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

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. To test the long‐term importance of salmon subsidies to riparian ecosystems, we measured soil nitrogen cycling in response to a 20‐yr manipulation where salmon carcasses were systematically removed from one bank and deposited on the opposite bank along a 2‐km stream in southwestern Alaska. Soil samples were taken at different distances from the stream bank along nine paired transects and measured for organic and inorganic nitrogen concentrations, and nitrogen transformation rates. Marine‐derived nitrogen was measured using ¹⁵N/¹⁴N for bulk soils, and NH4+">𝑁𝐻+4NH4+ and NO3−">𝑁𝑂−3NO3− 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 nitrogen isotope fractionation in soils that raises significant methodological issues with standard MDN assessments in riparian systems. Surprisingly, despite 20 yr of salmon supplementation, the presence of MDN did not cause a long‐term increase in soil nitrogen availability. This finding indicates the importance of MDN to ecosystem nitrogen biogeochemistry, and riparian vegetation may be overestimated for some systems. 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.