NUTRITIONAL ASSESSMENT OF A FOREST FERTILIZATION EXPERIMENT IN NORTHERN SWEDEN BY ROOT BIOASSAYS

Roots were excised from long-term nitrogen, phosphorus and potassium fertilisation experiments in a Pinus sylvestris plantation in northern Sweden, and tested for their ability to take up N-15, P-32 or Rb-86 from solutions of ammonium chloride, potassium dihydrogen phosphate and rubidium chloride. High levels of uptake in these tests indicate low nutrient status. At this site, lower uptake rates in the trees from the fertilised treatments showed they had responded to the addition of all three nutrients, although the nitrogen test could not detect a difference in root response between trees from the two higher nitrogen treatments, which, according to other criteria, were high enough to create nitrogen-saturated conditions. There was no evidence from the nitrogen uptake measured that this was affected by phosphorus or potassium fertiliser treatment, suggesting that correcting the supply of these elements did not induce further nitrogen demand. Conversely, trees receiving high nitrogen additions did not show any increased demand for phosphorus or potassium. This is in agreement with data on tree growth and nitrogen retention of the stands. Trees from plots treated with acid 16-21 years previously or lime 21 years previously did not respond differently from trees in control plots, unless nitrogen, phosphorus and potassium fertiliser were also added, when demand for the three nutrients was reduced. The results suggest that, in spite of difficulties in sampling tree roots in an area with a rich ground flora, the method is a useful test of the long-term nutrient status in forest fertilisation experiments.