The use of ?15N to examine past mangrove stand structures

Twin Cays, Belize, is dominated by Rhizophora mangle L. (red mangrove). Tall (>5m in height) R. mangle are located along the fringe of the island and dwarf R. mangle grow in the interior of the island. These stand structures can be differentiated using ?13C and ?15N analysis (mean tall ?13C = -28 ?, mean tall ?15N = 0?; mean dwarf ?13C = -25?, mean dwarf ?15N = -10?), which may also prove useful in examining past mangrove stand structures from sediment cores. 15N label was traced in R. mangle leaves in a laboratory and field experiment over three months. The 15N label was examined to determine distribution of nitrogen in various biochemical fractions of the leaf and to verify if nitrogen is fractionated in a predictable manner over time. This information could be beneficial in examining past mangrove stand structures. Experimental data indicate that nitrogen is mobile within each biochemical fraction of the R. mangle leaf over time and a measurable amount of nitrogen exists in each fraction after 3 months of incubation. Nitrogen immobilization was evident in each experiment, as the ?15N values decreased ~200? in each of the labeled fractions of the laboratory experiment, which was mirrored by an increase in ?15N in the control samples. The amount of nitrogen in the biochemical fractions of the field experiment varied over time either increasing or decreasing, which may be due to the various environmental conditions such as tidal fluctuation, temperature, oxygen concentrations and microbial activity. The ?15N signature of the residual nitrogen fraction (?15N = 87?) reflected that of the bulk fraction (?15N = 133?) in the laboratory experiment as well as in the field experiment (residual nitrogen ?15N = 759?, bulk ?15N = 770?). To use isotope analysis to examine past mangrove stand structures it is essential that the original signature be maintained over time. The results of this study indicated that the simple interpretation of nitrogen isotopes may not be useful in examining past mangrove stand structures due to the variation over time, although this type of analysis may be considered if coupled with additional proxies and diagenetic factors are taken into account.