Long-term N-15-nitrogen retention in tidal freshwater marsh sediment: Elucidating the microbial contribution

The long-term retention of nitrogen in sediment of a tidal freshwater marsh in the Scheldt estuary (Belgium) was investigated by an in situ N-15-labeling experiment. Sediment of an unvegetated creek bank and sediment vegetated by common reed (Phragmites australis) were labeled with N-15-enriched NH4+ after which N-15 was traced into pore-water dissolved NH4+, NO3-, and N-2; plant roots and leaves; and bulk sediment over a 1-yr period. Label retention in the sediment organic matter was further characterized by analyzing KCl-extracted sediment and hydrolyzable amino acids (including the bacterial biomarker D-alanine). Within weeks all added (NH4+)-N-15 was transformed and/or assimilated by the biota. Between 42% and 48% of the added label was recovered in plants, root, and sediment (collectively) after 3-6 months, whereas 24% remained after 1 yr. Transfer to plants and roots was rapid and although retention in leaves was transient, the N-15 assimilated by roots was retained for the entire 1-yr period. Root assimilation could account for 12-23% of the N-15 retained in vegetated plots. The majority of N-15 was retained in the organic matter pool within the sediment (represented by N-15 in the KCl extracted sediment and total hydrolyzable amino acids), primarily through efficient recycling of the N-15 within the microbial community. N-15 incorporation into D-alanine confirmed that bacteria were the major group of microorganisms responsible for the strong retention of N-15. This study shows the strong potential of reed roots and especially bacteria to retain nitrogen in tidal freshwater sediment over longer periods of time.