Assessment of soil organic carbon from various land use and land cover types within the Agno River Basin, the Philippines

In the Philippines, baseline assessments of soil organic carbon (SOC) are crucial for carbon sequestration assessments, particularly across one of the Philippines’ most intensively utilized river systems – the Agno River Basin (ARB). This research explores how land use and land cover (LULC), soil management practices, and physicochemical characteristics collectively influence the organic carbon storage of four land cover types: forest, agriculture, grassland, and built-up areas in the ARB. A total of 29 composite soil samples were collected from 0–20 cm (topsoil) and 20–50 cm (subsoil) depths to represent various LULC. Bulk density (BD) was obtained using undisturbed cores, and SOC was determined using the Walkey-Black method. The findings reveal variations in the percentage of SOCs across different land cover types. Built-up areas exhibit lower %SOC (0.86% in topsoil and 0.11% in subsoil), whereas forested regions display higher %SOC (2.58% in topsoil and 1.74% in subsoil). The study also found that %SOC decreased with depth, with higher values at 0–20 cm and lower values at 20–50 cm depth. The computed SOC storage (in tons/ha) decreased in the order of forest > grassland > agriculture > built-up. The study also establishes weak to moderate negative correlations between soil physicochemical properties (i.e. BD, PD or particle density, pH, %silt, and %sand) and %SOC, whereas %clay exhibits a weak positive correlation. This investigation contributes to a deeper understanding of the intricate relationships between soil properties and SOC dynamics, presenting insightful information for sustainable land management practices in the ARB.