Effect of biuret on growth and nutrition of Douglas-fir (Pseudotsuga menziesii (Mirb) Franco) seedlings

Effects of biuret concentrations on growth and nutrient uptake by two provenances of Douglas-fir (Pseudotsuga menziesii (Mirb) Franco) seedlings were examined experimentally in sand, solution culture and soil. In sand and solution culture, biuret stimulated the growth of Douglas-fir seedlings at lower concentrations (from 0.001 to 0.097 mM), but inhibited and even killed the seedlings at greater concentrations (from 0.971 to 9.709 mM). Nutrient uptake by seedlings and the net photosynthetic rate in needles increased at lower concentrations of biuret. Biuret stimulation of growth was greater at 7.143 than 2.857 mM of applied nitrogen (N). In soil, biuret applied at 2 and 10 (mu)g g^-1 (oven-dry) soil (equivalent to the rates of 5 and 25 kg biuret ha^-1) increased seedling height, basal diameter and stem volume while biuret at 100 (mu)g g^-1 soil (equivalent to 250 kg biuret ha^-1) inhibited height growth and killed the seedlings at 1000 (mu)g g^-1 soil (equivalent to 2500 kg biuret ha^1). The stimulation of Douglas-fir seedling growth at lower concentrations of biuret (2 and 10 (mu)g g^-1 soil) was associated with improvement of soil N availability while the exact causes of the inhibition of growth at high concentrations (100 and 1000 (mu)g g^-1 soil) are unknown but could include direct biuret toxicity or the indirect effect of the elevated H+ and/or Al3+. Provenance 93 was more responsive to lower concentrations of biuret but was more susceptible to biuret toxicity while provenance 98 was more tolerant of high concentrations of biuret but less stimulated by the lower concentrations of biuret. Yellow tip (chlorosis) in needles was a typical symptom of biuret toxicity.