EFFECTS OF NITROGEN-SOURCE AND SEASON OF APPLICATION ON THE NUTRITION AND GROWTH OF LODGEPOLE PINE

The effects of spring and fall applications of urea and ammonium nitrate fertilizer on the nutrition and growth of lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) were evaluated at three locations in the interior of British Columbia over a 6-year period. Ammonium nitrate was generally more effective at increasing 1st-year foliar nitrogen concentration than was urea, irrespective of season of application. Spring-applied ammonium nitrate was especially effective in increasing foliar nitrogen levels. Despite the apparent superiority of ammonium nitrate over urea in improving foliar nitrogen status, the basal area increment of fertilized trees in the three trials was not affected by nitrogen source. The effect of season of application on basal area increment was not consistent across all sites or between measurement periods. Spring-applied ammonium nitrate produced the smallest 0- to 3-year basal area and height increments in two of the trials. The negative effects of fertilization on foliar sulphur concentration may partially explain the poor basal area and height response following fertilization at one site and the relatively small response to spring-applied ammonium nitrate at another site. Because spring-applied ammonium nitrate is more effective at increasing foliar nitrogen concentration, it is most likely to create a nitrogen:sulphur imbalance, at least over the short term. Further studies are warranted to test the hypothesis that ammonium nitrate is better than urea at stimulating tree growth when accompanied by sulphur additions.