The Dual Role of NPP in Mediating and Moderating Climate-Soil Carbon Pathways Under Warming and Drought Across European Ecosystems.

Soil carbon dynamics in response to climate change hold critical implications for global carbon cycling and ecosystem resilience. Net primary productivity (NPP), the dominant source of soil carbon inputs, serves dual functions: it mediates climate effects via carbon input fluxes and moderates them by altering the intensity of climate impacts on soil carbon. In this study, we assembled a dataset of soil organic carbon (SOC; hereafter OC) and its fractions-particulate organic carbon (POC) and mineral-associated organic carbon (MAOC)-from 8147 repeated sampling sites across the European region in 2009 and 2018. Using structural equation modeling with joint mediation and moderation analysis across three major land uses, we identified two functionally distinct, context-dependent roles of NPP. NPP acts as a mediator: favorable climate increases belowground inputs and promotes OC accumulation, primarily through the formation and stabilization of MAOC. NPP also acts as a moderator: it amplifies temperature's negative impacts on soil carbon, with POC being most sensitive to warming. The effects of NPP on the relationship between climatic moisture and soil carbon exhibited a nonlinear reversal: under humid conditions, NPP mainly reduced decomposition-driven carbon loss, whereas under drought conditions, it enhanced input-driven carbon accumulation. In contrast, soil moisture consistently showed a stable positive influence, continuously supporting carbon inputs and accumulation. Cross-ecosystem comparisons show that forests rely more strongly on NPP-driven input and regulatory pathways, whereas in cropland the mediating and moderating roles of NPP are comparatively weaker, making soil carbon more directly influenced by climate. Overall, NPP plays a dual role in the climate-soil carbon pathway: as both a source of input enhancement and a marginal amplifier, representing a functional trade-off under climate change scenarios.