Residence Time Of Organic Carbon Research Articles

Soil Respiration and Carbon Sequestration

An increase in the concentration of carbon dioxide in the atmosphere is a trigger for the activation of all processes of the carbon cycle, including soil respiration (SR), since it causes not only an increase in the greenhouse effect of the atmosphere, but also its fertilization. The consequence of fertilization is the tendency to increase the world’s net primary production of photosynthesis (NPP) and heterotrophic respiration of soils (RH). The increase in global terrestrial carbon sink was accompanied by an increase in CO2 in the atmosphere. The global increase in RH is due to global losses of soil organic carbon and is confirmed by models according to which the average residence time of organic carbon in the soil pool has decreased by 4.4 years over the past century. To assess the level of C sequestration in soils, it is necessary to determine the balance between the RH of the soil and the amount of new soil C-sink in the form of net biome production (NBP) – resistant to mineralization. Carbon sink into net ecosystem production (NEP) determines short-term unsustainable carbon sequestration.

Roots are key to increasing the mean residence time of organic carbon entering temperate agricultural soils.

The ratio of soil organic carbon stock (SOC) to annual carbon input gives an estimate of the mean residence time of organic carbon that enters the soil (MRTOC ). It indicates how efficiently biomass can be transformed into SOC, which is of particular relevance for mitigating climate change by means of SOC storage. There have been few comprehensive studies of MRTOC and their drivers, and these have mainly been restricted to the global scale, on which climatic drivers dominate. This study used the unique combination of regional-scale cropland and grassland topsoil (0-30cm) SOC stock data and average site-specific OC input data derived from the German Agricultural Soil Inventory to elucidate the main drivers of MRTOC . Explanatory variables related to OC input composition and other soil-forming factors were used to explain the variability in MRTOC by means of a machine-learning approach. On average, OC entering German agricultural topsoils had an MRT of 21.5±11.6years, with grasslands (29.0±11.2years, n=465) having significantly higher MRTOC than croplands (19.4±10.7, n=1635). This was explained by the higher proportion of root-derived OC inputs in grassland soils, which was the most important variable for explaining MRTOC variability at a regional scale. Soil properties such as clay content, soil group, C:N ratio and groundwater level were also important, indicating that MRTOC is driven by a combination of site properties and OC input composition. However, the great importance of root-derived OC inputs indicated that MRTOC can be actively managed, with maximization of root biomass input to the soil being a straightforward means to extend the time that assimilated C remains in the soil and consequently also increase SOC stocks.

Diatom Dominance Enhances Resistance of Phytoplanktonic POM to Mesopelagic Microbial Decomposition

Particulate organic matter (POM) lability is one of the key factors determining the residence time of organic carbon (OC) in the marine system. Phytoplankton community composition can influence the rate at which heterotrophic microorganisms decompose phytoplankton detrital particles and thus, it controls the fraction of OC that reaches the ocean depths, where it can be sequestered for climate-relevant spans of time. Here, we compared the degradation dynamics of POM from phytoplankton assemblages of contrasting diatom dominance in the presence of mesopelagic prokaryotic communities during a 19-day degradation experiment. We found that diatom-derived POM exhibited an exponential decay rate approximately three times lower than that derived from a community dominated by flagellated phytoplankton (mainly coccolithophores and nanoflagellates). Additionally, dissolved organic matter (DOM) released during the degradation of diatom particles accumulated over the experiment, whereas only residual increases in DOM were detected during the degradation of non-diatom materials. These results suggest that diatom-dominance enhances the efficiencies of the biological carbon pump and microbial carbon pump through the relatively reduced labilities of diatom particles and of the dissolved materials that arise from their microbial processing.

Organic Inputs and Chemical Fertilizer on Carbon Mineralization From Two Ultisols

There are challenges that limit the use of organic inputs for soil fertility management. Amongst them is the limited knowledge of factors that affect rates of decomposition and nutrient release from different organic inputs. A study was conducted on surface soil samples of two Ultisols to determine factors affecting carbon (C) mineralization from selected organic inputs. A loamy sand (LS) from a Kandiustult and a sandy clay loam (SCL) from a Paleustult were used. Fine earth fractions of the soils mixed with organic inputs with and without chemical fertilizer were incubated for 13 weeks and the CO2 evolved was measured. Organic inputs used were biomasses of Cajanus cajan, Tephrosia vogelii, Crotalaria juncea, Mucuna pruriens, a mixture of native grasses and shrubs and composted cattle manure. The latter two inputs are traditionally used by farmers, while the leguminous plants were recommended by scientists. Treatments with chemical fertilizer only, representing the conventional farming practice, and a control with soil alone were included. Addition of organic inputs with or without fertilizer increased total CO2 emissions by 81 to 129% on the LS and by 18 to 34% on the SCL. Adding chemical fertilizer significantly (p < 0.05) increased C mineralization rate constant (k) by 116% on the LS and 48% on the SCL. The mean residence time of organic carbon from treatments grouped by input type followed the order: Control > Traditional > Legumes > Conventional on both soils. In general, the k on the LS was double that on the SCL. The type of organic input, soil texture and application of chemical fertilizer significantly affected C mineralization rates from the soils.

Millennial soil retention of terrestrial organic matter deposited in the Bengal Fan

The abundance of organic carbon (OC) in vegetation and soils (~2,600 PgC) compared to carbon in the atmosphere (~830 PgC) highlights the importance of terrestrial OC in global carbon budgets. The residence time of OC in continental reservoirs, which sets the rates of carbon exchange between land and atmosphere, represents a key uncertainty in global carbon cycle dynamics. Retention of terrestrial OC can also distort bulk OC- and biomarker-based paleorecords, yet continental storage timescales remain poorly quantified. Using “bomb” radiocarbon (14C) from thermonuclear weapons testing as a tracer, we model leaf-wax fatty acid and bulk OC 14C signatures in a river-proximal marine sediment core from the Bay of Bengal in order to constrain OC storage timescales within the Ganges-Brahmaputra (G-B) watershed. Our model shows that 79–83% of the leaf-waxes in this core were stored in continental reservoirs for an average of 1,000–1,200 calendar years, while the remainder was stored for an average of 15 years. This age structure distorts high-resolution organic paleorecords across geologically rapid events, highlighting that compound-specific proxy approaches must consider storage timescales. Furthermore, these results show that future environmental change could destabilize large stores of old - yet reactive - OC currently stored in tropical basins.

Influence of iron redox cycling on organo-mineral associations in Arctic tundra soil

Arctic tundra stores large quantities of soil organic matter under varying redox conditions. As the climate warms, these carbon reservoirs are susceptible to increased rates of decomposition and release to the atmosphere as the greenhouse gases carbon dioxide (CO2) and methane (CH4). Geochemical interactions between soil organic matter and minerals influence decomposition in many environments but remain poorly understood in Arctic tundra systems and are not considered in decomposition models. The accumulation of iron (Fe) oxyhydroxides and organo-iron precipitates at redox interfaces may be particularly important for carbon cycling given that ferric iron [Fe(III)] species can enhance decomposition by serving as terminal electron acceptors in anoxic soils or inhibit microbial decomposition by binding organic molecules. Here, we examine chemical properties of solid-phase Fe and organic matter in organic and mineral horizons within the seasonally thawed active layer of Arctic tundra on the North Slope of Alaska. Spectroscopic techniques, including micro-X-ray fluorescence (μXRF) mapping, micro-X-ray absorption near-edge structure (μXANES) spectroscopy, and Fourier transform infrared spectroscopy (FTIR), were coupled with chemical sequential extractions and physical density fractionations to evaluate the spatial distribution and speciation of Fe-bearing phases and associated organic matter in soils.Organic horizons were enriched in poorly crystalline and crystalline iron oxides, and approximately 60% of total Fe stored in organic horizons was calculated to derive from upward translocation from anoxic mineral horizons. Ferrihydrite and goethite were present as coatings on mineral grains and plant debris, and in aggregates with clays and particulate organic matter. Minor amounts of ferrous iron [Fe(II)] were present in iron sulfides (i.e., pyrite and greigite) in mineral horizon soils and iron phosphates (vivianite) in organic horizons. Concentrations of organic carbon in the organic horizons (28±5wt.% C) were approximately twice the concentrations in the mineral horizons (14±2wt.% C), and organic matter was dominated by base-extractable and insoluble organics enriched in aromatic and aliphatic moieties. Conversely, water-soluble organic molecules and organics solubilized through acid-dissolution of iron oxides comprised <2% of soil organic C and were consistent with a mixture of alcohols, sugars, and small molecular weight organic acids and aromatics released through decomposition of larger molecules. Integrated over the entire depth of the active layer, soils contained 11±4kgm−2 low-density, particulate organic C and 19±6kgm−2 high-density, mineral-associated organic C, indicating that 63±19% of organic C in the active layer was associated with the mineral fraction.We conclude that organic horizons were enriched in poorly crystalline and crystalline iron oxide phases derived from upward translocation of dissolved Fe(II) and Fe(III) from mineral horizons. Precipitation of iron oxides at the redox interface has the potential to contribute to mineral protection of organic matter and increase the residence time of organic carbon in arctic soils. Our results suggest that iron oxides may inhibit organic carbon degradation by binding low-molecular-weight organic compounds, stabilizing soil aggregates, and forming thick coatings around particulate organic matter. Organic matter released through acid-dissolution of iron oxides could represent a small pool of readily-degradable organic molecules temporarily stabilized by sorption to iron oxyhydroxide surfaces. The distribution of iron in organic complexes and inorganic phases throughout the soil column constrains Fe(III) availability to anaerobic iron-reducing microorganisms that oxidize organic matter to produce CO2 and CH4 in these anoxic environments. Future predictions of carbon storage and respiration in the arctic tundra should consider such influences of mineral stabilization under changing redox conditions.

Earth system transition during the Tonian–Cambrian interval of biological innovation: nutrients, climate, oxygen and the marine organic carbon capacitor

AbstractThe Tonian–Cambrian interval (c. 0.9–0.5 Ga) witnessed major tectonic, climatic and chemical changes to the Earth system and culminated in the Ediacaran–Cambrian radiation of animals. Negative carbon isotope (δ13C) excursions of extraordinary magnitude form the backdrop to all these events and are consistent with the presence of a vast marine organic carbon reservoir that changed its size due to periodic imbalances between organic production (as an oxidant source) and terrestrial oxidant sinks. Prior to both Cryogenian glaciations, this pool of long-lived organic carbon may have become substantially depleted, leading to a weakening of the regulation of climate and oxygen. The late Ediacaran Shuram anomaly probably represents a third depletion event, which was followed by oxygenation and the Ediacaran–Cambrian radiation. The evolutionary diversification of animals shortened the ocean residence time of organic carbon and introduced new carbon cycle feedbacks that together helped to mould the modern Earth system.

Does grassland introduction into cropping cycles affect carbon dynamics through changes of allocation of soil organic matter within aggregate fractions?

Implementation of ley grassland into crop rotation could have positive influence in soil ecosystem services such as C storage. The periodical changes of land-use plus the in situ labelling given by the introduction of maize crops under ley grassland induce differences in soil organic matter (SOM) that could be traced either by stable isotopes or by the characterization of plant biomarkers such as lignin derived phenols. Evaluation of SOM dynamics is often limited by the complexity of soil matrix. To override these limitations, a hierarchical approach to decompose the soil mosaic into aggregates has been proposed in this study. Soil and plant samples were collected from a long-term experimental area in Lusignan (western France). Soils from four different treatments (bare fallow, permanent maize, permanent grassland, and ley grassland based on 6years of grassland followed by 3years of maize) were sampled, fractionated into water stable aggregates, and finally analysed for carbon, nitrogen, and lignin contents, as well as for 13C isotopic signature.Soils under ley and permanent grassland stored higher amount of SOM in larger aggregates and preserved more efficiently the lignin stocks than the corresponding samples under permanent maize. Contemporary, finer fraction of ley grassland showed higher mean residence time of organic carbon, probably due to a legacy effect of the previous years under grassland. Even if maize derived SOM was identified, the grassland footprint was still dominating the ley grassland soils, as described by the principal component analysis. Strong correlation between these results and the quality and stoichiometry of the vegetal litter returned to soil were found, evidencing the needs for a comprehensive evaluation at a molecular level of all the parameters modified by land-use changes, including tillage, to understand the potential for carbon storage of different agroecosystems.

Investigating the influence of regional climate and oceanography on marine radiocarbon reservoir ages in southwest New Zealand

The New Zealand fjords are located at a latitude where distinct oceanic and atmospheric fronts separate carbon reservoirs of varying residence time. The marine radiocarbon reservoir age in this region is likely to deviate from the global average reservoir age over space and time as frontal boundaries migrate north and south. Here we present new estimates of modern radiocarbon reservoir age using the radiocarbon content of bivalve shells collected live before 1950. Multiple measurements from hydrographically distinct sites support the use of a ΔR, defined as the regional offset between measured and modeled marine radiocarbon reservoir age, of 59 ± 35 years for the New Zealand fjords. We also assess the radiocarbon content of bulk surface sediments throughout the fjord region. Sediment with a higher proportion of marine organic carbon has relatively less radiocarbon than more terrestrial sediment, suggesting a short residence time of organic carbon on land before deposition in the fjords. Additionally, we constrain reservoir age variability throughout the Holocene using coeval terrestrial and marine macrofossils. Although our modern results suggest spatial consistency in ΔR throughout the fjords, large deviations from the global average marine radiocarbon reservoir age exist in the paleo record. We find four ancient ΔR values, extending back to ∼10.2 cal kyr BP, to be negative or near zero. A likely cause of younger radiocarbon reservoir ages at select intervals throughout the Holocene is the increased influence of the Southern Hemisphere westerly winds, which cause extreme precipitation in the region that delivers terrestrial carbon, enriched in radiocarbon, to fjord basins. However, bivalve depth habitat may also influence radiocarbon content due to a stratified water column containing distinct carbon pools. This work highlights the need for thorough assessment of local radiocarbon cycling in similar regions of dynamic ocean/atmosphere frontal zones, especially fjords and other semi-restricted estuaries.

An Evaluation of Temporal Changes in Sediment Accumulation and Impacts on Carbon Burial in Mobile Bay, Alabama, USA

The estuarine environment can serve as either a source or sink of carbon relative to the coastal ocean carbon budget. A variety of time-dependent processes such as sedimentation, carbon supply, and productivity dictate how estuarine systems operate, and Mobile Bay is a system that has experienced both natural and anthropogenic perturbations that influenced depositional processes and carbon cycling. Sediments from eight box cores provide a record of change in bulk sediment accumulation and carbon burial over the past 110 years. Accumulation rates in the central part of the basin (0.09 g cm−2) were 60–80 % less than those observed at the head (0.361 g cm−2) and mouth (0.564 g cm−2) of the bay. Sediment accumulation in the central bay decreased during the past 90 years in response to both anthropogenic (causeway construction) and natural (tropical cyclones) perturbations. Sediment accumulation inevitably increased the residence time of organic carbon in the oxic zone, as observed in modeled remineralization rates, and reduced the overall carbon burial. Such observations highlight the critical balance among sediment accumulation, carbon remineralization, and carbon burial in dynamic coastal environments. Time-series analysis based solely on short-term observation would not capture the long-term effects of changes in sedimentation on carbon cycling. Identifying these relationships over longer timescales (multi-annual to decadal) will provide a far better evaluation of coastal ocean carbon budgets.

The rates of organic matter renewal in gray forest soils and chernozems

The rates of soil carbon renewal were determined by the method of natural 13C abundance in a chernozem under a 40-year-long monoculture of corn and in a gray forest soil after application of corn residues. The mean rate of soil carbon renewal in the chernozem reached 1271–1498 years, whereas in the gray forest soil it depended on the amount of carbon introduced with corn residues and varied from 19 to 63 years. The rate of organic carbon renewal in the chernozem decreased from 697 years in the upper horizon to 2742 years in the layer of 40–60 cm. The mean residence time of organic carbon generally increased with a decrease in the size of particle-size fractions.

Biophysical controls on organic carbon fluxes in fluvial networks

Rivers may be efficient environments for metabolizing terrestrial organic carbon that was previously thought to be recalcitrant, owing to pockets that provide geophysical opportunities by retaining material for longer, and to the adaptation of microbial communities, which has enabled them to exploit the energy that escapes upstream ecosystems. Metabolism of terrestrial organic carbon in freshwater ecosystems is responsible for a large amount of carbon dioxide outgassing to the atmosphere, in contradiction to the conventional wisdom that terrestrial organic carbon is recalcitrant and contributes little to the support of aquatic metabolism. Here, we combine recent findings from geophysics, microbial ecology and organic geochemistry to show geophysical opportunity and microbial capacity to enhance the net heterotrophy in streams, rivers and estuaries. We identify hydrological storage and retention zones that extend the residence time of organic carbon during downstream transport as geophysical opportunities for microorganisms to develop as attached biofilms or suspended aggregates, and to metabolize organic carbon for energy and growth. We consider fluvial networks as meta-ecosystems to include the acclimation of microbial communities in downstream ecosystems that enable them to exploit energy that escapes from upstream ecosystems, thereby increasing the overall energy utilization at the network level.

Radiocarbon of dissolved humic substances in river waters from the Chernobyl area

Radiocarbon (14C) was used to study the origin and transport of aquatic humic substances in river waters at the Chernobyl area, which received a pulse input of 14C as a consequence of the nuclear accident. Water samples were collected in April 1999 from the Pripyat and Sakhan Rivers, which flow through the radioactive contaminated area (30 km exclusion zone). The Δ14C values of humic and fulvic acids ranged from −68‰ to +75‰ and were ∼400‰ lower than those of non-contaminated environments. The aquatic humic substances may be derived mainly from those of bog, peat, and podzolic soil with older 14C age, and thereby reflect a larger proportion of older groundwater humic substances. Contribution of 14C by the Chernobyl accident appears to be small because of the long residence time of organic carbon at the surface soil.

Radiocarbon of dissolved humic substances in river waters from the Chernobyl area

Radiocarbon ( 14 C) was used to study the origin and transport of aquatic humic substances in river waters at the Chernobyl area, which received a pulse input of 14 C as a consequence of the nuclear accident. Water samples were collected in April 1999 from the Pripyat and Sakhan Rivers, which flow through the radioactive contaminated area (30 km exclusion zone). The Δ 14 C values of humic and fulvic acids ranged from −68‰ to +75‰ and were ∼400‰ lower than those of non-contaminated environments. The aquatic humic substances may be derived mainly from those of bog, peat, and podzolic soil with older 14 C age, and thereby reflect a larger proportion of older groundwater humic substances. Contribution of 14 C by the Chernobyl accident appears to be small because of the long residence time of organic carbon at the surface soil.

Estimating Paleoproductivity from Organic Carbon Accumulation Rates--Problem or Panacea?: ABSTRACT

Organic carbon accumulation rates (OCAR; g/cm/sup 2//ky), calculated as the product of the weight fraction of organic carbon (OC), dry bulk density (DBD; g/cc), and sedimentation rate (SR; cm/ky), are the most uniform method of expressing variations in OC flux and/or preservation in marine environments. Several factors influence OC preservation; these include rate of OC production (flux from surface waters or other sources), water depth (WD), rates of OC decomposition in the water column, and SR (i.e., residence time of OC at the sediment/water interface). Understanding the patterns and variations of paleoproductivity (PP) that resulted in accumulation of relatively OC-rich strata depends upon differentiating between factors that enhance OC preservation and OC flux. Assuming that OC preservation is not a function of dissolved oxygen (DO) at levels above about 0.5 ml/l, Mueller and Suess in 1979 related OCAR to PP in modern marine settings by correcting for the effects of OC preservation as a function of SR. Zahn et al in 1986 improved on the Mueller-Suess equation, using a more comprehensive data set and taking into account the progressive diminution of the OC flux during transit through the water column. They have tested the efficacy of these PP equations asmore » applied to several independently derived data sets for which OCAR could be calculated for the last 12 ky in cores from a variety of depths and a range of productivity in the Pacific and Atlantic Ocean basins. They also attempted to construct an equation that best predicted PP by fitting all available data in their data set for the modern ocean using stepwise multiple linear regression analysis of OC, DBD, SR, and WD data to predict PP and by using several methods of estimating productivity at each locale. Inclusion of larger data sets than previously used reduced the ability of any equation to predict PP.« less

Organic carbon dynamics and preservation in deep-sea sediments

Three methods of estimating the particulate organic carbon fluxes to the sediment-water interface of the deep Pacific Ocean agree to within the error of the measurements at MANOP sites M, H, and C. Sediment trap experiments, pore water results, and surface sediment organic carbon data suggest that a major fraction of the particulate organic carbon raining to abyssal depths at these locations is degraded within the surface sediments rather than at the sediment-water interface or in the nephloid layer. Organic carbon rain rates at the three sites are similar—within a factor of two; however, the preservation rate of organic carbon and the chemistry of sediment pore waters are very different. A model developed to describe the pore water oxygen and sedimentary carbon distributions indicates model developed to describe the pore water oxygen and sedimentary carbon distributions indicates that changes in the rate constant for organic matter degradation and the bioturbation rate may contribute significantly to the observed differences in character of both pore water and sediment chemistry at these locations. The implication with respect to interpreting the sedimentary record is that cycles of organic carbon and redox sensitive metals (i.e., manganese) are not simply related to particulate organic carbon flux or surface water primary productivity. The residence time of organic carbon with respect to degradation in the surface sediments is on the order of 15 to 150 y.