Abstract: Nutrient enrichment profoundly alters soil organic carbon (SOC) dynamics, yet the relative controls of plant versus microbial pathways on specific SOC fractions remains mechanistically unresolved across contrasting grasslands. We investigated the effects of 11-year nitrogen (N) and phosphorus (P) addition on SOC fractions in a semi-humid alpine meadow (AM) and an arid alpine steppe (AS) on the Northern Tibetan Plateau, aiming to disentangle the plant- and microbe-mediated mechanisms. In AM, where tight plant-microbial coupling prevailed, N addition increased SOC primarily by enhancing plant-derived particulate organic carbon (POC) through increased aboveground net primary productivity (ANPP), but reduced mineral-associated organic carbon (MAOC) via soil acidification-induced microbial suppression. Conversely, in plant-microbial decoupled AS, N addition had no effect on SOC but decreased dissolved organic carbon and slightly increased MAOC, likely mediated by NO₃^--N promoting microbial-residue stabilization. P addition reduced SOC in AM by lowering fungal diversity and accelerating POC decomposition, whereas it exerted negligible effects in AS despite increasing ANPP, likely offset by enhanced microbial respiration. N + P additions showed no additive effects on soil carbon fractions in either grassland. Overall, SOC persistence under nutrient enrichment is likely modulated by the balance between plant and microbial nutrient demands, as well as soil resource availability. This suggests that AM may functions as transient sinks while AS acts as slightly conservative reservoirs. Our study underscores the necessity of incorporating ecosystem-specific plant-soil-microbial decoupling mechanisms, driven by inherent water-nutrient coupling and pH shifts, to better understand carbon persistence.

Key words: Alpine grasslands, N and/or P additions, pH, plant carbon input, microbial diversity, plant-microbe interactions, SOC fractions