Denitrification in vadose zone material amended with dissolved organic matter from topsoil and subsoil

Abstract

Limited information is available on the potential of subsoil and underlying alluvial gravels (vadose zone) to denitrify leached nitrate, though it is recognized that C availability is an important constraint. In this laboratory study, we used samples (<4 mm fraction) from subsoil (0.4–0.85 m depth) and alluvial gravel layers (1.2–4.8 m) derived from a site in Canterbury, New Zealand, to quantify denitrification potential and examine the denitrification response to inputs of dissolved organic C (DOC). As C sources, we used DOC extracted in either cold (20 °C) or hot (80 °C) water from topsoil and subsoil (assumed to represent C substrates likely to be transported into the subsoil in drainage). Measurements of potential denitrification enzyme activity (DEA, via samples amended with glucose and nitrate-N) showed the alluvial gravel samples had denitrification capability although rates were low (<5 μg N₂O–N kg⁻¹ h⁻¹) compared with those reported for topsoils. The effect of the soil-derived DOC, at addition rates 16–215 mg kg⁻¹, on denitrification was determined in a 7-d anaerobic incubation using acetylene to block reduction of N₂O to N₂. There was a lag of 24–36 h, during which N₂O production was low, followed by a period of rapid increase (36–96 h), and a final phase (120–168 h) in which N₂O concentrations remained relatively stable. During the rapid phase, the N₂O production rate was very high (up to 440–500 μg N kg⁻¹ h⁻¹ at the highest DOC addition rate) and exceeded that measured in the 8-h DEA assay (a measure of indigenous denitrifier enzymes). The post-96 h decline in N₂O production rate was attributed to depletion of available C. The temporal pattern of CO₂–C production was similar to that of N₂O, though the lag period was shorter (12–24 h). The ratio of N₂O–N to CO₂–C increased with time; the maximum was circa 0.3:1 at the highest addition rate of hot water extractable DOC. Our results suggest that, with input of DOC substrate, alluvial gravel materials could generate enough enzyme during a 1–2 day anaerobic period to denitrify significant quantities of nitrate. However, in situ denitrification in alluvial gravels may be low because of biophysical limitations such as low C inputs of bioavailable C and anaerobicity mostly confined to localized zones or lenses.