Response of Organic Carbon Components and Stability to Long-Term Application of Low Doses of Biochar and Biochor-Based Fertilizers

Soil organic carbon (SOC) sequestration plays a vital role in sustaining soil productivity and mitigating climate change. Although biochar and charcoal-based fertilizers are known to enhance SOC sequestration, current understanding is predominantly derived from studies applying high doses. With the goal of elucidating the mechanisms through which long-term, low-dose biochar application influences SOC composition and stability, this study evaluated the long-term impacts of biochar and carbon-based fertilizers on SOC content, chemical structure, and microbial residual carbon assessed via amino sugar biomarkers. The following features are demonstrated by this study: (1) The application of biochar and carbon-based fertilizers significantly increased the contents of active organic carbon components (DOC, MBC, POC) and stable carbon components (MAOC, humic carbon) in the plow layer soil. Notably, the C50 treatment reduced the easily oxidizable organic carbon (EOC) content by 19.25% compared to the control. (2) Long-term application increased the relative abundance of aromatic functional groups in SOC, enhanced SOC decomposition resistance (as reflected by the F-index). Compared with NPK, the BBF treatment increased the F-index by 21.28% and 25.00% in the 0–20 cm and 20–40 cm soil layers. (3) The BBF treatment significantly increased both soil amino sugar content and the contribution of microbial residual carbon to SOC. Specifically, it elevated the levels of GluN, GalN, and MurN by 9.24% to 33.31% across soil layers. Fungal residual carbon constituted the dominant fraction across all treatments. In summary, the content and stability of SOC are enhanced by biochar and biochar-based fertilizers through synergistic mechanisms that involve altering its chemical composition and stimulating the accumulation of fungal residual carbon.