Effects of nitrogen fertilizer source and application rates on foliar nitrogen concentration, photosynthesis, and growth of Douglas-fir

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Abstract
The effects of nitrogen fertilization on foliage nitrogen concentration, rates of photosynthesis, and stem diameter growth were studied for Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) trees, with a range of fertilizer applications from 0 to 896 kg N/ha. Ammonium nitrate (AN) and urea were compared as nitrogen sources. AN provided for a higher foliage nitrogen concentration and better growth the 1st year, but there was no source effect thereafter. A significant relationship was obtained between foliage nitrogen concentration and rate of photosynthesis, with an optimum rate at 1.74% foliar nitrogen and with no effect of nitrogen source. Stem diameter growth was increased at all fertilization rates, although no effect of the two lowest rates of fertilization (112 and 224 kg N/ha) was found on foliar nitrogen concentration.