Seasonal plant-microbial coupling regulates carbon transformation pathways and soil organic carbon persistence in alpine grasslands

To elucidate the seasonal mechanisms underlying soil carbon cycling in alpine grasslands, we collected soil profile samples (0–50 cm) from the Qinghai-Tibet Plateau (QTP) in May, August, and November. Amino sugars and lignin phenols were used as molecular markers of microbial-derived and plant-derived carbon, respectively, and their ratio was used as a biomarker-based indicator of carbon transformation state. By integrating biomarker analysis with path modelling, we investigated the spatiotemporal patterns, controlling factors, and accumulation pathways of soil organic carbon (SOC). Our results showed that SOC remained relatively stable across seasons, whereas amino sugars, lignin phenols, and the amino sugar-to-lignin phenol ratio exhibited pronounced seasonal variation. SOC and biomarkers were generally concentrated in the topsoil (0–10 cm) and declined with depth, although amino sugars showed no significant depth-related variation in May. The ratio was highest in August (9.13 ± 0.42), compared with May (2.42 ± 0.36) and November (3.45 ± 0.13), and tended to be higher in deeper soil layers across seasons. Soil carbon transformation state was jointly associated with biotic and abiotic factors. Variance partitioning indicated that the independently explained variation in the May model was mainly associated with the unique abiotic fraction, whereas in the August and November models it was mainly associated with the unique biotic fraction. SOC accumulation followed three seasonal pathways: in May, it was primarily associated with physicochemical preservation of plant-derived carbon (path coefficient = 0.93) under constrained microbial activity; in August, enhanced carbon transformation promoted comparable plant- and microbial-derived contributions to SOC (0.45 and 0.51); and in November, SOC was more strongly linked to microbial residues (path coefficient = 0.69), suggesting a greater role of previously accumulated microbial-derived carbon. These findings provide a process-based framework for improving predictions of alpine grassland carbon sink dynamics under environmental change.