A map-based comparison of chemical characteristics in the surface horizon of arable acid and non-acid sulfate soils in coastal areas of Finland

Reclamation of acid sulfate (AS) soils for agriculture causes severe environmental problems and may expose the human population to increased metal intake in some cases. This is due to drainage-induced oxidation of reduced sulfide-rich sediments, and subsequent acid formation and metal mobilization in the soil. Most of the mobilized elements are lost through leaching, but some metals may be transported upwards in the soil profile. In this study, we compared the concentrations of total N and C, electrical conductivity (EC), and easily available (AAAc/AAAcEDTA-extractable) Al, Cd, Cr, Cu, Fe, Ni, Pb, S, and Zn in the surface horizon (0-15 cm) of arable AS and non-AS soils, classified on a map basis (mb), in the whole coastal area of Finland covered by the Litorina Sea stage of the Baltic Sea. Our aim was to assess whether the surface horizon of AS soils poses a risk of increased metal bioavailability. The study was conducted by combining two regional-scale datasets, monitoring data on the chemical status of Finnish cultivated soils, and a probability map of occurrence of AS soils in coastal areas of Finland. Mean total N and C concentration was 40% and 56% higher, respectively, in fine-textured AS(mp )soils, and 20% and 18% higher, respectively, in clay AS(mp) soils, than in the corresponding non-AS(mp) soils, although in clay soils the differences were not statistically significant. Electrical conductivity and easily available S and Ni concentrations were significantly higher in AS(mp) soils than in non-AS(mp) soils (29%, 56%, and 106% higher, respectively, in fine soils; 72%, 61%, and 47% higher, respectively, in clay soils). Mean Fe, Cr, Cu, and Zn concentrations were higher in AS(mp) than non-AS(mp) soils, but the difference was statistically significant only for Zn in clay soils (63%) and Cr in fine soils (68%). Mean Al, Cd, and Pb concentrations did not differ between AS(mp) and non-AS(mp) soils. These results are consistent with earlier findings in more spatially restricted studies. Enrichment of easily available metals in the surface horizon of AS soils could affect the quality of crops grown on these soils, an issue requiring further study.