NATURAL N-15 ABUNDANCE IN 2 NITROGEN SATURATED FOREST ECOSYSTEMS

Natural N-15 abundance values were measured in needles, twigs, wood, s oil, bulk precipitation, throughfall and soil water in a Douglas fir ( Pseudotsuga menziesii (Mirb.) and a Scots pine (Pinus sylvestris L.) s tand receiving high loads of nitrogen in throughfall (>50 kg N ha(-1) year(-1)). In the Douglas fir stand delta(15)N values of the vegetatio n ranged between -5.7 and -4.2 parts per thousand with little variatio n between different compartments. The vegetation of the Scots pine sta nd was less depleted in N-15 and varied from -3.3 to -1.2 parts per th ousand, delta(15)N. At both sites delta(15)N values increased with soi l depth, from -5.7 parts per thousand and -1.2 parts per thousand in t he organic layer to + 4.1 parts per thousand and + 4.7 parts per thous and at 70 cm soil depth in the Douglas fir and Scots pine stand, respe ctively. The delta(15)N values of inorganic nitrogen in bulk precipita tion showed a seasonal variation with a mean in NH4+-N of -0.6 parts p er thousand at the Douglas fir stand and +10.8 parts per thousand at t he Scots pine stand. In soil water below the organic layer NH4+-N was enriched and NO3-- N depleted in N-15, which was interpreted as being caused by isotope fractionation accompanying high nitrification rates in the organic layers. Mean delta(15)N values were very similar in the drainage water at 90 cm soil depth at both sites (-7.1 to -3.8 parts per thousand). A dynamic N cycling model was used to test the sensitiv ity of the natural abundance values for the amount of N deposition, th e N-15 ratio of atmospheric N deposited and for the intrinsic isotope discrimination factors associated with N transformation processes. Sim ulated delta(15)N values for the N saturated ecosystems appeared parti cularly sensitive to the N-15 ratio of atmospheric N inputs and discri mination factors during nitrification and mineralization, The N-satura ted coniferous forest ecosystems studied were not characterized by ele vated natural N-15 abundance values. The results indicated that the na tural N-15 abundance values can only be used as indicators for the sta ge of nitrogen saturation of an ecosystem if the delta(15)N values of the deposited N and isotope fractionation factors ale taken into consi deration. Combining dynamic isotope models and natural N-15 abundance values seems a promising technique for interpleting natural N-15 abund ance values found in these forest ecosystems.