Nitrogen cycling in the upland boreal shield forest: Response to an experimental addition of nitrate.

摘要

The emission of N gases by industrial and agricultural activity has increased the load of N by several-fold to many forested ecosystems during this century. Following a long-term elevated N input, the demand for N by plants and soil microorganisms may be satisfied and the terrestrial ecosystem may reach a state of "N saturation". Nitrogen saturation can cause freshwater acidification and forest decline. Presently, boreal and temperate Shield catchments efficiently retain mineral N inputs (NH4+ and NO 3-) and buffer downstream acid-sensitive freshwaters from N-based acidification.; The objectives of this study were (1) to describe the N cycle in small upland boreal Shield catchments at the Experimental Lakes Area (ELA), northwestern Ontario, and (2) to study the processes involved in N retention in this system using an experimental addition of NO3- to one catchment. The ELA Upland catchments are representative of sparsely vegetated rocky ridges common throughout acid-sensitive areas of the Canadian Precambrian Shield.; Under unmanipulated conditions, several aspects of the Upland catchments N cycle contradicted the traditional view for the boreal forest. As was expected for an unproductive conifer forest, mineral N inputs were efficiently retained. However, overall the catchments leaked more N than expected because of the export of dissolved organic N (DON). There was a striking contrast in internal N cycling in the different components of the landscape.; The response of U3 to the NaNO3 treatment indicated a variable potential in time and space to retain the elevated N input. Nitrate retention was limited during the snowmelt period when biological retention mechanisms were less active. However, during the growing season N retention by U3 remained similar to reference catchments. Forest islands and lichen patches responded in opposite ways to the increased NO3- input. On bedrock surfaces, net nitrification rates doubled in lichen patches and by the second year of addition N was no longer retained. Although fast hydrological flushing and low biomass must have limited N retention on bedrock surfaces, the intrinsic N saturation of lichen patch soil microorganisms was determinant in preventing NO3- retention.; On the short-term, the upland boreal Shield landscape has a limited potential to prevent N-based acidification of downstream ecosystems because of a weak potential for N retention during a part of the year and the intrinsic N saturation of a portion of the landscape. (Abstract shortened by UMI.)