Submitted to: American Society for Microbiology Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 5/19/2002
Publication Date: N/A
Technical Abstract: Nitrate removal from groundwater beneath riparian vegetation has been attributed to denitrification, but the carbon sources driving microbial metabolism in subsurface soils have not been investigated. Dissolved organic carbon (DOC) leaching from the root zone to deeper sediments is a likely source of C to support microbial metabolism. We compared the ability of natural DOC and other defined substrates to promote denitrification in four saturated subsurface sediments (9.35 to 10 m depth). Soils amended with 50 micro g of **15NO3-N per g soil were incubated for 89 days under anaerobic conditions. DOC extracted from shallow subsoil (1 to 2 m depth) and defined C substrates were added 39 days after nitrate addition at 30 micro g C/g soil. DOC addition caused 12 to 30% reduction in nitrate concentration with 90% loss of the DOC. DOC half-lives ranged from 6 to 14 days. Nitrate concentrations remained constant in subsoils without added carbon substrates and DOC concentrations in untreated subsoils ranged between 1 and 20 micro g C/g soil. Incorporation of **15N into microbial biomass or other organic forms was minimal. Denitrification was carbon limited; benzoate added at 300 micro g C/g soil caused complete removal of nitrate in three sediments, but did not support any denitrification in other sediment. The results show that DOC from soil can effectively support denitrification in subsoils below the root zone.