Depth-driven responses of soil organic carbon fractions to orchard cover crops across China: A meta-analysis

Cover crops (CC) have been widely recognized and implemented as one of the most effective agronomic practices for enhancing soil organic carbon (SOC) sequestration in orchard ecosystems. However, considerable uncertainty remains regarding the effect of CC on specific SOC fractions, posing challenges for accurate prediction of carbon (C) dynamics, which requires further comprehensive study at regional and national scales. Based on 615 paired- comparisons from 47 studies across China, we investigated the effects of CC management on SOC fractions, including microbial biomass C (MBC), dissolved organic C (DOC), particulate organic C (POC), easily oxidizable organic C (EOC), light fraction organic C (LFOC), and heavy fraction organic C (HFOC). In addition, we quantified the effects of various environmental factors (e.g., climatic conditions), soil properties (e.g., soil characteristics and depth), and agronomic variables (e.g., experiment duration, tree age, cover type, source and species of grass, cover pattern, mowing practices, and residue management) on the changes in SOC fractions. Compared to conventional clean (bare ground) tillage, CC significantly increases MBC (35.4 %), DOC (23.7 %), POC (36.2 %), EOC (18.4 %), LFOC (99.9 %), and HFOC (5.4 %). Random forest modeling demonstrates that soil depth is the dominant driver of SOC fractions responses to CC, and the CC effects are weakened with soil depth. It is therefore crucial to consider the various drivers of SOC fractions between soil depths in order to accurately forecast soil C dynamics and its potential feedback on global warming. Overall, this study systematically assessed the effects of CC on SOC fractions changes in China and identified CC as a promising practice for increasing SOC in orchards. These findings further indicate that the response of SOC fractions to CC is predominantly influenced by specific climatic, edaphic, and agronomic variables. These results not only reveal the ecological benefits of CC, but also highlight the importance of developing site-specific CC practices for the sustainability of agroecosystems.