Biomass and nitrogen dynamics in an irrigated hybrid poplar plantation

Abstract

A 3-year study measured the effects of ground cover treatments and N fertilization on biomass and nitrogen dynamics in an irridiated hybrid poplar ( Populus deltoides Bartr. X P. trichocarpa Torr. and Gray, clone NC-9922) plantation in northern Wisconsin, U.S.A. Annually fertilized (112 kg N ha −1 year −1) and unfertilized plots were maintained weed free (bare soil), allowed to revegetate with native weeds, or seeded to birdsfoot trefoil ( Lotus corniculatus L.). Biomass and N in trees and ground-cover vegetation were sampled before and after each growing season. Trees in bare-soil plots responded to fertilization primarily in the third growing season, but total biomass of 3-year-old trees was not increased by annual fertilization. In plots with a ground cover,fertilization increased tree growth but cover crop treatment had no effect. Ground cover biomass peaked during the second growing season, but declined thereafter, primarily due to reductions in below-ground biomass. Estimated recovery of fertilizer N was low in bare soil plots after 3 years, with 2% in the ‘perennial’ portion of the trees and 13% in the leaf litter. In contrast, recovery in the cover crop plots was 44%–51% in years 2–4. During that period, both biomass and N pool dominance shifted from primarily cover crop to primarily trees. The ground cover appeared to reduce tree growth in years 1–3, but total tree biomass after 4 years was greater in fertilized plots with ground cover (22.7 Mg/ha) than in fertilized bare soil plots (16.7 Mg/ha). Biomass production in fertilized trefoil plots in the fourth year (15.1 Mg ha −1 year −1, excluding leaves) exceeds that of local forests by 50%, and may be comparable to corn productivity in the area.