Regulation of nitrification in aquatic sediments by organic carbon

Nitrification, the microbial conversion of ammonium to nitrate, is an important transformation in the aquatic nitrogen cycle, but the factors regulating nitrification rates in freshwater ecosystems are poorly understood. We investigated the effects of organic carbon quantity and quality on nitrification rates in stream sediments. First, we hypothesized that when environmental C:N ratios are high, heterotrophic bacteria are subject to N limitation and will outcompete nitrifying bacteria for available NH4+, thereby reducing nitrification rates. In laboratory experiments, organic carbon amendments (30 mg C L-1, as glucose) to stream sediments completely inhibited nitrification with or without addition of NH4+ (P < 0.0001), whereas amendment with NH4+ only (0.75 mg N L-1) increased nitrification by 40% compared with unamended controls (P < 0.0001). Carbon amendments also increased microbial respiration rates over controls by 4-6 times. Therefore, organic carbon additions significantly decreased nitrification rates but increased total microbial activity. Second, we hypothesized that carbon of high quality would have a stronger negative effect on nitrification than would carbon of low quality. To stream sediments, we added organic carbon as either glucose (higher quality) or sugar maple leaf extract (lower quality). Nitrification rates were reduced by the addition of either organic carbon source but were more severely inhibited by glucose (P = 0.001). Our results suggest that organic carbon is an important regulator of nitrification rates and is of key importance in understanding N dynamics in freshwater ecosystems.