Elevated soil organic carbon induced by a one-time woody peat application persists through the fourth year: Microbial-driven divergent temporal changes in particulate and mineral-associated organic carbon

Knowledge regarding the multi-year dynamics of soil organic carbon (SOC) and its underlying microbial mechanisms after a one-time application of woody peat (WP) under field conditions remains limited. In this study, we investigated temporal changes in SOC and its fractions, particulate organic carbon (POC) and mineral-associated organic carbon (MAOC), using a time-series of adjacent fields (0, 1, 2, and 4 years post-application). Our findings show that a one-time WP application significantly increased SOC in the first year, driven by concurrent increases in both POC and MAOC. These increases were partially derived from the applied WP and its residual components, mediated by shifts in the microbial community. From the first to fourth year after application, the elevated SOC level remained stable through contrasting dynamics of POC and MAOC, namely, POC declined substantially over time, whereas MAOC continued to increase. The reduction in POC was primarily associated with decreases in bacterial necromass C and β-1,4-glucosidase activity, which were closely linked to a microbial succession from protistan and bacterial dominance in the first year, to a bacterial-fungal mixture in the second year, and finally to fungal dominance by the fourth year. In contrast, the increase in MAOC was mainly driven by rising fungal necromass C and phenol oxidase activity, which were associated with a microbial succession that progressed from protistan and bacterial dominance in the first year to distinct fungal dominance in the subsequent second and fourth years. This study demonstrates the long-term effect of a one-time WP application on SOC accumulation and provides a time-resolved microbial perspective, identifying specific bacterial, fungal, and protistan genera associated with POC and MAOC dynamics, thereby clarifying the microbial mechanisms governing SOC persistence. Our findings underscore the important role of protists in regulating both POC and MAOC, particularly during the initial stage after WP application.