Chernozemic soils of Canada: Genesis, distribution, and classification

Abstract

Pennock, D., Bedard-Haughn, A. and Viaud, V. 2011. Chernozemic soils of Canada: Genesis, distribution, and classification. Can. J. Soil Sci. 91: 719–747. Chernozemic soils in Canada have a characteristic biomantle that fully expresses the effect of organisms on soil formation. Additions of large amounts of below-ground biomass from grasses are transformed into complex organic compounds through the activities of meso- and macro-fauna, microbial degradation and combustion by fires. Degradation is regulated by (a) climatic influences on plant inputs and microbial activity, (b) the chemical and biochemical nature of the residues, (c) encapsulation of organic matter within aggregates by soil micro-faunal activities and freeze–thaw processes and (d) protection against decomposition by Ca2+and clay minerals. These organic compounds are mixed with the mineral matrix through the action of organisms from mites to badgers. Regional differences in the regulators cause differences in soil organic carbon (SOC) storage and the colour value of the surface Chernozemic A horizon. The storage of SOC is lowest in the Brown soil zone (≈60 to 80 Mg ha−1) and greatest in the Black soil zone (≈120 to 150 Mg ha−1); this corresponds to a decrease in the annual water deficit from ≈200 mm (Brown) to 70 to 100 mm (Black). Where soil CaCO3contents are high either through initial concentration in the parent material or by the precipitation of secondary CaCO3, substantially higher SOC storage than the regional norms can result. A repetitive catenary pattern occurs throughout the region. The primary controls on this pattern are hydrological – a lateral component to water flow in hillslopes leads to more developed horizonation downslope, and discharge surrounding wetlands causes precipitation of secondary carbonate minerals and more soluble salts in a fringe surrounding the wetlands. Chernozemic landscapes have been highly altered by humans through their conversion to agricultural production. Loss of the dense root network of the native grasslands causes a substantial decrease in SOC. This loss of carbon and reduction in A horizon thickness is accelerated by erosion; the effects of tillage erosion are now recognized as being ubiquitous through the agricultural region. The substantial amounts of SOC storage and our ability to increase storage through altered management practices make these soils a particular focus of interest in a future made more uncertain by the possibility of human-induced climatic change.