Pedogenic carbonate in Chernozemic soils and landscapes of southeastern Saskatchewan

Abstract

The objective of this study was to characterize soil carbonates and estimate their accumulation by using a method based on δ13C values of soil carbonate in pedons and landscapes. Dark Brown Chernozem soils of the Amulet Association (with mixed C3 and C4 plants) and native Black Chernozem soils of the Oxbow Association (with a pure C3 plants) were included. Six soil samples representing the Cca and Ck horizons of the Oxbow and Amulet Associations were chosen for detailed pedogenic carbonate studies, which include particle size fractionation, mineralogical identification, surface morphology examination, and stable carbon isotopic compositions analysis on the particle size fractions. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses of the fractionated particle sizes indicated that dolomite dominates the carbonate occurring in the Ck horizons, and is mainly in medium and coarse silt fractions. In the Cca horizons, carbonate in clay and fine silt fractions is mostly calcite, whereas dolomite dominates the carbonates in medium and coarse silt fractions. Calcitic coatings on the surfaces of both silt and sand particles were observed. Stable carbon isotope ratio (δ13C value) of carbonate was -8% in the clay fraction of Cca horizons in which carbonate is dominated by calcite. Carbonate in the fine particle size fractions of the Cca horizons is dominated by calcite and has the most depleted δ13C values, indicating that pedogenic carbonate occurs as calcite and mainly in the fine particle size fractions. In contrast, carbonates from coarse silts of Ck horizons were composed solely of dolomite and had the most enriched δ13C values (-1.1 to -1.4%), a character of lithogenic carbonate. On a landscape basis, carbonate occurs mainly in the Cca horizons in mid- and upper slope soils as a result of moisture movement and ground water fluxes. Soils in the depressions were more leached and contained small amounts of carbonate, mostly in the 60- to 90-cm depth. Calculations from the δ13C values of soil carbonates indicate that about 50-68% of the carbonate in the Cca horizons of middle slope soils is of pedogenic origin, which is equivalent to a CaCO3 storage of 136-188 kg m-2. In contrast, 5 to 39% (0-36 kg m-2 CaCO3) of carbonate stored in the lower slope soils is of pedogenic origin. Thirty-one to fifty percent of the carbonate (equivalent to 100-134 kg m-2 CaCO3) in upper slope soils is of pedogenic origin.