Abstract
Nitrogen (N) cycling in mangroves is complex, with rapid turnover of low dissolved N concentrations, but slow turnover of particulate N. Most N is stored in soils. The largest sources of N are nearly equal amounts of mangrove and benthic microalgal primary production. Dissolved N fluxes between the forests and tidal waters show net uptake, indicating N conservation. N2-fixation is underestimated as rapid rates measured on tree stems, aboveground roots and cyanobacterial mats cannot currently be accounted for at the whole-forest scale due to their extreme patchiness and the inability to extrapolate beyond a localized area. Net immobilization of NH4+ is the largest ecosystem flux, indicating N retention. Denitrification is the largest loss of N, equating to 35% of total N input. Burial equates to about 29% of total inputs and is the second largest loss of N. Total inputs slightly exceed total outputs, currently suggesting net N balance in mangroves. Mangrove PON export equates to ≈95% of PON export from the world’s tropical rivers, but only 1.5% of the entire world’s river discharge. Mangrove N2O emissions, denitrification, and burial contribute 0.4%, 0.5–2.0% and 6%, respectively, to the global coastal ocean, which are disproportionate to their small worldwide area.
Highlights
Nitrogen (N) is the most important nutrient element in fostering growth, reproduction, productivity, and other energetic and physiological processes that enable ecosystems to thrive
Dissolved organic and inorganic N in mangrove tidal waters are dominated by dissolved organic nitrogen (DON), followed in descending order by ammonium (NH4 + ), nitrate (NO3 − ) and nitrite (NO2 − )
Not included in the budget are: (1) direct inputs from groundwater and upstream; (2) marine and terrigenous particle flux and deposition at the soil surface; (3) pelagic and benthic production; (4) dry deposition; (5) consumption and assimilation by fauna and flora; (6) N2 -fixation on tree stems, cyanobacterial mats, aboveground roots, senescent leaves and litter; the latter are not included because of the inability to extrapolate these rates to more than a small area given the lack of knowledge of their areal coverage in a “typical” mangrove forest, and (7) rates of soil N transformations such as ammonification and denitrification likely account for only a part of total N flux in soils as most studies measured these rates only to soil depths of 5–20 cm
Summary
Nitrogen (N) is the most important nutrient element in fostering growth, reproduction, productivity, and other energetic and physiological processes that enable ecosystems to thrive Mangrove forests and their associated waterways are major coastal ecosystems that live along the world’s subtropical and tropical coastlines, requiring nitrogen and other nutrients like all other ecosystems [1]. Mangroves are an important ecological and economic resource, offering a wide variety of ecosystem goods and services, such as being important breeding sites and nursery grounds for birds, fish, crustaceans, amphibians, shellfish, reptiles, and mammals [2] These tidal forests are a potentially renewable resource of wood and accumulate sediment, carbon, contaminants and nutrients, such as N. We will assess the standing stocks of N in the forests and associated waterways, prior to examining functional processes, including the significance of N retention and the contribution of mangroves to N flow in the global coastal ocean
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