Forest soils exhibit a variety of complex biochemical nitrogen (N) reactions in which nitric oxide (NO) and nitrous oxide (N₂O) can be produced by coexisting processes that respond differently to the same environmental conditions. In general, two biochemical processes, (i) the oxidation of ammonia (nitrification) and (ii) the reduction of nitrate (denitrification), are known as the major sources of nitrogen oxides. Few reports indicated that a direct oxidation of soil organic N compounds (Norg) to NO and N₂O may also be significant in soils.

A triplet ¹⁵N tracer experiment (TTE) combined with an inverse abundance approach (IAA) was applied to quantify NO and N₂O formation in soil related to different but simultaneously utilized soil N sources (ammonium, nitrate, and N_org). In addition, the impact of oxic and hypoxic conditions (21 and 2% v/v O₂, respectively) on total soil NO/N₂O release and source composition was studied. Experiments were conducted with soil samples from 5 different Basque forest stands (mature beech, young beech, mature pine, young pine, and new pine plantation).

The release rates of NO and N₂O were higher in the soil samples from beech stands than in the samples from pine stands. The change from oxic to hypoxic conditions increased the NO release rate 2- to 14-fold and the N₂O release rate 3.6- to 25-fold. The study suggests that, under oxic conditions, N₂O formation based on N_org appears to be the dominant pathway of soil N₂O production (48–76% to total N₂O release). Under hypoxic conditions, the relative contribution of N_org significantly decreased, whereas its absolute contribution increased concomitantly. Denitrification was the dominant process of soil N₂O release under hypoxic conditions and served as the major pathway of soil NO release under both oxic and hypoxic conditions (40 and 60% of total soil NO release, respectively).

We conclude that the individual contribution of different soil N pools to the total soil N gas release and the impact of environmental parameters (e.g., O₂ availability) are site-specific. Nonetheless, further research is required to elucidate the impact of forest stands on soil NO and N₂O production, particularly N₂O formation directly based on N_org transformation.