Particulate Organic Carbon Concentrations Research Articles

Particle dynamics and fluxes in canyons and open slopes of the southern Cretan margin (Eastern Mediterranean)

The Samaria and Lithinon canyons at the southern Cretan margin (Eastern Mediterranean) were studied with respect to hydrography, deep currents, particle dynamics and settling fluxes by utilizing: (a) current-meter and sediment trap records from May 2005 to May 2006; and (b) hydrographic and optical measurements along with particulate matter and particulate organic carbon concentrations (PMC and POC, respectively) in two surveys (May and October 2005). The deep currents were very weak with mean velocity ∼3–4 cm s−1 and their direction was controlled by the local bottom topography. The entire area was characterized by very low PMC and POC, depicted both by transmissometry profiles and discrete bottle data. The fluxes of downward settling particles are comparable with the fluxes measured in the broader Eastern Mediterranean and hence point to an oligotrophic environment with low atmospheric and lateral inputs. A series of relatively high total mass fluxes (>1000 mg m−2 d−1) were recorded near the head of the Samaria canyon, at the trap closest to Crete, and was attributed to: (a) high temporal lithogenic inputs (mineral calcite) transferred to the sea by a small, intermittent stream flowing through the Samaria gorge, which was activated by short rainfall events and spring snowmelt; and (b) inputs of numerous coccoliths of E. huxleyi and calcite by shallow sediment resuspension generated by wind-induced waves. Apart from this local-scale event, there was very little evidence of matter transfer through the canyons, thus, during the one-year of observations, the canyons were dormant. Because of this lack of down-canyon transport during the study period, open slopes and canyons exhibited roughly similar characteristics in terms both of particle dynamics and fluxes. However, the presence of coarse-grained sediments along the canyon axes point to episodic canyon activity, occurring at unknown time scales and periodicity.

Freeze\u2013thaw processes and intense rainfall: the one-two punch for high sediment and nutrient loads from mid-Atlantic watersheds

Large runoff, sediment, and nutrient exports from watersheds could occur due to individual extreme climate events or a combination of multiple hydrologic and meteorological conditions. Using high-frequency hydrologic, sediment, and turbidity data we show that freeze–thaw episodes followed by intense winter (February) rainstorms can export very high concentrations and loads of suspended sediment and particulate organic carbon (POC) and nitrogen (PN) from mid-Atlantic watersheds in the US. Peak suspended sediment (> 5000 mg L−1), POC (> 250 mg L−1) and PN (> 15 mg L−1) concentrations at our 12 and 79 ha forested watersheds for the February rainfall-runoff events were highest on record and the fluxes were comparable to those measured for tropical storms. Similar responses were observed for turbidity values (> 400 FNU) at larger USGS-monitored watersheds. Much of the sediments and particulate nutrients likely originated from erosion of stream bank sediments and/or channel storage. Currently, there is considerable uncertainty about the contribution of these sources to nonpoint source pollution, particularly, in watersheds with large legacy sediment deposits. Future climate projections indicate increased intensification of storm events and increased variability of winter temperatures. Freeze–thaw cycles coupled with winter rain events could increase erosion and transport of streambank sediments with detrimental consequences for water quality and health of downstream aquatic ecosystems.

Organic carbon fluxes by precipitation, throughfall and stemflow in an olive orchard in Southern Spain

In forests and grasslands, canopy-derived carbon fluxes have been shown to be an important part of the carbon cycle, yet very few data are available for permanent agricultural crops. Concentration of total (TOC), dissolved (DOC) and particulate organic carbon (POC) was measured during an entire hydrological year in direct rainfall throughfall and stemflow in a mature olive orchard. Throughfall accounted for 68% of incoming rainfall, but TOC concentration was on average 14 times higher than in rainfall (7.63 vs. 106.12 mg/L). Stemflow on the other hand represented only a small fraction of gross precipitation, on average less than 1%, while its TOC concentration was on average 10-fold higher than in rainfall (74.13 mg/L). Dynamics of OC enrichment were mainly driven by precipitation patterns, as well as plant phenology, whereas a dramatic increase happened during flowering. Stemflow and throughfall were proven to be important sources of organic carbon supplying 13.5 g C/m2/year beneath the canopy of each tree, while 2.41 g C/m2/year reached the soil through gross precipitation. This large fraction of carbon is a mean of recirculating important carbon compounds that may help mobilize soil nutrients and maintain water holding capacity in the circumscribed area below olive canopies.

Food Composition for Blue Mussels (Mytilus edulis) in the Menai Strait, UK, Based on Physical and Biochemical Analyses

To clarify the composition of food ingested by blue mussels (Mytilus edulis) in the Menai Strait, where the largest mussel bed in the United Kingdom lies, hydrodynamic observations with organic carbon and nitrogen biochemical analysis were conducted. This strait is characterized by a narrow channel and wide tidal flats with large tidal mixing that influences the distribution of particulate organic matter (POM), which is food formussels. The maximum velocities of the tidal currents were estimated to be 0.5–1.0 m s-1 during northeastward flow and 0.6–1.3 m s-1 during southwestward flow. The residual current was southwestward and the water flux was estimated to be 510 m3 s-1. Field observations indicate that the inflow of pelagic phytoplankton into the strait mainly occurred in the northern region of the strait. Resuspended benthic primary production, however, also causes higher chlorophyll-a concentration in the central region of the strait near the mussel bed and its contribution to mussel growth was estimated to be 22%–50%. The concentrations of chlorophyll-a and particulate organic carbon indicated that the POM in the strait mainly originated from detritus. Benthic microalgae is, however, an important factor to understand the total primary production in the strait. The consumption of the primary producers was estimated to be approximately 6.3 t C day-1. In the phytoplankton community, pelagic and thycopelagic (living in both pelagic and benthic environments) species comprised 59.3%–77.2% and 12.8%–40.7% of the cells, respectively. The primary production by resuspended benthic species in the Liverpool Bay region was considerably larger than that in the Menai Strait region. As a conclusion, theMenai Strait is an area with complex hydrodynamic conditions causing horizontal advection and resuspension of both pelagic and benthic phytoplankton, and this complex would cause the most suitable condition for mussel production.

Photochemical degradation of dissolved organic matter reduces the availability of phosphorus for aquatic primary producers

In situ experiments were done to determine the effects of the photochemical degradation of dissolved organic matter (DOM) and subsequent formation of particulate matter on dissolved phosphorus (P) concentrations in surface waters. Filtered (1.2 or 0.4 μm) headwaters (DOM of 8.1–26 mg L−1; P of 22–43 μg L−1) were exposed to solar radiation in quartz bottles located 5 cm below the water surface for 7–10 days. Dark controls were wrapped in aluminum foil. After incubation, particulate organic carbon (POC) and particulate phosphorus (PP) were determined in both the filtrate and newly formed particles. The results revealed increasing concentrations of PP and POC in exposed samples with increasing exposure time (cumulative irradiation energy). At the end of experiments, PP concentrations were from 5 to 20 μg L−1 in the exposed samples. Based on an enumeration of bacteria in the samples, we estimated the contribution of biotic and abiotic processes to the PP production. The abiotic PP formation ranged from 56 to 83% and 50–95% of the total PP in the exposed and control samples, respectively. The remainder was assumed to be bacterial P uptake. Despite the overlapping intervals, biotic and abiotic PP productions were usually higher in exposed samples than in controls. The PP and POC production was affected by the properties of DOM, such as its humic content and freshness index. We hypothesize that the observed immobilization of dissolved P in bacteria and on photochemically-formed particles can contribute to a P limitation of primary production in headwater environments that receive waters rich in soil DOM.

A New Sample Preparation System for Micro-14C Dating of Glacier Ice with a First Application to a High Alpine Ice Core from Colle Gnifetti (Switzerland)

AbstractRadiometric dating of glacier ice is an essential tool where stratigraphic dating methods cannot be applied. This study focuses on Alpine glacier ice and presents a new sample preparation system for dating of glacier ice samples via radiocarbon (14C) dating of the microscopic particulate organic carbon (POC) fraction incorporated in the ice matrix. An adaptable, low-cost inline filtration-oxidation-unit (REFILOX) has been developed, which for the first time unifies all sample preparation steps from ice filtration to CO2quantification in one closed setup. A systematic14C investigation of modern European aerosol samples revealed that a POC combustion temperature of 340°C provides the best representation of the real sample age. A very low process blank of maximally 0.3±0.1 µgC now enables14C dating of high Alpine ice samples, where POC concentrations are generally low (typically 10–50 µgC/kg), in an ice sample mass range of 300–500 g. In a first successful application, the method was used to obtain age constraints for an ice core from the cold, high Alpine firn saddle Colle Gnifetti (Switzerland). Analysis of the bottom ice core sections revealed a basal age of 4171–3923 cal yr BP but also a so far enigmatic discontinuity in the age-depth relationship.

Model predictions of long-lived storage of organic carbon in river deposits

Abstract. The mass of carbon stored as organic matter in terrestrial systems is sufficiently large to play an important role in the global biogeochemical cycling of CO2 and O2. Field measurements of radiocarbon-depleted particulate organic carbon (POC) in rivers suggest that terrestrial organic matter persists in surface environments over millennial (or greater) timescales, but the exact mechanisms behind these long storage times remain poorly understood. To address this knowledge gap, we developed a numerical model for the radiocarbon content of riverine POC that accounts for both the duration of sediment storage in river deposits and the effects of POC cycling. We specifically target rivers because sediment transport influences the maximum amount of time organic matter can persist in the terrestrial realm and river catchment areas are large relative to the spatial scale of variability in biogeochemical processes.Our results show that rivers preferentially erode young deposits, which, at steady state, requires that the oldest river deposits are stored for longer than expected for a well-mixed sedimentary reservoir. This geometric relationship can be described by an exponentially tempered power-law distribution of sediment storage durations, which allows for significant aging of biospheric POC. While OC cycling partially limits the effects of sediment storage, the consistency between our model predictions and a compilation of field data highlights the important role of storage in setting the radiocarbon content of riverine POC. The results of this study imply that the controls on the terrestrial OC cycle are not limited to the factors that affect rates of primary productivity and respiration but also include the dynamics of terrestrial sedimentary systems.

Temporal variability in composition and fluxes of Yellow River particulate organic matter

AbstractThis study examines temporal variations of the abundance and carbon isotopic characteristics of particulate organic carbon (POC) and specific‐source compounds in the context of hydrological variability in the Yellow River. The content and bulk carbon isotopic characteristics (13C and 14C) of POC were relatively uniform over the hydrologic (seasonal) cycle. We attribute these temporally invariant geochemical characteristics to the dominant contribution of loess material to the suspended particulate matter (SPM). In contrast, molecular‐level signals revealed that hydrologic conditions exert a significant influence on the proportional contributions of petrogenic and especially fresh plant‐derived OC, while pre‐aged soil OC is mobilized via deeper erosion processes (e.g., gully erosion, mudslides) and is independent of hydrodynamics and surface runoff. A coupled biomarker‐isotope mixing model was applied to estimate the time‐varying supply of contemporary/modern biomass, pre‐aged soil, and fossil OC components to Chinese marginal seas from the Yellow River. We found that natural (e.g., precipitation) and human‐induced (e.g., water and sediment regulation) variations in hydrological regime strongly influence the flux with the magnitude of the corresponding annual fluxes of POC ranging between 0.343 ± 0.122 Mt yr−1 and 0.581 ± 0.213 Mt yr−1, but less strongly infleunce proportions of the different OC constituents. Inter‐annual differences in pre‐aged soil and fossil OC fluxes imply that extreme climate events (e.g., floods) modulate the exhumation and export of old carbon to the ocean, but the OC homogeneity in the pre‐aged mineral soil‐dominated watersheds facilitates robust predictions in terms of OC transport dynamics in the past (sediment cores) and in the future.

Spatial distribution of glomalin-related soil protein and its relationship with sediment carbon sequestration across a mangrove forest

Arbuscular mycorrhizal (AM) fungi produce a recalcitrant glycoprotein, (glomalin-related soil protein (GRSP)), which can contribute to soil carbon sequestration. Here we made a first study to characterize the spatial distribution of GRSP fractions in a mangrove forest at Zhangjiang Estuary, Southeastern China and to explore potential contributions of GRSP to sediment organic carbon (SOC) in this forest. We identified GRSP fractions in surface sediments, as well as those at a depth of 50 cm. The contents of easily extractable GRSP (EE-GRSP), total GRSP (T-GRSP), GRSP in particulate organic matter (POM-GRSP) and GRSP in pore water (PW-GRSP) ranged between 1.20–2.22mgg−1, 1.38–2.61mgg−1, 1.45–10.78mgg−1 and 10.35–39.65mgL−1 respectively, and these four GRSPs are significantly affected by sample sites and sediment layers. Carbon in GRSP accounted for 2.8–5.9% of SOC and its contributions can far exceed that of microbial biomass carbon (0.21–0.73%) in the 0–50cm sediment layers. Our data indicate that GRSP could be transported by pore water and accumulated in sediment profiles. The non-linear regression analysis revealed that as SOC and particulate organic carbon (POC) contents decrease, GRSP proportions increase, indicating the increase of the recalcitrant carbon offsetting the effects of mangrove carbon loss, especially labile C. Regression and ordination analyses indicated that GRSP fractions were mainly positively correlated with sediment carbon fractions and spore density but were negatively correlated with sand, pH. Strikingly, the unfavorable environmental factors for microbial organisms, especially AM fungi, prove to be able to promote the production or accumulation of GRSP. We propose that there are two different pathways for affecting the pool size of GRSP in mangrove ecosystems: (i) directly via indigenous AM fungi propagules; (ii) or via the GRSP transport and deposition by pore water and tides.

Particulate organic carbon is a sensitive indicator of soil degradation related to overgrazing in Patagonian wet and mesic meadows

Patagonian meadows are key for the development and sustainability of rural environments. However, they have been degraded due to a combination of weather conditions and overgrazing. Total soil organic carbon (TOC) has been found as a moderate indicator of meadow soil degradation by long-term heavy grazing. The aim of this study was to evaluate the particulate organic carbon (POC) (a more labile fraction of the soil organic matter) as an ecological indicator for assessing soil changes in natural Patagonian meadows, related to different long-term grazing managements. We worked in three locations along a precipitation gradient in Northern Patagonia. Each location has wet and mesic meadows with good and poor grassland condition, attributed to two long-term grazing pressures (light and heavy). We sampled soil throughout the plant-growing season following the natural soil moisture fluctuations, and quantified the POC content, the POC:TOC ratio, and the F>53 proportion. Along the different sampling dates, significant differences between good versus poor grassland condition were found in many cases in POC, a few in POC:TOC ratio, and none in F>53 proportion. The POC varies over time, in association with changes in the soil moisture (adjusted R2 vary between 0.76 and 0.99); therefore, the sampling date must be standardized (we recommend dry periods). The POC appears to be a sensitive indicator when assessing the effect of different soil management practices on soil quality, an important step for meadow conservation, restoration and sustainable use, but further research is needed to validate this exploratory study.

Physiological responses of coastal phytoplankton (Visakhapatnam, SW Bay of Bengal, India) to experimental copper addition

Trace amount of copper (Cu) is essential for many physiological processes; however, it can be potentially toxic at elevated levels. The impact of variable Cu concentrations on a coastal phytoplankton community was investigated along a coastal transect in SW Bay of Bengal. A small increase in Cu supply enhanced the concentrations of particulate organic carbon, particulate organic nitrogen, biogenic silica, total pigment, phytoplankton cell and total bacterial count. At elevated Cu levels all these parameters were adversely affected. δ13CPOM and δ15NPOC reflected a visible signature of both beneficial and toxic impacts of Cu supply. Skeletonema costatum, the dominant diatom species, showed higher tolerance to increasing Cu levels relative to Chaetoceros sp. Cyanobacteria showed greater sensitivity to copper than diatoms. The magnitude of Cu toxicity on the phytoplankton communities was inversely related to the distance from the coast. Co-enrichment of iron alleviated Cu toxicity to phytoplankton.

Validation and Intercomparison of Ocean Color Algorithms for Estimating Particulate Organic Carbon in the Oceans

Particulate Organic Carbon (POC) plays a vital role in the ocean carbon cycle. Though relatively small compared with other carbon pools, the POC pool is responsible for large fluxes and is linked to many important ocean biogeochemical processes. The satellite ocean-colour signal is influenced by particle composition, size, and concentration and provides a way to observe variability in the POC pool at a range of temporal and spatial scales. To provide accurate estimates of POC concentration from satellite ocean colour data requires algorithms that are well validated, with uncertainties characterised. Here, a number of algorithms to derive POC using different optical variables are applied to merged satellite ocean colour data provided by the Ocean Colour Climate Change Initiative (OC-CCI) and validated against the largest database of $\textit{in situ}$ POC measurements currently available. The results of this validation exercise indicate satisfactory levels of performance from several algorithms (highest performance was observed from the algorithms of \cite{stramski2008} and \cite{loisel2002}) and uncertainties that are within the requirements of the user community. Estimates of the standing stock of the POC can be made by applying these algorithms, and yield an estimated mixed-layer integrated global stock of POC between 0.77 and 1.3 Pg C of carbon. Performance of the algorithms vary regionally, suggesting that blending of region-specific algorithms may provide the best way forward for generating global POC products.

Patterns of Bacterial and Archaeal Gene Expression through the Lower Amazon River

Analysis of metatranscriptomic and metagenomic datasets from the lower reaches of the Amazon River between Obidos and the river mouth revealed microbial transcript and gene pools dominated by Actinobacteria, Thaumarchaeota, Bacteroidetes, Acidobacteria, Betaproteobacteria, and Planctomycetes. Three mainstem stations spanning a 625 km reach had similar gene expression patterns (transcripts gene copy-1) across a diverse suite of element cycling genes, but two tributary-influenced stations at the mouth of the Tapajos River and near the Tocantins River at Belem had distinct transcriptome composition and expression ratios, particularly for genes encoding light-related energy capture (higher) and iron acquisition and ammonia oxidation (lower). Environmental parameters that were useful predictors of gene expression ratios included concentrations of lignin phenols, suspended sediments, nitrate, phosphate, and particulate organic carbon and nitrogen. Similar to the gene expression data, these chemical properties reflected highly homogeneous mainstem stations punctuated by distinct tributary-influenced stations at Tapajos and Belem. Although heterotrophic processes were expected to dominate in the lower Amazon, transcripts from photosynthetic bacteria were abundant in tributary-influenced regions, and transcripts from Thaumarcheota taxa genetically capable of chemosynthetic ammonia oxidation accounted for up to 21% of the transcriptome at others. Based on regressions of transcript numbers against gene numbers, expression ratios of Thaumarchaeota populations were largely unchanged within the mainstem, suggesting a relatively minor role for gene regulation. These quantitative gene and transcript inventories detail a diverse array of energy acquisition strategies and metabolic capabilities for bacteria and archaea populations of the world’s largest river system.

Biogeochemical sensor performance in the SOCCOM profiling float array

AbstractThe Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) program has begun deploying a large array of biogeochemical sensors on profiling floats in the Southern Ocean. As of February 2016, 86 floats have been deployed. Here the focus is on 56 floats with quality‐controlled and adjusted data that have been in the water at least 6 months. The floats carry oxygen, nitrate, pH, chlorophyll fluorescence, and optical backscatter sensors. The raw data generated by these sensors can suffer from inaccurate initial calibrations and from sensor drift over time. Procedures to correct the data are defined. The initial accuracy of the adjusted concentrations is assessed by comparing the corrected data to laboratory measurements made on samples collected by a hydrographic cast with a rosette sampler at the float deployment station. The long‐term accuracy of the corrected data is compared to the GLODAPv2 data set whenever a float made a profile within 20 km of a GLODAPv2 station. Based on these assessments, the fleet average oxygen data are accurate to 1 ± 1%, nitrate to within 0.5 ± 0.5 µmol kg−1, and pH to 0.005 ± 0.007, where the error limit is 1 standard deviation of the fleet data. The bio‐optical measurements of chlorophyll fluorescence and optical backscatter are used to estimate chlorophyll a and particulate organic carbon concentration. The particulate organic carbon concentrations inferred from optical backscatter appear accurate to with 35 mg C m−3 or 20%, whichever is larger. Factors affecting the accuracy of the estimated chlorophyll a concentrations are evaluated.

Revisiting Ocean Color algorithms for chlorophyll a and particulate organic carbon in the Southern Ocean using biogeochemical floats

AbstractThe Southern Ocean (SO) ecosystem plays a key role in the carbon cycle by sinking a major part (43%) of the ocean uptake of anthropogenic CO2, and being an important source of nutrients for primary producers. However, undersampling of SO biogeochemical properties limits our understanding of the mechanisms taking place in this remote area. The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project has been deploying a large number of autonomous biogeochemical floats to study the SO (as of December 2016, 74 floats out of 200 have been deployed). SOCCOM floats measurements can be used to extend remote sensing chlorophyll a (chl a) and particulate organic carbon (POC) products under clouds or during the polar night as well as adding the depth dimension to the satellite‐based view of the SO. Chlorophyll a concentrations measured by a sensor embedded on the floats and POC concentrations derived from backscattering coefficients were calibrated with samples collected during the floats' deployment cruise. Float chl a and POC were compared with products derived from observations of MODIS and VIIRS sensors. We find the Ocean Color Index (OCI) global algorithm to agree well with the matchups (within 9%, on average, for the Visible Infrared Imaging Radiometer Suite (VIIRS) and 12%, on average, for the Moderate Resolution Imaging Spectroradiometer Aqua (MODIS)). SO‐specific algorithms estimating chl a are offset by ∼45% south of the Sea Ice Extent Front ( °S). In addition, POC estimates based on floats agree well with NASA's POC algorithm.

Physiological responses of coccolithophores to abrupt exposure of naturally low pH deep seawater.

Upwelling is the process by which deep, cold, relatively high-CO2, nutrient-rich seawater rises to the sunlit surface of the ocean. This seasonal process has fueled geoengineering initiatives to fertilize the surface ocean with deep seawater to enhance productivity and thus promote the drawdown of CO2. Coccolithophores, which inhabit many upwelling regions naturally ‘fertilized’ by deep seawater, have been investigated in the laboratory in the context of ocean acidification to determine the extent to which nutrients and CO2 impact their physiology, but few data exist in the field except from mesocosms. Here, we used the Porcupine Abyssal Plain (north Atlantic Ocean) Observatory to retrieve seawater from depths with elevated CO2 and nutrients, mimicking geoengineering approaches. We tested the effects of abrupt natural deep seawater fertilization on the physiology and biogeochemistry of two strains of Emiliania huxleyi of known physiology. None of the strains tested underwent cell divisions when incubated in waters obtained from <1,000 m (pH = 7.99–8.08; CO2 = 373–485 p.p.m; 1.5–12 μM nitrate). However, growth was promoted in both strains when cells were incubated in seawater from ~1,000 m (pH = 7.9; CO2 ~560 p.p.m.; 14–17 μM nitrate) and ~4,800 m (pH = 7.9; CO2 ~600 p.p.m.; 21 μM nitrate). Emiliania huxleyi strain CCMP 88E showed no differences in growth rate or in cellular content or production rates of particulate organic (POC) and inorganic (PIC) carbon and cellular particulate organic nitrogen (PON) between treatments using water from 1,000 m and 4,800 m. However, despite the N:P ratio of seawater being comparable in water from ~1,000 and ~4,800 m, the PON production rates were three times lower in one incubation using water from ~1,000 m compared to values observed in water from ~4,800 m. Thus, the POC:PON ratios were threefold higher in cells that were incubated in ~1,000 m seawater. The heavily calcified strain NZEH exhibited lower growth rates and PIC production rates when incubated in water from ~4,800 m compared to ~1,000 m, while cellular PIC, POC and PON were higher in water from 4,800 m. Calcite Sr/Ca ratios increased with depth despite constant seawater Sr/Ca, indicating that upwelling changes coccolith geochemistry. Our study provides the first experimental and field trial of a geoengineering approach to test how deep seawater impacts coccolithophore physiological and biogeochemical properties. Given that coccolithophore growth was only stimulated using waters obtained from >1,000 m, artificial upwelling using shallower waters may not be a suitable approach for promoting carbon sequestration for some locations and assemblages, and should therefore be investigated on a site-by-site basis.

Decomposition from legume and non-legume crop residues: Effects on soil organic carbon fractions under controlled conditions

Cover crop (CC) residues protect the soil from erosion and their permanence on the surface is largely influenced by their biochemical constituents. We performed a study under controlled conditions to investigate the dynamics of legume and non-legume CC residues decomposition and the transformations of the soil labile organic carbon fractions in the surface layer (0&lt;strong&gt;-&lt;/strong&gt;15 cm). The experiment was carried out on a Typic Argiudoll (clay loam, 27.4 g kg&lt;sup&gt;-1&lt;/sup&gt; soil organic matter, 14 mg kg&lt;sup&gt;-1&lt;/sup&gt; extractable phosphorus and 6.5 pH) placed in undisturbed pots (1570 cm&lt;sup&gt;3&lt;/sup&gt;) in a greenhouse under controlled conditions of temperature (25 ± 1 °C). We evaluated three CC species (oat, &lt;em&gt;Avena sativa&lt;/em&gt; L.; vetch, &lt;em&gt;Vicia sativa&lt;/em&gt; L.; Persian clover, &lt;em&gt;Trifolium resupinatum&lt;/em&gt; L.) and a no-CC control (fallow). Shoot residues were applied on the soil surface at 5.4, 5.4 and 2.7 g dry matter (equivalent to 6, 6 and 3 Mg ha&lt;sup&gt;-1&lt;/sup&gt; for oat, vetch and clover, respectively) and incubated for 362-days (eight sampling times). The water content in the pots was maintained periodically by weight at 60% of soil water-holding capacity. The soil samples were analyzed for particulate organic carbon (POC), and total and soluble carbohydrates (CHt and CHs, respectively). Oat and vetch residues decomposed faster than clover, with the decomposition rate constant (&lt;em&gt;k&lt;/em&gt;) values of 1.3, 1.4 and 1.9 year&lt;sup&gt;-1&lt;/sup&gt;, respectively. At the end of the experiment, POC concentration was lower in vetch (1.83 g kg&lt;sup&gt;-1&lt;/sup&gt;) and clover (1.96 g kg&lt;sup&gt;-1&lt;/sup&gt;) than in oat (2.21 g kg&lt;sup&gt;-1&lt;/sup&gt;) and fallow (3.00 g kg&lt;sup&gt;-1&lt;/sup&gt;), indicating a loss of 45&lt;strong&gt;-&lt;/strong&gt;64% from their initial values. Soil CHt was influenced by residue quality, where the periods of greatest residue decay (vetch 21-59 days and oat 93-130 days) corresponded to higher soil CHt. Hence, this organic carbon fraction is sensitive to residue decomposition and can be indicators of changes in soil organic matter over short periods of time.

Rapid acclimation of microbes to changing substrate pools in epipelagic waters of an Antarctic polynya during austral summer 2003

Antarctic polynyas are among the most productive areas in the ocean. They are subjected to strong temporal variations, having profound effects on the local ecosystems. We tested the changes occurring in several biogeochemical features [Chlorophyll a (Chl a), phaeopigment, dissolved and particulate organic carbon (DOC and POC) concentrations] as well as in biological dynamics (prokaryotic and nanoplanktonic abundance, heterotrophic carbon production and degradative activities) at the transition between late spring and summer in surface waters (upper 125 m) in the Terra Nova Bay polynya, in the Ross Sea. Samplings were performed from January 18 to 21 and, after an interval of almost 5 days, from January 25 to 27. The first period was characterised by spring conditions still maintained by the presence in the area of two mega-icebergs (B15 and C19), which caused a delay in the ice melting that started during the second survey. Chlorophyll a concentrations were highest during the first period, whereas phaeopigments/Chl a ratio increased during the following sampling time suggesting variations in phytoplankton biomass/physiology and a change in organic matter (OM) quality. No significant differences in DOC and POC concentration were detected between the two periods together with unvaried abundances of prokaryotes and small protists. The changes in OM over time were evidenced also by changing exoenzymatic activity patterns. Notwithstanding the general variations occurring during the surveys, heterotrophic production and prokaryotic growth were not substantially modified, suggesting a rapid (timescales of days) acclimation to changing organic matter quality for the maintenance of prokaryotic metabolic requirements.

The impact of adding organic carbon on the concentrations of total residual oxidants and disinfection by-products in approval tests for ballast water management systems

In the G8 and G9 approval tests for ballast water management systems, organic carbon additives are frequently supplemented into test water to satisfy the water quality requirements. Because organic additives can affect the approval test, the additive selected, and its use and validation should be included in the test report. This study assessed the effects of organic carbon additives on the concentration of total residual oxidants (TROs) and the formation of disinfection by-products (DBPs). The concentration of dissolved organic carbon (DOC) in test water containing additives varied depending on the type of additive, but all additives, except for methylcellulose, had concentrations similar to or higher than the theoretical values. There was a low concentration of particulate organic carbon (POC) compared to the amount of corn starch added. Over the course of the five-day holding time, TRO concentrations tended to decrease. In general, substances with a large molecular size had a higher DBP concentration than their counterparts with a smaller molecular size, some of which, however produced the highest DBP concentrations due to their molecular structure. The results suggest that the formation of DBPs is affected by the reaction with TROs, molecular size, and molecular structure in a complex manner.

Slow‐sinking particulate organic carbon in the Atlantic Ocean: Magnitude, flux, and potential controls

AbstractThe remineralization depth of particulate organic carbon (POC) fluxes exported from the surface ocean exerts a major control over atmospheric CO₂ levels. According to a long‐held paradigm most of the POC exported to depth is associated with large particles. However, recent lines of evidence suggest that slow‐sinking POC (SSPOC) may be an important contributor to this flux. Here we assess the circumstances under which this occurs. Our study uses samples collected using the Marine Snow Catcher throughout the Atlantic Ocean, from high latitudes to midlatitudes. We find median SSPOC concentrations of 5.5 μg L−1, 13 times smaller than suspended POC concentrations and 75 times higher than median fast‐sinking POC (FSPOC) concentrations (0.07 μg L−1). Export fluxes of SSPOC generally exceed FSPOC flux, with the exception being during a spring bloom sampled in the Southern Ocean. In the Southern Ocean SSPOC fluxes often increase with depth relative to FSPOC flux, likely due to midwater fragmentation of FSPOC, a process which may contribute to shallow mineralization of POC and hence to reduced carbon storage. Biogeochemical models do not generally reproduce this behavior, meaning that they likely overestimate long‐term ocean carbon storage.