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

Abstract

We introduced a 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 4, 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 redcedar (Thuja plicata), began to take up 15N tracer within 7 days. Root uptake continued through the winter. The 15N tracer content of soil organic matter reached a maximum of ~52%, 5 weeks after the application, and a relative equilibrium of ~40% within 5 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 1-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, disturbance, and may also be the result of a legacy effect, in which annual salmon nutrient addition may lead to increased efficiency of nutrient uptake and use.