Distribution of Cu in agricultural soils with different land uses through stable isotope analysis

Abstract

Conversion of paddy soil to abandoned land involves the change of the copper biogeochemical cycle. Both the excessive accumulation and deficit of this element can threaten soil sustainable development or food security. Copper (Cu) is one of the essential micronutrients for plants, yet little is known about how the conversion of paddy soil to abandoned land affects Cu isotope signature. This study investigated Cu isotope signatures in paddy land (PL) and abandoned paddy lands for 1, 3, and 5 years (NPL1, NPL3, NPL5) in the Mun River basin of a tropical zone. The results show higher [Cu] and positive δ65Cu values in profile PL, with a significant correlation between δ65Cu and the chemical index of alteration (CIA) value. The low [Cu] and positive δ65Cu values are in profile NPL1, while low [Cu] and negative δ65Cu values are in profiles NPL3 and NPL5. Owing to artificial and seasonal water saturation conditions in paddy soils (PL), the variation of δ65Cu values within the profile was controlled by the decomposition and rice uptake in the upper layer, and the transformation of Cu-bearing minerals with the long-term flooding (lower layer). After cropland abandonment, the distinct Cu isotope signatures among soil profiles might be due to the transformation of soil minerals. At the topmost of abandoned soils, rice harvest and the abandonment of paddy soil might result in 63Cu-deplete at the early abandonment times (1 ∼ 2 years), but the abandoned soil was restored with increased abandonment years and preferentially result in 65Cu-deplete. Our findings provide a comprehensive insight into Cu isotope signature in response to the conversion of land-use types, which will contribute to understanding the soil Cu geochemical cycle in terrestrial ecosystems.