Soil quality responses to extensive grazing use in subalpine pastures across the Pyrenees.

Subalpine pastures in the Pyrenees are part of a long-standing cultural landscape shaped by centuries of extensive free-range grazing and transhumance. Like other European mountain regions, these grasslands are biodiversity-rich socio-ecological systems whose persistence depends on continuous management. Their ecological and cultural value is increasingly threatened by land abandonment, shrub encroachment, and climate warming, which reduce forage quality, alter soil processes, and compromise ecosystem resilience. Understanding how grazing influences soil functioning is therefore essential for sustainable pastoral management. We tested the hypothesis that, within low-stocking extensive systems, areas with moderately higher grazing exhibit enhanced soil quality relative to lightly used areas through effects on vegetation, nutrient inputs, and biogeochemical functioning, while remaining within low-intensity stocking levels.

We assessed soil quality under two relative grazing uses, Higher grazing use (HG) and Lower grazing use (LG), in extensive free-range systems with very low absolute stocking densities. At the Spanish site, the grazing unit comprises ~8,000 ha used by a free-ranging herd of 30 cattle (~0.04 LU·ha⁻¹). GPS tracking of five collared cows revealed strong contrasts in site use: 3,496 minutes in HG areas versus 298 minutes in LG areas during July–September. Parcels were classified based on vegetation structure and field indicators of bovine activity. At each site (Spain, Andorra, France), two areas (HG, LG) were sampled, each with four replicated subplots. Soil cores were collected at 0–6 cm (bulk density, mesofauna) and 0–20 cm (physical, chemical, biological properties), and aboveground biomass was harvested in 40×40 cm quadrats.

Soil Quality Index (SQI) values were calculated using the Minimum Data Set approach (Andrews et al., 2002), normalized on a 0.1–1 scale. Mesofauna was incorporated through the Ecological–Morphological Index (Menta et al., 2018).

The highest-weighted SQI indicators were electrical conductivity (0.560), total glomalin (0.197), pH (0.197), cation exchange capacity (0.197), water saturation content (0.170), coarse fragments (0.170), Olsen-P (0.073), porosity (0.073), bulk density (0.073), and clay (0.073). SQI showed consistent regional patterns, with higher values in HG areas: Spain 0.780 ± 0.005 vs. 0.727 ± 0.017; France 0.624 ± 0.027 vs. 0.606 ± 0.008; Andorra 0.714 ± 0.034 vs. 0.692 ± 0.024.

Several high-weight indicators showed grazing-related changes. Aggregate stability increased under higher grazing in Andorra but decreased in France and Spain. Total glomalin was identical between HG and LG in Andorra and France, but lower under LG in Spain. Cation exchange capacity and pH were consistently higher in HG. Electrical conductivity remained slightly higher in HG, especially in Spain. Coarse fragments varied by site, but their contribution was moderate relative to conductivity and cation exchange capacity.

Overall, moderately higher grazing helps maintain soil structural stability, supports fungal contributions to soil carbon, preserves cation-exchange capacity and pH, and sustains electrical conductivity within functional ranges. Together, these processes enhance soil quality in extensive free-range systems. Our findings highlight intermediate grazing as a key driver of soil functioning and ecosystem resilience in subalpine Pyrenean pastures, emphasizing the integration of soil indicators, biological communities, and grazing patterns for sustainable management of high-mountain rangelands.