Continuously applying kitchen waste fertiliser more strongly promotes microbial-derived carbon accumulation in mineral-associated organic carbon than other fertilisers across the paddy soil profile in the Yangtze River Delta, China

Paddy soil plays a crucial role in terrestrial carbon turnover and climate change. The effects of the continuous application of various fertilisers on soil organic carbon (SOC) accumulation and stabilisation across the paddy soil profile remain inadequately understood. This study systematically compared the impact of four consecutive-year application of kitchen waste fertiliser (KWF), chicken manure fertiliser (CMF) and conventional inorganic fertiliser (CF) on total carbon stock and SOC composition at surface (0–20 cm), subsurface (20–40 cm) and deep (40–60 cm) layers of paddy soil in the Yangtze River Delta, China. KWF enhanced surface SOC and soil inorganic carbon (SIC) by 25 % and 19 %, respectively, outperforming other fertilisers in surface SOC and SIC accumulation. KWF also increased SIC throughout the soil profile, thereby substantially improving soil fertility and carbon stock. Both organic fertilisers raised the proportion of particulate organic carbon (POC) but reduced the proportion of mineral-associated organic carbon (MAOC) at the surface layer, while increasing MAOC content at the surface layer, with the most significant increase observed in KWF treatment. Unlike CMF and CF, KWF induced a marked dominance of microbial-derived carbon in MAOC and in SOC (i.e. MAOC + POC) across the soil profile by more effectively increasing bacterial necromass carbon (BNC), which constituted 72 %, 98 % and 99 % of microbial-derived carbon in MAOC at the surface, subsurface and deep layer, respectively. Furthermore, KWF increased the MAOC proportion and caused microbial-derived carbon (primarily consisting of BNC) to dominate in MAOC and in SOC (i.e. MAOC + POC) at the deep layer, thus facilitating deep-layer SOC stability despite no significant increase in deep-layer SOC content. These findings underscore the unique potential of KWF as a carbon-sequestering fertiliser for SOC accumulation and stabilisation across the paddy soil profile, by enriching recalcitrant microbial-derived carbon in MAOC and SOC, primarily through bacterial turnover pathways. This study has significant implications for optimising fertilisation practices for carbon sequestration to enhance paddy soil fertility and mitigate climate warming in urbanising regions.