Spawning salmon alleviate terrestrial nitrogen limitation by fertilizing early succession in a large river floodplain

Up to five million Pacific-salmon spawn per year in the Kol River (Kamchatka, RU), contributing 270,000 kg N yr^-1 to this large (7^th order) floodplain river. We sought to determine how this N subsidy fertilizes soils and vegetation during a 100-year succession chronosequence. We found that prior to the salmon migration, soils of early succession alluvial bars (0–5 yrs) were N-poor, containing 200 kg total N ha^-1 (to 10 cm depth). Net mineralization was also low, less than 0.05 kg N ha^-1 day^-1. In contrast, 80 year-old forest stands were N-rich; total mineral soil N and net mineralization were an order of magnitude higher.  Thus, as is in most terrestrial ecosystems, early succession should have been strongly N-limited. However, during September, flooding deposited salmon carcasses onto young alluvial bars, thereby contributing up to 150 kg of labile N ha^-1 yr^-1 to these otherwise infertile sites. Using sequential whole-plant harvests, we found early succession willows assimilated N and doubled in mass during carcass decomposition, even though this N-pulse occurred late in the growing season. Early succession foliage was enriched with the ¹⁵N isotope (d¹⁵N: 2 to 6, depending on species) relative to terrestrially fixed N (d¹⁵N: -1), confirming salmon were a major N-source in early succession. Vegetation across this chronosequence was composed of fast growing, N-rich (foliar C:N = 8 to 14 versus average for temperate forests: 35) species. Furthermore, foliar C:N ratios were consistently low regardless of soil N availability. Thus, we concluded vegetation growth was typically not N-limited.