Depth-dependent drivers of soil aggregate carbon across Tibetan alpine grasslands

Elucidating the effects underlying soil organic carbon (SOC) variation is imperative for ascertaining the potential drivers of mitigating climate change. However, the drivers of variations in various SOC fractions (e.g., macroaggregate C, microaggregate C, and silt and clay C) at different soil depths remain poorly understood. Here, we investigated the effects and relative contributions of climatic, plant, edaphic, and microbial factors on soil aggregate C between the topsoil (0-10 cm) and subsoil (20-30 cm) across alpine grasslands on the Tibetan Plateau. Results showed that the C content of macroaggregates, microaggregates, and silt and clay fractions in the topsoil was 128.6 %, 49.6 %, and 242.4 % higher than that in the subsoil, respectively. Overall, plant properties were the most determinants controlling soil macroaggregate, microaggregate, and silt + clay associated C for both two soil depths, accounting for 32.2 %, 37.4 %, and 38.8 % of the variation, respectively, followed by edaphic, microbial, and climatic factors. The aggregate C of both soil depths was significantly related with the climatic, plant, edaphic, and microbial factors, but the relative importance of these determinants was soil-depth dependent. Specifically, the effects of plant root biomass and microbial (e.g., microbial biomass carbon and fungal diversity index) factors on each aggregate C weakened with soil depth, but the importance of edaphic factors (e.g., clay content, pH, and bulk density) strengthened with soil depth, except for the weakened effect of bulk density on the microaggregate C. And the effects of climatic factor (e.g., mean annual precipitation) on macroaggregate and microaggregate C increased with soil depth. Our results highlight differential drivers and their impacts on soil aggregate C between the topsoil and subsoil, which benefits biogeochemical models for more accurately forecasting soil C dynamics and its feedbacks to environmental changes.