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 ( 2 O–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 N 2 O to N 2 . There was a lag of 24–36 h, during which N 2 O production was low, followed by a period of rapid increase (36–96 h), and a final phase (120–168 h) in which N 2 O concentrations remained relatively stable. During the rapid phase, the N 2 O 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 N 2 O production rate was attributed to depletion of available C. The temporal pattern of CO 2 –C production was similar to that of N 2 O, though the lag period was shorter (12–24 h). The ratio of N 2 O–N to CO 2 –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.