Soil Aggregate-Associated Carbon Fraction Dynamics during the Process of Tea (Camellia sinensis L.) Planting in Southern Guangxi, China

Revealing the variation in soil aggregate-associated organic carbon (C-org) in tea plantations of various planting ages is crucial to shed more light on the accumulation and decomposition of soil C-org in the tea-planting period. This study measured the concentrations of soil C-org, active carbon (C-act), and recalcitrant carbon (C-rec) in different-sized aggregates obtained from tea plantations of various planting ages (8, 17, 25, and 43 years old) at the soil depths of 0-20 and 20-40 cm in southern Guangxi, China. According to the wet-sieving approach, soil aggregates were classified as macro- (> 0.25 mm) and micro- (< 0.25 mm) aggregates, and the former were further divided into coarse (> 2 mm), medium (2-1 mm), and fine (1-0.25 mm) fractions. Based on the mean weight diameter (MWD), the stability of soil aggregates was the highest in the 17-year-old tea plantations, and it was closely related to the concentration of soil C-act (0-20 cm: R-2 = 0.9744, p < 0.05; 20-40 cm: R-2 = 0.8951, p < 0.05), but not C-org (0-20 cm: R-2 = 0.1532, p > 0.05; 20-40 cm: R-2 = 0.4538, p > 0.05), during the tea-planting process. In the 0-20 and 20-40 cm soil layers, the coarse and medium macro-aggregates had higher concentrations of C-org, C-act, and C-rec, regardless of the tea-planting age; meanwhile, the soil C-act/C-rec ratio, indicating the C-org availability, increased as aggregate size increased, implying that the soil C-org was younger and more labile in coarse macro-aggregates relative to finer aggregates. Moreover, the tea-planting age significantly affected the C-org, C-act, and C-rec reserves in both soil layers. To be specific, continuous tea planting facilitated the accumulation of soil C-org and C-rec, but their reserves' increase rates decreased over time; meanwhile, the soil C-act reserve increased during the early (from 8 to 17 years) tea-planting stage and later decreased. Therefore, during the middle (from 17 to 25 years) and late (from 25 to 43 years) tea-planting stages, maintaining the soil as an C-act pool plays a vital role in facilitating the formation and stabilization of soil aggregates in southern Guangxi, China.