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

Roots were excised from long-term nitrogen, phosphorus and potassium f ertilisation experiments in a Pinus sylvestris plantation in northern Sweden, and tested for their ability to take up N-15, P-32 or Rb-86 fr om solutions of ammonium chloride, potassium dihydrogen phosphate and rubidium chloride. High levels of uptake in these tests indicate low n utrient status. At this site, lower uptake rates in the trees from the fertilised treatments showed they had responded to the addition of al l three nutrients, although the nitrogen test could not detect a diffe rence in root response between trees from the two higher nitrogen trea tments, which, according to other criteria, were high enough to create nitrogen-saturated conditions. There was no evidence from the nitroge n uptake measured that this was affected by phosphorus or potassium fe rtiliser treatment, suggesting that correcting the supply of these ele ments did not induce further nitrogen demand. Conversely, trees receiv ing high nitrogen additions did not show any increased demand for phos phorus 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 dif ferently from trees in control plots, unless nitrogen, phosphorus and potassium fertiliser were also added, when demand for the three nutrie nts was reduced. The results suggest that, in spite of difficulties in sampling tree roots in an area with a rich ground flora, the method i s a useful test of the long-term nutrient status in forest fertilisati on experiments.