Effect of colonizing tree species on soil nutrient availability in a clay soil of the boreal mixedwood
Soil nutrient availability was assessed around stems of trembling aspen (Populustremuloides Michx.), white spruce (Piceaglauca (Moench) Voss), and paper birch (Betulapapyrifera Marsh.) that regenerated after fire 49, 77, and 123 years ago on a clayey soil of the southern boreal forest. For all stand ages, forest floor pH was greater by 0.5 unit under aspen than under other species, while the accumulation of organic matter and nutrients was generally greater in the forest floor of spruce. With time since fire, forest floor pH and mineral soil reserves of nitrogen (N) and exchangeable calcium declined significantly, while the C/N ratio increased, perhaps as a result of nutrient immobilization in the aggrading biomass. Net N mineralization and nitrification, measured by aerobic laboratory incubations, were higher under birch and aspen than under spruce in the forest floor and the mineral soil of the youngest stand (49 years old). However, in older stands, these parameters were significantly lower, and no significant differences were found between soils of deciduous and coniferous species. These results suggest that a decrease in N availability with time since fire was caused by factors other than the generally assumed succession to coniferous trees.