The fate of Zn in agricultural soils: a stable isotope approach to anthropogenic impact, soil formation and soil-plant cycling

The supplementation of Zn to farm animal feed and the excretion via manure leads to an unintended Zn input to agricultural systems, which might compromise the long-term soil fertility. The Zn fluxes at three grassland sites in Switzerland were determined by a detailed analysis of relevant inputs (atmospheric deposition, manure, weathering) and outputs (seepage water, biomass harvest) during one hydrological year. The most important Zn input occurred through animal manure (1,076 to 1,857 g ha-1 yr-1) and Zn mass balances revealed net Zn accumulations (456 to 1,478 g ha-1 yr-1). We used Zn stable isotopes to assess the importance of anthropogenic impacts and natural long-term processes on the Zn distribution in soils. Soil-plant cycling and parent material weathering were identified as the most important processes, over the entire period of soil formation (13,700 years), while the soil pH strongly affected the direction of Zn isotopic fractionation. Recent anthropogenic inputs of Zn only had a smaller influence compared to the natural processes of the past 13,700 years. However, this will probably change in the future, as Zn stocks in the 0-20 cm layer will increase by 22% to 68% in the next 100 years, if Zn inputs remain on the same level as today