Interactions between biogeochemical and management factors explain soil organic carbon in Pyrenean grasslands

Abstract. Grasslands are one of the major sinks of terrestrial soil organic carbon (SOC). Understanding how environmental and management factors drive SOC is challenging because they are scale-dependent, with large scale drivers affecting SOC both directly and through drivers working at detailed spatial scales. Here we addressed how regional, landscape and grazing management, soil properties and nutrients and herbage quality factors affect SOC in mountain grasslands in the Pyrenees. Taking advantage of the high variety of environmental heterogeneity in the Pyrenees, we fit a set of models with explicative purposes using data that comprise a wide range of environmental and management conditions. We found that temperature seasonality (MMT) was the most important geophysical driver of SOC in our study. MMT was positively related to SOC but only under certain local conditions: exposed hillsides, steep slopes and relatively highly grazed areas. High MMT conditions probably are more favourable for plant biomass production, but landscape and grazing management factors buffer the accumulation of this biomass into SOC. Concerning biochemical SOC predictors, we obtained some surprising, interactive effects between grazer type, soil nutrients and herbage quality. Soil N was a crucial factor modulating effects of livestock species and neutral detergent fibre content of plant biomass and herbage recalcitrance effects varied depending on grazer species. These results highlight the gaps in the knowledge about SOC drivers in grassland under different environmental and management conditions, and they may serve to generate testable hypothesis in latter studies directed to climate change mitigation policies.