Divergent changes in particulate and mineral-associated organic carbon under natural revegetation along a soil texture gradient in temperate grasslands of China

The natural revegetation of grasslands can stimulate belowground carbon inputs and facilitate the storage of soil organic carbon (SOC), which is conducive to the restoration of grassland ecosystems. Previous studies have investigated the dynamics of organic carbon (OC) in bulk soil, but paid little attention to OC fractions within aggregates. This study explored the response of OC changes in functionally distinct fractions, namely particulate organic matter (POM) and mineral-associated organic matter (MAOM), to natural restoration through systematic measurements of two grassland vegetation species at three study sites along a soil texture gradient in the Loess Plateau. In naturally restored grasslands, the SOC and OC contents of POM (POM_C) and MAOM (MAOM_C) within bulk soil and aggregates were significantly (p < 0.05) higher relative to bare land, with more effective improvement of POM_C (1.01−6.55 times) than MAOM_C (0.88−4.26 times). Compared to cover type (6.01 %−26.04 %), soil texture explained the variance of OC fractions to a greater extent (56.39 %−88.16 %). Soil texture had a larger contribution to MAOM_C in bulk soil with standardized path coefficients for total effect of 0.78 than POM_C, which had a coefficient of 0.67 (p < 0.05). Moreover, the growth rate of the MAOM_C pool exhibited a gradual decline (p < 0.001), which subsequently approached saturation level despite a persistent increase in total SOC in both bulk soil and aggregates. During soil carbon sequestration, aggregate fractions with relatively smaller particle size reached the MAOM_C saturation more easily than fractions with larger size. The findings of this study highlight the important role of soil texture in determining the saturation level of the MAOM_C pool at the regional scale. In the development of soil carbon sequestration strategies, more attention should be paid to the divergent behaviors of different SOM fractions and their distinct responses to natural restoration at larger spatiotemporal scales.