Scavenging of soluble organic matter from the prebiotic oceans

The existence of hot or cold nutrient broth or primeval soup is challenged on the basis of the recent geochemistry of soluble organic carbon in the oceans. Most of the dissolved organic carbon is recycled quickly by organisms, but the residual, biologically refractive, organic matter is efficiently scavenged from the oceans (residence time of 1000 to 3500 years) by nonbiologically mediated chemical and physical processes, such as adsorption on sinking minerals, polymerization and aggregation to humic type polymers or by aggregation to particulate matter through bubbling ans sinking of this material to the ocean bottom. Since there is no reason to believe that such nonbiological scavenging was not operative in the prebiotic oceans as well, then the prolonged existence of organic soup is very doubtful. The question of the orgin of life is thus assumed to be related to solid-liquid interfacial activity, and the answer may be associated with sediment-water interaction rather than with solution chemistry.     

Ecology of sea ice biota

Summary The sea ice does not only determine the ecology of ice biota, but it also influences the pelagic systems under the ice cover and at ice edges. In this paper, new estimates of Arctic and Antarctic production of biogenic carbon are derived, and differences as well as similarities between the two oceans are examined. In ice-covered seas, high algal concentrations (blooms) occur in association with several types of conditions. Blooms often lead to high sedimentation of intact cells and faecal pellets. In addition to ice-related blooms, there is progressive accumulation of organic matter in Arctic multi-year ice, whose fate may potentially be similar to that of blooms. A fraction of the carbon fixed by microalgae that grow in sea ice or in relation to it is exported out of the production zone. This includes particulate material sinking out of the euphotic zone, and also material passed on to the food web. Pathways through which ice algal production does reach various components of the pelagic and benthic food webs, and through them such top predators as marine mammals and birds, are discussed. Concerning global climate change and biogeochemical fluxes of carbon, not all export pathways from the euphotic zone result in the sequestration of carbon for periods of hundreds of years or more. This is because various processes, that take place in both the ice and the water column, contribute to mineralize organic carbon into CO₂ before it becomes sequestered. Processes that favour the production and accumulation of biogenic carbon as well as its export to deep waters and sequestration are discussed, together with those that influence mineralization in the upper ice-covered ocean.Authors are members of SCOR Working Group 86: Ecology of Sea Ice Biota     

Do soil aggregates really protect encapsulated organic matter against microbial decomposition?

Soil aggregates can provide an effective protection of organic matter against microbial decomposition as reported by several macroaggregate disruption studies. However, research on the role of aggregation for carbon mineralization was mainly focused on arable soils. In the present study we aim to clarify the impact of aggregation on organic matter protection by measuring carbon mineralization in terms of microbial respiration rates of intact macroaggregates (2–4 and 4–8 mm) and corresponding crushed aggregates from seven topsoil horizons from both arable and forest sites. For two arable and one forest soil we found a significantly (P < 0.001) lower carbon mineralization from intact aggregates as compared to the corresponding crushed material. The portion of aggregate protected carbon reached up to 30% for a grassland soil. For the other arable and forest soils no significant effect of aggregation was found. Similarly, no clear trend could be found for the protective capacity of different size fractions. We conclude that protection by aggregation is effective primarily for soils with a large pool of labile organic matter regardless of their usage as arable land or forest.     

Organic carbon transport and C/N ratio variations in a large tropical river: Godavari as a case study,India

This study gives an insight into the source of organic carbon and nitrogen in the Godavari river and its tributaries, the yield of organic carbon from the catchment, seasonal variability in their concentration and the ultimate flux of organic and inorganic carbon into the Bay of Bengal. Particulate organic carbon/particulate organic nitrogen (POC/PON or C/N) ratios revealed that the dominant source of organic matter in the high season is from the soil (C/N = 8–14), while in the rest of the seasons, the river-derived (in situ) phytoplankton is the major source (C/N = l–8). Amount of organic materials carried from the lower catchment and flood plains to the oceans during the high season are 3 to 91 times higher than in the moderate and low seasons. Large-scale erosion and deforestation in the catchment has led to higher net yield of organic carbon in the Godavari catchment when compared to other major world rivers. The total flux of POC, and dissolved inorganic carbon (DIC) from the Godavari river to the Bay of Bengal is estimated as 756 × 10⁹ and 2520 × 10⁹ g yr^–1, respectively. About 22% of POC is lost in the main channel because of oxidation of labile organic matter, entrapment of organic material behind dams/sedimentation along flood plains and river channel; the DIC fluxes as a function of alkalinity are conservative throughout the river channel. Finally, the C/N ratios (12) of the ultimate fluxes of particulate organic carbon suggest the dominance of refractory/stable soil organic matter that could eventually get buried in the coastal sediments on a geological time scale.     

Nitrate addition stimulates microbial decomposition of organic matter in salt marsh sediments

Salt marshes sequester carbon at rates more than an order of magnitude greater than their terrestrial counterparts, helping to mitigate climate change. As nitrogen loading to coastal waters continues, primarily in the form of nitrate, it is unclear what effect it will have on carbon storage capacity of these highly productive systems. This uncertainty is largely driven by the dual role nitrate can play in biological processes, where it can serve as a nutrient‐stimulating primary production or a thermodynamically favorable electron acceptor fueling heterotrophic metabolism. Here, we used a controlled flow‐through reactor experiment to test the role of nitrate as an electron acceptor, and its effect on organic matter decomposition and the associated microbial community in salt marsh sediments. Organic matter decomposition significantly increased in response to nitrate, even at sediment depths typically considered resistant to decomposition. The use of isotope tracers suggests that this pattern was largely driven by stimulated denitrification. Nitrate addition also significantly altered the microbial community and decreased alpha diversity, selecting for taxa belonging to groups known to reduce nitrate and oxidize more complex forms of organic matter. Fourier Transform‐Infrared Spectroscopy further supported these results, suggesting that nitrate facilitated decomposition of complex organic matter compounds into more bioavailable forms. Taken together, these results suggest the existence of organic matter pools that only become accessible with nitrate and would otherwise remain stabilized in the sediment. The existence of such pools could have important implications for carbon storage, since greater decomposition rates as N loading increases may result in less overall burial of organic‐rich sediment. Given the extent of nitrogen loading along our coastlines, it is imperative that we better understand the resilience of salt marsh systems to nutrient enrichment, especially if we hope to rely on salt marshes, and other blue carbon systems, for long‐term carbon storage.     

ELEVATED ATMOSPHERIC CARBON DIOXIDE INCREASES ORGANIC CARBON FIXATION BY EMILIANIA HUXLEYI (HAPTOPHYTA), UNDER NUTRIENT‐LIMITED HIGH‐LIGHT CONDITIONS1

Phytoplankton play a key role in determining the partitioning of CO₂ between the atmosphere and the ocean on seasonal, interannual, and millennial time scales. The magnitude of biological draw‐down of atmospheric CO₂ and C storage in the oceans is affected by concurrent changes in other environmental factors, like nutrient supply. Furthermore, variations in carbon‐to‐nitrogen (C:N) and carbon‐to‐phosphorus (C:P) assimilation ratios modify the oceanic CO₂ storage capacity. Here we show that increased atmospheric CO₂ concentration enhances CO₂ fixation into organic matter by a noncalcifying strain of Emiliania huxleyi (Lohmann) Hay & Mohler only under certain conditions, namely high light and nutrient limitation. Enhanced organic matter production was accompanied by marked deviations of the C:N:P ratio from the canonical stoichiometry of marine particulate matter of 106:16:1 (C:N:P) known as the Redfield ratio. Increased cell organic carbon content, C:N, and C:P were observed at high light when growth was either nitrogen or phosphorus limited. Elevated CO₂ led to further increases in the particulate C:N and C:P ratios. Enhanced CO₂ uptake by phytoplankton such as E. huxleyi, in response to elevated atmospheric CO₂, could increase carbon storage in the nitrogen‐limited regions of the oceans and thus act as a negative feedback on rising atmospheric CO₂ levels.     

On the possible role of organic melanoidin polymers as matrices for prebiotic activity

Summary One of the major diagenetic pathways of organic matter in recent sediments involves the condensation of cellular constituents, particularly amino acids and sugars, into insoluble melanoidin-type polymers. These polymers consist mainly of humic and fulvic acids and make up the major part of the organic carbon reservoir in recent sediments. We suggest that a similar set of reactions between abiotically formed amino acids and sugars, and more generally between aldehydes and amines, occurred on a large scale in the prebiotic hydrosphere. The rapid formation of this insoluble polymeric material would have removed the bulk of the dissolved organic carbon from the primitive oceans and would thus have prevented the formation of an "organic soup".Melanoidin polymers have several properties which make them attractive hypothetical precursors of contemporary oxidation-reduction coenzymes: 1. they contain heterocyclic nitrogen compounds similar to the nitrogenous bases; 2. they contain a high concentration of stable free radicals; and 3. they tend to concentrate those heavy metals which play prominent roles in contemporary enzymic redox processes. The prebiotic formation of similar polymers could, therefore, have provided the starting point for a basic class of biochemical reactions.We suggest that the prebiotic scenario involved chemical and protoenzymic reactions at the sediment-ocean interface in relatively shallow waters and under conditions not much different from those of the recent environment.On leave from the Isotope Department, Weizmann Institute of Sciences, Rehovot, Israel. This is the address for reprint requests.On leave from the Department of Cell and Molecular Biology, San Francisco State University, San Francisco, California.On leave from the Department of Biophysics, University of Houston, Houston, Texas.     

Total and hydrolizable particulate organic matter (carbohydrates, proteins and lipids) at a coastal station in Terra Nova Bay (Ross Sea, Antarctica)

We analysed quantity and quality of particulate organic matter during the austral summer 1994/1995 at a coastal station in Terra Nova Bay (Ross Sea, Antarctica). Our main aims were to investigate the origin and biochemical composition of particulate organic matter (POM), to measure its availability for consumers through the study of its digestible fraction (measured by using different enzymes separately) and to highlight the role of hydrolizable compounds in the organic matter diagenesis in the coastal waters at Terra Nova Bay. Temporal and spatial patterns of chlorophyll-a concentrations were reflected by the particulate organic carbon, nitrogen and total biopolymeric carbon concentrations, suggesting that most POM originated directly from phytoplankton. The most evident feature of POM in the coastal waters at Terra Nova Bay was the dominance of proteins (on average 57% of total biopolymeric particulate carbon), followed by carbohydrates (25%) and lipids (18%). We found that about 30% of the refractory particulate organic carbon (assumed to be present only after the complete exploitation of particulate organic nitrogen) did not originate from biopolymeric carbon (as sum of carbohydrate, protein and lipid carbon). This allows us to suggest the use of the digestible fraction of particulate biopolymeric carbon as a more accurate measure of the food availability of POM for consumers. In Terra Nova Bay coastal waters, most of the particulate protein pool was associated with large phytoplankton cells or phytodetritus. As a result, the protein pool appeared less available (i.e. less digestible) than the one present in oligotrophic waters where, conversely, most particulate organic nitrogen is sequestered into bacteria. The relative low availability of the protein pool, together with the rapid sinking of POM and the low remineralization rates of benthic heterotrophic microbes, are suggested as possible factors in determining the “inefficiency” in organic matter recycling of coastal waters at Terra Nova Bay, which behaves as a “loss type” system. Received: 17 June 1997 / Accepted 25 September 1997     

Mesopelagic microbial carbon production correlates with diversity across different marine particle fractions

The vertical flux of marine snow particles significantly reduces atmospheric carbon dioxide concentration. In the mesopelagic zone, a large proportion of the organic carbon carried by sinking particles dissipates thereby escaping long term sequestration. Particle associated prokaryotes are largely responsible for such organic carbon loss. However, links between this important ecosystem flux and ecological processes such as community development of prokaryotes on different particle fractions (sinking vs. non-sinking) are yet virtually unknown. This prevents accurate predictions of mesopelagic organic carbon loss in response to changing ocean dynamics. Using combined measurements of prokaryotic heterotrophic production rates and species richness in the North Atlantic, we reveal that carbon loss rates and associated microbial richness are drastically different with particle fractions. Our results demonstrate a strong negative correlation between prokaryotic carbon losses and species richness. Such a trend may be related to prokaryotes detaching from fast-sinking particles constantly enriching non-sinking associated communities in the mesopelagic zone. Existing global scale data suggest this negative correlation is a widespread feature of mesopelagic microbes.Subject terms: Microbial ecology, Biogeochemistry     

Modeling the carbon cycle in Lake Matano

Lake Matano, Indonesia, is a stratified anoxic lake with iron‐rich waters that has been used as an analogue for the Archean and early Proterozoic oceans. Past studies of Lake Matano report large amounts of methane production, with as much as 80% of primary production degraded via methanogenesis. Low δ¹³C values of DIC in the lake are difficult to reconcile with this notion, as fractionation during methanogenesis produces isotopically heavy CO₂. To help reconcile these observations, we develop a box model of the carbon cycle in ferruginous Lake Matano, Indonesia, that satisfies the constraints of CH₄ and DIC isotopic profiles, sediment composition, and alkalinity. We estimate methane fluxes smaller than originally proposed, with about 9% of organic carbon export to the deep waters degraded via methanogenesis. In addition, despite the abundance of Fe within the waters, anoxic ferric iron respiration of organic matter degrades <3% of organic carbon export, leaving methanogenesis as the largest contributor to anaerobic organic matter remineralization, while indicating a relatively minor role for iron as an electron acceptor. As the majority of carbon exported is buried in the sediments, we suggest that the role of methane in the Archean and early Proterozoic oceans is less significant than presumed in other studies.     

Algae-bacteria interactions and their effects on aggregation and organic matter flux in the sea

Aggregation of algae, mainly of diatoms, is an important process in marine pelagic systems, often terminating phytoplankton blooms and leading to the sinking of particulate organic matter in the form of marine snow. This process has been studied extensively, but the specific role of heterotrophic bacteria has largely been neglected, mainly because field studies and most experimental work were performed under non-axenic conditions. We tested the hypothesis that algae-bacteria interactions are instrumental in aggregate dynamics and organic matter flux. A series of aggregation experiments has been carried out in rolling tanks with two marine diatoms typical of temperate regions (Skeletonema costatum and Thalassiosira rotula) in an axenic treatment and one inoculated with marine bacteria. Exponentially growing S. costatum and T. rotula exhibited distinctly different aggregation behavior. This was reflected by their strikingly different release of dissolved organic matter (DOM), transparent exopolymer particles (TEP) and protein-containing particles (CSP), as well as their bacterial biodegradability and recalcitrance. Cells of S. costatum aggregated only little and their bacterial colonization remained low. Dissolved organic matter, TEP and CSP released by this alga were largely consumed by free-living bacteria. In contrast, T. rotula aggregated rapidly and DOM, TEP and CSP released resisted bacterial consumption. Experiments conducted with T. rotula cultures in the stationary growth phase, however, showed rapid bacterial colonization and decomposition of algal cells. Our study highlights the importance of heterotrophic bacteria to control the development and aggregation of phytoplankton in marine systems.     

Effects of organic matter on the growth of Thiobacillus intermedius

London, Jack (University of California, Los Angeles), and Sydney C. Rittenberg. Effects of organic matter on the growth of Thiobacillus intermedius. J. Bacteriol. 91:1062-1069. 1966.-Yeast extract, glucose, glutamate, and other organic materials stimulate the rate and extent of growth of Thiobacillus intermedius in thiosulfate broth. Growth did not occur in glucose or glutamate mineral salts medium in the absence of thiosulfate, although a stable variant was obtained which grows on yeast extract alone. Cells harvested from media supplemented with organic matter have a reduced rate of thiosulfate oxidation (20 to 30% of autotrophic), oxidize the organic supplement, and have an additive rate of oxidation in the presence of both the organic substrate and thiosulfate. Carboxydismutase synthesis is repressed, and the incorporation of bicarbonate carbon into cell material is almost completely eliminated by the presence of organic matter in the growth medium. It is concluded that the availability of organic matter eliminates the autotrophic assimilatory mechanisms of T. intermedius but not its autotrophic energy-generating system. The data are discussed in relation to the existence of "obligate" chemoautotrophic bacteria.     

Significance of ocean carbonate budgets for the global carbon cycle

Changes in the trace gas composition of the atmosphere over glacial–interglacial cycles are linked to changes in the oceanic carbon cycle. This paper examines the role of biologically driven fluxes of organic and inorganic carbon in modifying the carbon dioxide chemistry of the oceans, and the corresponding implications for the partitioning of CO₂ between the atmosphere and ocean. Relevant details of the marine carbon system are presented together with an assessment of the significance of remineralization and dissolution processes. Recent estimates of the marine carbonate fluxes show significant uncertainties and inconsistencies which must be resolved in order to assess fully the role of the oceans' biota in the marine carbon system. Various types of ocean carbon cycle models have been developed in order to interpret the changes in past atmospheric carbon dioxide. Some take account of the role of the oceans' biota, focussing in the main on the cycling of organic matter. Relatively few have considered the role of the carbonate pump and the subtle interactions between organic and inorganic carbon cycling. The significance of carbonate formation and dissolution, and of the effects of global change on the marine carbonate system, for air–sea fluxes of CO₂ are discussed. Finally some recommendations for future research are made in order to improve our understanding of how spatial and temporal variation in marine carbonate fluxes, in conjunction with processes determining the oxidation and burial of organic matter in the oceans, affect levels of CO₂ in the atmosphere.     

Bacterivorous grazers facilitate organic matter decomposition: a stoichiometric modeling approach

There is widespread empirical evidence that protist grazing on bacteria reduces bacterial abundances but increases bacteria-mediated decomposition of organic matter. This paradox has been noted repeatedly in the microbiology literature but lacks a generally accepted mechanistic explanation. To explain this paradox quantitatively, we develop a bacteria-grazer model of organic matter decomposition that incorporates protozoa-driven nutrient recycling and stoichiometry. Unlike previous efforts, the current model includes explicit limitation, via Liebig's law of minimum, by two possible factors, nutrient and carbon densities, as well as their relative ratios in bacteria and grazers. Our model shows two principal results: (1) when the environment is carbon limiting, organic matter can always be decomposed completely, regardless of the presence/absence of grazers; (2) when the environment is nutrient (such as nitrogen) limiting, it is possible for organic matter to be completely decomposed in the presence, but not absence, of grazers. Grazers facilitate decomposition by releasing nutrients back into the environment, which would otherwise be limiting, while preying upon bacteria. Model analysis reveals that facilitation of organic matter decomposition by grazers is positively related to the stoichiometric difference between bacteria and grazers. In addition, we predict the existence of an optimal density range of introduced grazers, which maximally facilitate the decomposition of organic matter in a fixed time period. This optimal range reflects a trade-off between grazer-induced nutrient recycling and grazer-induced mortality of bacteria.     

Short-term variations in particulate matter sedimentation off Kapp Norvegia,Weddell Sea,Antarctica: relation to water mass advection,ice cover,plankton biomass and feeding activity

Summary A multi-cup sediment trap was deployed at 250m in the shelf area off Kapp Norvegia, Weddell Sea (630 m water depth) to determine the relative importance of water mass advection, sea ice movement, phytoplankton biomass and plankton feeding. Short-term fluctuations in sedimentation were determined using a sampling frequency of 2.7 days over 54 days during January and February 1988. Three periods of enhanced sedimentation were associated with water mass exchange, settling of diatoms following break-up of ice cover and release of fecal matter by krill feeding on particulate matter derived from phytoplankton and ice algae. An initial sedimentation pulse (28 Jan) was mainly due to sinking pelagic diatoms and krill fecal strings containing algae released from sea ice passing over the trap position. The ¹³C-composition of the sedimented organic carbon was about-24. The isotope ratio decreased sharply by about 5.5 at the end of the first pulse indicating the source of sinking matter becoming pelagic diatoms of the retreating ice-edge. At this time the diatom Corethron criophilum contributed a very high proportion of the organic flux causing an increase of the opal/C_org ratios. The second pulse (6 Feb) was due to empty diatom frustules, minipellets and small planktonic aggregates. Much of the organic carbon was transported by round fecal pellets. During the third pulse (14 Feb), round fecal pellets transported even more; the percentage of C. criophilum to the diatom organic carbon flux was more than 80% (>2mg C m^–2 day^–1).Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Organic geochemical changes in Pliocene sediments of ODP Site 1083 (Benguela Upwelling System)

The intensification of the Northern Hemisphere Glaciation (INHG) was a major event in the development of the current climate state, and as one of the most productive regions in the world's oceans, the behaviour of the Benguela Upwelling System (BUS) following the INHG is of wide interest. To investigate post-INHG changes in productivity and organic matter accumulation, total organic carbon and biomarker accumulation rates were determined for sediments from ODP Site 1083 and compared to alkenone-derived sea surface temperatures and nitrogen isotopic compositions. These data indicate that the interval between 2.6 and 2.4 Ma was characterized by dramatic changes in upwelling intensity and organic carbon export on the northern edge of the modern BUS. The upwelling is reflected by significant changes in alkenone-derived SST estimates between glacial and interglacial intervals, with a total variability of 16 °C. The studied interval is also characterized by large changes in organic matter export as reflected by changes in TOC and biomarker accumulation rates, which show maxima during OIS 98 and during the transition from OIS 97 to 96. Intervals of elevated TOC are also characterized by elevated concentrations of sedimentary microbial biomarkers and lower %CaCO₃, suggesting that enhanced delivery of labile organic matter to the seafloor resulted in enhanced remineralisation with released CO₂ being consumed by CaCO₃ dissolution. However, in apparent contrast to recent Pleistocene sediments at the same site, organic matter export after the INHG was not solely driven by upwelling intensity. Of the three Pliocene glacial–interglacial cycles examined (OIS 101 to 96), each is unique with respect to the timing and magnitude of changes in organic matter accumulation. Each is also characterized by different algal assemblages as inferred from biomarker distributions, with OIS 97 and 96 particularly dominated by diatoms. We suggest that these differences reflect the important but evolving role of Southern Ocean waters in the Pliocene BUS: nutrient depletion of SO waters occurred during parts of Pliocene glacial intervals such that even intense upwelling did not persistently result in enhanced organic matter accumulation rates.     

The Fate of Marine Bacterial Exopolysaccharide in Natural Marine Microbial Communities

Most marine bacteria produce exopolysaccharides (EPS), and bacterial EPS represent an important source of dissolved organic carbon in marine ecosystems. It was proposed that bacterial EPS rich in uronic acid is resistant to mineralization by microbes and thus has a long residence time in global oceans. To confirm this hypothesis, bacterial EPS rich in galacturonic acid was isolated from Alteromonas sp. JL2810. The EPS was used to amend natural seawater to investigate the bioavailability of this EPS by native populations, in the presence and absence of ammonium and phosphate amendment. The data indicated that the bacterial EPS could not be completely consumed during the cultivation period and that the bioavailability of EPS was not only determined by its intrinsic properties, but was also determined by other factors such as the availability of inorganic nutrients. During the experiment, the humic-like component of fluorescent dissolved organic matter (FDOM) was freshly produced. Bacterial community structure analysis indicated that the class Flavobacteria of the phylum Bacteroidetes was the major contributor for the utilization of EPS. This report is the first to indicate that Flavobacteria are a major contributor to bacterial EPS degradation. The fraction of EPS that could not be completely utilized and the FDOM (e.g., humic acid-like substances) produced de novo may be refractory and may contribute to the carbon storage in the oceans.     

Complications in the determination of the nutrient status of the marine environment

How much of the organic matter in the oceans is truly bioavailable under in situ conditions? This bioavailability may be altered by perturbation, priming, bottle (surface or confinement) effects, and air contaminants. Measured free organic matter may not be free in situ but may depend on method of analysis. We have yet to determine the threshold energy for growth of microbes and their energy of maintenance under in situ conditions.     

Association of Soil Aggregation with the Distribution and Quality of Organic Carbon in Soil along an Elevation Gradient on Wuyi Mountain in China

Forest soils play a critical role in the sequestration of atmospheric CO₂ and subsequent attenuation of global warming. The nature and properties of organic matter in soils have an influence on the sequestration of carbon. In this study, soils were collected from representative forestlands, including a subtropical evergreen broad-leaved forest (EBF), a coniferous forest (CF), a subalpine dwarf forest (DF), and alpine meadow (AM) along an elevation gradient on Wuyi Mountain, which is located in a subtropical area of southeastern China. These soil samples were analyzed in the laboratory to examine the distribution and speciation of organic carbon (OC) within different size fractions of water-stable soil aggregates, and subsequently to determine effects on carbon sequestration. Soil aggregation rate increased with increasing elevation. Soil aggregation rate, rather than soil temperature, moisture or clay content, showed the strongest correlation with OC in bulk soil, indicating soil structure was the critical factor in carbon sequestration of Wuyi Mountain. The content of coarse particulate organic matter fraction, rather than the silt and clay particles, represented OC stock in bulk soil and different soil aggregate fractions. With increasing soil aggregation rate, more carbon was accumulated within the macroaggregates, particularly within the coarse particulate organic matter fraction (250–2000 μm), rather than within the microaggregates (53–250μm) or silt and clay particles (< 53μm). In consideration of the high instability of macroaggregates and the liability of SOC within them, further research is needed to verify whether highly-aggregated soils at higher altitudes are more likely to lose SOC under warmer conditions.     

Sources of organic carbon in mangrove sediments: variability and possible ecological implications

Mangrove sediments from three different mangrove ecosystems (Coringa Wildlife Sanctuary in the Godavari Delta, Andhra Pradesh, India, and Galle and Pambala, south-west Sri Lanka) were analysed for their organic carbon content, elemental ratios (C:N) and carbon stable isotope composition. Organic carbon content (0.6 – 31.7% dry weight), C/N ratios (7.0 – 27.3) and ¹³C (between –29.4 and –20.6) showed a wide range of values. Lower stocks of organic carbon coincided with low C/N (atom) ratios and less negative ¹³C values, indicating import of marine or estuarine particulate suspended matter. High organic carbon stocks coincided with high C/N ratios and ¹³C values close, but not equal, to those of the mangrove vegetation. The variations observed in this study and published literature data could be adequately described by a simple two-end mixing model, whereby marine/estuarine suspended matter and mangrove litter were taken as end members. Thus, while in some mangrove ecosystems or vegetation zones, organic carbon stocks can be very high and are almost entirely of mangrove origin, there also appear to be cases in which deposited estuarine or marine suspended matter is the dominant source of organic carbon and nitrogen in mangrove sediments. This situation is remarkably similar to that observed in temperate salt marsh ecosystems where the importance of local vascular plant production to the sediment organic carbon pool is equally variable. The observed high variability in organic matter origin is thought to have a major impact on the overall carbon dynamics in intertidal mangrove ecosystems.