Soil degradation and herbicide pollution by repeated cassava monoculture within Thailand’s conservation region

Abstract

In a national park in Northeast Thailand, agricultural land has been converted from natural forest by small-scale farmers. To investigate the impact of repeated cassava monoculture on soil, we conducted a five-year (2016–2020) study on the physical and chemical properties of soil in cassava farmland and examined the properties of forest soil as a control. Soil nitrogen, available phosphate and potassium, and bulk density were significantly higher in the farmland than in the forest, whereas organic matter and cation exchange capacity were the opposite. In the farmland, soil nitrogen, electrical conductivity, and bulk density increased significantly, whereas pH and organic matter decreased significantly, indicating that soil degradation progressed during the five years. The use of glyphosate and paraquat herbicides is prohibited within national parks. However, in 2020, glyphosate was detected in farmland soil at levels of up to 5.0 mg kg-1 during both the rainy and dry seasons, and glyphosate (2.5 mg l-1) was detected in stream water from the farmland during the dry season. The use of glyphosate in the farmland was suspected at least in 2020. Soil degradation and herbicide pollution may carry a high risk of causing irreversible changes to the terrestrial ecosystems.