Assimilation and short‐term processing of microphytobenthos nitrogen in intertidal sediments

Microphytobenthos (MPB) are an important nitrogen (N) sink in coastal systems, but little is known about the fate of N assimilated by MPB. We used an in situ 15 N pulse‐chase experiment in intertidal, nonpermeable, sandy mud to trace the assimilation, transformation, and loss from the sediment of MPB‐N over 31 d. Following assimilation, 15 N was tightly retained in surface sediments, with only up to 8.1% transported to 2–10 cm sediments over 31 d. MPB accounted for a considerable portion of the 15 N in surface sediments throughout the study (59% ± 13%). Bacteria rapidly assimilated 15 N but accounted for only up to 17% of the 15 N within surface sediments. Of the assimilated 15 N, 78.9% was lost from the sediment over 31 d. Resuspension was the dominant loss pathway (74.4%) and was primarily associated with minor flooding following a rainfall event at 8 d. Biological pathways for N export were far less important. Denitrification, during both sediment exposure and inundation, was the main biological pathway for 15 N loss (2.6% total), whereas there was little loss through ammonification and nitrification (NH 4 + or NO x efflux; 1.4% combined) and dissolved organic nitrogen efflux (0.5%). It was estimated that the N incorporated by MPB within one emersion period would take ~ 40 d to be entirely removed from the sediment. This study highlights the potential importance of MPB and intertidal sediments for the uptake and longer‐term storage of coastal N, and the need to better quantify the impact of episodic flooding on coastal N budgets.