Evidence for solum recarbonation following forest invasion of a grassland soil

Calcareous organic-matter-rich Black Chemozemic soils often persist under p oplar forests in Saskatchewan, not acquiring the Ae horizons and related pr operties more characteristic of Gray Luvisol soils. These are Rego Black Ch ernozems, locally termed "Wooded Calcareous" because of the occurrence of a dark, calcareous horizon (AC) at depths of 10 to 30 cm. We hypothesize tha t dark, calcareous horizons are former Bm horizons that have been recarbona ted because of intense biocycling of Ca by aspen (Populus tremuloides). The comparatively small amounts of carbonate occur mainly in the fine silt and clay fractions, and are dominantly calcite, indicating secondary origin, i n comparison to the carbonate minerals of the Ck horizons where both calcit e and dolomite occur, the latter more common in coarse fractions. The delta (13)C values of the carbonate minerals indicate that virtually all the carb onate in fine fractions of the upper horizons of the Wooded Calcareous soil is pedogenic. The dominance of pedogenic carbonate in the Ahk and AC horiz ons of the Wooded Calcareous soil is consistent with a secondary enrichment , a probable result of increased biocycling of Ca where aspen grows on site s with large amounts of soluble Ca in the subsoil. High concentrations of s oluble Ca2+ and SO42- in the LFH of the Wooded Calcareous are consistent wi th increasing biocycling of these ions, from a gypsum-rich subsoil.