Nitrogen deposition in forest and steppe in western Canada

Köchy, M., and S. D. Wilson (2001)

1. Atmospheric nitrogen (N) deposition has become the most important agent of vegetation change in densely populated regions. It may also contribute to forest expansion in grasslands due to its different effects on grasses and trees.
2. We measured N deposition and available soil N with ion-exchange resin over 2 yr in western Canada in six national parks in regions varying in population density and industrialization. N deposition was significantly higher in four parks close to agro-industrial land-use than in two remote parks surrounded by forest.
3. Available soil N was highest in the high-deposition parks and lowest in the low-deposition parks. Available soil N increased significantly with N deposition across all parks.
4. We measured the local variation of N deposition and availability within Elk Island, the park with the highest rate of N deposition. Locally, temporal variation of N deposition and soil N availability was much greater than the effects of soil type, fire history, grazing, or vegetation type.
5. We measured N mass and ¹⁵N:¹⁴N ratios (d¹⁵N) in vegetation and soil in two parks: Elk Island, receiving 22.1 kg N ha^-1 yr^-1, and Jasper, receiving 7.77 kg N ha^-1 yr^-1. Mass of N in vegetation per area increased by 74% in Elk Island but only by 26% in Jasper within five decades, due to forest expansion into grassland. d¹⁵N in forest vegetation was significantly lower in Elk Island than in Jasper, suggesting that anthropogenic sources contribute significantly to the high rates of N entering that ecosystem.
6. We determined the rate of forest expansion within the parks from aerial photographs. We found a strong positive relationship with N deposition. The strong relationships between N deposition, available soil N, N mass in vegetation, and forest expansion suggest that even comparatively low rates of N deposition accumulate over time and affect ecosystem composition on the landscape scale. Forest expansion into cold-temperate grasslands is usually attributed to fire control and elimination of bison. We conclude that it may be accelerated by fertilization from atmospheric N deposition.