Effects of Biochar-Based Organic Fertilizer on Soil Aggregates and Soil Organic Carbon

Biochar-based organic fertilizer is increasingly used to improve soil quality, yet its effects on soil aggregation and organic carbon (SOC) fractions remain poorly understood. This study aimed to elucidate the effects of biochar-based organic fertilizer on SOC accumulation and soil aggregation and their underlying mechanisms. A microcosm experiment was conducted using a Cambisol under four treatments: no addition, organic fertilizer alone, biochar alone, and biochar-based organic fertilizer. We determined aggregate size distribution and stability, SOC and its fractions, as well as microbial community composition. Biochar-based organic fertilizer increased the proportion of macroaggregates, geometric mean diameter, and SOC content (from 0.9% to 1.9%, p < 0.05), along with increases in mineral-associated organic carbon (MAOC) and particulate organic carbon (POC, an increase of 8.1 g kg−1, p < 0.05). The biochar component played a dominant role in promoting MAOC accumulation and aggregate stability, both of which were closely linked to shifts in bacterial community composition. In contrast, organic fertilizer primarily contributed to POC content associated with fungal communities. Overall, biochar-based organic fertilizer is effective for improving soil physical structure and SOC accumulation via biochar–organic fertilizer synergy. These findings have important implications for its use as a sustainable soil amendment for degraded arable soils.