||Nitrogen retention by salt marshes has been suggested as a means of mitigating the delivery of land-derived nitrogen loads to coastal waters. As land-derived nitrogen loads increase, it is unclear whether there is an upper limit to the amount of nitrogen retained by salt marshes. A long-term fertilization study in the Great Sippewissett Marsh on Cape Cod, USA, has been examining the changes to salt marsh vegetation and sediment processes as a result of increased nitrogen loading. To determine whether decadal-scale changes in nitrogen loading and sources are recorded in salt marsh sediments, we examined sediment delta N-15 and % N profiles from below low and high marsh vegetation in control and fertilized plots in Great Sippewissett Marsh. As expected, we found little change in delta N-15 values in control plots. Nitrogen burial, calculated using % N values in bulk sediments, was higher in fertilized plots, but did not increase over time. However, delta N-15 values in fertilized plots were higher than in control plots and increased over time, becoming heavier than the source fertilizer and continuing to increase linearly. The continuous increase in sediment delta N-15 values in fertilized plots over the fertilizer delta N-15 value suggests that de nitrifying bacteria are responding to the increased nitrogen load and fractionating the available nitrogen. The nitrogen that remained unaccounted for by burial led us to conclude that 47 to 80% of the fertilizer nitrogen was denitrified.