Abstract
Carbon cycling within the deep mangrove forest floor is unique compared to other marine ecosystems with organic carbon input, mineralization, burial, and advective and groundwater export pathways being in non-steady-state, often oscillating in synchrony with tides, plant uptake, and release/uptake via roots and other edaphic factors in a highly dynamic and harsh environment. Rates of soil organic carbon (CORG) mineralization and belowground CORG stocks are high, with rapid diagenesis throughout the deep (>1 m) soil horizon. Pocketed with cracks, fissures, extensive roots, burrows, tubes, and drainage channels through which tidal waters percolate and drain, the forest floor sustains non-steady-state diagenesis of the soil CORG, in which decomposition processes at the soil surface are distinct from those in deeper soils. Aerobic respiration occurs within the upper 2 mm of the soil surface and within biogenic structures. On average, carbon respiration across the surface soil-air/water interface (104 mmol C m−2 d−1) equates to only 25% of the total carbon mineralized within the entire soil horizon, as nearly all respired carbon (569 mmol C m−2 d−1) is released in a dissolved form via advective porewater exchange and/or lateral transport and subsurface tidal pumping to adjacent tidal waters. A carbon budget for the world’s mangrove ecosystems indicates that subsurface respiration is the second-largest respiratory flux after canopy respiration. Dissolved carbon release is sufficient to oversaturate water-column pCO2, causing tropical coastal waters to be a source of CO2 to the atmosphere. Mangrove dissolved inorganic carbon (DIC) discharge contributes nearly 60% of DIC and 27% of dissolved organic carbon (DOC) discharge from the world’s low latitude rivers to the tropical coastal ocean. Mangroves inhabit only 0.3% of the global coastal ocean area but contribute 55% of air-sea exchange, 14% of CORG burial, 28% of DIC export, and 13% of DOC + particulate organic matter (POC) export from the world’s coastal wetlands and estuaries to the atmosphere and global coastal ocean.
Highlights
IntroductionMangrove forests have the largest organic carbon (CORG ) stocks of any tropical terrestrial or marine ecosystem [1,2], with a global mean total forest stock of 738.9 ± 27.9 (±1 standard error, SE)
Mangrove forests have the largest organic carbon (CORG ) stocks of any tropical terrestrial or marine ecosystem [1,2], with a global mean total forest stock of 738.9 ± 27.9 (±1 standard error, SE)Mg CORG ha−1, of which 76.5% is stored in the soil, 14.8% vested in aboveground biomass and the remaining 8.7% vested in belowground roots [3]
This paper critically examines the traditionally measured rates of soil respiration and compares these data with rates of belowground dissolved inorganic carbon (DIC) production, highlighting any discrepancies to accurately estimate rates of CORG mineralization throughout the deep forest floor
Summary
Mangrove forests have the largest organic carbon (CORG ) stocks of any tropical terrestrial or marine ecosystem [1,2], with a global mean total forest stock of 738.9 ± 27.9 (±1 standard error, SE). After initial colonization of a mudflat, the forest develops and the floor builds up further, adjusting to sea-level, subsidence, and uplift, with the net result being several meters of soil [9] Over time, these deposits are penetrated further by mangrove trees and their extensive root systems, various other flora (e.g., microalgae) and fauna, especially burrowing crabs, and highly abundant and productive microbial communities [10]. This paper critically examines the traditionally measured rates of soil respiration and compares these data with rates of belowground DIC production, highlighting any discrepancies to accurately estimate rates of CORG mineralization throughout the deep forest floor This information is compared with recent estimates of DIC export via subsurface pathways to determine its significance in mangrove carbon cycling and the functional links between the mangrove forest floor and the adjacent coastal ocean. A revised carbon mass balance for the world’s mangroves is presented to identify the major and minor pathways of carbon flow as well as what further empirical measurements are needed to improve our knowledge of carbon balance in these tidal ecosystems
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