Fraction Of Particulate Organic Carbon Research Articles

Labile soil organic matter fractions as influenced by non-flooded mulching cultivation and cropping season in rice–wheat rotation

Labile soil organic matter (SOM) fractions are especially important because they are more vulnerable to disturbance and play a crucial role for nutrient and carbon cycling. Although water conservation has become increasingly important in rice–wheat rotation, the effects of non-flooded mulching cultivation on labile SOM fractions remain unknown. Based on a long-term field experiment (10 years), we analyzed the impact of non-flooded mulching cultivation and cropping season on labile SOM fractions in a rice–wheat rotation in Chengdu Plain, southwest China. Compared with traditional flooding (TF), the plastic film mulching (PM) and wheat straw mulching (SM) treatments increased dissolved organic carbon (DOC) (42% after rice season and 41% after wheat season), but decreased microbial biomass carbon (MBC) (19% after rice season and 28% after wheat season) and nitrogen (MBN) (17% after rice season and 24% after wheat season) in the 0–5 cm depth. SM increased particulate organic carbon (POC) and KMnO4-oxidizable C (KMnO4-C) contents after both the rice and wheat seasons. Microbial biomass and DOC concentrations were higher for all three cultivations after the rice season than after the wheat season. In contrast, POC contents under PM and SM were higher after the wheat season than after the rice season. In general, results in this study indicate that non-flooded mulching and cropping season significantly influenced labile SOM fractions. The DOC fraction was the most sensitive fraction affected by non-flooded mulching, while POC and PON fractions respond fast within the two cropping seasons.

Combining management based indices with environmental parameters to explain regional variation in soil carbon under dryland cropping in South Australia

Identifying drivers of variation in soil organic carbon (OC) at a regional scale is often hampered by a lack of historical management information. Focusing on red-brown-earth soils (Chromosol) under dryland agriculture in the Mid-North and Eyre Peninsula of South Australia, our aims were 2-fold: (i) to provide a baseline of soil OC stocks (0.3 m) and OC fractions (mid-infrared predictions of particulate, humus, and resistant OC in 0.1 m samples) in cropping and crop-pasture systems; and (ii) to evaluate whether the inclusion of management-based indices could assist in explaining regional-level variation in OC stocks and fractions. Soil OC stocks in both regions varied ~20 Mg ha–1, with higher OC stocks in the Mid-North (38 Mg ha–1) than the Eyre Peninsula (29.1 Mg ha–1). The humus OC fraction was the dominant fraction, while the particulate OC was the most variable. Environmental variables only partially explained soil OC variability, with vapour pressure deficit (VPD) offering the greatest potential and likely acting as an integrator of temperature and moisture on plant growth and decomposition processes. Differences between broad-scale cropping and crop–pasture systems were limited. In the Mid-North, variability in soil OC stocks and fractions was high, and could not be explained by environmental or management variables. Higher soil OC concentrations (0.1 m) in the Eyre Peninsula cropping than crop–pasture soils were largely accounted for in the particulate OC fraction and are therefore unlikely to represent a long-term stable OC pool. Use of the management data in index format added some explanatory power to the variability in OC stocks over the main environmental variables (VPD, slope) within the Eyre Peninsula cropping soils only. In the wider context, the management data were useful in interpreting differences between regional findings and highlighted difficulties in using uninformed, broad-scale management categories.

Land‐use change in subalpine grassland soils: Effect on particulate organic carbon fractions and aggregation

AbstractAbandonment of mountain grassland often changes vegetation composition and litter quantity and quality, but related effects on labile soil organic matter (SOM) are largely unknown. The aim of this study was to investigate the impacts of grassland management and abandonment on soil carbon distribution in light (< 1.6 g cm–3) particulate organic matter (POM) and aggregation along a gradient of management intensity including hay meadows, pastures, and abandoned grasslands. The reduction of management intensity is an interregional phenomenon throughout the European Alps. We therefore selected sites from two typical climate regions, namely at Stubai Valley, Austria (MAT: 3°C, MAP: 1097 mm) and Matsch Valley, Italy (MAT: 6.6°C, MAP: 527 mm), to evaluate effects of land‐use change in relation to climate. Free water‐floatable and free POM (wPOM, fPOM), and an occluded POM fraction (oPOM), were isolated from three water‐stable aggregate size classes (2–6.3 mm, 0.25–2 mm, < 0.25 mm) using density fractionation. Aggregate mean weight diameter slightly decreased with decreasing management intensity. In contrast to absolute POM‐C, fPOM‐C increased in aggregates at both sites with abandonment. Because the oPOM‐C was less affected by abandonment, the ratio of oPOM‐C : fPOM‐C shifted from > 1 to < 1 from meadow to abandoned grassland in aggregates at both sites and thus independent of climate. This suggests that in differently managed mountain grasslands free and occluded POM are functionally different SOM fractions. In bulk soil, the oPOM‐C : fPOM‐C ratio is better suited as an indicator for the response of SOM to management reduction in subalpine grasslands than the total soil C, absolute or relative POM‐C content.

Positive sampling artifacts of organic carbon fractions for fine particles and nanoparticles in a tunnel environment

The positive artifacts in particulate organic carbon fractions for fine particles (PM2.5) and nanoparticles (PM0.1) were characterized in a tunnel environment by using the QBQ (a quartz filter behind a quartz filter) method. The OC concentrations of the backup quartz filters ranged from 3.56 to 11.38μgm−3 with the average of 6.70μgm−3 for PM2.5, and from 2.62 to 7.27μgm−3 with the average of 4.64μgm−3 for PM0.1. The most abundant species on the backup quartz filters was OC1 for both PM2.5 and PM0.1, accounting for 56.9% and 41.1% of the measured organic carbon, respectively. Most of EC fractions (EC1–EC3) on the backup filters for PM2.5 and PM0.1 were below the minimum detection limit. Therefore, only OC contributed to positive artifacts distinctly with the average percentage of 21.7% and 48.0% for PM2.5 and PM0.1, respectively. The artifacts for four organic carbon fractions ranged from 36.1% (OC1) to 4.4% (OC4) for PM2.5 and from 68.0% (OC1) to 31.9% (OC4) for PM0.1. The uncorrected OC/EC ratios on the front quartz filters were higher by as much as 30% and 107% for PM2.5 and PM0.1 than those corrected for positive organic artifacts, respectively. That is, much higher percentage of positive artifacts was found for PM0.1 OC fractions on the front filters. The comparison of the present and previous studies shows that OC positive artifacts vary widely among various PM fractions and sampling sites attributing to many factors that are worth investigating in the future.

Base flow and stormwater net fluxes of carbon and trace metals to the Mediterranean sea by an urbanized small river

Base flow and storm flow events from a small, urbanized Mediterranean river located in the South of France were studied to evaluate net fluxes from the continent to the coastal sea water. Considered variables were: pH, conductivity, redox potential, temperature, dissolved O2, SPM (Suspended Particulate Matter), Cl−, NO3−, SO42−, Na+, K+, Mg2+, Ca2+, DOC (Dissolved Organic Carbon), Cu, Zn, Cd and Pb in the dissolved fraction and POC (Particulate Organic Carbon), Cr, Mn, Fe, Ni, Cu, Zn, Cd and Pb in the particulate fraction. If rainfall intensity and antecedent rainfall history were sufficient to explain many observed variations, the patterns of particles and solutes transport greatly varied from one storm event to another. SPM, POC, particulate Cu, Zn, Cd and Pb had similar behavior characterized by an immediate increase at the beginning of the storm flow and the highest values at the first high discharge peak. Among dissolved species, Cl−, SO42−, Na+, Ca2+ and Mg2+ had a behavior very similar one to the other. They exhibited high concentrations and enrichment factors at the beginning of the storm flow, due to fast leaching of highly labile species. Their concentrations decreased during posterior discharge peaks but positive enrichment factors indicated permanent sources for these ions. DOC, K+ and NO3− had different behaviors which indicated sources positively correlated with rainfall intensity. A 3D-fluorescence study showed that the humified organic matter contribution to DOC increased during storm flow. Relationships between DOC and dissolved Cu and Pb indicated differences in organic-matter binding properties between dry and humid periods. Storm flow were responsible for more than 90% of the annual output to the sea of SPM, POC and particulate Cu, Zn, Cd and Pb and more than 70% for dissolved Pb, Cd, NO3− and DOC. For the other dissolved species, outputs were balanced between base flow and storm flow. Contrary to what was observed in large rivers, organic carbon was mainly transported in the POC fraction. The average specific fluxes of Cu and Pb to the sea were 3.8 and 3.4 kg km−2 y−1, respectively, of the same order of magnitude than specific fluxes of other North-Mediterranean rivers, but outputs were more intense during shorter durations. The extrapolation of the Eygoutier River data to the Mediterranean non-desert coastline showed that the order of magnitude of Cu and Pb annually brought to the sea by the whole of small anthropized coastal rivers can be similar to the annual input by the Rhône or the Po River.

Soil organic carbon pools in particle-size fractions as affected by slope gradient and land use change in hilly regions, western Iran

This study was conducted to explore the effects of topography and land use changes on particulate organic carbon (POC), particulate total nitrogen (PTN), organic carbon (OC) and total nitrogen (TN) associated with different size primary particle fractions in hilly regions of western Iran. Three popular land uses in the selected site including natural forest (NF), disturbed forest (DF) and cultivated land (CL) and three slope gradients (0-10 %, S1, 10-30 %, S2, and 30-50%, S3) were employed as the basis of soil sampling. A total of 99 soil samples were taken from the 0-10 cm surface layer in the whole studied hilly region studied. The results showed that the POC in the forest land use in all slope gradients was considerably more than the deforested and cultivated lands and the highest value was observed at NF-S1 treatment with 9.13%. The values of PTN were significantly higher in the forest land use and in the down slopes (0.5%) than in the deforested and cultivated counterparts and steep slopes (0.09%) except for the CL land use. The C:N ratios in POC fraction were around 17-18 in the forest land and around 23 in the cultivated land. In forest land, the silt-associated OC was highest among the primary particles. The enrichment factor of SOC, EC,was the highest for POC. For the primary particles, EC of both primary fractions of silt and clay showed following trend for selected land uses and slope gradients: CL> DF> NF and S3 > S2> S1. Slope gradient of landscape significantly affected the OC and TN contents associated with the silt and clay particles, whereas higher OC and TN contents were observed in lower positions and the lowest value was measured in the steep slopes. Overall, the results showed that native forest land improves soil organic carbon storage and can reduce the carbon emission and soil erosion especially in the mountainous regions with high rainfall in west of Iran.

Particulate organic matter in soil under different management systems in the Brazilian Cerrado

The combination of the no-till planting system (NTS) and pasture (e.g. brachiaria grass, Urochloa sp.) for livestock production constitutes a crop–livestock integration (CLI) system. CLI systems significantly increase the total organic carbon (TOC) content of soil and the particulate organic carbon (POC) of soil organic matter (SOM). The present study evaluated TOC and the granulometric fractions of SOM under different management systems in a Cerrado area in the state of Goiás. Two areas applying crop rotation were evaluated, one using CLI (corn/brachiaria grass/bean/cotton/soybean planted sequentially) and the other NTS (sunflower/pearl millet/soybean/corn planted sequentially). A third area covered with natural Cerrado vegetation (Cerradão) served as a reference to determine original soil conditions. Soil was randomly sampled at 0–5, 5–10, 10–20, and 20–40 cm. The TOC, POC, and mineral-associated organic carbon (MOC) were assessed, and POC and MOC stocks calculated. The CLI system resulted in greater TOC levels than NTS (0–5, 5–10, and 10–20 cm). Compared with the Cerradão, CLI areas exhibited higher stocks of TOC (at 5–10 and 10–20 cm) and POC (at 0–40 cm). Results obtained for TOC and POC fractions show that land management with CLI was more efficient in increasing SOM than NTS. Moreover, when compared with NTS, the CLI system provided better POC stratification.

The isotopic composition of particulate organic carbon in mountain rivers of Taiwan

Small rivers draining mountain islands are important in the transfer of terrestrial particulate organic carbon (POC) to the oceans. This input has implications for the geochemical stratigraphic record. We have investigated the stable isotopic composition of POC (δ13Corg) in rivers draining the mountains of Taiwan. In 15 rivers, the suspended load has a mean δ13Corg that ranges from −28.1±0.8‰ to −22.0±0.2‰ (on average 37 samples per river) over the interval of our study. To investigate this variability we have supplemented suspended load data with measurements of POC in bedrock and river bed materials, and constraints on the composition of the terrestrial biomass. Fossil POC in bedrock has a range in δ13Corg from −25.4±1.5‰ to −19.7±2.3‰ between the major geological formations. Using coupled δ13Corg and N/C we have found evidence in the suspended load for mixing of fossil POC with non-fossil POC from the biosphere. In two rivers outside the Taiwan Central Range anthropogenic land use appears to influence δ13Corg, resulting in more variable and lower values than elsewhere. In all other catchments, we have found that 5‰ variability in δ13Corg is not controlled by the variable composition of the biomass, but instead by heterogeneous fossil POC. In order to quantify the fraction of suspended load POC derived from non-fossil sources (Fnf) as well as the isotopic composition of fossil POC (δ13Cfossil) carried by rivers, we adapt an end-member mixing model. River suspended sediments and bed sediments indicate that mixing of fossil POC results in a negative trend between N/C and δ13Corg that is distinct from the addition of non-fossil POC, collapsing multiple fossil POC end-members onto a single mixing trend. As an independent test of the model, Fnf reproduces the fraction modern (Fmod) in our samples, determined from 14C measurements, to within 0.09 at the 95% confidence level. Over the sampling period, the mean Fnf of suspended load POC was low (0.29 ± 0.02, n = 459), in agreement with observations from other mountain rivers where physical erosion rates are high and fossil POC enters river channels. The mean δ13Cfossil in suspended POC varied between −25.2±0.5‰ and −20.2±0.6‰ from catchment to catchment. This variability is primarily controlled by the distribution of the major geological formations. It also covers entirely the range of δ13Corg found in marine sediments which is commonly thought to derive from mixing between marine and terrigenous POC. If land-sourced POC is preserved in marine sediments, then changes in the bulk δ13Corg observed offshore Taiwan could instead be explained by changes in the onshore provenance of sediment. The range in δ13Corg of fossil organic matter in sedimentary rocks exposed at the surface is large and given the importance of these rocks as a source of clastic sediment to the oceans, care should be taken in accounting for fossil POC in marine deposits supplied by active mountain belts.

Oxygen and organic carbon fluxes in sediments of the Bay of Biscay

The relationship between particulate organic carbon (POC) concentrations measured in modern sediment and fluxes of exported POC to the sediment surface needs to be understood in order to use POC content as a proxy of paleo-environmental conditions. The objective of our study was to compare POC concentrations, POC mineralization rates calculated from O 2 consumption and POC burial rates. Benthic O 2 distributions were determined in 58 fine-grained sediment cores collected at different periods at 14 stations in the southeastern part of the Bay of Biscay with depths ranging from 140 to 2800 m. Depth-dependent volume-specific oxygen consumption rates were used to assess rates of aerobic oxidation of organic matter (OM), assuming that O 2 consumption solely was related to heterotrophic activity at the sediment–water interface. Heterogeneity of benthic O 2 fluxes denoted changes in time and space of fresh organic material sedimentation. The most labile fraction of exported POC engendered a steep decrease in concentration in the upper 5 mm of vertical O 2 profiles. The rupture in the gradient of O 2 microprofile may be related to the bioturbation-induced mixing depth of fast-decaying carbon. Average diffusive O 2 fluxes showed that this fast-decaying OM flux was much higher than buried POC, although diffusive O 2 fluxes underestimated the total sediment oxygen demand, and thus the fast-decaying OM flux to the sediment surface. Sedimentary POC burial was calculated from sediment mass accumulation rate and the organic carbon content measured at the top of the sediment. The proportion of buried POC relative to total exported POC ranged at the most between 50% and 10%, depending on station location. Therefore, for a narrow geographic area like the Bay of Biscay, burial efficiency of POC was variable. A fraction of buried POC consisted of slow-decaying OM that was mineralized within the upper decimetres of sediment through oxic and anoxic processes. This fraction was deduced from the decrease with depth in POC concentration. At sites located below 500 m water depth, where the fast-decaying carbon did not reach the anoxic sediment, the slow-decaying pool may control the O 2 penetration depth. Only refractory organic material was fossilized in sedimentary records at locations where labile OM did not reach the anoxic portion of the sediment.

Importance of biogenic opal as ballast of particulate organic carbon (POC) transport and existence of mineral ballast‐associated and residual POC in the Western Pacific Subarctic Gyre

We determined particulate organic carbon (POC) and mineral ballast (biogenic opal: Opal, CaCO3, lithogenic material) fluxes and estimated mineral ballast carrying coefficients (CCs) using time‐series sediment trap (ST) data from multiple depths in the Western Pacific Subarctic Gyre (WSG). The deep sea POC flux was highly correlated with mineral ballast flux, which accounted for almost 100% of the POC flux. About 70% was associated with Opal flux, suggesting its importance to vertical POC transport in the WSG unlike previous reports documenting importance of CaCO3. Opal had the highest CC at all depths, but CC decreased with decreasing depth, as did the fraction of “mineral ballasts‐associated” POC (M‐POC). Conversely, the residual POC fraction (R‐POC) increased with decreasing depth, from about at most 10% at ca. 5000 m to 80% at ca. 150 m. Thus, total POC flux includes M‐POC and R‐POC, and decomposition rate are greater in R‐POC than in M‐POC.

Relative contribution of autochthonous and allochthonous carbon to limnetic zooplankton: A new cross-system approach

We measured the 13 C signature of epilimnetic CO 2 , particulate organic carbon (POC), and zooplankton in 27 lakes in late summer, and used the relationships between the POC and zooplankton signatures and the CO 2 signature to estimate the average autochthonous contribution to these fractions of the plankton and the average photosynthetic fractionation. Using this novel approach, average autochthonous contribution to epilimnetic POC was between 62 and 75 %. Epilimnetic zooplankton were between 77 and 91 % autochthonous, indicating that zooplankton bias their feeding towards the autochthonous fraction of POC. On average, zooplankton were 1.2‰ enriched in 13 C relative to POC, but their biased feeding on phytoplankton means that they can be depleted relative to POC in lakes where POC is highly depleted in 13 C. The relationship between 13 C-POC and 13 CO 2 allowed us to estimate average photosynthetic fraction as ―15.9‰. This estimate is independent of how much allochthonous C contributes to POC. Given the strength of the relationships between 13 C of POC or zooplankton with 13 CO 2 signature, we infer that variation in photosynthetic fractionation was not a major contributor to differences among lakes in POC and zooplankton 13 C signature. Allochthonous C is an important, although clearly secondary, source of C to zooplankton of these lakes in late summer.

Impacts of particulate organic carbon and dissolved organic carbon on removal of polycyclic aromatic hydrocarbons, organochlorine pesticides, and nonylphenols in a wetland

The potential of wetlands for controlling point- and nonpoint-source pollution in surface water has attracted increasing interest. The partitioning process of organic contaminants between water, particulate organic carbon (POC) and dissolved organic carbon (DOC), impacts their behaviors in the aquatic environments. Meantime, the partitioning process of organic contaminants is closely related to their physicochemical properties, such as hydrophobicity (or K ow), and their fates in wetlands may vary greatly depending on physicochemical properties. The aim of this study was to examine fates and removals of polycyclic aromatic hydrocarbons (PAHs), organochlorine pesticides (OCPs) and nonylphenols (NPs) in a wetland in Beijing, China, and provide useful information for ecological remediation. Water samples, collected at five sites from inlet to outlet of the wetland once a month in summer 2006, were immediately filtered within 2 days through 0.45-μm glass fiber prefilters and enriched by solid-phase extraction. The filtered particulates were collected as the total suspended particulates (TSPs), freeze-dried, and Soxhlet-extracted. After extraction, samples were purified following a clean-up procedure and analyzed by GC-MS. TSPs could be removed efficiently with a removal rate of 97.4%, and DOC could be moderately removed with a removal rate of 44.7% from inlet to outlet. The total removals of target contaminants varied widely from null to 82.0%. A good correlation between logK ow and logK oc (organic-carbon-normalized suspended-particulate partition coefficient) was observed (r 2 = 0.84 for PAHs and r 2 = 0.86 for OCPs, p < 0.01). Ratios of the POC-bound fraction of target contaminants (or DOC-bound fraction) to the freely dissolved fraction increased with their K ow values. The removal of the POC fraction contributed more than 50% to the total removal for the contaminants with logK ow > 5.0. Only a small portion of the removal was attributed to the removal of the freely dissolved fraction. Hydrophobic compounds such as PAHs and OCPs with higher K ow values would show stronger POC or DOC preference. Their removal depended greatly on their K ow values and the removal of total suspended particulates. On the other hand, concentrations of NPs decreased little in the wetland, probably due to their production through degradation of their precursors and relatively low hydrophobicity. POC and DOC play essential roles on the fates and removals of hydrophobic organic contaminants in the wetland. The removal of target contaminants with a high K ow should be mainly through association with the suspended particulates which were precipitated and retained in the wetland. The fates of the organic contaminants in the wetland greatly depended on their hydrophobicities. Further work should be done to study the influence of hydraulic retention time and some other environmental factors, e.g., temperature, on removals and fates of organic contaminants. Behavior of NPs and their precursors in the wetland should also be investigated more thoroughly.

Oxygen production and carbon fixation in oligotrophic coastal bays and the relationship with gross and net primary production

Planktonic primary production and respiration in 2 coastal oligotrophic sites of the Northwest Mediterranean Sea were examined. Primary production was quantified using 3 methods (light and dark changes in dissolved O2, 18 O-labeling and 14 C uptake technique) using in situ bottle incubations. Gross primary production (GPP) based on the O2 light-dark technique was not signifi- cantly different from that using the 18 O-labeling technique, indicating that the former technique pro- vides accurate estimates of GPP in these environments. Respiration in the dark was not significantly different from respiration in the light. Total 14 C uptake (including the dissolved and particulate organic carbon fractions) during the whole duration of the light period approached GPP and conse- quently overestimated net primary production.

Inputs of organic carbon from Ria de Aveiro coastal lagoon to the Atlantic Ocean

Land/ocean boundaries constitute complex systems with active physical and biogeochemical processes that affect the global carbon cycle. An example of such a system is the mesotidal lagoon named Ria de Aveiro (Portugal, 40°38′N, 08°45′W), which is connected to the Atlantic Ocean by a single channel, 350 m wide. The objective of this study was to estimate the seasonal and inter-tidal variability of organic carbon fluxes between the coastal lagoon and the Ocean, and to assess the contribution of the organic carbon fractions (i.e. dissolved organic carbon (DOC) and particulate organic carbon (POC)) to the export of organic carbon to the Ria de Aveiro plume zone. The organic carbon fractions fluxes were estimated as the product of the appropriate fractional organic carbon concentrations and the water fluxes calculated by a two-dimensional vertically integrated hydrodynamic model (2DH). Results showed that the higher exchanges of DOC and POC fractions at the system cross-section occurred during spring tides but only resulted in a net export of organic carbon in winter, totalling 85 t per tidal cycle. Derived from the winter and summer campaigns, the annual carbon mass balance estimated corresponded to a net export of organic carbon (7957 = 6585 t yr −1 POC + 1372 t yr −1 DOC). On the basis of the spring tidal drainage area, it corresponds to an annual flux of 79 g m −2 of POC and 17 g m −2 of DOC out of the estuary.

Carbonyl Emissions from Gasoline and Diesel Motor Vehicles

Carbonyls from gasoline-powered light-duty vehicles (LDVs) and heavy-duty diesel-powered vehicles (HDDVs) operated on chassis dynamometers were measured by use of an annular denuder-quartz filter-polyurethane foam sampler with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine derivatization and chromatography-mass spectrometry analyses. Two internal standards were utilized based on carbonyl recovery: 4-fluorobenzaldehyde for < C8 carbonyls and 6-fluoro-4-chromanone for > or = C8 compounds. Gas- and particle-phase emissions for 39 aliphatic and 20 aromatic carbonyls ranged from 0.1 to 2000 microg/L of fuel for LDVs and from 1.8 to 27 000 microg/L of fuel for HDDVs. Gas-phase species accounted for 81-95% of the total carbonyls from LDVs and 86-88% from HDDVs. Particulate carbonyls emitted from a HDDV under realistic driving conditions were similar to concentrations measured in a diesel particulate matter (PM) standard reference material. Carbonyls accounted for 19% of particulate organic carbon (POC) emissions from low-emission LDVs and 37% of POC emissions from three-way catalyst-equipped LDVs. This identifies carbonyls as one of the largest classes of compounds in LDV PM emissions. The carbonyl fraction of HDDV POC was lower, 3.3-3.9% depending upon operational conditions. Partitioning analysis indicates the carbonyls had not achieved equilibrium between the gas and particle phases under the dilution factors of 126-584 used in the present study.

Terrestrial and marine POC fluxes derived from 234Th distributions and δ13C measurements on the Mackenzie Shelf

Water column deficits of 234Th relative to 238U in the Mackenzie Shelf, Cape Bathurst Polynya, and Amundsen Gulf were used to estimate sinking fluxes of particulate organic carbon (POC) in these areas. The 234Th fluxes were converted to marine and terrestrial POC fluxes using the POC/Th ratio on filterable particles &gt;70 μm and δ13C measurements to determine the fraction of marine and terrestrial POC. In June/July 2004, the greatest 234Th deficits (0–100 m: 56–95 dpm m−2) were observed in the Mackenzie outer shelf. Deficits in the upper 100 m ranged from 3 to 59 dpm m−2 in the Cape Bathurst Polynya. The δ13C values of POC in the &gt;70‐μm particles filtered in situ pumps ranged from −25.1‰ to −28‰. Using a two‐end‐member mixing model with marine POC = −21.4‰ and terrestrial POC = −28‰ shows that terrestrial POC is most evident at the Mackenzie Shelf stations but is present throughout the region. The fraction of marine POC ranged from 0 to 59% in the area in June/July 2004, with highest values in the Cape Bathurst Polynya. Fluxes of marine POC in the polynya average ∼5 mmol C m−2 d−1 at 50 m in June 2004 and increase to ∼12 mmol C m−2 d−1 in July. Comparable fluxes are observed at 100 m in June, but values decrease to ∼6 mmol C m−2 d−1 at 100 m in July. These fluxes are greater than estimates of organic carbon remineralization and burial in sediments of the polynya (∼3 mmol m−2 d−1), suggesting that POC may be exported out of the area, effectively remineralized by microbial activity in the twilight zone or incorporated into biomass.

Modelling carbon overconsumption and the formation of extracellular particulate organic carbon

Abstract. During phytoplankton growth a fraction of dissolved inorganic carbon (DIC) assimilated by phytoplankton is exuded in the form of dissolved organic carbon (DOC), which can be transformed into extracellular particulate organic carbon (POC). A major fraction of extracellular POC is associated with carbon of transparent exopolymer particles (TEP; carbon content = TEPC) that form from dissolved polysaccharides (PCHO). The exudation of PCHO is linked to an excessive uptake of DIC that is not directly quantifiable from utilisation of dissolved inorganic nitrogen (DIN), called carbon overconsumption. Given these conditions, the concept of assuming a constant stoichiometric carbon-to-nitrogen (C:N) ratio for estimating new production of POC from DIN uptake becomes inappropriate. Here, a model of carbon overconsumption is analysed, combining phytoplankton growth with TEPC formation. The model describes two modes of carbon overconsumption. The first mode is associated with DOC exudation during phytoplankton biomass accumulation. The second mode is decoupled from algal growth, but leads to a continuous rise in POC while particulate organic nitrogen (PON) remains constant. While including PCHO coagulation, the model goes beyond a purely physiological explanation of building up carbon rich particulate organic matter (POM). The model is validated against observations from a mesocosm study. Maximum likelihood estimates of model parameters, such as nitrogen- and carbon loss rates of phytoplankton, are determined. The optimisation yields results with higher rates for carbon exudation than for the loss of organic nitrogen. It also suggests that the PCHO fraction of exuded DOC was 63±20% during the mesocosm experiment. Optimal estimates are obtained for coagulation kernels for PCHO transformation into TEPC. Model state estimates are consistent with observations, where 30% of the POC increase was attributed to TEPC formation. The proposed model is of low complexity and is applicable for large-scale biogeochemical simulations.

The prediction of soil carbon fractions using mid-infrared-partial least square analysis

This paper describes the application of mid-infrared (MIR) spectroscopy and partial least-squares (PLS) analysis to predict the concentration of organic carbon fractions present in soil. The PLS calibrations were derived from a standard set of soils that had been analysed for total organic carbon (TOC), particulate organic carbon (POC), and charcoal carbon (char-C) using physical and chemical means. PLS calibration models from this standard set of soils allowed the prediction of TOC, POC, and char-C fractions with a coefficient of determination (R2) of measured v. predicted data ranging between 0.97 and 0.73. For the POC fraction, the coefficient of determination could be improved (R2 = 0.94) through the use of local calibration sets. The capacity to estimate soil fractions such as char-C rapidly and inexpensively makes this approach highly attractive for studies where large numbers of analyses are required. Inclusion of a set of soils from Kenya demonstrated the robustness of the method for total organic carbon and charcoal carbon prediction.

Source(s) and cycling of the nonhydrolyzable organic fraction of oceanic particles

A major fraction of particulate organic carbon (POC) in the deep ocean remains molecularly uncharacterized. In an effort to determine the chemical characteristics and source(s) of sinking POC, we studied a nonhydrolyzable fraction of sinking POC using 13C NMR (nuclear magnetic resonance) spectroscopy and analytical pyrolysis. 13C NMR spectra and products from analytical pyrolysis of the nonhydrolyzable fraction exhibit a strongly aliphatic character that is distinct from that of bulk POC. The aliphatic nature of this fraction is consistent with its low stable carbon isotope values. We hypothesize that the nonhydrolyzable fraction derives to a significant extent from a refractory component of organisms that selectively accumulates, resulting in its manifestation as a major part of POC sinking to the deep ocean and in underlying sediments.

Particulate carbon in the atmosphere of a Finnish forest and a German anthropogenically influenced grassland

Aerosol samples from a boreal forest in Finland (Hyytiälä) and an anthropogenically influenced grassland in Germany (Melpitz) were analyzed for carbonaceous content by a thermal-optical transmission method. In order to assess the adsorption of semi-volatile compounds on the quartz fiber filters at both locations, a backup quartz filter was placed beneath the filter used for the particulate matter collection. The fraction of particulate organic carbon with a secondary origin in gas/particle conversion of volatile organic compounds was estimated from the minimum ratio between particulate organic and black carbon, which occurred during periods of reduced photochemical activity. Values calculated by this method were 71% for the Finnish aerosol, whilst in Melpitz only 56% could be observed. Especially in Hyytiälä, the fraction of secondary organic carbon showed a maximum during daytime and minimum during night time, indicating a local or regional formation of secondary aerosol. It was verified that the mass of carbon adsorbed in most backup filters is not significantly different from the mass adsorbed in a blank filter.