Landscape-scale assessment of the contribution of improved Urochloa humidicola pastures to soil organic carbon stocks enhancement in the Colombian Llanos

Tropical grasslands can sequester substantial soil organic carbon (SOC), yet the temporal dynamics of SOC across soil layers—particularly in deep horizons—remain poorly understood at landscape scale. This study presents the first large-scale, stratified assessment of SOC stocks under Urochloa humidicola pastures in the Colombian Llanos. We evaluated SOC stocks and related soil properties (soil texture and bulk density) across pasture age classes (Uh 1, Uh 2, Uh 3, and Uh > 3 years), and compared them to conventionally managed, regularly burned savanna areas (CBS). Soil samples were collected from 111 georeferenced field sites at four depths (0–10 cm, 10–30 cm, 30–50 cm, and 50–100 cm), and SOC stocks were calculated using the equivalent soil mass (ESM) approach. Results revealed a consistent increment in both SOC concentration and SOC stocks with increasing pasture age. Total SOC stocks (0–100 cm) were up to 35% higher in older pastures (Uh > 3 years) compared to CBS, corresponding to an average gain of ~27 Mg C ha⁻¹. These gains were observed in both soil layers, with ~8.5 Mg C ha⁻¹ (19.5%) in the topsoil (0–30 cm) and ~14.8 Mg C ha⁻¹ (39%) in the subsoil (30–100 cm). Accumulated SOC tends to be more stable in these deeper soil layers. SOC concentration patterns align with these trends, supporting its use as an early indicator of soil improvement. Soil texture and hydrological (drainage) conditions influenced SOC concentration and stock changes, with clay-rich and water-retentive soils accumulated significantly more SOC than sandy-texture soils. These results highlight the climate mitigation potential of improved tropical pastures and the importance of stratified, depth-resolved monitoring of SOC stocks. U. humidicola pastures show progressive, spatially heterogeneous SOC gains, supporting the strategy of sustainable intensification of tropical grazing systems and their integration into carbon crediting mechanisms.