Elucidation of nitrate reduction pathways in anaerobic bioreactors using a stable isotope approach

Abstract

Leachate recirculation allows an increase of moisture content and the enhancement of the anaerobic digestion of wastes in landfill. Since there is no ammonia elimination process in landfill when leachate is recirculated, NH R 4 may accumulate. One strategy for NH R 4 removal is to treat aerobically the leachate outside the landfill to convert NH R 4 into NO S 3 . When nitrified leachate is recirculated, denitrification should occur in the waste. We have previously shown that wastes have a large capacity to convert nitrate into N 2 . Nevertheless, in some cases we observed nitrate reduction without gaseous nitrogen production. Using a stepwise multiple regression models, H 2 S concentration was the unique parameter found to have a negative effect on N 2 production. We then suspected that dissimilatory nitrate reduction to ammonium (DNRA) occurred in the presence of H 2 S. In order to verify this hypothesis, 15 N nitrate injections were performed into microcosms containing different H 2 S concentrations. The ammonium 15 N enrichment was measured using an elemental analyser coupled to an isotope ratio mass spectrometer. In the two microcosms containing the highest H 2 S concentrations, the ammonium was 15 N enriched and at the end of the experiment all the added nitrate was converted into ammonium. For the two microcosms containing the lowest H 2 S concentrations, no 15 N enrichment of ammonium was observed. This isotopic approach has allowed us to demonstrate that, in the presence of significant concentrations of H 2 S, denitrification is replaced by DNRA.

Impact of nitrate-enhanced leachate recirculation on gaseous releases from a landfill bioreactor cell

“…The nitrate injected can be converted into nitrogen gas via heterotrophic denitrification or into ammonia (NH 3 ) via a dissimilatory nitrate reduction to ammonium (DNRA) (Mazeas et al, 2008). Denitrification is often the prevalent process and the potential of MSW for rapid denitrification has been confirmed in a prior study by enumeration of denitrifying bacteria (El-Mahrouki and Watson-Craik, 2004).…”

The microbial nitrogen cycle

“…It is mostly assumed that denitrification is the major nitrate removal pathway in many anoxic ecosystems, and thus unfortunately in many studies (including this issue) little attention is paid to ammonium production by DNRA. Luckily by renewed application of isotopic and molecular methods the quantification of DNRA, denitrification and anammox in natural and man‐made ecosystems is possible and can thus be used in more studies to understand the factors controlling these processes (Christensen et al ., 2000; Strohm et al ., 2007; Tomaszek and Gruca‐Rokosz, 2007; Mazeas et al ., 2008).…”

The environmental controls that govern the end product of bacterial nitrate respiration

“…Two microbial processes compete for nitrate as an electron acceptor: denitrification (including anammox) and ammonification (including dissimilatory nitrate reduction to ammonium). The carbon/nitrogen ratio (2-5), pH (5), nitrite versus nitrate concentration (5-7), soil sand content (5), availability of fermentable carbon compounds (4,8), temperature (7,9), and sulfide concentration (10)(11)(12) are potentially important environmental controls on this competition. The lack of consensus regarding which factors are most important, and in which environments, is likely due to the complex and highly variable structure of natural microbial communities.…”