Soil microbial diversity, community structure and denitrification in a temperate riparian zone

Nitrate (NO3-) removal in riparian zones bordering agricultural areas occur s via plant uptake, microbial immobilisation and bacterial denitrification. Denitrification is a desirable mechanism for removal because the bacterial conversion of NO3- to N gases permanently removes NO3- from the watershed. A field and laboratory study was conducted in riparian soils adjacent to C arroll Creek, Ontario, Canada, to assess the spatial distribution of denitr ification relative to microbial community structure and microbial functiona l diversity. Soil samples were collected in March, June, and August 1997 at varying soil depths and distances from the stream. Denitrification measure ments made using the acetylene block technique on intact soil cores were hi ghly variable and did not show any trends with riparian zone location. Micr obial community composition and functional diversity were determined using sole carbon source utilization (SCSU) on Biolog(R) GN microplates. Substrat e richness, evenness and diversity (Shannon index) were greatest within the riparian zone and may also have been influenced by a rhizosphere effect. A threshold relationship between denitrification and measures of microbial c ommunity structure implied minimum levels of richness, evenness and diversi ty were required for denitrification.