Anthropogenic nitrogen accumulation potential of Okinawa mangroves in Japan

The extent of the stable nitrogen (N) isotope ratio (δ15N) of mangrove leaves reflects the anthropogenic N accumulation potential of mangroves. Therefore, the present study was designed to investigate the N accumulation potential of Okinawa mangroves in Japan using three ecological indicators from four mangrove watersheds. The dissolved inorganic nitrogen (DIN) concentration in mangrove creeks, the leaf δ15N and the soil δ15N are considered immediate indicators, short-term indicators, and long-term indicators, respectively. The four mangrove watersheds are classified into two groups, human-affected and forested mangroves, based on the relative land use ratio (%) of the watersheds. The observed values of the ecological indicators were subsequently compared between two groups of watersheds to determine the relative ecosystem conditions. The results showed that both the leaf δ15N (0 to 9 ‰) and the soil δ15N (1.5 to 8.0 ‰) values are significantly greater in human-affected mangroves than in forested mangroves. The DIN of creek water samples does not indicate an immediate risk of excess N input from human perturbation in mangroves. However, the significant relationships among the indicators reflect the anthropogenic N accumulation potential of mangroves in Okinawa, Japan. These findings are highly important, especially for policymakers, environmentalists, and related stakeholders, for initiating conservation and management practices for mangroves. Controlled, limited, and/or restricted human perturbation; proper management of municipal wastes; and planned use of agrochemicals upon necessity may help reduce anthropogenic N inputs in mangrove ecosystems in Okinawa.