Nutrient dynamics and carbon stock in Andisols affected by forest-to-vegetable farm conversion in Leyte, Philippines

Forest-to-vegetable farm conversion is a primary ecological concern in the Philippines. This study evaluated changes in soil fertility and soil organic carbon (SOC) stocks in Andisols under forest and vegetable farming systems. Soil profiles were opened and samples from genetic horizons were collected to assess depth-dependent changes in physical and chemical properties. Conversion of forest to vegetable farming was associated with increased bulk density and reduced porosity, water holding capacity, soil organic matter (SOM), and total nitrogen (N), particularly in surface horizons. Available phosphorus (P) was higher in cultivated soils due to fertilization, while P retention remained consistently high (>90%) across all depths. SOC stocks to 100 cm were noticeably greater in forest soils (25.11 kg m−2) than in cultivated soils (4.85 to 19.03 kg m−2). Pearson correlation analysis showed a moderate, positive significant relationship between SOM and available P (p < 0.05) and a strong, positive significant relationship between exchangeable aluminum (Al) and available P (p < 0.01), highlighting the roles of SOM and Al oxides in regulating P dynamics. Principal component analysis explained 64.9% of the total variance, with SOM, available P, and texture emerging as the dominant factors differentiating land use systems. Land conversion increased compaction and erosion risk and reduced SOM- and total N-associated nutrient pools while elevating available P and exchangeable base cations in cultivated soils due to repeated manure and fertilizer inputs. These results emphasize the need for sustainable management practices to maintain the fertility and SOC sequestration potential of Andisols under intensive vegetable production.