Agricultural drainage ditches are often polluted with nitrogen, which contributes to eutrophication of the lakes, rivers and reservoirs in which they drain. Denitrification permanently removes nitrogen from these systems. However, despite a considerable research effort, factors affecting denitrification are still poorly understood. In this study we tested how denitrifier community structure and various environmental factors relate to denitrification rates. Furthermore, we studied which factors regulate abundance and richness of denitrifying microorganisms in drainage ditches. Denitrification was measured in-situ, in 13 Dutch drainage ditches using the 15N isotope pairing technique. Denitrifier community structure and abundance were analysed by denaturing gradient gel electrophoresis (DGGE) and quantitative-PCR of the nirK gene.
Denitrification rates varied widely between the ditches, and significantly differed among sediment types (fine sand, clay, peat) and vegetation types (no plants, floating plants, submerged plants). Ditches with fine, sandy sediments, that were covered by duckweed and contained high nitrate concentrations, were denitrification hotspots. Highest rates were found in ditches next to arable land, which also received highest nitrogen loads. Ditches of similar sediment and vegetation types had similar denitrifying communities. Interestingly, denitrification rates correlated to nitrate concentrations, but not to denitrifier (nirK) abundance or richness, whereas denitrifier abundance correlated to organic matter content of the sediment but not to nitrate concentration. Our results indicate that availability of organic material in the sediment influences denitrifier abundance, whereas nitrate concentrations determine instantaneous denitrification rates, creating denitrification hotspots in nitrate-rich drainage ditches.