The Importance of Marine Nutrient Subsidies in Mountainous Riparian Forests

Prior to blockage of fish passage in the Columbia River Basin migration of anadromous fish from the Pacific Ocean would have brought with it large stores of marine derived nitrogen (N) and carbon (C) to central Idaho, USA. In a region dominated by nutrient poor soils and complex topography, anadromous fish may have been a primary nutrient input to riparian forests. To examine the importance of this subsidy, anadromous fish carcasses and analog pellets (a carcass substitute) were placed within riparian forests of a watershed void of anadromous fish for nearly a century. Soil samples 0-10cm collected beneath these treatments revealed rapid development of nutrient “hot-spots”. Within 10 days of treatment, inorganic N (NH₄⁺ + NO₃^-) significantly (p<0.05) increased from 2.2 mg/kg ± 0.3 SE to 1245 mg/kg ± 75 SE and dissolved organic carbon (DOC) significantly (p<0.05) increased from 205 mg/kg ± 27 SE to 510 mg/kg ± 30 SE. DOC and IN concentrations began to decline following snow melt and had returned to near background concentrations by one year post treatment. However, soil respiration measurements two years post treatment found significantly (p<0.05) elevated respiration rates suggesting microbial activity remained elevated. Lack of elevated C and N pools in conjunction with elevated respiration and isotopic evidence of rapid vegetative N utilization would suggest that supply and demand had reached a steady state. Foliar isotope analysis of conifer seedlings found significant (p<0.05) enrichment in δ¹⁵N from ­0.3‰ ± 0.06 SE pre-treatment to 4.7‰ ± 0.03 SE one year post treatment. Foliar %N did not significantly increase suggesting N uptake responsible for the shift in δ¹⁵N was quickly converted to additional biomass. Rapid utilization of N as well as the magnitude of soil C and N responses to these treatments demonstrates the vital nature of this nutrient subsidy to oligotrophic systems.