Contrasting soil carbon and nitrogen dynamics under forest and pasture land covers in mediterranean mountain ecosystems

Land use and land cover (LULC), together with forest characteristics, strongly influence soil organic carbon (SOC) and nitrogen (N) dynamics in Mediterranean mountain regions. Despite their ecological importance, data on SOC storage remain scarce, especially for young forests following land abandonment. This study evaluated SOC and N stocks, alongside key soil properties, across five LULC types— Pinus sylvestris, Fagus sylvatica, Quercus pyrenaica, Quercus ilex, and a pasture—in La Rioja, Spain. Soil and litter samples were collected from the forest floor and mineral soil (0–40 cm), and a soil quality index (SQI) was developed. 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 . Pasture soils stored the largest mineral horizon stocks, associated with higher organic matter, finer textures, and lower bulk density. Coniferous forests accumulated greater SOC and N stocks in the forest floor, reflecting slow litter decomposition and elevated C:N ratios. SQI values were highest in pasture (0.47), and lowest in Fagus sylvatica (0.32). SOC and N correlated positively with soil organic matter and clay content, and negatively with bulk density and pH. Pastureland and coniferous forests act as effective carbon sinks through distinct mechanisms: pasturelands through continuous belowground carbon inputs via root turnover and mineral soil incorporation, coniferous forests through surface accumulation of recalcitrant litter. For climate mitigation, land managers should combine fire-resistant reforestation in mixed stands with grassland conservation, considering species-specific traits and site conditions to sustain soil functioning under environmental change. • Land cover controls carbon stocks in Mediterranean mountain soils. • Pastures exceed even coniferous forests in mineral soil carbon storage. • Clay content and organic matter drive soil carbon and nitrogen storage. • Integrated management of grasslands and forests maximizes carbon storage.