NITROGEN SATURATION IN A HIGH-ELEVATION NEW-ENGLAND SPRUCE-FIR STAND

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 o f total combined plant and microbial nutritional demand) and spruce mo rtality during the 1980s. To test this hypothesis, N addition plots we re established in 1988, in a high elevation spruce-fir forest in south eastern 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 in fluence forest growth and N-cycling, four replicated treatment plots r eceived either NH4Cl-N, NaNO3-N, or a combination of both N forms at r ates ranging from 15.7 to 31.4 kg N ha(-1) year(-1). The N additions w ere applied in three equal doses each year between June and August fro m 1988 to 1994. In addition to N treatments, two control plots were al so established. Between 1988 and 1990, annual in situ net N mineraliza tion 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 an d net nitrification potential in the forest floor, and foliar and fore st floor elemental concentration were again measured on all plots. Int er-treatment and intra-treatment basal area growth changed substantial ly between 1988 and 1994. Spruce, fir, and birch trees on the N additi on plots receiving < 20 kg N ha(-1) year(-1) had the highest rate of g rowth between 1988 and 1990 and then had the highest rate of decline b etween 1991 and 1994, Spruce, fir, and birch trees on the N addition p lots receiving > 25 kg N ha(-1) year(-1) showed moderate rates of decl ine 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 seed lings were observed. In July 1994, net N mineralization potential was highest on the control plots and net nitrification potential of the fo rest 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 n et nitrification potential. Foliar %N was positively correlated with a dded N and negatively correlated with the change in net basal area gro wth. Foliar Ca:Al concentrations may also be negatively related to cha nges in net basal area growth. Our results suggest that N saturation h as caused foliar nutrient imbalances on the N addition plots, and that the stands may be changing in species composition and structure. No l ong-term effects of N-form additions on N saturation and forest health were observed. Continued N additions may change the stands from a slo w growing and slow N-cycling coniferous forest, to a fast N-cycling an d fast growing deciduous forest.