Labile Particulate Organic Carbon Research Articles

Investigating the complementarity of thermal and physical soil organic carbon fractions

Abstract. Partitioning soil organic carbon (SOC) in fractions with different biogeochemical stability is useful to better understand and predict SOC dynamics and provide information related to soil health. Multiple SOC partition schemes exist, but few of them can be implemented on large sample sets and therefore be considered relevant options for soil monitoring. The well-established particulate organic carbon (POC) vs. mineral-associated organic carbon (MAOC) physical fractionation scheme is one of them. Introduced more recently, Rock-Eval® thermal analysis coupled with the PARTYSOC machine learning model can also fractionate SOC into active (Ca) and stable SOC (Cs). A debate is emerging as to which of these methods should be recommended for soil monitoring. To investigate the complementarity or redundancy of these two fractionation schemes, we compared the quantity and environmental drivers of SOC fractions obtained on an unprecedented dataset from mainland France. About 2000 topsoil samples were recovered all over the country, presenting contrasting land cover and pedoclimatic characteristics, and analysed. We found that the environmental drivers of the fractions were clearly different, the more stable MAOC and Cs fractions being mainly driven by soil characteristics, whereas land cover and climate had a greater influence on more labile POC and Ca fractions. The stable and labile SOC fractions provided by the two methods strongly differed in quantity (MAOC/Cs=1.88 ± 0.46 and POC/Ca=0.36 ± 0.17; n=843) and drivers, suggesting that they correspond to fractions with different biogeochemical stability. We argue that, at this stage, both methods can be seen as complementary and potentially relevant for soil monitoring. As future developments, we recommend comparing how they relate to indicators of soil health such as nutrient availability or soil structural stability and how their measurements can improve the accuracy of SOC dynamics models.

Greater variation of soil organic carbon in limestone- than shale-based soil along soil depth in a subtropical coniferous forest within a karst faulted basin of China

Lithology strongly influences soil microbial traits and edaphic factors and in turn soil organic carbon (SOC) dynamics. However, the effect of lithology on microbial traits, edaphic factors and resulting SOC physical fractions variation along soil depth remains inadequately understood in karst faulted basin of China. This understanding is critical for improving SOC stability. By separating SOC into labile particulate organic carbon (POC) and stable mineral-associated organic carbon (MAOC) over karst limestone and non-karst shale soil in a subtropical coniferous forest, we aimed to assess potential regulatory mechanisms underlying lithology-associated SOC stability variations across soil depth by integrating soil nutrients, mineralogical characteristics, and microbial traits. We found that SOC and its fractions were higher in limestone than in shale soil, which implying vegetation restoration effects on SOC and its fractions partly depending on lithology. Additionally, we found that the effects of soil depth on SOC and its fractions were greater in limestone soils than shale soils, and the ratios of MAOC to SOC (MAOC:SOC) and MAOC to POC (MAOC:POC) show a opposite trend in response to soil depth between two the lithologies. Variation partitioning and random forest analyses revealed that among multiple factors, the variation of SOC stability assessed via MAOC:SOC was mainly explained by microbial traits than soil nutrients and mineral properties. Contrast to soil depth, structural equation modeling analyses showed that lithology was the primary factor controlling the SOC stability when microbial traits, soil nutrients and mineralogical characteristics were controlled as conditional variables. Overall, these results highlight the crucial role of lithology in regulating the SOC stability along soil depth, which improve our understanding and management of soil carbon (C) pool in karst faulted basin of southwest China.

Downstream alteration of the composition and biodegradability of particulate organic carbon in a mountainous, mixed land-use watershed

Despite increasing recognition of storm-induced organic carbon (C) export as a significant loss from the terrestrial C balance, little is known about the biodegradation and chemical transformation of particulate organic carbon (POC) in mountainous river systems. We combined analyses of C isotopes, solution-state 1H NMR, and lipid biomarkers with biodegradable dissolved organic C (BDOC) measurements to investigate downstream changes of POC composition and biodegradability at a mountainous, mixed land-use watershed in South Korea. Water and suspended sediment (SS) samples were collected in a forested headwater stream, a downstream agricultural stream, and two downstream rivers during peak flow periods of four storm events, using either sequential grab sampling from the headwater stream to the most downstream river within a few hours around the peak flow or sediment samplers deployed during the whole storm event. DOC concentrations exhibited relatively small changes across sites, whereas POC concentrations were highest in the agricultural stream, and tapered along downstream reaches. The δ13C and δ15N of SS in the agricultural stream were distinct from up- and downstream signatures and similar to those for erosion source soils and lake bottom sediment, although increases in radiocarbon age indicated continuous compositional changes toward the lake. 1H NMR spectra of SS and deposited sediment exhibited downstream decreases in carbohydrates and lignin but enrichment of organic structures related to microbial proteins and plant wax. The downstream sediments had more microbial n-alkanes and lipid markers indicating anthropogenic origin such as coprostanol compared to the forest soil n-alkanes dominated by plant wax. While the BDOC concentrations of filtered waters differed little between sites, the BDOC concentrations and protein- to humic-like fluorescence ratios of DOC leached from SS during a 13-day incubation were higher in downstream rivers, pointing to contribution of labile POC components to the enhanced biodegradation. Overall, inputs of microbial and anthropogenic origin, in interplay with deposition and mineralization, appear to substantially alter POC composition and biodegradability during downstream transport, raising a question on the conventional view of mountainous river systems as a passive conduit of storm pulses of POC.

Temporal and depth-related differences in prokaryotic communities in abyssal sediments associated with particulate organic carbon flux

Particulate organic carbon (POC) flux is hypothesized to be the most important parameter influencing activity and biomass of prokaryotic and faunal communities in the abyssal seafloor, but there is little evidence of POC-related changes in community composition of prokaryotes. This hypothesis was tested by 16S rRNA-gene-based analysis of prokaryotic DNA and RNA extracted from abyssal seafloor sediments during periods of low and high POC flux. Fingerprint analysis of prokaryotic communities indicated that approximately 50% of the phylotypes were identical at each sediment horizon, regardless of the temporal variations in POC flux. However, phylotypes were also detected that represented a relatively dynamic component of these communities and were probably strongly influenced by the prevalent POC flux regime. These patterns were also detected in deeper sediment horizons. DNA- and RNA-based community profiles differed, although both approaches had similar community dynamics. Crenarchaeota showed the strongest shift in community composition in response to availability of labile POC, indicating that POC flux may have a more pronounced impact on crenarchaeal communities than on bacterial communities. The high number of phylotypes common to each sample time suggests that both standing stock and active prokaryotic communities are stable.

Impact of Hurricane Katrina (2005) on shelf organic carbon burial and deltaic evolution

Sediment cores from the continental shelf adjacent to the Mississippi River delta immediately after the passage of Hurricane Katrina were used to examine the magnitude, and implications for the carbon budget, of sediment and particulate organic carbon (POC) remobilized by the storm on the river‐dominated continental shelf. POC was sourced from incision of the innermost continental shelf (<25 m water depth) and from surge ebb advection from adjacent wetlands and shallow estuaries, and was re‐deposited in deeper water on the shelf. This pulse of young (<1,600 yBP) labile POC, mixed with relict (>5000 yBP) POC eroded from the seafloor, has major implications for the remineralization versus burial of POC in deltas. The scale of erosional deflation of the shelf in water depths beyond seasonal wave‐current conditions suggests that, over millennia, tropical cyclones may be responsible for partly removing prodeltaic strata from the geologic record in low‐to‐mid latitude deltas.

Wetland‐driven shifts in suspended particulate organic matter composition of the Hudson River estuary, New York

Elemental carbon and nitrogen, stable isotopes, and lignin phenols were quantified for suspended particulate material exchanging between Tivoli Bays wetlands and the main stem of the freshwater‐tidal Hudson River estuary (HRE) across a tidal cycle, seasonally in 2006 and monthly in 2007 during the ice‐free portion of the year. Temporal shifts in organic matter (OM) composition ranged from 33.1 to 247.6 mg g−1 particulate organic carbon (POC), ‐34.7‰ to ‐27.2‰ stable carbon isotopic signature of POC (δ13CPOC), ‐3.6‰ to 13.6‰ nitrogen isotopic signature of particulate nitrogen (δ15NPN), and 0.8 to 25.0 mg per 100 mg OC lignin phenols. From May to August, the wetlands transferred abundant (up to 247.6 mg g−1), chemically distinctive (δ13CPOC as depleted as ‐34.7‰) particulate OM to the HRE, particularly Tivoli South Bay. We estimated a net export of > 60 Mg km−2 yr−1 POC from Tivoli Bays into the HRE. Mixing model iterations indicated that planktonic OM was the dominant source (50% to 86%) during summertime, while contributions from allochthonous and planktonic OM during other seasons were roughly equivalent (~ 38%). Taken together, both geochemical data and mixing modeling underscore the function and value of HRE tidal wetlands as major generators of compositionally unique and labile POC for the estuarine carbon budget. In a broader context, lateral aquatic connections with tidal wetlands at seasonal or localized scales may overshadow internal or upland influences on OM composition, production, and processing in estuaries; a latent but key consideration when deciphering temporal and longitudinal trends.

Kinetic pathways for the anaerobic decomposition of Ludwigia inclinata

This study is aimed at using kinetic modeling to investigate the yield of mineralization products from anaerobic decomposition of the aquatic macrophyte Ludwigia inclinata. The initial hypothesis was that the decay of different fractions of detritus and the kinetics of gases productions are both positively correlated to temperature. The plants and water samples were collected from a tropical oxbow lake; the anaerobic decomposition was described using incubations maintained at controlled temperatures. The methane and carbon dioxide productions were determined daily by gas chromatography. The mass loss of detritus owing to leaching and chemical oxidation was also measured, with the results being used to develop a mathematical (kinetic) model considering the heterogeneity of the resource and three mineralization pathways: (i) oxidation of the labile particulate organic carbon; (ii) formation of dissolved organic carbon leached from the detritus, and subsequent oxidation of those compounds; and (iii) oxidation of refractory particulate organic carbon (RPOC). The temperature did not affect the leaching rate constant or the mineralization of labile and dissolved fractions. On the other hand, the mineralization of RPOC was improved with increasing temperatures. The yield of CO2 formation was higher at 20.1°C and decreased with increasing temperatures. The methanogenesis was significantly affected by the temperature; it abbreviated the beginning of the process and increased the CH4 yield. Conversely as hypothesized, the results suggest that the increase in temperature improved the decay rates of RPOC, but did not affect the leaching process and the subsequent leachate oxidation. Furthermore, the rising temperatures had positive correlation with methanogenesis and negative with CO2 production.

Radiocarbon-based ages and growth rates of Hawaiian deep-sea corals

The radial growth rates and ages of three different groups of Hawaiian deep-sea 'corals' were determined using radiocarbon measurements. Specimens of Corallium secundum, Gerardia sp., and Leiopathes glaberrima, were collected from 450 {+-} 40 m at the Makapuu deep-sea coral bed using a submersible (PISCES V). Specimens of Antipathes dichotoma were collected at 50 m off Lahaina, Maui. The primary source of carbon to the calcitic C. secundum skeleton is in situ dissolved inorganic carbon (DIC). Using bomb {sup 14}C time markers we calculate radial growth rates of {approx} 170 {micro}m y{sup -1} and ages of 68-75 years on specimens as tall as 28 cm of C. secundum. Gerardia sp., A. dichotoma, and L. glaberrima have proteinaceous skeletons and labile particulate organic carbon (POC) is their primary source of architectural carbon. Using {sup 14}C we calculate a radial growth rate of 15 {micro}m y{sup -1} and an age of 807 {+-} 30 years for a live collected Gerardia sp., showing that these organisms are extremely long lived. Inner and outer {sup 14}C measurements on four sub-fossil Gerardia spp. samples produce similar growth rate estimates (range 14-45 {micro}m y{sup -1}) and ages (range 450-2742 years) as observed for the live collected sample. Similarly, with a growth rate of < 10 {micro}m y{sup -1} and an age of {approx}2377 years, L. glaberrima at the Makapuu coral bed, is also extremely long lived. In contrast, the shallow-collected A. dichotoma samples yield growth rates ranging from 130 to 1,140 {micro}m y{sup -1}. These results show that Hawaiian deep-sea corals grow more slowly and are older than previously thought.

Major outputs of the recent multidisciplinary biogeochemical researches undertaken in the Aegean Sea

The main outputs of a multidisciplinary and integrated studies are summarised. The results incorporate the latest biogeochemical researches, at basin scale, in the Aegean Sea (including thermohaline circulation studies, SPM dynamics, mass and energy fluxes, acknowledge biochemical processes in the euphotic and the benthic layer and benthic response to downward fluxes). The data were acquired within five (seasonal) research cruises, during 1997–1998. Data analysis and evaluation hence provided important new information on the functional processes of the Aegean ecosystem. In terms of water circulation, no new deep water formation in the Aegean Sea was observed, during 1997–1998, but rather intermediate water, due mainly to the mild winter conditions. All the biochemical parameters of the euphotic zone (nutrients, Particulate Organic Carbon (POC), chlorophyll- a, phytoplankton, primary and bacterial production), although high in the N. Aegean Sea reflect clearly the highly oligotrophic character of the Aegean Sea. In the N. Aegean, microbial food web was the main pathway of carbon, whereas in the S. Aegean, the food web could be classified as multivorous. An important Black Sea Water (BSW) signal was observed in the dissolved phase; this was especially pronounced in the Dissolved Organic Carbon (DOC), Mn and to a lesser degree to Cd, Cu and Ni concentrations. The downward material fluxes are higher in the N. Aegean, relative to the S. Aegean. Substantially higher values of near-bottom mass fluxes were measured in the deep basins of the N. Aegean, implying significant deep lateral fluxes of POM. The N. Aegean could be classified as a “continental margin” ecosystem, whilst the S. Aegean is a typical “oceanic margin” environment. There is a close relationship and, consequently, coupling between the near-bottom mass fluxes and the accumulation rates of organic matter (OM), with the near-bottom mineralisation, bioturbation, redox potential, oxygen consumption rates, the sediment microbiological and enzymatic activity and the meio-macro- and mega fauna abundances in the Aegean Sea. The N. Aegean is characterised by higher fluxes of labile POC and higher rates of benthic mineralisation and displays much higher benthic productivity and diversity relatively to the S. Aegean. The deep isolated N. Aegean basins should be regarded as “hot spots” of organic matter accumulation, benthic abundances and diversity. There is a general N–S gradient in the biogeochemical and benthic processes in the Aegean that is especially pronounced during winter–early spring, implied mainly by the of Black Sea Water (BSW) inflow (through the dissolved phase) and the lateral marginal inputs.

The Nature of Organic Matter in the Danube River Particles and North-western Black Sea Sediments

Suspended matter and sediments from the Danube River and the adjacent north-western Black Sea were analysed for organic carbon, nitrogen and biogenic carbonate and silica, as well as for different fractions of organic matter such as amino acids and carbohydrates. In addition, the nitrogen isotopic composition of surficial sediments of the NW Black Sea was examined. Particulate organic carbon (POC) contents decrease with total suspended solid (TSS) concentration due to dilution by mineral matter derived from upstream and riverbed erosion. The contribution of labile constituents such as amino acids and carbohydrates to POC remains uniform across the measured TSS contents. This suggests that any possible decrease in labile POC (LPOC) from dilution with refractory organic matter is masked by inputs of fresh organic matter from tributaries downstream. Comparison of TSS, POC and LPOC leaving the Iron Gate I barrier at the head of the Danube delta shows that the bulk of material currently entering the Black Sea is derived from inputs from tributaries and riverbed erosion, downstream of the Iron Gate I. The nature and distribution of organic matter in the surface sediments of the Black Sea are influenced by mixing between Danube-derived material and freshly produced organic matter on the shelf and in the open sea, on the one hand, and by oxic degradation during transport from the shelf to the abyssal Black sea, on the other. Preliminary data on the stable isotopic composition of nitrogen in surface sediments of the Black Sea further suggest that organic matter production on the shelf and the adjacent north-western Black Sea is fuelled by different nitrogen sources.

What controls the mixed‐layer depth in deep‐sea sediments? The importance of POC flux

The depth of biogenic particle mixing, i.e., the mixed‐layer depth (L), is fundamental in models of organic‐matter recycling and paleoreconstructions for deep‐sea sediments. Factors postulated to control L in the oxygenated deep sea include particulate organic carbon (POC) flux, oxygen penetration into the sediment, and a balance between the downward mixing and decay of labile POC. We explore the dependence of L on biogeochemical characteristics by compiling, from 36 sites in three oceans, an internally consistent set of deep‐sea estimates of L, POC flux, biogenic mixing intensity (Db), and POC reactivity. We use excess 210Pb as a tracer for L and Db to avoid the confounding effects of tracer‐dependent mixing. We find that L, estimated from the penetration depth of excess 210Pb, varies systematically with POC flux, with an asymptotic function explaining 88% of the variance in L. Stepwise multiple regression suggests that the penetration depth of excess 210Pb (and estimated L) is much more likely to be controlled by POC flux than by (1) the sediment inventory of excess 210Pb or (2) biogenic mixing intensity (Db). In addition, L is negatively related to oxygen penetration into the sediment (r = ‐0.629) and not significantly related to predictions of L from a recent mixing/POC‐decay model. We conclude that in the food‐poor deep sea, POC flux substantially controls the size and activities of the sediment‐mixing benthos and, in turn, the thickness of the biogenic mixed layer. Thus, in contrast to previous suggestions, average mixed‐layer depth is not environmentally invariant but rather responds predictably to ecologically important parameters such as POC flux.

Abundance and relationship of bacteria with transparent exopolymer particles during the 1996 summer monsoon in the Arabian Sea

Bacterial abundance and production, numbers, sizes and concentrations of transparent exopolymer particles (TEP) and total organic carbon (TOC) were measured during the 1996 summer monsoon to understand the relationship between TEP, the most labile particulate organic carbon, and bacteria. While high regional variability in the vertical distribution of TOC was discernible, TEP concentrations were high in surface waters at 18–20°N along 64°E with concentrations well over 25 mg alginic acid equivalents I−1 due to upwelling induced productivity. Their concentrations decreased with depth and were lower between 200 and 500 m. Bacterial concentrations were up to 1.99 × 108 I–1 in the surface waters and decreased by an order of magnitude or more at depths below 500 m. A better relationship has been found between bacterial abundance and concentrations of TEP than between bacteria and TOC, indicating that bacterial metabolism is fueled by availability of TEP in the Arabian Sea. Assuming a carbon assimilation of 33%, bacterial carbon demand (BCD) is estimated to be 1.017 to 4.035 g C m–2 d–1 in the surface waters. The observed TEP concentrations appear to be sufficient in meeting the surface and subsurface BCD in the northern Arabian Sea.

Chlorophyll-a and pheopigments as tracers of labile organic carbon at the central equatorial Pacific seafloor

High rates of primary production and fast sinking rates lead to the deposition of phytodetritus and highly reactive organic matter to the sediments of the central equatorial Pacific. These substances are responsible for driving important chemical fluxes and fueling benthic organisms. Chloropigments have proven useful as tracers of similar highly reactive organic carbon components in lacustrine and nearshore marine sediments. In this study we investigate the degradation of chlorophyll-a and pheopigments-a at four abyssal sites on the JGOFS equatorial Pacific transect along 140°W and explore the usefulness of these chloropigments as tracers of the most-reactive component of the deep-sea particulate organic carbon (POC) rain. First-order reaction rate constants ( k = 1–75 yr −1, half-life 3–250 d) and relative reactivities (chlorophyll-a > allomer and pheophorbide-a > pheophytin-a) derived from most of the sedimentary profiles are similar to those found in laboratory and other field (lake and coastal marine) studies. However, in some profiles, the rate constants determined by fitting data below 0.5 cm are slower by up to 3 orders of magnitude despite an apparent abundance of bacteria, macrofauna, and porewater oxygen. Model results assuming multi-G kinetics suggest that these chloropigments degrade as two components: one, which accounts for at least 99% of the degradation and 11–57% of the sediment inventory, degrades with a half-life of 4–120 days. The other component degrades with a half-life of up to 440 years. These results suggest that some otherwise labile POC may be protected and escape rapid degradation near the sediment-water interface. If phytodetritus is deposited continuously throughout the year, our model-calculated chlorophyll-a fluxes indicate that it could account for 25–100% of the annual POC flux at sites close to the equator.

Seasonal variations in production and consumption rates of dissolved organic carbon in an organic-rich coastal sediment

Dissolved organic carbon (DOC) concentrations in anoxic marine sediments are controlled by at least three processes: 1. (1) production of nonvolatile dissolved compounds, such as peptides and amino acids, soluble saccharides and fatty acids, via hydrolysis of particulate organic carbon (POC). 2. (2) conversion of these compounds to volatile fatty acids and alcohols by fermentative bacteria. 3. (3) consumption of volatile fatty acids and alcohols by terminal bacteria, such as sulfate reducers and methanogens. We monitored seasonal changes in concentration profiles of total DOC, nonacid-volatile (NAV) DOC and acid-volatile (AV) DOC in anoxic sediment from Cape Lookout Bight, North Carolina, USA, in order to investigate the factors that control seasonal variations in rates of hydrolysis, fermentation, and terminal metabolism. During the winter months, DOC concentrations increased continuously from 0.2 mM in the bottomwater to ~4 mM at a depth of 36 cm in the sediment column. During the summer, a large DOC maximum developed between 5 and 20 cm, with peak concentrations approaching 10 mM. The mid-depth summertime maximum was driven by increases in both NAV- and AV-DOC concentrations. Net NAV-DOC reaction rates were estimated by a diagenetic model applied to NAV-DOC concentration profiles. Depth-integrated production rates of NAV-DOC increased from February through July, suggesting that net rates of POC hydrolysis during this period are controlled by temperature. Net consumption of NAV-DOC during the late summer and early fall suggests reduced gross NAV-DOC production rates, presumably due to a decline in the availability of labile POC. A distinct subsurface peak in AV-DOC concentration developed during the late spring, when the sulfate depletion depth shoaled from 25 to 10 cm. We hypothesize that the AV-DOC maximum results from a decline in consumption by sulfate-reducing bacteria (due to sulfate limitation) and a lag in the development of an active population of methanogenic bacteria. A diagenetic model that incorporates a lag period in the sulfate reducer-methanogen transition successfully simulates the timing, magnitude, depth and shape of the AV-DOC peak.