Soil organic carbon dynamics: Influences of land-use change in natural and plantation forests of the Western Ghats, India.

Soil organic carbon (SOC) is a fundamental component of the global carbon cycle, underpinning ecosystem health, climate regulation, and sustainable land management worldwide. The conversion of natural forests to plantation systems in humid tropical regions has emerged as a critical global issue, leading to significant reductions in SOC stocks and compromising the carbon sequestration potential of soils. To assess these impacts, we compared SOC concentrations in plantation and natural forests across five humid tropical zones, including the globally significant Southern Western Ghats (SWG) of India- a recognized biodiversity hotspot and one of the world's most complex forest ecosystems. A stratified random sampling design was used across five agroecological zones to select paired natural forests and adjacent long-rotation teak (Tectona grandis) plantations (40-50 years old). Soil samples were collected from four horizons (O, A, B, and C) within 1 m depth profiles. SOC concentration (CHNS analyzer), bulk density, texture (hydrometer method), cation exchange and pH were determined. SOC stocks were calculated using bulk density and horizon depth. Our analysis shows that natural forests maintain substantially higher average SOC concentrations (16.61 g/kg) than plantation forests (11.82 g/kg). In natural forests, SOC ranged from 9.53 g/kg to 26.09 g/kg, while plantation forests ranged from 6.93 g/kg to 17.73 g/kg, reflecting similar trends observed in the SWG and other tropical regions. SOC concentrations were significantly greater in the surface layers of natural forests compared to deeper layers (P < 0.05), with more than 70% of SOC typically stored in the upper 30 cm. Correlation analysis showed a significant negative relationship between SOC and soil pH in natural forests (r = -0.37, P < 0.05), whereas plantation soils exhibited a positive relationship (r = 0.03, P < 0.05). Forest soils also showed a positive correlation between SOC and clay content (r = 0.16, P < 0.05) and a weak negative correlation with sand content (r = -0.04, P < 0.05). These findings underscore a global challenge: land use change from natural forest to plantation reduces SOC stocks, alters soil health, and diminishes the resilience of tropical soils to environmental change. Maintaining and restoring natural forests-both globally and in biodiversity hotspots like the SWG-is essential for maximizing soil carbon sequestration, supporting soil fertility, and achieving climate mitigation targets. This study provides a scientific foundation for sustainable land management and carbon storage strategies in tropical regions globally.