LONG-TERM RECOVERY IN THE SOIL-PROFILE OF N-15 FROM DOUGLAS-FIR NEEDLES DECOMPOSING IN THE FOREST FLOOR

To follow the movement and transformations of litter-fall N in a fores t ecosystem, Douglas-fir (Pseudotsuga menziesii (Mirb.) France) foliag e labelled with N-15 was mixed into the forest floor and left to decom pose in steel cylinders of 15 cm diameter x 60 cm length. There were f our treatments: N-15-labelled foliage only, foliage plus 200 kg N . ha (-1) as urea or NH4NO3, and unamended control. The addition of fertili zer N had no significant effects on N-15 recovery or distribution in t he soil profile. The overall recovery of N-15 to 60 cm depth was 53.7% , 24.9%, and 19.9% after 1, 3, and 7.5 years, respectively. After the first year almost all of the recovered N-15 was in the L layer. At 3 y ears, a higher proportion was found in the FH layer, and by 7.5 years, approximately one-third of the N-15 was found in mineral soil horizon s. There was very little recovery of N-15 in inorganic form (1% or les s) after 1 year; in subsequent years inorganic N was found at backgrou nd levels. Recoveries in soil after 1 and 3 years were similar to thos e reported for N-15 added as inorganic fertilizer in the absence of pl ant uptake. However, very low losses in the next sampling period (3 to 7.5 years) indicated stabilization of N-15 in increasingly recalcitra nt forms.