Nitrogen Cycling and Mass Balance in the World’s Mangrove Forests

• Published in: Nitrogen (Basel, Switzerland) Vol. 1; no. 2; pp. 167 - 189
• Main Author: Alongi, Daniel M.
• Format: Journal Article
• Language: English
• Published: MDPI AG 01.12.2020
• Subjects: biogeochemistry; denitrification; ecosystem; mangrove forests; mass balance; nitrogen
• ISSN: 2504-3129; 2504-3129
• DOI: 10.3390/nitrogen1020014

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.