Regulation of nitrification in aquatic sediments by organic carbon

Nitrification, the microbial conversion of ammonium to nitrate, is an impor tant transformation in the aquatic nitrogen cycle, but the factors regulati ng nitrification rates in freshwater ecosystems are poorly understood. We i nvestigated the effects of organic carbon quantity and quality on nitrifica tion rates in stream sediments. First, we hypothesized that when environmen tal C:N ratios are high, heterotrophic bacteria are subject to N limitation and will outcompete nitrifying bacteria for available NH4+, thereby reduci ng nitrification rates. In laboratory experiments, organic carbon amendment s (30 mg C L-1, as glucose) to stream sediments completely inhibited nitrif ication with or without addition of NH4+ (P < 0.0001), whereas amendment wi th NH4+ only (0.75 mg N L-1) increased nitrification by 40% compared with u namended controls (P < 0.0001). Carbon amendments also increased microbial respiration rates over controls by 4-6 times. Therefore, organic carbon add itions significantly decreased nitrification rates but increased total micr obial activity. Second, we hypothesized that carbon of high quality would h ave a stronger negative effect on nitrification than would carbon of low qu ality. To stream sediments, we added organic carbon as either glucose (high er quality) or sugar maple leaf extract (lower quality). Nitrification rate s were reduced by the addition of either organic carbon source but were mor e severely inhibited by glucose (P = 0.001). Our results suggest that organ ic carbon is an important regulator of nitrification rates and is of key im portance in understanding N dynamics in freshwater ecosystems.