In this research, the effect of Natural abundance of N stable isotopes used in combination with concentrations is studied because possibly those may be useful indicators of N-cycling in wetlands. Concentrations and 15N signatures of nitrate and ammonium ions, and sediment organic nitrogen (SON) were measured in two impacted coastal gulf area retention ponds and two natural marshes. Limited NO3- was detected in natural site surface water or pore water, but both isotopic signature and concentrations of NO3- in surface water of impacted sites indicated anthropogenic inputs. In natural sites, NH4+ 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 NH4+ released from SON mineralization than impacted sites. In NO3--limited systems, neither concentrations nor 15N natural abundance was able to provide information on N-cycling, while processes associated with NH4+ were better elucidated by using both concentrations and 15N natural abundance.