Nitrogen management in a Fraser fir (Abies fraseri [Pursh] Poir.) Christmas tree plantation: effects of fertilization on tree performance and nitrogen leaching

There is increasing concern over the effects of nitrogen (N) pollution resulting from fertilizer inputs to food and fiber production systems. Christmas tree production is one segment of the forestry sector that uses substantial fertilizer inputs. However, we know very little about the environmental costs and production benefits of N fertilization in Christmas tree plantations. I studied the costs and benefits of 2 years of N applications (as ammonium sulfate) at 0, 47, 95, and 140 kg N ha-1 year-1 to 4-year-old Fraser fir (Abies fraseri) Christmas trees. These rates correspond to 0, 50, 100, and 150% of the amount recommended for 4-year-old trees, and 33, 67, and 100@ of recommendations for 5-year-old trees. There were no significant effects of fertilization rate on tree growth or morphology. In contrast, foliar N concentrations increased with increasing rates of N application, and there was a trend toward improved foliage health (indicated by chlorophyll fluorescence) at the lowest application rate. Mineral N (NH+4 + NO-3) concentrations in water leaching below the rooting zone increased markedly with increasing N application rates. Lysimeter NO-3 concentrations in the highest application treatment were consistently 20-30 times greater than the safe drinking water standard of 10 mg NO-3-N/l. Together, these results suggest that conventional guidelines for fertilization of Michigan Fraser fir plantations greatly exceed the capacity of trees and surface soils to store N, provide no benefit to tree growth, and create the potential for significant N pollution of ground and surface waters. Furthermore, these results indicate that applying N at a lower rate of 47 kg N ha-1 year-1 substantially reduced NO-3 leaching with no decline in growth rates or tree quality.