Evaluating geomorphic and temporal factors in regulating nutrient availability from alder to streams in southwestern Alaska

Background/Question/Methods

Nitrogen (N)-fixing alder (Alnus spp.) provides key nutrient subsidies to ecosystems it grows in and adjacent to. The delivery of alder-derived nutrients to such ecosystems is modulated by geomorphic and temporal factors. In southwestern (SW) Alaska, alder is a prevalent constituent of the regional vegetation, and may contribute dissolved nutrients to adjacent aquatic systems. This importance is expected to increase as alder cover expands under climate warming, and as salmon harvesting reduces marine-derived nutrients (MDN) in salmon-spawning habitats. We investigate the roles of geomorphic and temporal factors in affecting alder-derived N availability in this region at 26 streams in 13 watersheds in the Togiak National Wildlife Refuge (TNWR) in SW Alaska. Measurements of alder cover and watershed features were extracted from satellite images of the TNWR in ArcGIS. Lysimeters were installed in soils under alder-dominated and non-alder communities to assess ammonium (NH₄⁺) and nitrate (NO_x) and leaching from alder. Water samples from streams were collected in the summer and fall from 2010 to 2012 for nutrient analyses, including NH₄⁺, NO_xtotal dissolved N (TDN) and total dissolved phosphorus (TDP). 

Results/Conclusions

In 2011 and 2012, alder patches leached approximately 25-35kg NO_x ha^-1 yr^-1 more than non-alder patches (F=29.2 and F=10.7 respectively, p<0.01). However, surface fluxes of NH₄ were 7.74kg NH₄-N ha^-1 yr^-1 lower in alder patches than non-alder patches (F=15.6, p<0.01) in 2012, likely due to high nitrification rates associated with inter-annual differences in temperature. Stream TDN and NO_x concentrations increase non-linearly where alder cover exceeds approximately 40%, indicating a possible threshold effect. The strength of this threshold effect increases from 2010 to 2012, probably because of the successively later timing of snowmelt in this region over the last three years. Stream N is strongly correlated with alder cover across seasons and years (NO_x; r=0.73 (alder), TDN; r=0.52 (alder), p<0.01, n=162). Stream N:P is positively related to alder cover (r=0.30, p<0.01, n=109), possibly reflecting that alder increases stream N availability and P limitation. Analysis of stream nutrient fluxes show similar relationships between N and alder (NOx; r=0.64, TDN; r=0.49, p<0.01, n=102). Measures of inter-annual differences in precipitation, temperature and growing season length will be analyzed to further assess the factors controlling variation in stream nutrient availability.