Effect of Climate Change on C:N Pools in a Boreal Watershed Ecosystem: 1993 Progress Report and 1994 Work Plan (Research Report 58)

Stottlemyer, Robert

The following report summarizes research findings, principally for Isle Royale (ISRO), from field and laboratory studies conducted June 1992, through June 1993. The data collected since have not been fully analyzed. Our study objective for 1993 was to assess if possible changes in the nitrogen cycle, as might occur from global climate scenarios for temperature and moisture, could complement inorganic precipitation nitrogen (N) inputs to the boreal forests of Isle Royale. Total atmospheric N input (>6 kg ha - yr -1) exceeds the ecological requirements of major forest species. A decade of ecosystem study shows strong N retention (92%). There are significant differences in mineral soil N pools (492, 564, and 1153 kg ha- l for birch, spruce, and alder respectively). Annual soil N03 - N concentrations varied with vegetation (p <0.05) being higher beneath alder (11.6 mg N kg-I) than spruce and birch (1.1 and 0.9 mg N kg- l respectively). Soil NH4 + concentrations marginally increased with time (p = 0.07), but did not significantly differ among vegetation types (5.4, 6.8, and 8.1 mg N kg- l for birch, spruce and alder). Field net N mineralization was inversely related (p <0.001) to soil temperature, but laboratory incubations showed a gain (p <0.03) in mineralization over temperatures from 100 to 200C. Field net N mineralization varied with incubation period (p < 0.01) and species (p = 0.05). Annual net mineralization was 35, 38, and 84 kg N ha -1 y -1 for birch, spruce, and alder respectively. There was no apparent relationship between field net mineralization, net nitrification, and initial mineral N concentrations and soil moisture. A comparison of most mineral N sources (vegetation blowdown, atmospheric inputs, herbivory) indicates that a 2 - 5° C increase in summer soil temperature would significantly increase available N more than from these other sources.