The constraint of soil Zn isotope compositions by diverse land utilizations: Evidence from geochemical fingerprint in a typical karst area

Stable isotopes are increasingly employed to identify anthropogenic disturbances and geogenic processes of zinc (Zn) in terrestrial ecosystems. To reveal how land use affects the transformation of soil Zn, profile samples were collected from abandoned cropland (AC), grazing shrubland (GS), and natural land (secondary forest land, SF) in a typical karst area, and analyzed the Zn content and isotopic composition. Results exhibited that soil Zn contents in forest (SF: 72.50 ∼ 131.20 mg/kg) were more variable than those in agricultural lands (AC: 74.53 ∼ 112.20 mg/kg; GS: 48.26 ∼ 82.57 mg/kg). Rock weathering was the main contributor to soil Zn in each profile. Differences among profiles arose from partially assimilative Zn returning to the forest through plant debris, whereas leaching processes coupled with the consumption of crop production and livestock led to Zn loss. The δ66Zn varied in the largest range in SF (−0.26 ‰ to 0.04 ‰), and the extremums were both in the upper soil (0 to 80 cm). After the decomposition of plant debris, vegetation preferentially assimilated lighter δ66Zn, while retaining heavier δ66Zn in the surface soil. Moreover, the soil solution also had a lighter δ66Zn composition that leached downward. The narrowest range of δ66Zn composition was in AC (−0.14 ‰ to −0.02 ‰). The assimilation of crops exhausted the lighter δ66Zn fraction during cultivation, and the heavier δ66Zn fraction continued to be lost by leaching after stopping tillage. The δ66Zn composition ranged from −0.18 ‰ to 0.00 ‰ and decreased with depth in the GS. Animal excrement supplied unintentionally heavier δ66Zn compositions during grazing and obviously disturbed soil Zn behavior under the effects of vertical leaching. Both the content and isotopic composition showed that soil Zn dynamics were highly sensitive to agricultural activities. This study reveals a potential feedback mechanism for soil Zn behavior during agricultural disturbances, which is beneficial for optimizing land management in karst areas.