BIOMASS AND NITROGEN DYNAMICS IN AN IRRIGATED HYBRID POPLAR PLANTATION (COVER CROPS, NITRATE LEACHING, WEED CONTROL)
Abstract
A 3-year study measured the effects of ground cover treatments and nitrogen fertilization on biomass and nitrogen dynamics in an irrigated hybrid poplar (Populus deltoides Bartr. x P. trichocarpa Torr. and Gray, clone NC-9922) plantation in northern Wisconsin. Annually fertilized (112 kg N/ha/yr) and unfertilized plots were either maintained weed-free (bare soil), allowed to revegetate with native weeds, or seeded to birdsfoot trefoil (Lotus corniculatus L.). Biomass and nitrogen pools in trees and the ground cover vegetation were estimated by sampling before and after each growing season. Soil solution was monitored during each growing season using cup lysimeters. Samples of the Ap horizon were evaluated for mineralizable N before and after plot establishment. 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. High nitrate-nitrogen concentrations in the soil solution suggested significant leaching in both unfertilized and fertilized bare soil plots in the first growing season, and in fertilized plots the second season. Nitrate-nitrogen concentrations declined sharply in fertilized bare soil plots during the third growing season. Cover crop biomass was greatest in the second year and declined thereafter due to declines in below-ground components. Fertilization increased tree growth in these plots, but cover crop treatments had no effect. Fertilization increased soil solution nitrate in the surface horizon, but no evidence of substantial leaching occurred until the fall of the fourth year. Mineralizable soil nitrogen was greater in the fertilized trefoil plots compared to that in unfertilized trefoil or native weed plots. However, fertilization did not increase mineralizable soil nitrogen in bare soil plots. Results of this study suggest that, under irrigated conditions, a cover crop can substantially reduce leaching losses of nutrients and serve as a slow-release pool of nitrogen after the trees achieve crown closure. Fertilization is not recommended in these plantations until the second growing season if a cover crop is present and the third growing season if complete weed control is practiced.
Degree
Ph.D.
Subject Area
Forestry
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