Organic Carbon Quality Impacts Benthic Microbial Nitrate Reduction
Résumé
Microbial nitrate reduction in wetland soils and sediments plays an important role mitigating the excess of nitrogen released into the environment. The reduction of nitrate to its reduced gaseous form, i.e. denitrification, is tightly linked to the oxidation of organic carbon. In intertidal wetlands, the organic carbon derived from aquatic or terrestrial primary producers strongly differs in composition, which in turn might affect nitrate reduction rates. In the current study the impact of carbon quality on benthic nitrate reduction in a temperate intertidal wetland sediment was determined. To this end, nitrate reduction rates were determined over a three-week period using an anoxic controlled flow-through reactor approach using sediments amended with carbon from different vegetation types. Organic carbon from algal or plants was mixed with the sediment at the same absolute quantity of carbon. A significant initial increase in nitrate reduction rates was observed in the presence of algal (microphytobenthos) derived organic carbon, most likely due to its labile structure; however, rates decreased rapidly, most likely due to a depletion of the labile source. This indicates the dependence of denitrifiers to the microalgae turnover in non-vegetated sediment. Despite a delay, the addition of plant derived organic carbon, increased nitrate reduction rates with a higher amount of total nitrate reduced over the duration of experiment compared to the microphytobenthos addition. These results highlight that presence of both labile and recalcitrant carbon sources in sediment is essential to support an efficient nitrogen biogeochemical cycle.
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