Ecosystem carbon gains from afforestation in the Boreal Transition ecozone of Saskatchewan (Canada) are coupled with the devolution of Black Chernozems

A field study was conducted to assess the long-term effects of growing trees on ecosystem and soil carbon (C) pools in a pasture landscape situated in the Boreal Transition ecozone of Saskatchewan that is characterized by Black Chernozemic soils. Tree species consisted of white spruce (Picea glauca (Moench) Voss) and Siberian larch (Larix sibirica Lebed.) and had been growing for 50 years before soils were sampled and analyzed for C and N concentrations, particle size distribution, and bulk density. Above- and belowground (roots) biomass were also determined respectively by allometry and soil washing. Ecosystem C gains in the forested plots were evident when compared to pasture plots. Most of this gain was attributed to the standing biomass and to lesser extent to forest floor development. However, C pools in the Black Chernozemic A horizons were decreased under trees and were apparently redistributed to the B horizons. Multiple linear models developed to explain C levels in the mineral soil at different depths indicated a significant shift from soil C being driven by root litter quality in the Ah horizons to being driven by root litter quantity in the B horizons. The model of C redistribution was consistent with other studies and is closely tied to root production and quality. Whether a new steady-state C level in the Chernozemic A horizons was reached still needs to be elucidated, but previous research in the region suggest that tree invasion on well-drained Black Chernozems could lead to “dechernozemization” through acidification and C loss.