Influence of labile dissolved organic matter on nitrate reduction in a seepage face.

Seepage faces, the outer rim of subterranean estuaries, are an important reaction node for SGD-borne nitrate (NO3-) on a global scale. Labile dissolved organic matter (DOM) has been suggested to be a key factor constraining the NO3- removal rate in aquifer systems. To determine whether and to what extent the availability of labile DOM affects benthic NO3- reduction in seepage faces, a series of flow-through reactor (FTR) experiments with sandy sediment collected from a seepage face was conducted under oxic conditions. Experimental results revealed that the addition of labile DOM (glucose) to porewater did not trigger a significant enhancement in NO3- reduction rate. In contrast, the aerobic respiration was boosted from ca. 50 to 90 μmol dm-3 sediment h-1 by glucose amendments, accounting for approximately 70% consumption of the labile DOM pool. This rapid consumption may increase the NO3- reducing capability within the sediment, but only indirectly. Together with fluorescent DOM (FDOM) analyses, it can be inferred that NO3- reducers tend to choose sediment organic matter the prime electron donor under the experimental conditions. As a result, enrichment of DOM in seepage faces, depending on composition, might only stimulate aerobic respiration and nitrification, thus promoting the increase of ensuing NO3- fluxes to adjacent coastal waters.