Persistence Of Organic Carbon Research Articles

Nanoscale mechanisms of carboxyl carbon preservation during Fe(II)-induced ferrihydrite transformation

The persistence of organic carbon (OC) in natural environments is widely attributed to OC associations with minerals such as iron (Fe) minerals. Carboxyl, a critical structure of OC, can form strong complexes with Fe minerals, but it is still largely unknown about the effects of carboxyl groups on the transformation of Fe oxides and the stabilization mechanisms of OC on Fe oxides at nanoscales. In this study, four carboxylic acids with varying numbers of carboxyl groups (2, 3, 4, and 27 carboxyls) and molecular weight (ranging from 100 to 2000 Da) were added during the coprecipitation of Fe oxides and OC. This approach was taken to systematically elucidate the nanoscale distribution and sequestration mechanisms of carboxyl-containing OC on Fe oxides over different aging periods. Results suggested that ferrihydrite transformed quickly into lepidocrocite, goethite, and magnetite with the presence of Fe(II). The transformation rate of lepidocrocite to goethite slowed with increasing carboxyl number in OC molecules for the low molecular weight carboxylic acids (100–300 Da), but the high molecular weight carboxylic acid (∼2000 Da) promoted ferrihydrite transformation into goethite. Meanwhile, the particle sizes of produced magnetite decreased with increasing carboxyl number in OC molecules. During the mineral transformation, the release of OC from Fe oxides to aqueous solution decreased with increasing numbers of carboxyl groups. Spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) coupled with electron-energy-loss spectroscopy (EELS) suggested that carboxyl-rich OC promoted the formation of abundant defective or porous structures in goethite, resulting in OC accumulation in the bulk areas of goethite, which provided an effective way to sequester OC. The aggregation of high molecular weight OC containing more carboxyl groups with magnetite nanoparticles also played an important role in the preservation of OC. Furthermore, Fourier-transform infrared spectroscopy (FTIR) and near edge X-ray absorption fine structure spectroscopy (NEXAFS) indicated that OC may form bidentate binding with Fe oxides and the binding strength of Fe(III) and OC may change with varying numbers of carboxyl groups. Results in this study provided a comprehensive understanding of the dynamic interactions between OC with different numbers of carboxyl groups and Fe oxides, and shed insights into the nanoscale mechanisms of OC stabilization during Fe oxide transformation process.

Accumulation of radiocarbon in ancient landscapes: A small but significant input of unknown origin

The persistence of organic carbon (C) in soil is most often considered at timescales ranging from tens to thousands of years, but the study of organic C in paleosols (i.e., ancient, buried soils) suggests that paleosols may have the capacity to preserve organic compounds for tens of millions of years. However, a quantitative assessment of C sources and sinks from these ancient terrestrial landscapes is complicated by additions of geologically modern (~ 10 Ka) C, primarily due to the infiltration of dissolved organic carbon. In this study, we quantified total organic C and radiocarbon activity in samples collected from 28- to 33-million-year-old paleosols that are naturally exposed as unvegetated badlands near eastern Oregon’s “Painted Hills”. We also used thermal and evolved gas analysis to examine the thermodynamic stability of different pools of C in bulk samples. The study site is part of a ~ 400-m-thick sequence of Eocene–Oligocene (45–28 Ma) paleosols, and thus we expected to find radiocarbon-free samples preserved in deep layers of the lithified, brick-like exposed outcrops. Total organic C, measured in three individual profiles spanning depth transects from the outcrop surface to a 1-m depth, ranged from 0.01 to 0.2 wt% with no clear C-concentration or age-depth profile. Ten radiocarbon dates from the same profiles reveal radiocarbon ages of ~ 11,000–30,000 years BP that unexpectedly indicate additions of potentially modern organic C. A two-endmember mixing model for radiocarbon activity suggests that modern C may compose ~ 0.5–2.4% of the total organic C pool. Thermal and evolved gas analysis showed the presence of two distinct pools of organic C, but there was no direct evidence that C compounds were associated with clay minerals. These results challenge the assumption that ancient badland landscapes are inert and “frozen in time” and instead suggest they readily interact with the modern C cycle.

The role and fate of organic carbon during aging of ferrihydrite

The persistence of organic carbon (OC) in natural environments is widely attributed to mineral protection, especially by iron (Fe) (oxyhydr)oxides. The effect of OC binding strength on the aging of Fe (oxyhydr)oxides and the mobility and fate of OC during aging however, is unknown. Here we investigate how OC binding strength controls the aging of ferrihydrite (Fh) and subsequent retention or release of the associated OC. We focus on carboxyl-rich OC coprecipitated with Fh and track the physiochemical properties and OC stability as a function of carboxyl-richness over time. In agreement with previous work we find that during carboxyl-rich OC coprecipitation with Fh, OC is adsorbed to the Fh particle surfaces and that increasing carboxyl-richness results in an increasing number of carboxylate-Fe bonds between the OC and the mineral particles and thus increasing OC binding strength. We show that OC substantially retards the aging of Fe (oxyhydr)oxide from Fh to more crystalline Fe minerals and that this retardation increases with increasing OC binding strength. We also show that the total amount of OC decreases during aging and that the proportion of the remaining OC that is non-desorbable with 0.1 M NaOH decreases during aging for OC with relatively low binding strength but increases during aging for OC with relatively high binding strength. Our results therefore indicate that OC with higher binding strength coprecipitated with Fh becomes proportionally more stable with the solid phase and thus less mobile during aging in natural environments. We suggest that our work might offer a deeper mechanistic insight into the processes responsible for OC persistence with minerals and thus the long-term preservation of OC in natural environments.

Mineralogical control on methylotrophic methanogenesis and implications for cryptic methane cycling in marine surface sediment

Minerals are widely proposed to protect organic carbon from degradation and thus promote the persistence of organic carbon in soils and sediments, yet a direct link between mineral adsorption and retardation of microbial remineralisation is often presumed and a mechanistic understanding of the protective preservation hypothesis is lacking. We find that methylamines, the major substrates for cryptic methane production in marine surface sediment, are strongly adsorbed by marine sediment clays, and that this adsorption significantly reduces their concentrations in the dissolved pool (up to 40.2 ± 0.2%). Moreover, the presence of clay minerals slows methane production and reduces final methane produced (up to 24.9 ± 0.3%) by a typical methylotrophic methanogen—Methanococcoides methylutens TMA-10. Near edge X-ray absorption fine structure spectroscopy shows that reversible adsorption and occlusive protection of methylamines in clay interlayers are responsible for the slow-down and reduction in methane production. Here we show that mineral-OC interactions strongly control methylotrophic methanogenesis and potentially cryptic methane cycling in marine surface sediments.

Carboxyl-richness controls organic carbon preservation during coprecipitation with iron (oxyhydr)oxides in the natural environment

The coprecipitation of organic carbon with iron minerals is important for its preservation in soils and sediments, but the mechanisms for carbon-iron interactions and thus the controls on organic carbon cycling are far from understood. Here we coprecipitate carboxylic acids with iron (oxyhydr)oxide ferrihydrite and use near-edge X-ray absorption fine structure spectroscopy and wet chemical treatments to determine the relationship between sequestration mechanism and organic carbon stability against its release and chemical oxidative remineralisation. We show that organic carbon sequestration, stabilisation and persistence increase with an increasing number of carboxyl functional groups. We suggest that carboxyl-richness provides an important control on organic carbon preservation in the natural environment. Our work offers a mechanistic basis for understanding the stability and persistence of organic carbon in soils and sediments, which might be used to develop an overarching relationship between organic functional group-richness, mineral interactions and organic carbon preservation in the Earth system.

Aluminous clay and pedogenic Fe oxides modulate aggregation and related carbon contents in soils of the humid tropics

Abstract. Aggregation affects a wide range of physical and biogeochemical soil properties with positive effects on soil carbon storage. For weathered tropical soils, aluminous clays (kaolinite and gibbsite) and pedogenic Fe (oxyhydr)oxides (goethite and hematite; termed “Fe oxides”) have been suggested as important building units for aggregates. However, as aluminosilicates, aluminum hydroxides, and Fe oxides are part of the clay-sized fraction it is hard to separate how certain mineral phases modulate aggregation. In addition, it is not known what consequences this will have for organic carbon (OC) persistence after land-use change. We selected topsoils with unique mineralogical compositions in the East Usambara Mountains of Tanzania under forest and cropland land uses, varying in contents of aluminous clay and Fe oxides. Across the mineralogical combinations, we determined the aggregate size distribution, aggregate stability, OC contents of aggregate size fractions, and changes in aggregation and OC contents under forest and cropland land use. Patterns in soil aggregation were rather similar across the different mineralogical combinations (high level of macroaggregation and high aggregate stability). Nevertheless, we found some statistically significant effects of aluminous clay and pedogenic Fe oxides on aggregation and OC storage. An aluminous clay content > 250 g kg−1 in combination with pedogenic Fe contents < 60 g kg−1 significantly promoted the formation of large macroaggregates > 4 mm. In contrast, a pedogenic Fe content > 60 g kg−1 in combination with aluminous clay content of < 250 g kg−1 promoted OC storage and persistence even under agricultural use. The combination with low aluminous clay and high pedogenic Fe contents displayed the highest OC persistence, despite conversion of forest to cropland causing substantial disaggregation. This indicates that aggregation in these tropical soils is modulated by the mineralogical regime, causing moderate but significant differences in aggregate size distribution. Nevertheless, aggregation was little decisive for overall OC persistence in these highly weathered soils, where OC storage is more regulated by direct mineral–organic interactions.

Iron oxides and aluminous clays selectively control soil carbon storage and stability in the humid tropics

Clay minerals and pedogenic metal (oxyhydr)oxides are the most reactive soil mineral constituents controlling the long-term persistence of organic carbon (OC) in terrestrial ecosystems. However, their co-occurrence in most soils complicates direct assessment of their individual contribution to OC persistence. Making use of unique mineralogical combinations in soils located in the East Usambara Mountains of Tanzania, we disentangled the contribution of clay-sized aluminous minerals (kaolinite, gibbsite) and pedogenic Fe (oxyhydr)oxides (predominant goethite and hematite) on OC storage and stabilization under natural forests and croplands. Topsoil samples, varying in contents but not types of aluminous clays and pedogenic Fe (oxyhydr)oxides, were identified by selective extractions, X-ray diffraction, and Mössbauer spectroscopy. Associated abundance of particulate and mineral-associated organic matter (OM) was quantified by density fractionation and their changes during land-use conversion were determined as a measure of OC persistence. Additionally, we assessed the resistance of OC to chemical oxidation as well as microbial decomposition in a 50-day laboratory incubation. We found that the ratio of pedogenic Fe to aluminous clay is more consequential for OC storage and stabilization than their individual contents, despite the fact that Fe (oxyhydr)oxides generally exert a stronger impact on OC than aluminous clays. Conjunction of large amounts of Fe (oxyhydr)oxides with low aluminous clay contents caused the strongest accumulation of mineral-associated OC, a low soil respiration, high OC stability against chemical oxidation, and high OC persistence during land-use change. Our study suggests that certain mineralogical combinations in the humid tropics alleviate OM losses during land conversion because of the strong and selective mineral control on OC stabilization, particular if the weight ratio of pedogenic Fe to aluminous clay exceeds the threshold range of 0.44‒0.56.

Drivers of organic carbon allocation in a temperate slope-floodplain catena under agricultural use

The distribution of soil organic carbon (OC) in the landscape shows high variability between landform positions. Soils in depositional sites are characterized by a highly diverse vertical structure and can contain significant amounts of OC over the whole soil profile. However, anthropogenic modifications of the landscape such as land use and artificial drainage can significantly alter the OC balance in a catchment. Here, we analyse the spatial distribution of OC stocks along an agriculturally used catena in a catchment, tributary to the Danube River in the Bavarian Forest in Southern Germany. The combination of geomorphic aspects with highly detailed soil data based on 16 soil profiles along a catena allowed us to closely examine the relationship between soil OC and the factors of landform position, soil inherent properties and grassland vs. cropland land use. We determined bulk density, total carbon (TC) and inorganic carbon (IC) for each soil horizon in order to quantify OC stocks and picture the vertical OC distribution in the soils. Pedogenic oxides were analysed as an indicator for the redox conditions as well as the soil development. Furthermore, we analysed the clay fraction as well as poorly crystalline iron oxides (FeOX) as potential binding agents for OC. The persistence of OC in the landform positions could be designated by the proportion of 14C content of the bulk soil OC. Our data revealed that the amount of OC stored in the topsoil is mostly determined by land use, but subsoil OC stocks highly depend on landform position. Most OC of this catena is stored in the floodplain featuring particularly high amounts of OC in their subsoils. Radiocarbon signatures of the floodplain OC indicated a young 14C age compared to the footslope subsoils. We assume that these high OC stocks were due to 1.) formerly high input of OC-rich sediments, 2.) preservation of this OC from decomposition due to high water saturation and 3.) current land use (grassland). Dropping of the ground water table due to artificial drainage measures left detectable footprints within a few decades in the depth distribution of pedogenic iron oxides and indicated currently dominant oxic conditions within the uppermost 100 cm of the floodplain soils. We suspect that under these conditions the OC stocks are now vulnerable to OC losses. Even though we found no correlation between FeOX and OC, it can be assumed that the interplay between FeOX and OC will play a key role for the stabilization of the floodplain OC in the future. Today most floodplains in Central Europe are not in their natural state, but drained and under agricultural land use. Therefore this work points to the necessity of monitoring the OC pools in these landform positions and to consider quick responses when it comes to changes in the hydrologic regime and/or land-use system.

An experimental study on the effects of nutrient enrichment on organic carbon persistence in the western Pacific oligotrophic gyre

Abstract. Carbon sequestration in the ocean is of great concern with respect to the mitigation of global warming. How to hold the fixed organic carbon in the presence of tremendous numbers of heterotrophic microorganisms in marine environments is the central issue. We previously hypothesized that excessive nutrients would ultimately decrease the storage of organic carbon in marine environments. To test this, a series of in situ nutrient enrichment incubation experiments were conducted at a site (17.59° N, 127.00° E) within the western Pacific oligotrophic gyre. Five treatments were employed: glucose (Glu), algal exudation organic material (EOM), nitrate (N) and phosphate (P), N and P in combination with glucose and a control with no added nutrients. The results showed that the dissolved organic carbon consumption rates and bacterial community specific growth rates were enhanced by inorganic nutrient enrichment treatments during the initial 48 h incubation. At the end of 14 days of incubation, about one-third (average 3.3 μmol C kg−1) more organic carbon was respired in the glucose-enriched incubation with the addition of inorganic nutrients compared to that without. In contrast, when nutrients were limiting, glucose could not be efficiently used by the bacteria and thus it remained in the environment. These results suggest that repletion of inorganic nutrients could facilitate microbial consumption of organic carbon and thus has a significant impact on carbon cycling in the environment.

Persistence of organic carbon in heated aerosol residuals measured during Tropical Composition Cloud and Climate Coupling (TC4)

The Particle Analysis by Laser Mass Spectrometry (PALMS) single particle mass spectrometer was used to analyze the composition of the nonvolatile fraction of atmospheric aerosol in a number of different environments. The mass spectra of individual particles sampled through an inlet section heated to 300°C were compared to unheated particles during flights of the NASA DC‐8 aircraft during the Tropical Composition Cloud and Climate Coupling (TC4) mission. Comparisons are presented of measurements made in the marine boundary layer, the free troposphere, and the continental boundary layer over the Colombian jungle. The heated section completely removed sulfate from the aerosols except for sodium sulfate and related compounds in sea salt particles. Organic material in sea salt particles was observed to be less volatile than chlorine. Biomass burning particles were more likely to survive heating than other mixed sulfate‐organic particles. For all particle types, there was a significant contribution to the residues from carbonaceous material other than elemental carbon. These results demonstrate the remaining compositional complexity of aerosol residuals that survive heating in a thermal denuder.

Biogeochemistry of anaerobic lacustrine and paleosol sediments within an aerobic unconfined aquifer

The geochemistry and the distribution and abundance of anaerobic bacteria were determined for sediments sampled in a deep borehole in south‐central Washington. The sampled sediments consisted of a 12‐m‐thick lacustrine sequence underlain by 8 m of paleosol grading into 5 m of silty sands, within an aerobic unconfined aquifer otherwise composed of transmissive sands and gravels. Concentrations of porewater sul‐fate varied systematically with depth, reaching a minimum of 3.9 mg L ‐1 in the central portion of the lacustrine sequence. Lacustrine sediments contained up to 1 wt% total organic carbon, whereas other sediments contained less than 0.2 wt% organic carbon. Fermentative bacteria were present throughout the sampled sequence, and were assumed to be responsible for primary degradation of organic carbon. Dissimilatory iron‐reducing bacteria (DIRB) were at maximum abundance where bioavailable Fe(III) and organic carbon were present at favorable combined concentrations. Sul‐fate‐reducing bacteria were culturable in two zones with few DIRB, where sulfate and organic carbon were sufficiently available. Evidence of iron oxidizers or methanogens was lacking in all samples. The fine‐grained nature of the lacustrine sediments, their low hydraulic conductivities, the persistence of organic carbon within them for 5–8 million years, and the concentration gradients of electron acceptors in lacustrine porewaters indicated that bacterial oxidation of sediment organic matter was ongoing within these sediments, albeit at very low rates. Low hydraulic conductivity may have constrained the mobility of bacteria and electron donors and acceptors. Microenvi‐ronments within the sediments, within which bacterial activity was limited by the flux of fermentation by‐products and the availability of electron acceptors, could thus explain the continued presence of organic carbon and bioavailable Fe(III).