Biogeochemistry of methane emission in mangrove ecosystem - Review
Goutam Kumar- &AL. Ramanathan
School of Environmental Sciences, Jawaharlal Nehru University, New Delhi-ll0067, India.
[E-mail: goutamses@gmail.com ]
Received 17December 2012; revised I May 2014
Mangrove plays a pivotal role in the nutrient biogeochemical processes by behaving as both source and sink for nutrients and other materials. Litter from mangroves added major part ofthe carbon and fresh water sources from estuary enrich them with nutrients. Microbial processes playa vital role in the dynamics of nutrients and its cycling. In the anoxic condition microbes convert organic carbon into methane and other gases. Spatial and temporal variation of methane emission is dependent on complex biogeochemical processes as well as the climatic conditions existing in the mangrove ecosystem. Methane emission from mangrove ecosystems is perceived to be the major source of green house gases in the environment.
[Keywords: Mangroves ecosystem, Methane emission, Soil biogeochemistry, Primary productivity]
Introduction
Tropical coastal mangrove ecosystems are one of the most productive ecosystems of the world' and are characterized by high turnover rates of organic matter and nutrient recycling between the marine and terrestrial ecosystems". The degree of estuarine processing is related to the estuarine freshwater residence time and additional organic flux may fuel intense microbial 02 depletion". It gives rise to important gaseous by-products including methane (CH4), which consequently are available for sea-to- air-gas transfer', Organic matter plays a significant role on soil redox potential along with ferrous iron.
The potential thus obtained is vital for the conversion of substrates like H2; CO2, acetate and methanol, to methane. Under anoxic condition and high temperature, mangrove sediments are strong source of atmospheric methane". Spatial and temporal variation of methane emission is found with respect to numerous natural and anthropogenic sources and wetlands lying between 200N to 300S latitude released
*For correspondence:
approximately 60% of total emission of natural wetlands". According to the last report Intergovernmental Panel on Climate Change (IPCCY, the atmospheric concentration of methane has increased by 2.5 times since the pre-industrial era, although, during the last 2 decades, annual increase in atmospheric methane concentrations has declined from 1 to 0.5% 8. Methane is one of the major atmospheric trace gases and it contributes significantly (15%) to global warming due to its radiative forcing. Several authors reported methane emission from mangroves 5,9-16. Methane (CH4) emission from mangrove sediments are potential sources of greenhouse gas to the atmosphere and as such may contribute to global climate change 5,17-19.
Nutrients in the sediments in the mangrove area The distribution of Total Carban (TC) in mangrove sediments is a function of microbial activities". The high amount of Organic Carbon (OC) in mangrove sediments is due to the presence of fine
mol C m-2 year-129-30. Through an alternate approach, the estimation of Net Production(NP) is done by analyzing root and wood production which varies longitudinally and a global average of wood production is 67 mol C m-2 year-1 29 and root production is 44 mol C m-2 year-1 31-32. It has been reported that micro benthos in the mangrove ecosystems account benthic primary production and range between 7 and 73 mol C m-2 year-1 33-35. The inputs from microalgae are generally considered to be low due to light limitation or inhibition by tannins36. Some studies indicate that they may contribute significantly under certain conditions (110–118 mol C year-2 d-1 for lagoon systems) 37.
Origin of mangrove organic carbon
High input of organic carbon to some mangrove sediments may be from dense microbial mates.38 Algal material can represent a significant fraction of sedimentary organic carbon, particularly in the early stages of mangrove forest39-40. The large majority of mangrove sediments have POC/PN ratio more than 10, which is typical for subtidal marine sediments.
The large fraction of having relatively high POC/PN ratio indicates that mangrove sediments contain a significant input of mangrove litter 41.
Soil biogeochemistry and methane emissions Soil geochemistry
The soil Eh at the swamp remained low due to continuously inundated conditions. Soil O2 concentration generally showed a similar pattern as the soil Eh measurement during each hydrological season with a statistically significant correlation between the soil Eh values and O2 concentrations. The only insignificant relation (P = 0.07) was found at the swamp where the Eh and O2 concentrations were at the extreme lower ends of the scale42. Redox potential (Eh) is a convenient measure to state whether the soils are in reducing or anaerobic condition.
Oxygen supply becomes limited as it is rapidly consumed by bacterial respiration. Organic matter plays a significant role on the soil redox potential along with ferrous iron. Potential thus obtained is vital grained particles which have more surface area and
consequently leads to increase in sorption of more Organic Matter21 (OM). Further, the water residence time in interior mangrove lagoon is higher than the adjacent two estuaries22. The phenomenon of co- deposition of organics and clay particles to their close hydraulic equivalence and/or to the relatively high absorptive capacity of clays for organic molecules.23 Total phosphorus (TP) levels in the mangrove sediments are within the global limits (0.1–16 mg/
g)24. Phosphorus (P) levels in this ecosystem are controlled by the weathering of the phosphate nodules present in the drainage area25 and agricultural runoff, i.e. the adjacent agricultural fields are applied with di-ammonium phosphate and NPK fertilizers.26-27 The correlation coefficient between OC vs. TP is very low (R2 = 0.03, P < 0.05) which shows that P is the limiting factor for biomass production. Sulfur concentration levels is higher in the interior mangrove sediments than the estuarine sediments28. In the mangrove sediment, sulfate reduction is the dominant mineralization processes up to the depth of 1 m20. The carbon dynamics in the mangrove ecosystem is influenced by the sulfate reduction.
Organic carbon inputs in the mangrove ecosystem Mangrove ecosystem productivity
The annual litter fall is the proxy estimate of mangrove productivity, its latitudinal variation shows that highest is being close to the equator 29. Generally, global average litterfall rates are in the order of ~38
for the conversion of substrates like H2, CO2, acetate and methanol, to methane. In the estuarine environment, soils are mostly acidic in nature with low-tide period; methane concentration follows the trend of redox potential of the soil43.
Soil Biogeochemistry
In estuaries, methane production occurs that pre vail due to presence of typical anaerobic conditions due to flooding which alters microbial flora in the soils to enrich facultative or anaerobic microorganisms. Under such conditions, fermentation becomes one of the major biochemical processes responsible for organic matter decomposition, wherein oxidized compounds other than molecular oxygen act as an electron acceptor. Therefore, the occurrence of organic matter is an essential requirement for generation of methane. Main products of fermentation are ethanol, acetate, propionate, H2, N2, CH4 and CO2. Two major pathways are commonly observed in submerged soils to produce methane44. They are
(i) decarboxylation of acetic acid
CH3COO- + H+ CH4 + CO2 (ÄGo = -36 kJ mol-1) and (ii) reduction of CO2 with H2 (originated from organic litter/matter) to form methane.
The process of reduction of CO2 to CH4 with H2 or formate ion, the electron donor can be represented as follows:
4 H2 + CO2 CH4 + 2H2O (ÄGo = -103.4 kJ mol-1) 4 HCOO- + 4H+ CH4 + 3CO2 +2 H2O (ÄGo = -119.5 kJ mol-1)
The reducing condition in soils is due to the slow downward diffusion of oxygen and the presence of large quantities of Organic Matter44. During tidal mixing, the flooded soils are buffered by Fe, Mn and Al oxides/hydroxides and carbonates. Anaerobic condition initially favours conversion of Mn4+ to Mn2+
and also NO3 to N2 (gaseous). After these processes, where Mn4+ and NO3- are completely consumed, Fe3+
is reduced to Fe2+ and so on, until the soil eventually reaches a highly anaerobic state. This is the condition, which favours reduction of carbon dioxide to methane. Estuarine soil chemistry represents high iron, nitrogen and manganese species which favours higher methane production44. It has been reported that sulfate reduction is dominant mineralization process upto 1 m depth which influence the carbon dynamics in the mangrove sediment20
Decomposition of organic matter in anoxic mangrove sediments mostly through sulphate- reduction with participation of sulfate reducing bacteria43,46. Desulfovibrio, Desulfotomaculu, Desulfosarcina, and Desulfococcus are primary decomposers mangrove sediments, which largely
of SO4-2, HCO3-, and Cl-, either singly or in combination, inhibited CH4 production significantly over that of unamended control 52. Sulphate is known to inhibit methanogenesis even at a very low concentration of 0.1%53. Sulphate-reducing bacteria out compete methanogens for methanogenic substrates like acetate and H2/CO2 and thereby inhibit the production of CH454-55. Mechanism might be osmotic imbalance by affecting the cation-anion equilibrium that in turn may adversely affect the activity of methanogens and/or microorganisms producing substrates for methanogens. Sodium is required for amino acid transport, growth, methanogenesis, and internal pH regulation in methanogenic bacteria. Most methanogens possess an inwardly directed gradient of sodium ions. Thus the inhibitory effect of NaCl as recorded could be due to Cl- content of the salt52. Bicarbonate ion (HCO3-) can adversely affect the uptake and metabolism of nutrients in higher plants and also affect the Na+/K+ ratio. It is possible that soil amendment with HCO3- also adversely affect soil microflora including methanogens and their activities52. Anion-mediated salinity responsible for inhibition of CH4 production followed the order of SO4- 2>HCO3->Cl-52.
Temperature
Soil temperature significantly affects the activity of soil microorganisms and resulting affects CH4 production56-57. A significant positive correlation between soil temperature and CH4 emission was also reported58. A 3–5oC temperature rise with waterlogged condition is likely to increase methane emissions 2 fold. The causative factors for this rise are (i) chemical destruction by hydroxyl radical (OH) and (ii) microbial intervention in anaerobic decomposition 59-60.
Tidal variation
Tidal variation of dissolved methane also show increasing concentrations during high tide. This indicate the occurrence of tidal flushing of mangrove swamp for exporting laterally large quantities of methane to the estuarine water 50,61.
control iron, phosphorus and sulphur dynamics in mangrove ecosystem43. Prevailing water-logged, anaerobic and rich in organic carbon environment is suitable for growth of sulfate reducing bacteria47. Competition between sulfate reducing bacteria and methanogens exist for the utilization of hydrogen and acetate to the extent that they lower the rate of methane production. In muddy, water-logged conditions, conversion of sulphate to sulphide causes toxicity to both sulfate reducing bacteria and methanogens48.
Factors affecting methane emission Input of carbon and microbial activities
Leaf decay rates of dominant mangrove species differ from species to species, which makes input of carbon for methane production besides of other components. Also, soils rich in OM exhibit increased microbial activity and thus intensify reduction to form methane. The high OC present in soil facilitates rapid microbial decomposition leads to produce high methane during winter season. Methane emission was proportional to the OC present in the soil49.
Soil salinity
Methane concentrations in the estuarine waters generally showed a decreasing trend from fresh to salt water when there was a dominant river input followed by an emission and oxidation49-51. Addition
Seasonal variation
The methane concentration is higher in late winter due to relatively high level of water vapour in the troposphere could be accounted for low dispersion in the ambient air. Also, short day lengths, low solar intensity, and low ambient temperature would reduce photochemical destruction of methane62. The dry season is observed from December to April due to less precipitation in the equatorial region. During dry season of the year, advection of air pollutants from urban areas causes atmospheric oxidation (due to reaction of (OH), hence minimum conc. of CH4 observed in April14. In the Adyar estuary, there is intense methanogenesis fueled by the high input rate of OC (largely domestic organic waste) and increased residence time of river discharge resulting exceptionally high level of dissolved methane5,63. Methane production and consumption
Methane production
Significant CH4 production can only occur under strongly reducing conditions. Critical Eh values for CH4 production in homogenous soils are normally below -150 mV 64-65. Soil CH4 concentrations reached higher than the atmospheric level when the soil Eh was lower than +300 mV, and then increased greatly as the soil Eh decreased further42. Unlike dispersed soil conditions, the strictly reducing conditions favorable for methanogenesis can be developed in inner soil microenvironment when the surrounding soil Eh is still high.
Methane consumption
Only part of the methane produced is emitted to the atmosphere. Considerable amounts are consumed by methanotrophic bacteria66-67. Re-oxidation of methane is mainly confined to the zone close to the water table, where neither the supply of oxygen nor of methane is limited. Similarly, methane consumption occurs in the oxygenated zone surrounding plant roots. Potential for methane oxidation by methanotrophics is typically an order of magnitude larger than the potential for methane production by methanogens. As a result, methanotrophic bacteria can limit the amount of methane that is released to the atmosphere substantially.
Table 1—Annual ranges of methane emission from different mangrow ecosystems.
Name of Mangrove ecosystem Annual Range of methane emission Sunderban mangrove water14. (1.97–134.6)×103 (m mol m-2 d-1) Sagar Island, NE coast, Bay of Bengal12. -5.83 – 8.88 µg m-2s-1
Shenzhen and Hong Kong mangroves, South China16. 10.10–5168.62µmol m-2 h-1 Muthupet mangrove, South India 72. 49.22–97.73 µmol m-2 h-1 Bhitarkanika mangrove, East India 69. 5.73–201.88 µmol m-2 h-1 Pichavaram mangrove, South India70. 39.17–62.06 µmol m-2 h-1 South East Queensland mangrove, Australia13. 0.19–1087.50 µmol m-2 h-1 Ranong mangrove, Thailand71. 0.49–0.35 µmol m-2 h-1
Methane emission in different mangroves
Methane emission in the mangrove ecosystem depend upon the natural and anthropogenic factors.
Organic carbon added in the mangrove ecosystem through litters and microbial biomass in the sediments. Mangrove litters under anoxic condition of the sediments and at high temperatures could be a strong source of atmospheric methane5. Conversion of organic carbon into methane depends on the organic carbon content in the sediments t45. In the mangrove dominated estuaries, high methane emission rate is resulted by nutrient from mangrove forest and riverine source, microbial processes in waters and sediments, tidal exchanges with swamps and flats and lateral movement of pore water.68
Different mangrove ecosystems having its own characteristics and anthropogenic influence upon them. Hence, the methane emission with different rates in various mangroves is noticed as under:
Perspectives and research directions
A large number of studies have significantly increased our knowledge on methane emission in mangrove systems and on the importance of various biogeochemical processes. We still lack, however, a complete understanding of the underlying mechanisms controlling the spatial and temporal variability of these processes as a function of changes in environmental conditions. Vegetation type, faunal composition, microbial processes and sediment structure changes along tidal elevation gradients, and range from more marine influenced communities near the seaward edge to a significant terrestrial imprint at the higher elevations. The variability in carbon transformations and transport conditions among mangrove environments is affected by specific local conditions with respect to climate, degree of exposure to strong water movement, the vicinity of river discharges, soil and bedrock composition in the neighboring terrestrial system, the local vegetation and fauna. Due to such inherent environmental variability combined with the rather limited data available, generalizations on a global scale become difficult. It needs further studies at different levels
and conditions to establish the correlation between environmental condition and methane emission in the mangrove ecosystem.
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