Subsurface Denitrification in a Forest Riparian Zone: Interactions between Hydrology and Supplies of Nitrate and Organic Carbon

• Published in: Biogeochemistry Vol. 51; no. 2; pp. 193 - 223
• Main Authors: Hill, Alan R., Devito, Kevin J., Campagnolo, S., Sanmugadas, K.
• Format: Journal Article
• Language: English
• Published: Heidelberg Kluwer Academic Publishers 01-11-2000; Springer
• Subjects: Alkynes; Aquifers; Canada, Ontario; Carbon; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geochemistry; Groundwater; Hydrogeology; Hydrology. Hydrogeology; Mineralogy; Nitrates; Peat; Piezometers; Riparian areas; Riparian soils; Sediments; Silicates; Water geochemistry; Canada; Ontario; Eastern Canada; chlorides; experimental studies; nitrous oxide; nitrogen; ground water; concentration; transport; nitrates; North America; halides; denitrification; in situ; floodplains; drainage basins; sand; depth; aquifers; hydrochemistry; dissolved materials; peat; acetylene; injection; organic carbon
• ISSN: 0168-2563; 1573-515X
• DOI: 10.1023/A:1006476514038

The influence of hydrology and patterns of supply of electron donors and acceptors on subsurface denitrification was studied in a forest riparian zone along the Boyne River in southern Ontario that received high nitrogen inputs from a sand aquifer. Two hypotheses were tested: (1) subsurface denitrification is restricted to localized zones of high activity; (2) denitrification zones occur at sites where groundwater flow paths transport NO3- to supplies of available organic carbon. A plume of nitrate-rich groundwater with concentrations of 10-30 mg N L-1 flowed laterally at depths of 1.5-5 m in sands beneath peat for a horizontal distance of 100-140 m across the riparian zone to within 30-50 m of the river. In situ acetylene injections to piezometers revealed that significant denitrification was restricted to a narrow zone of steep NO3- and N2O decline at the plume margins. The location of these denitrification sites in areas with steep gradients of groundwater DOC increase supported hypothesis 2. Many of these denitrification "hotspots" occurred near interfaces between sands and either peats or buried river channel deposits. Field experiments involving in situ additions of either glucose or NO3- to piezometers indicated that denitrification was C-limited in a large subsurface area of the riparian zone, and became N-limited beyond the narrow zone of NO3- consumption. These data suggest that denitrification may not effectively remove NO3- from groundwater transported at depth through permeable riparian sediments unless interaction occurs with localized supplies of organic matter.