High-quality fine particulate organic matter drives SOM stocks: Insights from Mollisols in Northeast China

How the quantity and quality of particulate organic matter (POM) affect the mineral-associated organic matter (MAOM) accumulation and the aggregate stability has not been systematically studied, which limits the in-depth understanding of the relationship between POM and SOM stocks. This study investigated the changes in the biomass and C:N ratio of fine roots, the C contribution of POM to SOM and the C:N ratio of POM within different particle sizes in native grassland, plantations and croplands in the Mollisols region of Northeast China. Then, the influence of fine roots and POM characteristics on MAOM accumulation and aggregate stability was analyzed. The results showed that (1) the C contribution of fine POM (fPOM, 0.053-0.25 mm) to SOM and the C:N ratio of fPOM in the native grassland was significantly higher and lower than that in the other sites (P < 0.05), respectively. (2) The C contribution of MAOM to SOM and the mean weight diameter (MWD) of water-stable aggregates were significantly positively correlated with the C contribution of fPOM to SOM (P < 0.01), and negatively correlated with the fPOM C:N ratio (P < 0.01). (3) The effect of fPOM C:N ratio on MWD of water-stable aggregates was stronger compared to the C contribution of fPOM to SOM. (4) The fine roots biomass showed a significant indirect effect on the C contribution of MAOM to SOM and the MWD of water-stable ag-gregates by influencing the C contribution of fPOM to SOM (P < 0.05). In general, both the quantity and quality (C:N ratio) of fPOM were lower in other land use types than that in native grassland, which significantly affected the SOM stocks. At the same time, the fPOM was also a medium for the fine roots to affect the soil characteristics. Promoting the accumulation of fine POM and focusing on improving the quality of fine POM could be a promising measure to ensure food production and increase soil carbon stocks. Conservation tillage practices such as no-till may be effective.