An assessment of the relationship between potential chemical indices of nitrogen saturation and nitrogen deposition in hardwood forests in southern Ontario |
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Authors: | Shaun A Watmough |
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Institution: | 1. Environmental and Resource Studies, Trent University, 1600 West Bank Drive, Peterborough, ON, K9J 7B8, Canada
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Abstract: | Southern Ontario receives the highest levels of atmospheric nitrogen (N) deposition in Canada and there are concerns that forests in the region may be approaching a state of ‘N saturation’. In order to evaluate whether potential chemical indices provide evidence of N saturation, 23 hardwood plots were sampled along a modeled N-deposition gradient ranging from 9.3 to 12.8 kg/ha/year. All plots were dominated by sugar maple (Acer saccharum Marsh.) and foliar N and foliar δ15N were positively correlated with modeled N deposition. However, forest floor N content and the C:N ratio were unrelated to N deposition, but were instead related to soil pH and annual temperature; lower C:N ratios and higher N content in the forest floor were found at the most acidic sites in the cooler, northern part of the study region despite lower N deposition. Likewise, δ15N values in surface mineral soil and the 15N enrichment factor of foliage (δ15N foliage ? δ15N soil) are correlated to soil pH and temperature and not N deposition. Further, potential N mineralization, ammonification, and nitrification in Ontario maple stands were highest in the northern part of the region with the lowest modeled N deposition. Nitrogen cycling in soil appears to be primarily influenced by the N status of the forest floor and other soil properties rather than N deposition, indicating that chemical indices in soil in these hardwood plots may not provide an early indicator of N saturation. |
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