Natural abundance of N stable isotopes used in combination with concentrations may be useful indicators of N-cycling in wetlands. Concentrations and 15N signatures of NO −3 , NH + 4 , and sediment organic nitrogen (SON) were measured in two impacted coastal golf course retention ponds and two natural marshes. Limited NO − 3 was detected in natural site surface water or pore water, but both isotopic signature and concentrations of NO − 3 in surface water of impacted sites indicated anthropogenic inputs. In natural sites, NH + 4 concentrations were greatest in deeper pore water and least in surface water, suggesting diffusion predominates. The natural sites had greater %SON, and 15N indicated that the natural sites also had greater NH + 4 released from SON mineralization than impacted sites. In NO − 3 -limited systems, neither concentrations nor 15N natural abundance was able to provide information on N-cycling, while processes associated with NH + 4 were better elucidated by using both concentrations and 15N natural abundance.