Denitrification and organic carbon availability in riparian wetland soils and subsurface sediments

The influence of organic C quantity and quality on denitrification in riparian environments is poorly understood. We measured denitrification potential (DNP), organic matter, and several fractions of organic C in surface soils and subsurface sediments in a river riparian zone. Surface soils in conifer forest peat, mixed forest, and marsh sites had similar DNP, although mean organic matter ranged from 9.4% (marsh) to 19.6% (mixed forest) and 36.6% (peat). These soils also differed widely in organic C, water-extractable C, and anaerobic mineralizable C. Mean DNP in peat at depths of 0.8 to 1.4 m was four times lower than in the surface peat. Mean organic matter and organic C were significantly greater in the deep peat than at the surface, whereas the other C fractions were similar. Mean organic matter content of buried channel sediments at depths of 2 to 3 m was 3.6%; however, mean DNP was 75 to 80 times lower than in the surface mixed forest and marsh soils. When the three surface soil sites were considered separately, anaerobic mineralizable C showed the highest correlation with DNP in the marsh soils (r = 0.87) and the conifer peat soil (r = 0.82). Water-extractable C was also highly correlated with DNP in the marsh soils (r = 0.81). Correlations between DNP and either organic matter or the three C fractions were not significant in the deep peat, whereas the former channel sediments showed a significant relationship between DNP and both organic matter (r = 0.81) and water-extractable C (r = 0.81). These results show that C quantity and quality influence DNP, but no single index was a good predictor for all soil types.