Rapid decline in soil organic carbon stocks following forest-to-maize field conversion within a watershed in Northern Thailand

• Soil organic carbon (SOC) is consistently high in upper topsoil across all soil types, with the highest levels observed in forests. • SOC negatively correlates with bulk density, sand particles, and the duration since forest-to-maize conversion, while positively correlating with pH, silt, and clay particles. • SOC levels decreased by 34.42% over 15 years after forest conversion to maize cropping. • SOC depletion rate is 1.01 MgC/ha annually. Soil organic carbon (SOC) content was measured to assess the impact of land-use change from forests to maize cultivation across different soil types in Mae Tha Subwatershed, Lampang Province, Northern Thailand, over a 15-year period (2007-2022). A Geographic Information System (GIS) was used to determine sampling points, followed by field surveys and topsoil analysis in converted maize fields. The results revealed that both maize and forest soils exhibited a similar pattern of higher SOC in the upper topsoil. However, forest soils had significantly higher SOC content compared to maize fields. The average SOC content in the upper topsoil of forests was 44.58 ± 14.88 MgC/ha, significantly higher than the average of 30.38 ± 10.57 MgC/ha found in maize fields. Land-use change resulted in an estimated SOC loss of approximately 34.42% over 15 years, pointing to a decrease of 1.01 MgC/ha per year. Additionally, SOC content generally showed a negative correlation with bulk density, sand particles, and years since conversion. Conversely, a positive correlation was observed between SOC and pH, silt particles, and clay particles. The decline in SOC stocks due to the conversion of forests to maize cropping threatens soil fertility and increases reliance on chemical fertilizers, which endangers drinking water resources. This degradation reduces soil resilience to climate change impacts, exacerbating food security challenges. Urgent action is needed to implement sustainable practices, such as carbon farming and agroforestry, to enhance soil health and mitigate climate change effects.