Potential influence of poorly crystalline minerals on soil chemistry in Podzols of southwestern Canada

Concepts of soil chemistry were established, in the main, for agricultural soils with a fine texture and near-neutral pH. These concepts often need to be modified when considering acidic forest soils. For example, studies conducted in northeastern US forests challenged the idea of a universal positive relationship between base saturation (Bsat), effective cation exchange capacity (CECe) and pH. The objective of this study was to measure soil chemistry variables in Podzols of southwestern Canada and determine their interrelationships as well as the potential influence of surface-active iron (Fe) and aluminium (Al) species. We measured exchangeable cation concentration in each pedogenic horizon and investigated their relation to pH, soil organic carbon, silt and clay and pyrophosphate-extractable, oxalate-extractable and dithionite-extractable Al and Fe. We found that the chemistry of the forest floor was different from that of the mineral soil because of the limited extent of mixing between the two layers. In the forest floor, biological cycling maintained a large Bsat and large calcium saturation but a low pH. In the subsoil, pH had a positive correlation with Bsat but a negative correlation with CECe. The formation of organo-mineral complexes between soil organic matter and short-range order Al and Fe phases could explain the anomalous relationship between pH and CECe. This study provides new insight into mechanisms controlling forest soil chemical properties and should hence contribute to our ability to manage forests for long-term productivity.