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

Abstract:

Hydroxylamine (HA) as a crucial intermediate in the microbial oxidation of ammonium to nitrite (nitrification) is a potential precursor of abiotic N₂O formation in the soil. 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 135 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). Meanwhile, 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 0.3–44.6 μg N kg^-1 dry soil and 0.02–16.2 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. For example, HA concentration was greater at locations with a thick litter layer or at locations close to the stream. The average N₂O emission rate in the Oh layer was 0.38 μg N kg^-1 dry soil h^-1, 10-fold larger than in the Ah layer. Interestingly, N₂O emission rate exhibited high correlation with soil HA content in the Oh (R² = 0.65, p < 0.01) and Ah (R² = 0.45, p < 0.05) layer. The results demonstrated that HA is a crucial component for aerobic N₂O formation and emission in spruce forest soils. Moreover, 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. Mn content was the most important factor for HA recovery at the specific extraction conditions. Further studies should focus on the effects of soil organic matter, Mn content, and pH on the production of N₂O from HA under aerobic conditions.