The impact of relict organic materials on the denitrification capacity in the unsaturated-saturated zone continuum of three volcanic profiles

Abstract

The denitrification capacity of wetlands, riparian zones, and aquifers in glacial outwash areas is well documented, but little or no information exists for volcanic profiles, particularly those containing relict organic matter contained in or on top of paleosols (old soils buried by volcanic deposits) below the groundwater table. Relict carbon contained in these layers could provide the necessary electrons to fuel heterotrophic denitrification. To the best of our knowledge, this is the first study investigating the denitrification capacity in both the unsaturated and saturated zone of volcanic profiles. Samples from three profile types with differing organic matter distribution were amended with ¹⁵N-enriched nitrate (NO₃⁻) and incubated in the laboratory under anaerobic conditions. Dinitrogen (N₂) dominated the ¹⁵N gas fluxes; averaged across all samples, it accounted for 96% of the total ¹⁵N (nitrous oxide [¹⁵N₂O] and ¹⁵N₂) gas fluxes. Dinitrogen fluxes were generally highest in the A horizon samples (4.1-6.2 nmol N g⁻¹ h⁻¹), but substantial fluxes were also observed in some paleosol layers (up to 0.72 nmol N g⁻¹ h⁻¹). A significant correlation (p < 0.001) was found between the concentration of extractable dissolved organic carbon and the total ¹⁵N gas flux produced in samples from below the A horizon, suggesting that heterotrophic denitrification was the dominant NO₃⁻ attenuation process in this study. Extrapolation of lab-derived denitrification capacities to field conditions suggests that the denitrification capacity of profiles containing relict soil organic matter in the saturated zone exceeds the estimated N leaching from the root zone.