Variation in Foliar ẟ15N Reflects Anthropogenic Nitrogen Absorption Potential of Mangrove Forests

Abstract

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.

An Assessment of Mangrove Ecosystem Condition by Foliar Stable Nitrogen Isotope Ratio Index in Okinawa, Japan

“…A part of the land-use maps of the watersheds, presented in Fig. 1 to 3, was adopted from the supplementary data in Tanu et al [23] and a part of those was outlined by using ArcMap Geographic Information Services (GIS) software (ArC Map 10.4.1). The source data for the land-use were collected from the National Land Policy Bureau, Japan Ministry of Land, Infrastructure, Transport, and Tourism.…”

“…Based on the foliar δ 15 N values mangroves on three islands named Okinawa, Ishigaki and Iriomote in Japan have previously been reported as intensively anthropogenically impacted, moderately anthropogenically impacted and pristine mangroves, respectively [23]. Furthermore, the land-use showed distinct influence on leaf and water N in mangrove watersheds on Ishigaki Is.…”

“…As well, a complex setting of ecological and environmental processes [16][17][18][19] coupling between land and estuaries have made it difficult to firmly conclude the responses of mangroves to nutrient cycling specifically nitrogen (N) and land use [20][21]. However, being generally oligotrophic in nature and capable of capturing excess N from the environment, mangroves have been using as long-term monitoring tool for eutrophication trends in the ecosystems for decades [22][23][24][25][26]. In contrast, few studies have also found that mangroves are able to add nitrogen (N) to estuaries [27].…”

“…Incorporation of 15 N-enriched nitrogenous ions leads to generally higher values of 15 N ratio (δ 15 N) in terrestrial and aquatic plants [25,[29][30][31][32]. Hence, foliar δ 15 N composition of coastal plants have been using as useful indicator for assessing the differences between pristine and anthropogenically impacted ecosystems [22][23][24][25]27,[29][30][31][32][33][34][35][36]. However, such an assessment is not sufficient to understand the degree of changes in anthropogenically impacted mangroves from the natural condition.…”

“…Hence, to clarify the degree of impacts is essential to undertake conservation actions in those mangrove watersheds on priority basis. In previous studies, mangrove stands (regardless of species variation) having an average foliar δ 15 N up to 3‰ have been considered as natural mangrove sites [22][23][24][25][26]37,38]. Henceforth, this value could be the base of reference site to which other mangroves would be compared.…”