Spatial distribution of hydroxylamine and its role to aerobic N2O formation in a Norway spruce forest soil

Hydroxylamine (HA) is potentially involved in soil N2O formation as a crucial intermediate in the oxidation of ammonium to nitrite. However, the determination of HA concentration in natural soil samples has not been reported until now. Here, we determined the HA concentrations in organic (Oh) and mineral (Ah) layers of 110 soil samples collected from a spruce forest (Wüstebach, Eifel National Park, Germany) using a novel approach, based on the fast extraction of HA from the soil at a pH of 1.7, the oxidation of HA to N2O with Fe3+, and the analysis of produced N2O using gas chromatography (GC-ECD). In a second step, N2O emission rates were determined by means of aerobic laboratory incubations of 3 g soil in 22-mL vials. Subsequently, the spatial distribution of soil HA concentrations and N2O emission rates in the Oh and Ah layers of the whole sampling area were analyzed using a geostatistical approach. The correlations among soil HA, N2O emission rate, pH, soil C, N, Fe, Mn and soil water content (SWC) were further explored. The HA concentrations ranged from 0.3–37.0 μg N kg-1 and 0.02–11.4 μg N kg-1 dry soil in the Oh and the Ah layer, respectively. The spatial distribution of HA was similar in both layers, with substantial spatial variability dependent on soil type, tree density and distance to a stream, e.g., HA concentration was greater at locations with a thick litter layer or at locations close to the stream. N2O emission rates showed a similar pattern as soil HA concentrations, with higher rates in the Oh layer than in the Ah layer. N2O emission rate exhibited the highest correlation with soil HA content in the Oh layer, while soil NO3- content explained N2O emissions best in the Ah layer, associated with SWC, Mn and C content. HA concentration was negatively correlated with pH and positively correlated with SWC in the Oh layer, while positively correlated with C and N as well as NO3- content in the Ah layer. Moreover, Mn content was the most important factor for HA recovery at the specific extraction conditions. The results demonstrated that HA is a crucial component for aerobic N2O formation and emission in spruce forest soils. Mn may also play a key role to the aerobic N2O emission due to the chemical reaction with HA. Further studies should focus to the relationships between HA, Mn and aerobic N2O emission in other ecosystems