Patterns of nitrogen mineralization and nitrification in floodplain successional soils along the Tanana River, interior Alaska

Abstract

Cold climatic conditions govern the productivity of taiga forests, yet within a successional sequence the microclimatic and biogeochemical variations also have a major effect on soil microbial activities, thus affecting plant productivity through nutrient availability. Nitrogen mineralization and nitrification were measured in primary-successional floodplain forests of interior Alaska. Forest floor and mineral soils from an early (open willow), middle (poplar–alder), and late (white spruce) successional stage were used. The effects of temperature, moisture, and NH4+ were tested in the laboratory for each of the successional stages. Potential nitrification was estimated using the chlorate-inhibition technique. Surface mineral soils and white spruce forest floor had low to undetectable rates of nitrogen mineralization and nitrification (<1–3 μg N•g−1•d−1). The poplar–alder forest floor had the most pronounced seasonal patterns and the highest rates of net NH4+ mineralization (<1–7 μg N•g−1•d−1) and net nitrification (<1–21 μg N•g−1•d−1). Temperature was limiting in early and midsuccessional stages, and both moisture and temperature were limiting in the later white spruce stage. Ammonium additions increased nitrification only in the poplar–alder forest floor, suggesting the NH4+ is not limiting in the other successional stages. The chlorate inhibition assay indicated that a considerable portion of the nitrification in the poplar–alder forest floor may be due to heterotrophic activity.