BASE CATION CYCLING IN A PRISTINE WATERSHED OF THE CANADIAN BOREAL FOREST

Abstract

Base cation cycling was investigated on the Canadian Shield in a pristine catchment of the boreal forest. An input–output budget (atmospheric deposition – stream exports) for the whole watershed indicated that nitrogen (N) was strongly retained, while sulphur (S) and base cations (K, Ca, Mg) were lost. The S losses are attributed to internal sources, such as SO4 desorption and/or mineralisation of soil organic-S. The base cation losses at the catchment scale, however, might be contributed by weathering of the till below the rooting zone, which has little ecological significance. For this reason, a detailed budget of basic cations, including atmospheric deposition, tree uptake, mineral weathering and leaching losses was constructed for the soil rooting zone of a stand representative of the forest covering the catchment. Different scenarios of tree uptake (whole tree and stem-only harvesting), weathering rates (PROFILE modelled values, PROFILE modelled values corrected for soil Na leaching) and soil leaching losses (depth of 22 and 81 cm) were considered. The element budgets were summed for the 1997–2003 (7 years) period. In every scenario, K was lost (−35.5 to −1.2 kg ha−1) from the exchangeable reservoir. Gain or losses of Ca were observable, depending of the scenario considered (28.8 to −20.1 kg ha−1) while Mg (−0.8 to 13.3 kg ha−1) generally accumulated over the 7-year period. The absolute K losses are very important, given the very small K soil exchangeable reservoir (55.2 kg ha−1) at the site. Because the amount of K sequestered in biomass (110 kg ha−1) is twice the amount found in the exchangeable soil reservoir, this forest is very sensitive to commercial forestry operations and forest harvesting appears as the main pathway of base cation losses. This raises important questions regarding the sustainability of the boreal forest that lies on K-poor soils of the Canadian Shield.