FATE OF NITROGEN IN RIPARIAN FOREST SOILS AND TREES: AN¹⁵N TRACER STUDY SIMULATING SALMON DECAY

Abstract

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 approximately 52%, five weeks after the application, and a relative equilibrium of approximately 40% within five months. Six months after the addition, in spring 2004, at least 37% of the 15N tracer was found in tree tissues: approximately 23% in foliage, approximately 11% in roots, and approximately 3% in stems. Within the stems, xylem and phloem sap contained approximately 96% of the tracer N, and approximately 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 approximately 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.

Nitrogen assimilation and short term retention in a nutrient-rich tidal freshwater marsh – a whole ecosystem <sup>15</sup>N enrichment study

“…Especially deeper-rooted species, such as the dominant reed, likely assimilate N from deeper layers, which because of the short term nature of our study is much less enriched than the overlying water. Thus macrophyte retention is likely underestimated and macrophyte and tree tissues may be more important for long-term (months) retention (Drake et al, 2006).…”

The flocculation feedback loop: Delivery of marine derived nutrients in Pacific salmon streams.

“…Figure 30: Average carbon to nitrogen ratio for infiltration bag samples collected from the recirculating channel during a baseline period and then over 14 days following exposure to salmon + clay (n = 3, error bars represent one standard error of the mean) 102 Tables Table 1: Salmon-floc feedback loop stages, corresponding study parameters and their location within the thesis 9 Pacific salmon play a significant role in the nutrient cycle of their natal watersheds because they deliver substantial quantities of marine derived nutrients during spawning events (Bilby et al, 1996;Naiman et al, 2002;Schindler et al, 2003). The MDN delivered can support riparian zone vegetation and terrestrial organisms (Drake et al, 2006;Hocking and Reimchen, 2006), benthic macroinvertebrates (Chaloner et al, 2002), algae (Johnston et al, 2004), and fish populations (Naiman et al, 2002). The quantity of MDN delivered to spawning streams varies annually with the mass of spawning salmon that return.…”

The delivery, seasonal storage, and trophic transfer of marine-derived nutrients within a stream-riparian ecotone

“…Nitrification favors the oxygen-rich hyporheic zone, whereas the riparian zone has high potential for denitrification due to its anoxic environment (Casey et al 2001 (Kline et al 1990, Bilby et al 1996, Chaloner et al 2002, Hicks et al 2005, Scheuereii et al 2007. Drake et al 2006, Pinay et al 2009, Kiernan et al 2010.…”

“…Prior to senescence trees will reabsorb N from the leaves, so a decrease in total Nand an increased C:N is to be expected (Drake et al 2006 within 7 days and that 37% of the 15 N-NH/ tracer was contained within tree tissues after six months.…”

“…Despite an apparent contrast in run timing and plant growth, biological uptake of MDN can occur quickly and be stored within riparian soil and plants (Drake et al 2006) and stream organisms (Bilby et al 1996 ). There are still questions, however, especially in regard to the environmental factors that limit uptake and persistence of MDN in individual species and in the riparian forest as a whole (Drake et al 2011, Devries 2012, Rinella et al 2013 (Bilby et al 1996, Ben-David et al 1998, Reimchen et al 2002, Bilby et al 2003).…”