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).