Soil, pH and N availability effects on net nitrification in the forest floors of a range of boreal forest stands

Abstract

Net nitrification in forest soils is potentially problematic as it can promote nitrogen and cation losses as well as soil acidification and favor competing vegetation in regenerating forests. Rates of net nitrification have been associated with vegetation and soil types. However, this phenomenon is difficult to predict. Our objectives were to describe the relationships between stand types and net nitrification in the forest floor of five stand types (Paper birch: Betula papyrifera, trembling aspen: Populus tremuloides, white spruce: Picea glauca, jack pine: Pinus banksiana, white cedar: Thuja occidentalis) typical of the boreal mixedwood of northwestern Quebec and to experimentally verify the effect of pH and ammonium availability as well as the occurrence of persistent allelopathic inhibitors on net nitrification rates. All samples came from well-drained lacustrine clay soils and all forest stands originated from wildfires. The net accumulation of nitrate and ammonium was measured during the course of 6-week laboratory incubations. Nitrate accumulation was highest under aspen and birch, low under white spruce and cedar and not detectable under jack pine. The forest floor of jack pine had the lowest pH and exchangeable bases and the widest C:N ratio. Ammonium addition did not promote a significant increase in net nitrification. Increase of forest floor pH had a positive effect on net nitrification while acidification depressed it. The absence of net nitrate production in jack pine forest floor appeared to be related to the absence of nitrifiers as it was stimulated by the addition of a small amount of nitrifying forest floor. Our results suggest that jack pine and aspen, which occupy the same habitat, may have the capacity to change soil conditions in order to favor or depress nitrification. Observed rates of nitrification under these species is coherent with their preference for a particular form of mineral N. Although pH appeared as an important regulator of net nitrification, the effects of other factors are yet to be established.