Dynamics of foliar N in Douglas-fir after spring and fall application of ammonium nitrate and urea

We investigated foliar N dynamics after spring and fall application of ammonium nitrate and urea at 200 kg N•ha−1 to an intermediate productivity 38-year-old Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) stand growing in Humo-Ferric Podzols on Vancouver Island. Douglas-fir responded by increasing: the N concentration of existing foliage and both the concentration and content of N in new shoots. Six months after spring fertilization, N concentration in current foliage averaged 30 and 21 % above that of the control for ammonium nitrate and urea, respectively. Nitrogen concentration of current foliage was increased after 2 years in the ammonium nitrate treatment only. When ammonium nitrate was applied in fall, peak N concentration in current foliage occurred after 6 months (in midspring) at 25% above that of the control. Fall application of urea did not affect current foliage N concentration until the next fall when a 19% increase over that of the control was observed. At the first fall after fertilization (seasons of application combined), ammonium nitrate caused a 26% increment over control in N content of current foliage, whereas urea caused a 13% increase. This superiority of ammonium nitrate over urea was attributed to the nitrate ion. The hypothesis that N uptake into foliage from ammonium nitrate would be greater when applied in spring was not supported.