N-15-LABELED UREA FERTILIZATION OF A TAMARACK BLACK SPRUCE MIXED STAND ON A DRAINED MINEROTROPHIC PEATLAND - N-15 IN SOIL AND TREE UPTAKE

N-15-Labelled urea fertilizer was used to examine initial uptake patte rns of nitrogen (N) into foliage of tamarack (Larix laricina (Du Roi) K. Koch.) and black spruce (Picea mariana (Mill.) BSP.), and movement of N from point of placement through live moss and organic soil on a d rained minerotrophic peatland in central Alberta, Canada. The site was ditched in Fall 1987 and N-15-labelled urea fertilizer was applied on 1 June 1989. Tree foliage and soil samples were taken periodically du ring the 1989 growing season. A greater proportion of fertilizer N was retained (immobilized) in peat residue of surface soil layers (0-20 c m depth) and this did not change during the study period. However, min eral-N-15 concentration decreased from 76 to 19 mg N-15 kg(-1) between Week 2 and Week 13 following fertilizer application. Foliar %NDFF (th e percentage of total N derived from fertilizer) and N-15 content (amo unt of N-15 per needle) increased rapidly for both species during the growing season. A higher proportion of fertilizer N taken up by trees was allocated to tissues of high metabolic activity, i.e. current need les. The foliar %NDFF was higher for black spruce compared with that o f tamarack. This may be related to a greater size (amount of foliage a nd associated photosynthetic organelle) and strength (rate of biologic al activity) of the sink for N associated with black spruce relative t o that of tamarack.