| Summary: | Floodplain vegetation communities are mosaics of succession stages caused by erosion/redeposition as river channels migrate throughout their floodplains. Typically, plants colonizing alluvial deposits are severely N-limited, but N accumulates during succession, and this process determines long term fertility. The Kol River (Kamchatka, RU), received large annual N-subsidies from salmon and we sought to determine how salmon-N contributed to fertility during succession. We constructed a vegetation chronosequence model and made N-fertility measurements within replicate succession stages before, during and after salmon runs. Natural abundance of 15N was used as a tracer of salmon-N. We found that new alluvial deposits were N-poor, containing <200 kg-N ha-1 (to 10cm). However, soil-N increased more than 10X within 30 years and soils were N-rich henceforth. Net N-mineralization on young alluvial bars only provided a small fraction of the colonizing forest’s N requirement, whereas soils in older forests provided abundant N. Negative correlation between foliar C:N and soil N during the first 20 years of succession indicated that the youngest succession stages were N-limited. The salmon run commenced in midsummer and caused river water N to increase 3X. Subsequent late season flooding deposited an average of 25 kg-N ha -1 as salmon carcasses onto young alluvial bars during 2006, but deposition rates were 10 to 30X higher on other years. The N-pulse created by decomposing salmon on young alluvial bars was brief because subsequent flooding flushed these coarse soils, but colonizing willows assimilated N rapidly during this time and high foliar δ15N (3 to 5‰) confirmed that salmon were a major N-source in early succession. Foliar δ 15N was abnormally high throughout the floodplain (1 to 5‰) indicating that older forests recycled salmon-N that accumulated during early succession. All plant species that occurred during the first several centuries of succession had N-rich foliage (mean C:N 12 to 22). We conclude that salmon fertilized otherwise N-poor early succession annually, and built ecosystem N-pools during early succession, thereby allowing nitrophilic plant species to proliferate.
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