Nitrogen saturation in a high elevation New England spruce-fir stand☆

Abstract High rates of nitrogen (N) deposition were first postulated as a cause of N saturation (i.e. the availability of NH4N and NO3N in excess of total combined plant and microbial nutritional demand) and spruce mortality during the 1980s. To test this hypothesis, N addition plots were established in 1988, in a high elevation spruce-fir forest in southeastern Vermont, an area of relatively low N deposition (5.4 kg N bulk deposition ha−1 year−1). To test how the form of applied N may influence forest growth and N-cycling, four replicated treatment plots received either NH4ClN, NaNO3N, or a combination of both N forms at rates ranging from 15.7 to 31.4 kg N ha−1 year−1. The N additions were applied in three equal doses each year between June and August from 1988 to 1994. In addition to N treatments, two control plots were also established. Between 1988 and 1990, annual in situ net N mineralization and net nitrification in the forest floor, litterfall and forest floor mass and elemental concentration, foliar elemental concentration, and basal area growth by species were measured on each plot. In July 1994, basal area growth by species, net N mineralization potential and net nitrification potential in the forest floor, and foliar and forest floor elemental concentration were again measured on all plots. Inter-treatment and intra-treatment basal area growth changed substantially between 1988 and 1994. Spruce, fir, and birch trees on the N addition plots receiving 25 kg N ha−1 year−1 showed moderate rates of decline from 1988 to 1994. Numerous birch and maple sprouts were noted on the sites with the highest rates of decline, but no spruce or fir seedlings were observed. In July 1994, net N mineralization potential was highest on the control plots and net nitrification potential of the forest floor was highest on the plots receiving 15.7 kg N ha−1 year−1. A strong positive correlation existed between forest floor %N and net nitrification potential. Foliar %N was positively correlated with added N and negatively correlated with the change in net basal area growth. Foliar Ca:Al concentrations may also be negatively related to changes in net basal area growth. Our results suggest that N saturation has caused foliar nutrient imbalances on the N addition plots, and that the stands may be changing in species composition and structure. No long-term effects of N-form additions on N saturation and forest health were observed. Continued N additions may change the stands from a slow growing and slow N-cycling coniferous forest, to a fast N-cycling and fast growing deciduous forest.

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