An Integration of Forest and Lake Fertilization: Transport and Transformations of Fertilizer Elements

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Abstract
Forest fertilization (435 kg urea∙ha−1) on the Mohun drainage, northern Vancouver Island, during late fall 1979 resulted in a combined increase in urea, ammonia, and nitrate concentrations to 8000 μg N∙L−1 above control levels in streams not protected with 50-m-wide leave (i.e. unfertilized) strips and 350 μg N∙L−1 in those having leave strips. By spring 1980, dissolved nitrogen loads to Mohun Lake increased by a minimum of 55% in the largest basin and 1924% in the smallest basin. This corresponded to fertilizer losses of 2.1% of the total applied from drainages that had leave strips and up to 5.2% from those where the treatment included direct application to stream channels. Nitrogen transport was dominated by reduced nitrogen species and lasted up to 144 d. Relatively small changes in nitrate transport were attributed to low temperatures, which reduced rates of nitrification. Duringthe3-yr prior to forest fertilization, nitrogen and phosphorus were added to the lake at an N:P ratio of 4.5 (wt/wt) and a load of 1.4–2.2 mg P∙m−2∙wk−1. Spring overturn phosphorus concentrations were 4–5 μg∙L−1 throughout the study and, except during the spring following forest fertilization, N:P ratios (NO3-N:TP) at spring overturn were 3–6 (wt/wt). Despite the low ratios, nitrogen-fixing phytoplankton were not dominant in Mohun Lake. After forest fertilization, average epilimnetic chlorophyll concentrations were 1.8 times higher than in 1979, the last year of lake fertilization. The increase is attributed to the much higher spring overturn N:P ratio that occurred after forest fertilization and to phosphorus recycling processes within the extensive littoral zone of the lake.