Natural ¹⁵N abundance in two nitrogen saturated forest ecosystems

Natural ¹⁵N 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^-1 year^-1). In the Douglas fir stand δ¹⁵N 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 ¹⁵N and varied from -3.3 to -1.2‰δ¹⁵N. At both sites δ¹⁵N 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 δ¹⁵N values of inorganic nitrogen in bulk precipitation showed a seasonal variation with a mean in NH₄⁺-N of -0.6‰ at the Douglas fir stand and +10.8‰ at the Scots pine stand. In soil water below the organic layer NH₄⁺-N was enriched and NO₃^--N depleted in ¹⁵N, which was interpreted as being caused by isotope fractionation accompanying high nitrification rates in the organic layers. Mean δ¹⁵N values of NH₄⁺ and NO₃^- 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 ¹⁵N ratio of atmospheric N deposited and for the intrinsic isotope discrimination factors associated with N transformation processes. Simulated δ¹⁵N values for the N saturated ecosystems appeared particularly sensitive to the ¹⁵N 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 ¹⁵N abundance values. The results indicated that the natural ¹⁵N abundance values can only be used as indicators for the stage of nitrogen saturation of an ecosystem if the δ¹⁵N values of the deposited N and isotope fractionation factors are taken into consideration. Combining dynamic isotope models and natural ¹⁵N abundance values seems a promising technique for interpreting natural ¹⁵N abundance values found in these forest ecosystems.