Environmental abiotic predictors
All the variables included in this study are listed in table S1. Climate variables were determined from Worldclim 2.0 (Fick & Hijmans 2017). Based on previous modeling of SOC in grasslands in the Pyrenees (Rodríguez et al. 2020), we selected Mean Annual Temperature (MAT), Mean Annual Precipitation (MAP) and Mean Summer Precipitation (MSP). In addition, we included in the model the intra-annual difference of temperature (Temperature Seasonality Index of Sebastià (TSIS); Rodríguez et al., 2020), calculated as the difference between mean summer temperature (MST) and mean annual temperature.
Bedrock type was determined in the field and confirmed from the Geological Map (ICGC. 2007). Bedrock was categorized into three categories: basic (marls and calcareous rocks), acidic (mostly sandstones and slates) and mixed.
Topographical variables included: slope, aspect, macrotopography and microtopography. Slope and aspect were determined in the field by clinometer and compass respectively. Macrotopography included exposed (South- and East-facing slopes mountain tops) and protected (North- and West-facing slopes, valley bottoms) positions, while microtopography considered three positions: smooth areas, convexities and concavities.
Regarding grazing management variables, detailed surveys were carried out among farmers, shepherds and land managers. Two management variables were considered: grazing intensity and grazer type. Grazing intensity was determined by estimating livestock stocking rates measured as livestock units/ha (LU/ha), and treated as semi-quantitative variable with three categories (Sebastià et al. 2008): low (1: <0.2 LU/ha), medium (2: 0.2–0.4 LU/ha) and high (3: up to 0.4 LU/ha); a few samples corresponded to abandoned grasslands (0 LU/ha). Grazer type was categorized into three main types: sheep grazing, cattle grazing and mixed grazing. Mixed grazing included associations comprising small and big livestock, mainly sheep and cattle, and more rarely horses. Sheep flocks always included a few goats (Rodríguezet al. 2018).