Fluxes of dissolved organic matter and nitrate and their contribution to soil acidification across changing permafrost landscapes in northwestern Canada

Climate warming is predicted to change the fluxes of dissolved organic matter and nitrate by increasing active layer thickness, plant productivity, and organic matter decomposition. These changes are hypothesized to increase mineral weathering and soil acidification rates. We investigated whether acidification rates and solute leaching fluxes are variable between permafrost-affected soils with different active layer thicknesses. We compared the fluxes of dissolved organic carbon (DOC) and total dissolved N and the ion fluxes associated with solute leaching and plant uptake to calculate proton budgets in the soils that differed in slope positions (upper slope and lower slope) and soil texture (clayey and sandy soils) in NW Canada. We found the wide variation in DOC and nitrate-N fluxes, depending on slope positions and soil texture. The nitrate-N fluxes were higher at the lower slope position of the sandy soil, compared to the upper slope position. Compared to sandy soils, the DOC fluxes from the organic horizons were higher in the clayey soils on shallower permafrost table. Organic acids were major proton sources in the organic horizons at all sites, but acidity was also contributed by nitrate (sandy and clayey soils at lower slope position) and carbonic acid (clayey soil at upper slope position). The weathering by carbonic acid lead to accumulation of short-range-order minerals in the clayey soils, while incipient podzolization of the sandy soils included the weathering of illite to vermiculite and the dissolution of short-range-order minerals. The shallower active layer thickness at the lower slope position resulted in lower plant productivity and acidification rates. In the permafrost-affected soils, the active layer thickness, the DOC and nitrate-N fluxes, and their contribution to acidification are dependent on the local variation in slope position and soil texture, as well as climate change.