We measured fluxes of NH 4 + and NO 3 − and δ15N of NH 4 + , sediment, and porewater NH 4 + from incubated sediment cores along a nitrate gradient and in different seasons from Childs River, MA. NH 4 + flux was low at the downstream site with the lowest concentration of organic matter (high salinity) but otherwise did not differ along the estuary. The δ15N of regenerated NH 4 + ranged from +6.1‰ to +15.3‰ but did not vary significantly with season or salinity; the mean for the entire estuary was +10.4 ± 0.5‰. Based on differences between the δ15N of regenerated NH 4 + and sediment, and expected isotopic fractionation due to remineralization, we concluded that nitrification occurred after remineralization of NH 4 + . Differences between the δ15N of regenerated NH 4 + and the δ15N of porewater NH 4 + provided further evidence of nitrification. We estimated that 11% to 48% of remineralized NH 4 + underwent coupled nitrification–denitrification before release into the water column. In spite of losses to denitrification, NH 4 + flux released 1.4 mol N m−2 year−1 to the water column and could provide 42% of phytoplankton nitrogen requirements.