Spatial Distributions of Soil Chemical Conditions in a Serpentinitic Wetland and Surrounding Landscape

摘要

Soils formed from serpentinite contain an abundance of Fe, Mn, Cr, Ni, and Mg, and low concentrations of the plant-essential nutrients Ca and K. The resulting vegetation is commonly xeromorphic and characteristically stunted. This study was conducted to (i) determine the spatial distributions of heavy metals and exchangeable cations (M_e) in an ultramafic wetland and surrounding landslide terrain, and (ii) to interpret the distributions relative to environmental conditions and pedogenic processes on the component landscape positions. Distributions of dithionite-extractable metals (M_d) and M_e in surface soils (0–15 cm depth) were assessed by kriging and by landscape units, characteristic landscape position, soils, and vegetation. Abundance of M_es ranked in the following order: Mg > Ca K > Mn > Na > Ni. The Ca/Mg ratios range from 0.13 to 3.77 (mean 0.43), with the highest ratios in a landscape unit with nonserpentine metamorphic colluvium over serpentinitic residuum. Exchangeable cations are concentrated within the wetland relative to surrounding terrain. Dithionite-extractable Fe, Mn, and Ni are concentrated in soils on the oxidizing, nonhydric lower landscape positions, near the hydrologic discharge point of the wetland. Chromium and Al are concentrated in the nonhydric upper landscape positions. Due to reducing conditions, the wetland contains low concentrations of M_d relative to the surrounding nonhydric terrain. Large vegetation differences between moisture class coupled with moderate vegetation differences between landscape units within the same moisture class, suggest that vegetation occurrence within the study area is controlled primarily by hydrology, and secondarily by elemental conditions.