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Biogeochemical cycles: Global approaches and perspectives

Abstract:

This chapter uses a literature review to advance our understanding of the biogeochemical functioning of mangrove wetlands and to elucidate biogeographic differences. We identify potential sources of variation in biogeochemical processes among various locations and analyze current advances in evaluating transformations of carbon (C) and other critical elements (e.g., N, P, S, Fe, Mn) in the context of mangrove conservation and management priorities. We also identify knowledge gaps and research priorities across biogeographic regions and latitudes. Mangrove forests inhabit unique locations at the interface between marine and terrestrial environments and, as such, possess attributes that characterize both environments. Because of their transitional position in coastal regions, mangrove forests around the world are increasingly vulnerable to anthropogenic activities (e.g., deforestation and urban settlement) and associated environmental impacts. This threatens the critical ecosystem services provided by mangrove wetlands through their biogeochemical functions, including climate change mitigation, flood regulation, and water purification. Key differences in mangrove functionality among regions are currently difficult to explain due to limited data availability, further confounded by variations within and among mangrove forests depending on hydrological regime, mangrove ecotype, and local geomorphology. Rates and pathways of microbial C and nutrient transformation in mangrove sediments are dependent on a number of fundamental factors including organic matter input, electron acceptor availability, bioturbation activity, and presence/density of tree roots, as well as local hydroperiod. The spatial heterogeneity of redox processes caused by burrows and roots (oxygen pumping) is much more complex and variable in intertidal mangrove sediments than in adjacent coastal and oceanic sediments. Nitrogen and phosphorus are critical nutrients that regulate the magnitude and spatial distribution of both net primary productivity and structural properties of mangrove wetlands. The waterlogged and anoxic mangrove sediments promote slow decomposition rates allowing significant C sequestration and long-term organic C accumulation in the sediments. Accordingly, mangrove wetlands have a strong climate change mitigation function and thus act as sinks for atmospheric CO2. In addition, emissions of other greenhouse gases, methane and nitrous oxide, from mangrove sediments are usually low because of their oxidation before reaching the sediment-air interface. High rates have only been reported under excessive anthropogenic nutrient inputs.