-1) soil), and 100.2% (171 mg C kg(-1) soil) compared to forest soil (228 mg C kg(-1) soil) after tea had been grown for 30, 50 and 100 years, respectively; however, cumulative CO2 emissions did not differ significantly between the 30- and 50-year-old plantations. The rate of SOC mineralization was positively related to particulate organic carbon (POC), water-soluble organic carbon (WSOC), microbial biomass C (MBC), and O-alkyl C contents, as well as beta-glucosidase/cellobiohydrolase activities and GH48/cbhI abundance; by contrast, the SOC mineralization rate was negatively correlated with the aromatic C content. More importantly, bacteria and fungi related to SOC mineralization, such as WPS-2 and Acidobacteria, and Sordariomycetes, Tremellomycetes, Mortierellomycetes and Agaricomycetes, respectively, had high relative abundances. Our results indicate that replacing forests with tea plantations enhanced both SOC stocks and mineralization rates and that this effect was positively correlated with tea cultivation time. We reveal that an increased length of the tea planting period was conducive to increasing SOC stocks, and mitigating C losses in tea plantation soils is crucial for establishing an ecologically low-C tea plantation system.