Land-use conversion controls on the mobility of Zn in paddy soils revealed by stable Zn isotopes

Understanding Zn biogeochemical cycling is necessary for monitoring Zn supply for plants and life during land use conversion, which is critical for environmentally sustainable development. But little is known about how the conversion of paddy soil to abandoned land affects the Zn isotope signature. A comparative field observation was conducted in northeast Thailand to investigate the Zn isotope footprint of paddy soils and abandoned paddy soils (PL and NPL). Our results show that Zn (τ_Zn^int : 0.04) slightly retains on PL, but is lost from NPL (τ_Zn^int from - 0.81 to - 0.24) to the river during weathering. Compared to PL (Δ⁶⁶Zn_parent-soil: -0.29 ‰), more ⁶⁶Zn isotopes might enter the river when rice cultivation ceases in NPL (Δ⁶⁶Zn_parent-soil from -0.26 ‰ to -0.47 ‰). Rice harvest and then root decay might result in heavy ⁶⁶Zn isotopes accumulating at the topmost soil in PL (δ⁶⁶Zn: 0.14 ‰) and short-term abandonment (1-2 years) in paddy soils (NPL1 δ⁶⁶Zn: 0.18 ‰). The release of assimilated Zn, and then the high adsorption of Zn in the Fe-SOM-metal(loid)s ternary system positively contribute to the high [Zn] in PL, while this was not observed in NPL. Our findings provide a comprehensive insight into the Zn isotope signature in response to the conversion of land-use types, which is beneficial for understanding the terrestrial Zn geochemical cycle.