Drake Deanne C, Naiman I Robert J, Bechtold J Scott
School of Aquatic and Fisheries Sciences, University of Washington, Seattle 98195, USA.
Ecology. 2006 May;87(5):1256-66. doi: 10.1890/0012-9658(2006)87[1256:fonirf]2.0.co;2.
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.
我们将一种(^{15}N - NH_4^+)示踪剂引入到一条有鲑鱼的溪流（美国华盛顿州肯尼迪溪）的河岸森林中，以量化鲑鱼氮素在季末脉冲中的循环和归宿，并最终确定调节鲑鱼数量与树木生长之间潜在联系的机制。(^{15}N)示踪剂模拟了向四个(50)平方米的地块中投放(7.25)千克鲑鱼（鲜重）的沉积情况。2003年10月下旬，正值当地鲑鱼产卵期，我们向土壤中添加了(NH_4^+)（鲑鱼尸体分解的初始产物）以及尸体提供的其他重要养分（磷、硫、钾、镁、钙）。我们对(^{15}N)示踪剂在土壤和树木中的情况进行了为期一年的追踪。(^{15}N)示踪剂的生物吸收很快：在14天内，(64%)的(^{15}N)示踪剂被土壤微生物群落固定，而主要河岸树种西部红雪松（Thuja plicata）的根系在7天内就开始吸收(^{15}N)示踪剂。根系吸收在整个冬季持续进行。施用后五周，土壤有机质中的(^{15}N)示踪剂含量最高达到约(52%)，五个月内达到约(40%)的相对平衡。添加六个月后，即2004年春季，至少(37%)的(^{15}N)示踪剂存在于树木组织中：约(23%)在叶片中，约(11%)在根系中，约(3%)在茎干中。在茎干中，木质部和韧皮部汁液中含有约(96%)的示踪氮，约(4%)在结构性木质部氮中。一年后，至少(28%)的(^{15}N)示踪剂仍存在于树木中，地块中的损失仅约(20%)。秋季吸收的大部分示踪氮在次年春季重新分配到叶片和茎干中，这为树木对鲑鱼养分的一年滞后生长反应提供了机制证据。几个世纪以来，每年秋季鲑鱼养分都会沉积在肯尼迪溪系统中，但该系统没有出现养分饱和的迹象。氮的吸收和保留速率是场地历史和干扰的函数，也可能是遗留效应的结果，即每年添加鲑鱼养分可能导致养分吸收和利用效率提高。
