Natural N Abundance in Two Nitrogen Saturated Forest Ecosystems

Natural N abundance values were measured in needles, twigs, wood, soil, bulk precipitation, throughfall and soil water in a Douglas fir (Pseudotsuga menziesii (Mirb.) and a Scots pine (Pinus sylvestris L.) stand receiving high loads of nitrogen in throughfall (>50 kg N ha year). In the Douglas fir stand δN values of the vegetation ranged between -5.7 and -4.2‰ with little variation between different compartments. The vegetation of the Scots pine stand was less depleted in N and varied from -3.3 to -1.2‰δN. At both sites δN values increased with soil depth, from -5.7‰ and -1.2‰ in the organic layer to +4.1‰ and +4.7‰ at 70 cm soil depth in the Douglas fir and Scots pine stand, respectively. The δN values of inorganic nitrogen in bulk precipitation showed a seasonal variation with a mean in NH-N of -0.6‰ at the Douglas fir stand and +10.8‰ at the Scots pine stand. In soil water below the organic layer NH-N was enriched and NO-N depleted in N, which was interpreted as being caused by isotope fractionation accompanying high nitrification rates in the organic layers. Mean δN values of NH and NO were very similar in the drainage water at 90 cm soil depth at both sites (-7.1 to -3.8‰). A dynamic N cycling model was used to test the sensitivity of the natural abundance values for the amount of N deposition, the N ratio of atmospheric N deposited and for the intrinsic isotope discrimination factors associated with N transformation processes. Simulated δN values for the N saturated ecosystems appeared particularly sensitive to the N ratio of atmospheric N inputs and discrimination factors during nitrification and mineralization. The N-saturated coniferous forest ecosystems studied were not characterized by elevated natural N abundance values. The results indicated that the natural N abundance values can only be used as indicators for the stage of nitrogen saturation of an ecosystem if the δN values of the deposited N and isotope fractionation factors are taken into consideration. Combining dynamic isotope models and natural N abundance values seems a promising technique for interpreting natural N abundance values found in these forest ecosystems.