Riparian soil nitrogen cycling and isotopic enrichment in response to a long-term salmon carcass manipulation experiment

Ecosphere
By: , and 

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Abstract

Pacific salmon acquire most of their biomass in the ocean before returning to spawn and die in coastal streams and lakes, thus providing subsidies of marine-derived nitrogen (MDN) to freshwater and terrestrial ecosystems. Recent declines in salmon abundance have raised questions of whether managers should mitigate for losses of salmon MDN subsidies. The ecological importance of salmon subsidies for direct consumers (e.g., bears, birds, insects) is well established; however, many studies examining the broader role of salmon nutrients have documented the presence of MDN, without demonstrating its ecological importance. This knowledge gap is particularly pronounced in terrestrial systems. To test the long-term importance of salmon to riparian ecosystems, a 20-year manipulation was performed where salmon carcasses were systematically removed from one bank and deposited on the opposite bank along the full length of a 2 km stream in southwestern Alaska. We examined the long-term effect of this manipulation on riparian soil fertility. Soil samples were taken from nine paired transects along the stream at distances 1, 3, 6, 10, and 20 m from the bank and measured for organic and inorganic nitrogen concentrations ([NH4+], [NO3-], [NOrg]) and nitrogen transformation rates (net mineralization and net nitrification). The potential presence of MDN was also measured using 15N/14N for bulk soils as well as NH4+ and NO3- soil pools. Stable isotope analyses confirmed 15N/14N was elevated on the salmon enhanced bank compared to the salmon depleted bank. However, 15N/14N values of plant-available inorganic nitrogen exceeded the 15N/14N of salmon inputs, highlighting N isotope fractionation in soils that raises significant methodological issues with standard MDN assessments. Surprisingly, despite 20 years of salmon supplementation, the presence of MDN did not cause a long-term increase in soil N availability measured immediately prior to salmon return during the peak vegetative growth season. This finding raises questions about the contribution of MDN subsidies to riparian vegetation and indicates its importance to ecosystem N biogeochemistry may be overestimated. Our findings further suggest that other site factors such as soil moisture and stand demography may influence riparian forest growth. Given that essential nutrients can also be pollutants, we urge more critical analyses of the role of MDN to inform compensatory mitigation programs targeting salmon nutrient enhancement.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Riparian soil nitrogen cycling and isotopic enrichment in response to a long-term salmon carcass manipulation experiment
Series title Ecosphere
DOI 10.1002/ecs2.2958
Volume 10
Issue 11
Year Published 2019
Language English
Publisher Ecological Society of America
Contributing office(s) Forest and Rangeland Ecosystem Science Center
Description e02958