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

Ferdouse Zaman Tanu,Ko Hinokidani,Yasuhiro Asakura,Yasuhiro Nakanishi,Satoshi Takahashi

doi:10.3390/f11020133

Ferdouse Zaman Tanu, Ko Hinokidani + Show 3 more

Open Access

https://doi.org/10.3390/f11020133

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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.

Highlights

Mangrove forests are ecologically important in trapping and storage sediments, nutrients, and heavy metals from inland runoff [1,2,3,4] and protect the surrounding ocean ecosystems from eutrophication [5,6]
Investigation of the site-specific responses of mangrove forests to anthropogenic disturbances is important for understanding the changes of the natural condition and how the mangroves are effective as ecological indicators of the changes of the ecosystems
The mean ± standard error (SE) of foliar N, carbon to nitrogen (C/N) ratio, and δ15 N for the mangrove and the non-mangrove leaf samples used in this study are provided in Supplementary Table S2

Summary

Introduction

Mangrove forests are ecologically important in trapping and storage sediments, nutrients, and heavy metals from inland runoff [1,2,3,4] and protect the surrounding ocean ecosystems from eutrophication [5,6]. The ongoing increase of anthropogenically produced reactive nitrogen (N) in the environment is one of the major causes of impairment to the natural condition and the valuable ecosystem services of mangrove forests [7,8,9,10,11]. Investigation of the site-specific responses of mangrove forests to anthropogenic disturbances is important for understanding the changes of the natural condition and how the mangroves are effective as ecological indicators of the changes of the ecosystems

Methods

Results

Discussion

Conclusion