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

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. 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 2km 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 15 N/ 14 N for bulk soils, and NH þ 4 and NO À 3 soil pools. Stable isotope analyses confirmed 15 N/ 14 N was elevated on the salmon-enhanced bank compared to the salmon-depleted bank. However, 15 N/ 14 N values of plant-available inorganic nitrogen exceeded the 15 N/ 14 N 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.

Spawning salmon density influences fruit production of salmonberry (Rubus spectabilis)

“…Continued) v www.esajournals.org have rhizomes as long as 5 to 18 m, depending on the forest type (Tappeiner et al 1991). Furthermore, previous research has demonstrated that nutrient availability in riparian areas can be highly variable within a watershed, regardless of the presence of salmon-derived nutrients (Feddern et al 2019). In salmon-bearing streams, bears are primary drivers of changes in nutrient cycling, while adding to the spatial inequality of carcass deposition (Holtgrieve et al 2009).…”

Links between fluctuations in sockeye salmon abundance and riparian forest productivity identified by remote sensing

“…2002, Feddern et al. 2019). In the southern distribution of Pacific salmon in North America, most salmon populations spawn in the late summer or early fall, near the end of the active growing season.…”

“…2011, Feddern et al. 2019). In general, additions of nitrogen fertilizer increase the availability and accumulation of nitrogen in soils, but concurrent increases in rates of nitrogen emission and leaching limit the benefits of fertilization to plant uptake over time (Lu et al.…”

“…Over a longer timescale, soils can accumulate nitrogen after successive years of salmon spawning events (Morris and Stanford 2011; but see Feddern et al. 2019), suggesting that saturation of nitrogen in soils might limit the interannual response of riparian plants to fluctuations in salmon abundance. Given that growing season varies with latitude and that the response of riparian vegetation to salmon is likely mediated by local factors such as geomorphology and historical salmon returns, the degree to which plants respond to salmon between geographic locations and within and among years remains generally unclear.…”

“…While stable isotopes can be helpful tracers of marine‐derived nutrients, their interpretation can be complicated by biogeochemical and micro‐topographical differences between study sites, as well as by isotope fractionation processes in soils (Feddern et al. 2019, Walsh et al. 2020).…”