Soil Organic Carbon Storage and Soil Quality across Forest Types and Pastureland in Mediterranean Mountain Ecosystems

Land use and land cover (LULC) significantly influence terrestrial carbon cycling and soil functioning in Mediterranean mountain regions, yet the relative contribution of different vegetation types to soil organic carbon (SOC) storage remains poorly quantified. Understanding these relationships is essential for developing effective land-based climate mitigation and adaptation strategies that balance carbon sequestration with broader ecosystem services. This study quantifies SOC and nitrogen (N) stocks across five contrasting LULC types in La Rioja, northern Spain: Pinus sylvestris, Fagus sylvatica, Quercus pyrenaica, Quercus ilex, and a pastureland.

We assessed SOC and N stocks in both forest floor and mineral soil layers (0-40 cm), alongside key physicochemical properties. A soil quality index (SQI) was developed to evaluate overall soil functioning beyond carbon storage alone.

Results demonstrate substantial variation in carbon storage mechanisms among LULC types. Total SOC stocks ranged from 42.9 Mg ha⁻¹ (Quercus ilex) to 112.9 Mg ha⁻¹ (pasture), while N stocks varied from 4.0 to 10.3 Mg ha⁻¹. Pastureland stored the highest mineral soil stocks, associated with elevated organic matter content, finer texture, and lower bulk density. Coniferous forests accumulated substantial SOC and N in surface organic layers, reflecting slow litter decomposition and high C:N ratios, but lower mineral soil stocks. SQI values ranged from 0.32 (Fagus sylvatica) to 0.47 (pasture), indicating significant differences in overall soil functioning.

These findings reveal important considerations for land management decisions: while forest expansion provides carbon sequestration through distinct accumulation pathways, well-managed pastureland systems can achieve comparable or superior total carbon storage with additional co-benefits for soil quality. Our results highlight that vegetation-specific effects on SOC distribution, soil properties, and nutrient dynamics must inform land-based climate strategies. An integrated management approach considering species-specific traits, soil characteristics, and multiple ecosystem services is essential for optimizing carbon storage and maintaining soil functioning in Mediterranean mountain environments under environmental change.

This research project is supported by the FORWARD project (PID2024-161314OB-I00) funded by the MICINN-FEDER.