Use of the inverse abundance approach to identify the sources of NO and N2O release from Spanish forest soils under oxic and hypoxic conditions

Abstract

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 Norg). 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 Norg appears to be the dominant pathway of soil N2O production (48–76% to total N2O release). Under hypoxic conditions, the relative contribution of Norg 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 Norg transformation.