© 2003 Heron Publishing—Victoria, Canada
Growth response of cottonwood roots to varied NH4:NO3 ratios in enriched patches
Walter T. M. Woolfolk (1) and Alexander L. Friend (2, 3)
1. Department of Forestry, College of Forest Resources / Forest and Wildlife Research Center, Mississippi State University, P.O.
Box 6722, Mississippi State, MS 39762-7622, USA / 2. USDA Forest Service, North Central Research Station, 410 MacInnes Drive, Houghton, MI 49931, USA / 3. Author to whom correspondence should be addressed (afriend@fs.fed.us) / Received March 21, 2002; accepted October 19, 2002; published online March 17, 2003
Summary
Maximization of short-rotation forest plantation yield requires frequent applications of nutrients, especially nitrogen (N).
Whole-plant growth is known to be sensitive to the proportion of ammonium to nitrate (NH4:NO3). However, the extent to which N form affects root growth, branching and morphology is poorly understood, and these variables
may have substantial impacts on plant nutrient and water acquisition. We used rooted cuttings of cottonwood (Populus deltoides Bartr. ex Marsh.) to investigate the effect of various NH4:NO3 ratios on root growth in N-enriched patches. A sand culture study with split-root systems was carried out in which 1–3% of
the total root system of each cutting was supplied with 2 mM N at NH4:NO3 ratios of 0:100, 20:80, 40:60, 60:40, 80:20 and 100:0 (molar basis), with the rest of the plant supplied with 0 mM N, resulting
in the whole plant becoming N deficient. During the experiment, whole-plant growth was unaffected by the treatments. Of the
NH4:NO3 ratios tested, greatest total root length, specific root length, and root N concentration of roots in enriched patches occurred
in the 20:80 NH4:NO3 treatment. The largest response of roots in enriched patches was third- and fourth-order root production. We conclude that
N form has a profound effect on root development and morphology in enriched patches.
Keywords:
ammonium:nitrate ratio preference, ammonium nutrition, nitrate nutrition, nitrogen-enriched patches, root foraging, root morphology,
root proliferation, sand culture.
