Biogeochemical Drivers of Location‐Specific Nitrogen Cycling Processes in Salt Marshes

Abstract Salt marshes remove nitrogen (N) from tidal water via denitrification (DNF) but simultaneously retain N in sediments via dissimilatory nitrate reduction to ammonium (DNRA). The rate of N removal may be governed by NO 3 − supply through nitrification and tidal exchange and the relative partitioning of NO 3 − to DNF or DNRA. Although both DNF and nitrification are inhibited by sulfide (H 2 S), DNRA may be favored by high H 2 S and dissolved organic carbon to nitrate (DOC:NO 3 − ) ratios, conditions often found in pore water of low‐elevation interior marshes. We compared seasonal rates of DNF and DNRA in control and fertilized plots across three locations with varying geochemical characteristics. Fertilization stimulated DNF regardless of location, whereas DNRA responses to fertilization were variable. DNRA rates ranged 0%–90% and averaged 21.6% of total nitrate reduction but varied by season, location, and N enrichment status. Structural equation modeling suggested that DNF rates were directly influenced by fertilization, DOC:NO 3 − ratio, and temperature, but DNRA rates were promoted primarily by temperature and sulfide. This indicates that high sulfide concentrations may promote DNRA as an important pathway for NO 3 − reduction. This study highlights how nitrogen removal (via DNF) and nitrogen retention (via DNRA) are regulated by distinct biogeochemical factors. This indicates that salt marshes do not universally act as nitrogen sinks; their function may be location‐specific and influenced by environmental conditions such as temperature, sulfide concentrations, and DOC:NO 3 − ratios.