Impact of Sundarban mangrove biosphere on the carbon dioxide and methane mixing ratios at the NE Coast of Bay of Bengal, India

Abstract: We review 72 published articles to elucidate characteristics of biomass allocation and productivity of mangrove forests and also introduce recent progress on the study of mangrove allometry to solve the site- and species-specific problems. This includes the testing of a common allometric equation, which may be applicable to mangroves worldwide. The biomass of mangrove forests varies with age, dominant species, and locality. In primary mangrove forests, the above-ground biomass tends to be relatively low near the sea and increases inland. On a global scale, mangrove forests in the tropics have much higher above-ground biomass than those in temperate areas. Mangroves often accumulate large amounts of biomass in their roots, and the above-ground biomass to below-ground biomass ratio of mangrove forests is significantly low compared to that of upland forests (ANCOVA, P

Abstract: Besides water vapour, greenhouse gases CO2, CH4, O3 and N2O contribute ~60%, 20%, 10% and 6% to global warming, respectively; minor contribution is made by chlorofluorocarbons and volatile organic compounds (VOC). We present CO2, CH4 and N2O fluxes from natural and relatively unmanaged soil–plant ecosystems (the ecosystems minimally disturbed by direct human or human-induced activities). All natural ecosystems are net sinks for CO2, although tundra and wetlands (including peatlands) are large sources of CH4, whereas significant N2O emissions occur mainly from tropical and temperate forests. Most natural ecosystems decrease net global warming potential (GWP) from –0.03 ± 0.35 t CO2-e ha–1 y–1 (tropical forests) to –0.90 ± 0.42 t CO2-e ha–1 y–1 (temperate forests) and –1.18 ± 0.44 t CO2-e ha–1 y–1 (boreal forests), mostly as CO2 sinks in phytobiomass, microbial biomass and soil C. But net GWP contributions from wetlands are very large, which is primarily due to CH4 emissions. Although the tropical forest system provides a large carbon sink, the negligible capacity of tropical forests to reduce GWP is entirely due to N2O emissions, possibly from rapid N mineralisation under favourable temperature and moisture conditions. It is estimated that the natural ecosystems reduce the net atmospheric greenhouse gas (GHG) emissions by 3.55 ± 0.44 Gt CO2-e y–1 or ~0.5 ppmv CO2-e y–1, hence, the significant role of natural and relatively unmanaged ecosystems in slowing global warming and climate change. However, the impact of increasing N deposition on natural ecosystems is poorly understood, and further understanding is required regarding the use of drainage as a management tool, to reduce CH4 emissions from wetlands and to increase GHG sink from the restoration of degraded lands, including saline and sodic soils. Data on GHG fluxes from natural and relatively unmanaged ecosystems are further compounded by large spatial and temporal heterogeneity, limited sensitivity of current instruments, few and poor global distribution of monitoring sites and limited capacity of models that could integrate GHG fluxes across ecosystems, atmosphere and oceans and include feedbacks from biophysical variables governing these fluxes.

Abstract: The atmospheric fluxes of N2O, CH4 and CO2 from the soil in four mangrove swamps in Shenzhen and Hong Kong, South China were investigated in the summer of 2008. The fluxes ranged from 0.14 to 23.83 μmol m−2 h−1, 11.9 to 5168.6 μmol m−2 h−1 and 0.69 to 20.56 mmol m−2 h−1 for N2O, CH4 and CO2, respectively. Futian mangrove swamp in Shenzhen had the highest greenhouse gas fluxes, followed by Mai Po mangrove in Hong Kong. Sha Kong Tsuen and Yung Shue O mangroves in Hong Kong had similar, low fluxes. The differences in both N2O and CH4 fluxes among different tidal positions, the landward, seaward and bare mudflat, in each swamp were insignificant. The N2O and CO2 fluxes were positively correlated with the soil organic carbon, total nitrogen, total phosphate, total iron and NH4+-N contents, as well as the soil porosity. However, only soil NH4+-N concentration had significant effects on CH4 fluxes.

Abstract: The Sundarban mangrove forest (4,264 km 2 ) constitutes about 3% of the total area of the world mangrove. We measured diurnal and seasonal variations of air‐water CO2 exchange in relation to the occurrence of phytoplankton during January‐December 2001. Diurnal variations of airflows showed that the minimum and maximum CO2 flux of 216.2 mmol m 22 h 21 and 49.9 mmol m 22 h 21 , respectively, occurred during the higher sea breeze. The average ratio of dissolved inorganic nitrogen (DIN 5 13.85 6 7.19 mmol L 21 ) to dissolved inorganic phosphorus (DIP 5 1.23 6 0.57 mmol L 21 ) was 11 6 4 and the surface water was undersaturated with respect to dissolved oxygen. The mean value of 0.1 6 0.08 for the ratio of phytoplankton production (P) to community respiration (R) indicated that the ecosystem was heterotrophic. The saturation of dissolved carbon dioxide with respect to the atmosphere varied seasonally between 59% and 156%, with minimum levels in postmonsoon and maximum levels in premonsoon/early monsoon (June/July). Out of the 36 genera of diatoms, 1 blue green alga, and 3 dinoflagellates that occurred throughout the year, only 6 reached bloom proportions in postmonsoon, when mangrove water was a sink of atmospheric CO2. Although 59.3% of the emitted CO2 was removed from the atmosphere by biological processes, on an annual basis, the Sundarban mangrove forest supplies 13.8 kg C ha 21 yr 21 of CO2 from water surface to the atmosphere. Even though it is important to compare all in and out fluxes, there is no direct link between CO 2 emission and the later CO2 removal by biological processes.

Abstract: Seasonal and spatial variation of dissolved and atmospheric methane (CH 4 ) was measured in the estuaries of the Sundarban mangrove ecosystem from January to December 2003. This unique mangrove forest ecosystem (9630 km 2 ), a part of the Hooghly–Matla estuarine system (NE coast of Bay of Bengal) comprises about 3% of the total area of the world's mangroves. Dissolved methane concentrations in the mangrove dominated estuarine water (Muriganga, Saptamukhi and Thakuran) were observed between 11.0 and 129.0 nmol L − 1 with emission rates between 1.97 and 134.6 μmol m − 2 d − 1 . Methane emission showed maximum rate during post-monsoon when phytoplankton reached blooming proportion. During post-monsoon materials transported from the mangrove forest could stimulate phytoplankton bloom when transparency of water column was increased and river input of nutrient was low. However, dissolved methane concentrations in the Hooghly estuary, the main river water flushing channel showed lower values between 10.3 and 59.25 nmol L − 1 with emission rates between 0.88 and 148.6 μmol m − 2 d − 1 . Methane concentrations in the salinity gradient zone of the Hooghly estuary did not show any decreasing trend with increased salinity due to its lateral transport from the mangrove forest situated at the lower stretch of the estuary. Dissolved methane concentration in the pore water collected from the virgin mangrove forest showed a maximum concentration of 5769 nmol L − 1 which was considerably lower compared to that of coastal salt marsh, tidal fresh water wetland. This indicated intense oxidation or out competition of methanogenic bacteria by sulfate and nitrate reducing bacteria. Methane emission from the adjacent forest ecosystem (18.36 mmol m − 2 d − 1 ) was higher compared to that in the mangrove water. Considering methane emission rates from the aquatic and forest ecosystems, an area average rate of 47.28 × 10 5 mol km − 2 yr − 1 for Sundarban mangrove environment was obtained. Extrapolating over the global area covered by mangrove forest with area average soil methane emission rate without considering its emission from water could yield inaccurate estimation.

Abstract: This extensive report entitled “Climate Change 1995: The Science of Climate Change” is the most comprehensive and up-to-date assessment available for scientific understanding of human influences on the past present and future climate. Its aim is to provide objective information on which to base global climate change that will ultimately meet the aim of the UN Framework Convention on Climate Change. The report includes an overview of the factors governing climate and climate change and quantification of the sources of globally important greenhouse gases and other pollutants arising from human activities. A review of the chemical and biological processes governing their removal from the atmosphere is presented. Also included is an assessment of recent trends in climate during the industrial era which has witnessed the ever-growing impact of human activities on the global environment. The strengths and weaknesses of various climate mathematical models used by researchers for understanding the past and present climate and for calculating possible future climates are assessed. Furthermore the report discusses research aimed at the detection of human influence on the climate of the last century and presents future change projections in global climate and sea level based on a range of scenarios of future emissions of pollutants due to human activity. Finally a list of research and observational priorities needed to improve scientific understanding in key areas is presented.

Abstract: Relationships between wind speed and gas transfer, combined with knowledge of the partial pressure difference of CO2 across the air-sea interface are frequently used to determine the CO2 flux between the ocean and the atmosphere. Little attention has been paid to the influence of variability in wind speed on the calculated gas transfer velocities and the possibility of chemical enhancement of CO2 exchange at low wind speeds over the ocean. The effect of these parameters is illustrated using a quadratic dependence of gas exchange on wind speed which is fit through gas transfer velocities over the ocean determined by the natural-14C disequilibrium and the bomb-14C inventory methods. Some of the variability between different data sets can be accounted for by the suggested mechanisms, but much of the variation appears due to other causes. Possible causes for the large difference between two frequently used relationships between gas transfer and wind speed are discussed. To determine fluxes of gases other than CO2 across the air-water interface, the relevant expressions for gas transfer, and the temperature and salinity dependence of the Schmidt number and solubility of several gases of environmental interest are included in an appendix.

Abstract: Methane is the most abundant organic chemical in Earth's atmosphere, and its concentration is increasing with time, as a variety of independent measurements have shown. Photochemical reactions oxidize methane in the atmosphere; through these reactions, methane exerts strong influence over the chemistry of the troposphere and the stratosphere and many species including ozone, hydroxyl radicals, and carbon monoxide. Also, through its infrared absorption spectrum, methane is an important greenhouse gas in the climate system. We describe and enumerate key roles and reactions. Then we focus on two kinds of methane production: microbial and thermogenic. Microbial methanogenesis is described, and key organisms and substrates are identified along with their properties and habitats. Microbial methane oxidation limits the release of methane from certain methanogenic areas. Both aerobic and anaerobic oxidation are described here along with methods to measure rates of methane production and oxidation experimentally. Indicators of the origin of methane, including C and H isotopes, are reviewed. We identify and evaluate several constraints on the budget of atmospheric methane, its sources, sinks and residence time. From these constraints and other data on sources and sinks we construct a list of sources and sinks, identities, and sizes. The quasi-steady state (defined in the text) annual source (or sink) totals about 310(±60) × 1012 mol (500(±95) × 1012 g), but there are many remaining uncertainties in source and sink sizes and several types of data that could lead to stronger constraints and revised estimates in the future. It is particularly difficult to identify enough sources of radiocarbon-free methane.

Abstract: The terrestrial biosphere plays an important role in the global carbon cycle. In the 1994 Intergovernmental Panel Assessment on Climate Change (IPCC), an effort was made to improve the quantification of terrestrial exchanges and potential feedbacks from climate, changing CO2, and other factors; this paper presents the key results from that assessment, together with expanded discussion. The carbon cycle is the fluxes of carbon among four main reservoirs: fossil carbon, the atmosphere, the oceans, and the terrestrial biosphere. Emissions of fossil carbon during the 1980s averaged 5.5 Gt y−1. During the same period, the atmosphere gained 3.2 Gt C y−1 and the oceans are believed to have absorbed 2.0 Gt C y−1. The regrowing forests of the Northern Hemisphere may have absorbed 0.5 Gt C y−1 during this period. Meanwhile, tropical deforestation is thought to have released an average 1.6 Gt C y−1 over the 1980s. While the fluxes among the four pools should balance, the average 198Ds values lead to a ‘missing sink’ of 1.4 Gt C y−1 Several processes, including forest regrowth, CO2 fertilization of plant growth (c. 1.0 Gt C y−1), N deposition (c. 0.6 Gt C y−1), and their interactions, may account for the budget imbalance. However, it remains difficult to quantify the influences of these separate but interactive processes. Uncertainties in the individual numbers are large, and are themselves poorly quantified. This paper presents detail beyond the IPCC assessment on procedures used to approximate the flux uncertainties. Lack of knowledge about positive and negative feedbacks from the biosphere is a major limiting factor to credible simulations of future atmospheric CO2 concentrations. Analyses of the atmospheric gradients of CO2 and 13 CO2 concentrations provide increasingly strong evidence for terrestrial sinks, potentially distributed between Northern Hemisphere and tropical regions, but conclusive detection in direct biomass and soil measurements remains elusive. Current regional-to-global terrestrial ecosystem models with coupled carbon and nitrogen cycles represent the effects of CO2 fertilization differently, but all suggest longterm responses to CO2 that are substantially smaller than potential leaf- or laboratory whole plant-level responses. Analyses of emissions and biogeochemical fluxes consistent with eventual stabilization of atmospheric CO2 concentrations are sensitive to the way in which biospheric feedbacks are modeled by c. 15%. Decisions about land use can have effects of 100s of Gt C over the next few centuries, with similarly significant effects on the atmosphere. Critical areas for future research are continued measurements and analyses of atmospheric data (CO2 and 13CO2) to serve as large-scale constraints, process studies of the scaling from the photosynthetic response to CO2 to whole-ecosystem carbon storage, and rigorous quantification of the effects of changing land use on carbon storage.

Abstract: Ecologists are expected to play an important role in future studies of the biosphere/atmosphere exchange of materials associated with the major biogeochemical cycles and climate. Most studies of material exchange reported in the ecological literature have relied on chamber techniques. Micrometeorological techniques provide an alternative means of measuring these exchange rates and are expected to be used more often in future ecological studies, since they have many advantages over the chamber techniques. In this article we will provide an overview of micrometeorological theory and the different micrometeorological techniques available to make flux measurements.