Abstract
Nitrogen based eutrophication of ecosystems is a global problem that gains momentum through a growing global population. The water quality of nitrate or ammonium contaminated rivers and streams cannot always be amended in centralized waste water treatment plants. Field denitrification plants were suggested as a solution for a decentralized reduction of nitrate to dinitrogen. Here, stable and cheap organic carbon sources serve as carbon and electron source for a microbial community. Still, our knowledge on the impact of these organic carbon sources on the development and diversity of these cultures is sparse. Moreover, the stability of these denitrification plants at different nitrate loading rates especially in the higher concentration regime were not tested so far. In this study, we compare the fate of carbon and nitrogen as well as the microbial community of wood pellet (WP) (pressed sawdust), wheat straw, and wood chips (WC) based laboratory denitrification reactors. Our study reveals that the diversity and composition of the community is strongly dependent on the carbon source. The diversity decreased in the order WC, wheat straw, and WPs. The three reactor types were characterized by different nitrate reduction kinetics and were affected differently by high nitrate loading rates. While the nitrate reduction kinetics were negatively influenced by higher nitrate doses in the wheat straw reactors, WPs as carbon source sustained the opposite trend and WC lead to an overall slower but concentration independent nitrate reduction rate. Counterintuitively, the concentration of soluble organic carbon was highest in the WP reactors but methane emission was not detectable. This is corroborated by the microbial diversity data in which methanogenic species were highly underrepresented compared to the other two reactor types. In contrary, the methane emissions in the wheat straw and WC reactors were comparable to each other.
Highlights
Microorganisms, especially bacteria participate in important steps of the terrestrial nitrogen cycle
We revealed that the microbial community changed depending on the nitrate loading rate and that high nitrate loading rates could lead to a diversification of anaerobic nitrate reduction toward the production of ammonium through dissimilatory nitrate reduction to ammonium (DNRA), which contradicts to textbook knowledge claiming that DNRA should be favored under high Corg/NO3− ratios
Compared to Wood chips (WC) as carbon source, the wheat straw and Wood pellets (WP) operated reactors were characterized by higher maximum contents of dissolved total organic carbon (TOC) (Figure 1)
Summary
Microorganisms, especially bacteria participate in important steps of the terrestrial nitrogen cycle. To nitrogen fixation and nitrification one essential process is denitrification In this part of the nitrogen cycle nitrate is reduced over several intermediates (NO2, NO, and N2O) to atmospheric nitrogen. Denitrification is necessary to prevent an imbalance in the nitrogen cycle in favor of Denitrifying Microbial Communities nitrogen input This imbalance is mainly caused by the industrial NH4+ production via the Haber–Bosch process, which leads to a nitrogen fixation of 9.7 × 1012 mol year−1 (Canfield et al, 2010), whereof a significant portion is introduced into the environment (Erisman et al, 2008). Mainly urea as a component of manure is used in the agricultural sector, which is a main contributor to water pollution. One consequence of high inputs of fertilizer is the spreading of nutrients (nitrate, ammonium, and phosphate) into fresh and ground water, which causes eutrophication and a loss of biodiversity (Vitousek et al, 1997)
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