Document: JEN-3-33-28

Comparison of N inputs among forested ecosystems by two Alnus spp. within adjacent upland and floodplain landscapes of interior Alaska.

MITCHELL, J.S.*, R.W.RUESS and D.D.ULIASSI

University of Alaska Fairbanks, Fairbanks, AK, USA 99775 ¹

Abstract:
Nitrogen cycling in successional floodplain and upland ecosystems near the Tanana River in interior Alaska is affected by the presence of N₂-fixing Alnus tenuifolia and Alnus crispa, respectively. We compared species-specific effects on N cycling within these 2 landscape regions by measuring 1) rates of N₂ fixation using a modified acetylene reduction assay (ARA), and 2) nodule biomass. Measurements of soil temperature and soil moisture were taken concurrent with ARA assays during the course of 2-year field studies. Within floodplain forests, fixation by A. tenuifolia at midsummer averaged 24.98 7.91 mmol C₂H₄ g dwt nodule^-1 hr^-1 and 17.5 3.05 mmol C₂H₄ g dwt nodule^-1 hr^-1 within alder- and poplar-dominated stands, respectively. In contrast, midsummer fixation by A. crispa averaged 6.46 .47 mmol C₂H₄ g dwt nodule^-1 hr^-1 across upland stands, with little variation among ecosystem types. We combined values for specific ARA with measurements of nodule biomass to calculate annual N₂ fixation inputs by both species. N input by A. crispa was confined to, and calculated for, beneath shrub canopies (2.33 0.95 g N m^-2 yr ¹). When expressed at the landscape scale, these estimates drop by an order of magnitude due to low alder density (2647 1264 stems ha^-1). Higher alder stem densities (7,839 1226 stems ha^-1) and thus, less patchy nodule distribution yielded substantially higher stand-level estimates of N inputs in alder- (5.87 1.08 g N m^-2 yr ¹) and poplar-dominated (3.79 1.13 g N m^-2 yr ¹) floodplain forests. Soil temperature accounted for most of the seasonal variation in ARA for both species. We found no significant interannual variability in rates of N₂ fixation despite marked differences in total annual precipitation between the 2 sampling years for each species, suggesting that soil water availability played only a minor role in regulation of N₂-fixation. Differences in fixation rates between the two species appear to be genetic, and closely parallel other interspecific growth trait characteristics, such as growth rate, leaf nitrogen content, defensive chemical production, and tolerance/response to herbivory.

Keywords: Alnus Alaska nitrogen-fixation

This abstract is being presented at: 3:30 PM in session:
Poster Session #15: Nutrient Cycling.