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Fate of nitrogen in riparian forest soils and trees: an 15n tracer study simulating salmon decay

Drake, Deanne C., Naiman, Robert J., Bechtold, J. Scott
Ecology 2006 v.87 no.5 pp. 1256-1266
salmon, Oncorhynchus keta, dead animals, biodegradation, riparian forests, forest soils, forest trees, Thuja plicata, streams, nitrogen, stable isotopes, environmental fate, nutrient uptake, roots, leaves, stems, soil nutrients, nutrient availability, Washington
We introduced an ¹⁵N‐NH₄ ⁺ tracer to the riparian forest of a salmon‐bearing stream (Kennedy Creek, Washington, USA) to quantify the cycling and fate of a late‐season pulse of salmon N and, ultimately, mechanisms regulating potential links between salmon abundance and tree growth. The ¹⁵N tracer simulated deposition of 7.25 kg of salmon (fresh) to four 50‐m² plots. We added NH₄ ⁺ (the initial product of salmon carcass decay) and other important nutrients provided by carcasses (P, S, K, Mg, Ca) to soils in late October 2003, coincident with local salmon spawning. We followed the ¹⁵N tracer through soil and tree pools for one year. Biological uptake of the ¹⁵N tracer occurred quickly: 64% of the ¹⁵N tracer was bound in soil microbiota within 14 days, and roots of the dominant riparian tree, western red cedar (Thuja plicata), began to take up ¹⁵N tracer within seven days. Root uptake continued through the winter. The ¹⁵N tracer content of soil organic matter reached a maximum of ~52%, five weeks after the application, and a relative equilibrium of ~40% within five months. Six months after the addition, in spring 2004, at least 37% of the ¹⁵N tracer was found in tree tissues: ~23% in foliage, ~11% in roots, and ~3% in stems. Within the stems, xylem and phloem sap contained ~96% of the tracer N, and ~4% was in structural xylem N. After one year, at least 28% of the ¹⁵N tracer was still found in trees, and loss from the plots was only ~20%. The large portion of tracer N taken up in the fall and reallocated to leaves and stems the following spring provides mechanistic evidence for a one‐year‐lagged tree‐growth response to salmon nutrients. Salmon nutrients have been deposited in the Kennedy Creek system each fall for centuries, but the system shows no evidence of nutrient saturation. Rates of N uptake and retention are a function of site history and disturbance and also may be the result of a legacy effect, in which annual salmon nutrient addition may lead to increased efficiency of nutrient uptake and use.