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 and plant samples were collected relative to these treatments until the onset of snow and from snow melt to one year post treatment. Nutrient “hot-spots” developed in the soil as the treatments decomposed. Prior to treatment, the inorganic N (ammonium + nitrate) concentration of surface soil (0-10cm) was 3.1 μg/g ± 0.1 SE. Thirty days following nutrient addition inorganic N concentration beneath the amendments had significantly increased (p<0.002) to 1473 μg/g ± 530 SE and 1256 μg/g ± 140 SE for the carcass and analog treatments respectively. Inorganic N concentrations began to decline following snow melt with the onset of the growing season and had returned to near background concentrations by one year post treatment. Dissolved organic carbon (DOC) increased significantly (p=0.006) for both treatments from 116 μg/g ± 10 SE to 462 μg/g ± 73 SE two weeks post treatment and remained elevated through the onset of snow. Following snow melt DOC for both treatments had declined to background concentrations. However, soil respiration measurements two years post treatment found significantly (p=0.005) elevated respiration rates suggesting that microbial activity remained high. The lack of elevated N pools in conjunction with this elevated respiration and evidence of rapid vegetative N utilization would suggest that supply and demand had reached a steady state. Evidence of vegetative N utilization was derived from foliar isotope analysis of conifer seedlings with a significant (p=0.005) enrichment in foliar δ15N from -0.3‰ ± 0.06 SE pre-treatment to 4.7‰ ± 0.03 SE one year post treatment. Further a significant shift in foliar %N was not observed suggesting the N uptake responsible for the shift in δ15N was quickly converted to additional biomass. Isotopic evidence from this and other studies supports that additional riparian plant species may also benefit from anadromous fish in a similar manner. This 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.