Organic Carbon Preservation Research Articles

Where they can't breathe

Ocean Anoxia Climate warming is causing the expansion of marine oxygen-deficient zones, which are regions in which dissolved oxygen concentrations are so low that many marine animals cannot survive. This phenomenon also might affect the global cycles of carbon, sulfur, nitrogen, and trace metals in the oceans. Raven et al. show how ocean anoxia affects microbial sulfur processing in sinking marine particles. They observed cryptic microbial sulfate reduction, which forms organic sulfur that is resistant to acid hydrolysis, a process that could enhance carbon preservation in sediments underlying oxygen-deficient water columns. This may help explain some of the more extreme episodes of organic carbon preservation associated with marine anoxia in Earth's history. Science , this issue p. [178][1] [1]: /lookup/doi/10.1126/science.abc6035

A fine detail physico-chemical depositional model for Devonian organic-rich mudstones: A petrographic study of the Hare Indian and Canol Formations, Central Mackenzie Valley, Northwest Territories

The Hare Indian and Canol Formations, which are part of the Horn River Group in the Northwest Territories, Canada, primarily consist of organic-rich mudstones deposited during the Middle to Late Devonian. The formations were previously considered to represent marine basin fill accumulated in an oxygen-starved distal shelf setting, evidenced by the organic-rich character, pyrite content, and lack of macro-scale bioturbation. The depositional model, paleo‑oxygenation interpretations, and methods of organic carbon preservation presented in this study are in contrast to previous assumptions of the Horn River Group mudstones. Detailed petrographic, sedimentological, and ichnological analyses were carried out on thin sections taken from several cored Horn River Group intervals. These organic-rich mudstone units contain eight distinct microfacies, representing four main sedimentation processes acting on an interpreted distal shelf setting: (1) pelagic suspension settling, (2) plug-like sediment-gravity flows, (3) surge and surge-like low-density turbidity currents, and (4) debrites. Pelagic suspension-settling dominated in distal, quiet waters out of the reach of persistent storm influence. Debrites, plug-like flows, and low-density turbidite processes represent a continuum, wherein storm influence was the dominant driver in sediment delivery. Several morphologically distinct, microscopic biogenic-sedimentary structures (i.e. ichnofossils) have been identified throughout the mudstone intervals, indicative of sediment pore waters that were at least periodically partially oxygenated. Evaluation of total organic carbon content against bioturbation and microfacies interpretation suggest that persistent anoxia was not the dominant factor in organic carbon preservation, but rather a result of a combination of heightened sedimentation and burial rates and possible amplified rates of primary productivity. The results of this study may be applied to evaluations of other organic rich mudstones to enhance paleo-depositional interpretations.

Iron-bound carbon increases along a freshwater−oligohaline gradient in a subtropical tidal wetland

Globally, a vast extent of tidal wetlands will be threatened by sea-level-rise-induced salinization. Because ferric (hydro)oxides [Fe(III)] play a crucial role in soil organic carbon (SOC) preservation, understanding the responses of the Fe-bound C pool to increasing salinity could assist in accurate prediction of the changes in C stocks in the tidal wetland soils facing imminent sea-level rise. In this study, we investigated pools of Fe-bound C and SOC, C-degrading enzyme activity, Fe species contents and Fe-cycling bacteria, and plant properties along a salinity gradient from freshwater (0.0 ± 0.1 ppt; part per thousand) to oligohaline (2.6 ± 0.6 ppt) in a subtropical tidal wetland. Overall, the belowground biomass and the content of root Fe(III) plaque (a proxy of root oxygen loss potential) rose with the increasing salinity. Along the salinity gradient, the abundance of Gallionella (Fe-oxidizing bacteria) increased, but the abundance of Geobacter (Fe-reducing bacteria) decreased. The Fe(II):Fe(III) ratios decreased as salinity increased, implying that more Fe(II) was oxidized and immobilized into Fe(III) closer to the sea. Fe sulfides contents also elevated close to sea. The co-existence of Fe(III) and Fe sulfides at the oligohaline sites implied a high spatial heterogeneity of Fe distribution. During the growing season, the SOC pool generally decreased with increasing salinity, probably due to a reduction in aboveground-C input and enhanced activity of the C-degrading enzyme. The Fe-bound C pool was positively affected by the amorphous Fe(III) content and negatively related to the activity of phenol oxidase. The Fe-bound C pool generally rose along the salinity gradient, with the importance of Fe-bound C to SOC increasing from 18% to 29%. Altogether, our findings implied that when the imminent sea-level-rise-induced salinization occurs, the total soil C stock may generally decrease, but Fe-bound C will become increasingly important in protecting the rest of the C stocks in tidal wetland soils.

Microbial sulfate reduction and organic sulfur formation in sinking marine particles.

Climate change is driving an expansion of marine oxygen-deficient zones, which may alter the global cycles of carbon, sulfur, nitrogen, and trace metals. Currently, however, we lack a full mechanistic understanding of how oxygen deficiency affects organic carbon cycling and burial. Here, we show that cryptic microbial sulfate reduction occurs in sinking particles from the eastern tropical North Pacific oxygen-deficient zone and that some microbially produced sulfide reacts rapidly to form organic sulfur that is resistant to acid hydrolysis. Particle-hosted sulfurization could enhance carbon preservation in sediments underlying oxygen-deficient water columns and serve as a stabilizing feedback between expanding anoxic zones and atmospheric carbon dioxide. A similar mechanism may help explain more-extreme instances of organic carbon preservation associated with marine anoxia in Earth history.

Preservation of organic carbon in dolomitized Cambrian stromatolites and implications for microbial biosignatures in diagenetically replaced carbonate rock

Stromatolites have been a major focus in the search for ancient microbial life, however, the organic carbon biosignatures of dolomitized stromatolites have not yet been fully characterized or correlated with their dolomitizing conditions. Although dolomitization rarely preserves microbial morphology, the presence of organic carbon can provide valuable information for characterization of fossils' biogenicity, syngenicity, and indigeneity to their host rock. The Cambrian Allentown Formation in New Jersey, USA, is an excellent example of dolomitized stromatolites and thrombolites containing diagenetically modified microbial biosignatures. Based on XRD and EPMA data, the dolomite composition is typically stoichiometric, with varying degrees of cationic ordering. The outcrop underwent early dolomitization in a marginal-marine setting and later burial diagenesis resulting in multi-generational dolomite formation: (1) microspar dolomite formed by early diagenetic replacement at or near the surface, (2) zoned dolomite formed penecontemporaneously with the microspar phase as rhombohedral crystals by infilling primary pore spaces within the microspar matrix. The rhombic crystals continued to grow outward in alternating stages of Fe-enriched and -depleted fluids, which were preserved in zoned rims and revealed by cathodoluminescence, and (3) saddle dolomite formed during late stage deep burial with Fe- and Mn-rich fluids, and occurs as a void-filling, high-temperature phase. Organic carbon, characterized using confocal Raman microscopy, has an exclusive distribution within the microspar dolomite, and the D and G bands' characteristics reveal similar thermal alteration to the host rock, indicating that the mapped organic carbon is indigenous and syngenetic with the Cambrian carbonates. The findings presented in this study reveal organic matter found within microspar of various dolomitized facies deriving from different source pools of organic carbon. This study sheds light on biosignatures in secondary dolostones and may aid biosignature detection in older carbonate rocks on Earth and Mars.

Correction to: Organic carbon preservation promoted by aromatic compound-iron complexes through manure fertilization in red soil

The article “Organic carbon preservation promoted by aromatic compound-iron complexes through manure fertilization in red soil”, written by Yunjie Wen et al., was originally published electronically on the publisher’s internet portal (currently SpringerLink) on 12 September 2020 with open access.

Organic carbon preservation promoted by aromatic compound-iron complexes through manure fertilization in red soil

This study was aimed at evaluating the effect of manure fertilization on organo-mineral complexes based on unfertilized (CK) and 5-year manure fertilization (M) in red soils (ultisols). Sequential density fractionation was adopted to obtain particular organic matter (POM, 2.6 (MOM> 2.6)), which were demineralized by hydrofluoric acid (HF). The HF-dissolved organic carbon (OC) was identified as mineral bound OC, and was characterized by comparing the band area of Fourier transform infrared spectroscopy before and after 10% HF demineralization. Manure fertilization significantly increased the OC and nitrogen contents of bulk soil, POM, MOM1.8–2.2, and MOM2.2–2.6, but did not affect MOM> 2.6. About 82.9–86.1% soil OC was retained by MOM1.8–2.2 and MOM2.2–2.6 dominated with iron (Fe) and aluminum oxides and phyllosilicates, in which HF-dissolved OC was significantly positively correlated with poorly crystalline (Feo) (R2 = 0.61, P 2.6 was composed of aromatic C, amide N, and polysaccharides, not being affected by fertilization. Our results suggested that manure fertilization promotes organo-mineral association, particularly for aromatic compound-Fe complexes, contributing to OC preservation in red soils.

Organic Matter Preservation in Ancient Soils of Earth and Mars.

The emerging field of astropedology is the study of ancient soils on Earth and other planetary bodies. Examination of the complex factors that control the preservation of organic matter and other biosignatures in ancient soils is a high priority for current and future missions to Mars. Though previously defined by biological activity, an updated definition of soil as planetary surfaces altered in place by biological, chemical or physical processes was adopted in 2017 by the Soil Science Society of America in response to mounting evidence of pedogenic-like features on Mars. Ancient (4.1–3.7 billion year old [Byr]) phyllosilicate-rich surface environments on Mars show evidence of sustained subaerial weathering of sediments with liquid water at circumneutral pH, which is a soil-forming process. The accumulation of buried, fossilized soils, or paleosols, has been widely observed on Earth, and recent investigations suggest paleosol-like features may be widespread across the surface of Mars. However, the complex array of preservation and degradation factors controlling the fate of biosignatures in paleosols remains unexplored. This paper identifies the dominant factors contributing to the preservation and degradation of organic carbon in paleosols through the geological record on Earth, and offers suggestions for prioritizing locations for in situ biosignature detection and Mars Sample Return across a diverse array of potential paleosols and paleoenvironments of early Mars. A compilation of previously published data and original research spanning a diverse suite of paleosols from the Pleistocene (1 Myr) to the Archean (3.7 Byr) show that redox state is the predominant control for the organic matter content of paleosols. Most notably, the chemically reduced surface horizons (layers) of Archean (2.3 Byr) paleosols have organic matter concentrations ranging from 0.014–0.25%. However, clay mineralogy, amorphous phase abundance, diagenetic alteration and sulfur content are all significant factors that influence the preservation of organic carbon. The surface layers of paleosols that formed under chemically reducing conditions with high amounts of iron/magnesium smectites and amorphous colloids should be considered high priority locations for biosignature investigation within subaerial paleoenvironments on Mars.

Efficient preservation of young terrestrial organic carbon in sandy turbidity-current deposits

Abstract Burial of terrestrial biospheric particulate organic carbon in marine sediments removes CO2 from the atmosphere, regulating climate over geologic time scales. Rivers deliver terrestrial organic carbon to the sea, while turbidity currents transport river sediment further offshore. Previous studies have suggested that most organic carbon resides in muddy marine sediment. However, turbidity currents can carry a significant component of coarser sediment, which is commonly assumed to be organic carbon poor. Here, using data from a Canadian fjord, we show that young woody debris can be rapidly buried in sandy layers of turbidity current deposits (turbidites). These layers have organic carbon contents 10× higher than the overlying mud layer, and overall, woody debris makes up >70% of the organic carbon preserved in the deposits. Burial of woody debris in sands overlain by mud caps reduces their exposure to oxygen, increasing organic carbon burial efficiency. Sandy turbidity current channels are common in fjords and the deep sea; hence we suggest that previous global organic carbon burial budgets may have been underestimated.

The impacts of land plant evolution on Earth's climate and oxygenation state – An interdisciplinary review

The Paleozoic emergence of terrestrial plants has been linked to a stepwise increase in Earth's O2 levels and a cooling of Earth's climate by drawdown of atmospheric CO2. Vegetation affects the Earth's O2 and CO2 levels in multiple ways, including preferential organic carbon preservation by decay-resistant biopolymers (e.g. lignin) and changing the continental weathering regime that governs oceanic nutrient supply and marine biological production. Over shorter time scales (≤1 Myr), land plant evolution is hypothesized to have occasionally enhanced P weathering and fertilized the oceans, expanding marine anoxia and causing marine extinctions. Oceanic anoxia would eventually become limited by oceanic O2 uptake as oxygen accumulates in the atmosphere and surface oceans when excess organic carbon is buried in marine sediments. Here, we review hypotheses and evidence for how the evolving terrestrial ecosystems impacted atmospheric and oceanic O2 and CO2 from the Ordovician and into the Carboniferous (485–298.9 Ma). Five major ecological stages in the terrestrial realm occurred during the prolonged time interval when land was colonized by plants, animals and fungi, marked by the evolution of 1) non-vascular plants, 2) vascular plants with lignified tissue, 3) plants with shallow roots, 4) arborescent and perennial vegetation with deep and complex root systems, and 5) seed plants. The prediction that land vegetation profoundly impacted the Earth system is justified, although it is still debated how the individual transitions affected the Earth's O2 and CO2 levels. The geological record preserves multiple lines of indirect evidence for environmental transitions that can help us to reconstruct and quantify global controls on Earth's oxygenation and climate state.

Spatial and temporal patterns in soil organic carbon, microbial biomass and activity under different land-use types in a long-term soil-monitoring network

Preservation of soil organic carbon (SOC) requires knowledge concerning the quantity and quality of both the SOC and the SOC-decomposing microbial community. In northern Germany, this information is assessed as part of Schleswig-Holstein’s long-term soil-monitoring programme, in which topsoils have been analysed since 1995. In this study, we evaluated long-term data from Schleswig-Holstein’s monitoring sites and compared the content of SOC and microbial biomass carbon (MBC) among croplands, grasslands and forests. We distinguished the microbial fractions as total MBC (chloroform fumigation extraction method) and glucose-responsive MBC, and we used the MBC/SOC ratio and soil basal respiration (SBR) to obtain a qualitative view of the SOC and its turnover potential. Additionally, we performed a temporal comparison for SOC and MBC between the periods 1995–2002 and 2005–2015. The results showed that median SOC content is the highest in forest soils (41 g kg−1), followed by grasslands (33 g kg−1), and croplands (12 g kg−1). Different ascending orders occurred for the total MBC (cropland < forest < grassland), however, and for the glucose-responsive MBC (forest < cropland < grassland), indicating differences in SOC quality and the microbial community structure. The different ratios between MBC and SOC showed that SOC in croplands and grasslands is characterised as easily degradable, but more stable in grasslands under no-till conditions. Forest SOC appears almost non-degradable, and thus is most stable in the long term. We concluded that land-use-specific carbon characteristics, including the functional microbial community structure, are beneficial for evaluation of SOC preservation potential. From the temporal perspective, a significant increase (45–114 %) was observable for the total MBC on cropland and grassland sites, whereas the changes in SOC were not significant. The remarkable growth of the total MBC in relation to the glucose-responsive MBC indicates shifts in the microbial community towards a K-strategy metabolism.

Early diagenetic pyrite cementation influences molecular composition of sedimentary organic matter in the Dongying Depression, China

Micro-variations of lipid hydrocarbons and biomarkers in well-laminated shales have not been extensively documented, especially within different rock textures, fabrics and mineralogies. Comparison of a pyrite cemented zone and its surrounding sediment may offer a new perspective on mineral control on preservation of hydrocarbons and organic carbon. A black well-laminated shale comprising two adjacent and contrasting calcareous (light) and pyrite (dark) laminae at the millimetre scale was selected from the Eocene Shahejie Formation, Dongying Depression, Bohai Bay Basin. It was horizontally sliced to separate a pyrite cemented concretion from the unaltered laminated shale, and investigated using geochemical techniques to assess the heterogeneity of elements, minerals, microfossils and hydrocarbons within the two slices. Micro-scale changes in source inputs were noted. For instance, the 4α-methylsteranes, dinosteranes and the triaromatic dinosteranes show varying distributions between the two slices, attributed to slight differences in dinoflagellate contributions. Similarly, variation in the amount of bacterial sulfate reduction is indicated by variations in the aryl isoprenoid ratio, relative amount of isorenieratane, long-chain n-alkane carbon isotopic composition and other molecular indicators including the relative amounts of dibenzothiophene, dibenzofuran, methyldibenzothiophene and methyltrimethyltridecylchromans. The primary control on these variations appears to be different euxinic water conditions, but diagenesis has also influenced the distributions. Differential anaerobic bacterial reworking of organic matter and pyrite concretion protection are responsible for organic geochemical variations on a millimetre-scale in the shale. The pyrite concretion has an altered record of its thermal history, but has a more accurate record of the organic geochemistry of microfossils and water column conditions. This study shows the potential for future investigations of closely-spaced vertical variation in organic geochemical parameters that may shed light on rapid temporal variation in organic matter input and depositional environment.

Examining bulk and iron-associated organic carbon through depth in margin sea sediments (China) under contrasting depositional settings: Chemical and NEXAFS spectral characterization

Abstract Marine sediments are the largest sink for organic carbon (OC) on Earth, and iron (Fe) oxides play an important role in stabilization of sedimentary OC. However, the roles of Fe oxides in OC stabilization during prolonged burial, for example, up to tens of thousands of years or more are still poorly constrained. In this study, we used traditional chemical extraction and near-edge X-ray absorption fine structure (NEXAFS) spectroscopic technique to characterize bulk OC and Fe-associated OC (Fe-OC) through depth in gravity cores collected from three sites near the Yangtze River Estuary (YRE), in the South Yellow Sea (SYS), and in the middle Okinawa Trough, which have contrasting depositional environments. Results show that depositional environments have exerted quite different influences on sources and burial of sedimentary OC, and thus on OC degradation during prolonged burial at the three sites. Reactive Fe (FeR) contents at the three sites are greatly influenced by sediment sources, the history of its transport, and its reworking intensity, with FeR contents near the highly dynamic YRE much higher than at the central SYS and the middle Okinawa Trough. The fractions of Fe-OC in total OC (fFe-OC) displayed no clear or consistent trends with depth or by site, probably due to the dual roles of Fe redox cycling in OC protection versus its oxidation. As indicated by the fFe-OC, reactive Fe plays a limited role in OC preservation in margin sea sediments of East China. A combination of NEXAFS spectra and isotopic compositions of bulk OC and Fe-OC indicates that main OC functionalities have not experienced differential alterations and/or no specific OC moieties have been selectively stabilized/released during prolonged burial in the three contrasting depositional environments.

Organic chemistry insights for the exceptional soil carbon storage of the seagrass Posidonia australis

The high organic carbon (OC) stores in seagrass meadows have led to their recognition as significant Blue Carbon sinks, though the diagenetic conditions that enable OC retention in seagrass soils remain poorly understood. In this study, seagrass soils were sampled from a Posidonia australis meadow in Oyster Harbour (Albany; south-western Australia) to investigate the preservation of sedimentary OC. We analysed soil characteristics (colour, grain size and redox potential), radiocarbon age, and characterised the soil organic matter (OM) using solid state CP/MAS 13C NMR spectroscopy to examine the preservation of OM down the soil profile. There was minimal change in organic composition over 1,700 years of accumulation, indicating long-term OM preservation. Primarily, this preservation appears to be driven by the recalcitrance of seagrass detritus buried in anoxic soils. The majority (70–83%) of total sedimentary OM comprised components directly attributable to seagrass origins (lignin, carbohydrate and black carbon-like matter), while the remainder consisted mostly of protein, some of which may have been present in seagrass biomass, along with likely contributions from algae and microbes. Although black carbon originates from organic matter combustion, here we provide evidence that the 13C NMR signal identified as black carbon-like matter in our soils is possibly associated with seagrass-derived organic matter consisting of degraded lignin products or other non-pyrogenic aromatics. The increase in the relative abundance of this black carbon-like matter with aging suggests its selective preservation. The relative abundances of carbohydrates significantly decreased with age down core (i.e. they appeared to be selectively decomposed), while lignin and protein did not show any quantitative changes in relative abundance (non-selective preservation). These findings demonstrate the exceptional preservation of P. australis derived OC, which contributes to our understanding of the higher OC storage capacity of Posidonia compared to other seagrass species.

Detailed facies analysis of Cenomanian–Turonian organic‐rich mudstones: Implications for depositional controls on source rocks

AbstractUnderstanding mudstone depositional processes, including both traction transport and dynamic mechanisms, requires a re‐assessment of how these processes relate to the accumulation and preservation of organic carbon in fine‐grained successions. This relationship was addressed using facies described in detail from a Cenomanian–Turonian aged, organic‐rich, marine mudstone‐dominated succession from the Albertan Western Interior Seaway. All facies exhibit a high degree of heterogeneity at the millimetre‐to‐decimetre scale, contain evidence of bioturbation and display similar ranges of sedimentary structures including wave ripples and current ripples, sharp‐based graded beds and starved ripples. Described facies vary in grain size as well as relative abundances of biogenic versus siliciclastic grains. Siliciclastic grains are dominantly composed of quartz silt as well as silt‐sized to sand‐sized clay mineral–rich aggregates while the biogenic grains are dominantly composed of silt‐sized to sand‐sized calcareous pellets, bivalve fragments and foraminifera. Although the succession is relatively clay mineral‐rich, the majority of clay minerals occur as mud aggregates that were transported by traction processes. The sedimentary structures present are diagnostic of sea floor reworking by waves and currents, indicating that the basin was relatively shallow (i.e. not hundreds of metres deep) with frequent reworking of the sea floor sediment with suspension settling deposits being rare. The abundance of bioturbation and in situ bivalves indicates that there was sufficient oxygen at the sea floor and that widespread bottom water anoxia was not persistent at the time of deposition of the organic‐rich interval of interest. The accumulation of organic‐rich sediment appears to be a preservational phenomenon caused by sediment being buried more rapidly than the organic carbon can oxidize in the surficial layers due to episodic inputs of siliciclastic material during large storm events.

Spatial and seasonal variations of organic carbon distributions in typical intertidal sediments of China

Intertidal zones are the main sites for land-ocean interaction and play an important role in transporting and accumulating organic carbon (OC). Understanding sources and preservation of sedimentary OC are vital to better evaluate the processes controlling the OC distribution in intertidal zones. In this study, we present bulk parameters of total organic carbon (TOC), δ13Corg, grain size and biomarker contents in surface sediments of 14 typical intertidal zones along China’s coastline during both the wet season (WS) and the dry season (DS). Statistical analysis of our data set showed that TOC and biomarker contents were generally high in small estuaries, intermediate but more variable in large estuaries and low in non-estuaries; and these distribution patterns broadly correlated with sedimentary clay contents. TOC and biomarker contents were generally higher in the WS than the DS reflecting both OC inputs and preservation. A binary mixing model based on δ13Corg and biomarker proxies revealed that large estuaries were characterized by higher terrestrial OC fractions (62–80%) except for the Yellow River Estuary (24%), while lower terrestrial OC fractions occurred in non-estuaries (13–15%). Variable OC contributions were found in small estuaries, with most sites characterized by low terrestrial OC contributions (3–35%), but high terrestrial OC contributions were found at the Qiantang River Estuary site (59%) and the Jiulong River Estuary site (67%). Carbon stocks in Chinese intertidal sediments are comparable with those in marine sediments from shelf areas and the Exclusive Economic Zone (EEZ). This study suggests that intertidal sediments are potentially an important carbon sink.

Runoff-driven export of terrigenous particulate organic matter from a small mountainous river: sources, fluxes and comparisons among different rivers

The land-to-ocean export and subsequent preservation of terrestrial particulate organic carbon (POC) in oceans represents a major carbon sink in the global carbon cycle. Small mountainous rivers (SMRs) are hotspots of this process; however, how the sources and fluxes of POC change during rain events in low-gradient SMRs and how they compare with those in steep SMRs are unknown. Here, we combined the bulk properties and lignin phenols to trace changes in POC sources and fluxes in a low-gradient SMR (stream gradient: 0.6%) in southeastern China. Our results were further compared with other SMRs to understand the geomorphological influences on the POC export from SMRs. The results indicated that POC sources changed from aquatic organisms and deep soils during low-flow periods to surface soils and plant debris during high-flow periods. The lignin concentration showed a significant positive relationship with the runoff, suggesting that hydrology played an important role in exporting terrigenous POC in low-gradient SMRs. We further estimated that approximately 22%, 56% and 79% of annual water, POC and particulate lignin were exported during high-flow periods, respectively. Furthermore, we found that with increasing water discharge, the magnitude of the lignin increase was greater in SMRs with smaller stream gradients. Our results suggest that the export of terrigenous POC from low-gradient SMRs is driven by hydrology and is more sensitive to rain events than that in steep SMRs.

Preservation of organic carbon promoted by iron redox transformation in a rice-wheat cropping system

Mineral protection regulates long-term global preservation of soil organic carbon (SOC), but little research has been conducted on the iron (Fe) oxide availability for carbon binding regulated by seasonal variation of wetting and drying and fertilization in paddy soils. The objective of this study was to explore the effects of five-year rice-wheat rotation and organic fertilization on the production of short-range-ordered (SRO) Fe oxides and SOC sequestration. Five fertilization treatments were examined: (1) no fertilizer control (CK), (2) chemical nitrogen, phosphorus and potassium (NPK), (3) 50% NPK plus manure (NPKM), (4) 100% NPK plus straw (NPKS), and (5) 30% NPK plus manure organic-inorganic compound fertilizer (NPKMOI). Soil samples were collected from 2013 to 2015 after each wheat and rice harvest. A split-plot analysis of variation was used to investigate the respective role of fertilizer treatments (main-plot) and crop seasons (sub-plot) in the transformation of SRO-Fe oxides. Variation partitioning analysis was used to quantify the contribution of SRO-Fe oxides to the accumulation of SOC. Five-year rice-wheat rotation without any fertilization (CK) increased the oxalate extractable Fe oxides (Feo) by 55.06% compared with the initial soil. The Feo concentration was higher (P < 0.05) in the NPKM, NPKS and NPKMOI treatments than in the NPK and CK treatments, and was higher after the rice harvest than after the wheat harvest. Furthermore, the Feo concentration was positively (P < 0.001) correlated with the SOC content and negatively correlated with the specific carbon mineralization rate (SCMR, rate per unit SOC). Moreover, the Feo could explain 52.57% and 40.5% of the variation in the SOC content and the SCMR, respectively. In addition, the partial humification coefficient (PHC, the percentage of SOC change per unit of exogenous carbon input) of the crop straw was 12.92% but amounted to 41.45% to 46.35% for different types of compost. These results indicate that the reductive dissolution and subsequent oxidative precipitation of Fe oxides induced by seasonal alternation of wetting and drying can be further promoted via organic fertilization possibly by forming organo-Fe complexes, which contribute to the accumulation of SRO-Fe oxides and the preservation of organic carbon in paddy soils.

Authigenic metastable iron sulfide minerals preserve microbial organic carbon in anoxic environments

The burial of organic carbon (OC) in sedimentary environments promotes long-term carbon sequestration, which allows the release of oxygen in the atmosphere. Organo-mineral interactions that form between terrigenous minerals and OC during transport to and deposition on the seabed enhance OC preservation. Here, we propose an authigenic mechanism for the coupled preservation of labile OC and metastable iron sulfide minerals under anoxic conditions. Sulfate-reducing microorganisms (SRM) are ubiquitous in anoxic environments and produce the majority of free sulfide in marine sediments, leading to the formation of iron sulfide minerals in situ. Using high spatial resolution microscopy, spectroscopy and spectro-microscopy, we show that iron sulfide biominerals precipitated in the presence of SRM incorporate and adsorb organic molecules, leading to the formation of stable organo-mineral aggregates that could persist for years in anoxic environments. OC/iron sulfide assemblages consist of the metastable iron sulfide mineral phases mackinawite and/or greigite, along with labile organic compounds derived from microbial biomass or from organic molecules released extracellularly by SRM. Together these results underscore the role that a major group of anoxic microbes play in OC preservation and illustrate the value of the resulting authigenic metastable iron sulfide minerals mackinawite and greigite in protecting labile organic molecules from degradation over time.

Comparison of sedimentary organic carbon loading in the Yap Trench and other marine environments

Knowledge about organic carbon loadings (ratio of sedimentary organic carbon (SOC) content to specific surface area (SSA)) and the fate of organic carbon (OC) is critical to understand the marine carbon cycle. We investigated the variations in the patterns of OC loadings and the preservation capacities of sedimentary OC in the Yap Trench and other marine environments. The average OC loading in sediment cores from various marine environments decreases with increasing water depth at a rate of ~0.06 mg OC/(m2·km) (R2 =0.23, P<0.01). Distinct low OC loadings (0.09±0.04 mg OC/m2) were observed in the Yap Trench, with the lowest values as ~0.02 mg OC/m2. A further comparative analysis indicated that OC/SSA=0.2 mg OC/m2 is a good indicator to distinguish between oxic deep-sea regions and suboxic energetic deltaic areas. Regression analysis between OC loading and bulk carbon isotope compositions indicates that marine OC (δ13C ~−20.4‰ to −18.6‰) dominates the lost OC within the Yap Trench and does not differ from that of the abyssal zone. In contrast, terrestrial OC with δ13C values of approximately −27.4‰ to −20.5‰ was the major source of remineralized OC in the sublittoral zone. The ratios of OC loadings in the bottom layer relative to those in the top layers of sediment cores indicate that the preservation capacities of hadal trenches are much lower than those of other environments, and only approximately 30% of the SOC deposited in hadal trenches is finally buried. The value is equivalent to 0.066% of the primary production-derived OC and much lower than the global ocean average (~0.3%). Overall, the hadal zone exhibits the lowest OC loading and preservation capacity of SOC of the different marine environments investigated, despite the occurrence of a notable funneling effect.