Research Highlights: Mangrove forests are absorbing anthropogenically produced excess nitrogen under moderate to intensive human interaction in the study sites, further indicating the degree of deviation from the natural ecosystem condition. Background and Objectives: Mangrove species, when directly connected to anthropogenic activities such as sewage disposal, agricultural inputs, and receiving of animal manure, absorb excess nutrients from the systems and act as ecological indicators of long-term natural changes. However, there is a paucity of examples of how the mangroves respond to a land-use gradient comparing to the non-mangrove plants under indirect anthropogenic impacts. Materials and Methods: In this investigation, foliar total nitrogen (N), carbon to nitrogen (C/N) ratio, and δ15N of mangrove and non-mangrove species collected from 15 watersheds on three islands in Okinawa, Japan, have been compared. The land-use areas in the study watersheds were delineated by ArcGIS software, and the correlation between the foliar traits and the human-affected area ratios were examined. Results: Foliar δ15N of the mangroves, which was significantly different from those of the non-mangroves on each island, showed significantly higher values (5‰ to 14‰) in human-affected forests, whereas the values were up to 3‰ in pristine forests. Furthermore, the significant positive relationship between foliar nitrogenous traits and the human-affected area ratios suggested that the anthropogenic N might be regulating foliar N content and δ15N signature on the sites. Conclusion: Different degrees of foliar isotopic fractionation with the land-use gradient have clarified that mangroves can be a powerful tool for monitoring ecosystem conditions under anthropogenic disturbances.