Particulate Organic Carbon Loss Research Articles

Response of erosion-induced carbon loss to rainfall characteristics is forest type dependent

The paucity of studies on the response of erosion-induced carbon (C) loss to rainfall characteristics hinders our ability to estimate how changes in rainfall regime may affect C budget of forests, particularly in regions prone to water erosion. In the current study, we monitored the losses of dissolved organic carbon (DOC) and particulate organic carbon (POC) in relation to rainfall depth, average intensity, maximum 5 min intensity (I5), erosivity (EI30), and vegetation cover in three young forests during the first three years of reforestation. The three forest types include an assisted naturally regenerated (ANR) forest, a young Castanopsis carlessi plantation (YCP), and a young Cunninghamia lanceolata (Chinese fir) plantation (YCF). Our results indicate that POC was the dominant form of C loss, and erosion-induced C loss was lower in the ANR than the two plantations, likely due to its high vegetation cover. Erosion-induced C loss decreased through time across forest types as vegetation cover increased, highlighting the role of vegetation cover in mitigating C loss. The response of C loss to rainfall characteristics varied among different forest types. Vegetation cover and rainfall depth had similar contributions in predicting DOC flux in the ANR, while rainfall depth was more important in the YCP and vegetation cover was more important in the YCF. The most important factor predicting POC loss was vegetation cover in the ANR, I5 in the YCP, while in the YCF vegetation cover and I5 were of similar importance. Overall, DOC loss was mainly driven by rainfall depth, while POC loss was more related to I5. Our results suggest that projected increases in storm intensity associated with climate change are likely to increase erosion-induced C loss. And ANR should be widely promoted as an effective management practice of C conservation.

Particle cycling rates at Station P as estimated from the inversion of POC concentration data

Particle cycling rates in marine systems are difficult to measure directly, but of great interest in understanding how carbon and other elements are distributed throughout the ocean. Here, rates of particle production, aggregation, disaggregation, sinking, remineralization, and transport mediated by zooplankton diel vertical migration were estimated from size-fractionated measurements of particulate organic carbon (POC) concentration collected during the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) cruise at Station P in summer 2018. POC data were combined with a particle cycling model using an inverse method. Our estimates of the total POC settling flux throughout the water column are consistent with those derived from thorium-234 disequilibrium and sediment traps. A budget for POC in two size fractions, small (1–51 µm) and large (> 51 µm), was produced for both the euphotic zone (0–100 m) and the upper mesopelagic zone (100–500 m). We estimated that POC export at the base of the euphotic zone was 2.2 ± 0.8 mmol m−2 d−1, and that both small and large particles contributed considerably to the total export flux along the water column. The model results indicated that throughout the upper 500 m, remineralization leads to a larger loss of small POC than does aggregation, whereas disaggregation results in a larger loss of large POC than does remineralization. Of the processes explicitly represented in the model, zooplankton diel vertical migration is a larger source of large POC to the upper mesopelagic zone than the convergence of large POC due to particle sinking. Positive model residuals reveal an even larger unidentified source of large POC in the upper mesopelagic zone. Overall, our posterior estimates of particle cycling rate constants do not deviate much from values reported in the literature, i.e., size-fractionated POC concentration data collected at Station P are largely consistent with prior estimates given their uncertainties. Our budget estimates should provide a useful framework for the interpretation of process-specific observations obtained by various research groups in EXPORTS. Applying our inverse method to other systems could provide insight into how different biogeochemical processes affect the cycling of POC in the upper water column.

Mineral-enhanced biological pump—A strategy based on mineral-microbe interactions for increasing carbon sink in water

<p indent=0mm>Addressing the threats posed by global climate change by reducing the number of greenhouse gases is currently one of the most important research goals of the scientific community. Ocean iron fertilization (OIF) is a geoengineering strategy aimed at mitigating global warming through human intervention. The OIF concept is based on the idea that artificial adding of iron nutrients into iron-depleted regions of the ocean will lead to a rapid boost of phytoplankton populations and the mass sinking of organic matter that ultimately leads to the sequestration of significant amounts of atmospheric CO₂ in the deep sea and sediments. The OIF experiments have been conducted in past over the time and the space scales of weeks and kilometres in polar, subpolar and tropical areas of the oceans. The results have generated new knowledge and a better understanding of the role of iron in regulating marine ecosystems and have confirmed that the iron supply is one of the key controls on the dynamics of phytoplankton blooms, such as diatom as the base of the marine food chain, which in turn affect the whole biogeochemical cycling of carbon. However, the OIF as an effective carbon control strategy is not adequately supported with concerns about potential significant impacts on marine ecosystems. The iron fertilization also did not significantly strengthen the vertical carbon export to the deep sea and thereby the sequestration of CO₂. The efficacy by which the OIF vertically exports organic carbon associated with diatom blooms to the deep sea remains constrained due to the remineralization and the loss of organic carbon during the sinking process of diatomaceous biogenic silica (BSi). Therefore, to address these limitations, the next generation of the IOF geoengineering experiments should be designed to improve the low efficiency of the vertical export of bloomed phytoplankton, which will reduce the loss of organic carbon during the sinking of biomass. In our recent studies, we observed a noticeable dissolution-inhibition effect of Al-bearing lake diatomaceous BSi, which is caused by the high content of Al incorporation in the structure of the BSi. This finding inspired a new idea about the possibility of incorporating naturally occurring, environmentally safe and Al-rich minerals as an Al source for the geoengineering experiments. We hypothesize that clay minerals, with characteristically high contents of Al and Si, might be good ingredients for mineral fertilization in geoengineering experiments to enhance the efficiency of the vertical carbon export of the biological pump. This new proposed mineral-aided process is termed mineral-enhanced biological pump (MeBP). In this paper, the concept, background and theoretical basis of MeBP for CO₂ sequestration in water bodies as a new and inexpensive geoengineering strategy are explained. The basic idea of the MeBP concept is to use finely grained powders of minerals, such as clay minerals as nutrients for geoengineering or ecological engineering experiment to enhance the efficiency of the complexes of microbial organic matter and minerals and to hinder the loss of particulate organic carbon by adjusting the mineral ratios and regulating the particle properties. If so, the improved carbon sequestration efficiency of the biological pump and the CO₂ sink enhancement in water bodies could be achieved. The MeBP offers several potential advantages such as application simplicity, low-cost, scalability, environmental friendliness, and flexibility, and it could be applied alone or in combination with other carbon sequestration strategies. Also, the approaches for the research and application of MeBP, the environmental and ecological risks of applying MeBP and the possible ways of avoiding these risks are discussed. These discussions aim to stimulate new thinking toward the development of new technologies of increasing carbon sink in water bodies that are sustainable, ecologically acceptable and scalable, and can be translated into real applications and engineering practices.

Antecedent soil moisture and rain intensity control pathways and quality of organic carbon exports from arable land

Under conditions of climate change, the severity and probability of storm events and droughts are expected to increase, this may affect the partitioning into surface and subsurface runoff. However, there is still a debate on how such extreme weather conditions affect the leaching of soil organic carbon. The aim of this study was to ascertain the impact of rainfall intensity (63 vs 79 mm h−1) and antecedent soil moisture (Matric potential: −10 hPa vs −450 hPa) on flow partitioning in the transportation (fluxes and timing) of dissolved (DOC) and particulate organic carbon (POC). Rainfall simulation experiments using undisturbed soil monoliths (1 m × 0.5 m × 0.3 m) allowed to monitor the transport of water, sediments, DOC, and POC via surface and shallow subsurface runoff. In addition, the composition of the leached organic matter was analysed using photometric and fluorometric methods.Most of the organic carbon was transported as POC via surface runoff. In the surface runoff, the influence of antecedent soil moisture on POC loss was larger than rainfall intensity, while rainfall intensity had a greater influence on DOC losses. Under initially unsaturated conditions, surface sealing reduced the infiltration rate, thus fuelling the total surface runoff, which, in turn, enhanced the transported total sediment and POC load of the surface runoff. While POC dominated surface runoff, DOC played a crucial role in the subsurface runoff with higher DOC concentrations under unsaturated conditions. In contrast to other treatments, DOC exported from unsaturated soils rained with high intensity had a unique composition with the highest proportion of humic-like peaks (A, M, C), protein-like peaks (T) and components (C4), suggesting a concentrating effect on DOC at those soils. Altogether, this research acquires new insights into how flow partitioning under extreme weather conditions affects organic carbon losses and which transport mechanism might pose a risk for aquatic systems.

Decoupling of ΔO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;∕Ar and particulate organic carbon dynamics in nearshore surface ocean waters

Abstract. We report results from two Lagrangian drifter surveys off the Oregon coast, using continuous shipboard sensors to estimate mixed-layer gross primary productivity (GPP), community respiration (CR), and net community production (NCP) from variations in biological oxygen saturation (ΔO2∕Ar) and optically derived particulate organic carbon (POC). At the first drifter survey, conducted in a nearshore upwelling zone during the development of a microplankton bloom, net changes in ΔO2∕Ar and [POC] were significantly decoupled. Differences in GPP and NCP derived from ΔO2∕Ar (NCPO2/Ar) and POC (NCPPOC) time series suggest the presence of large POC losses from the mixed layer. At this site, we utilized the discrepancy between NCPO2/Ar and NCPPOC, and additional constraints derived from surface water excess nitrous oxide (N2O), to evaluate POC loss through particle export, DOC production, and vertical mixing fluxes. At the second drifter survey, conducted in lower-productivity, density-stratified offshore waters, we also observed offsets between ΔO2∕Ar and POC-derived GPP and CR rates. At this site, however, net [POC] and ΔO2∕Ar changes yielded closer agreement in NCP estimates, suggesting a tighter relationship between production and community respiration, as well as lower POC loss rates. These results provide insight into the possibilities and limitations of estimating productivity from continuous underway POC and ΔO2∕Ar data in contrasting oceanic waters. Our observations support the use of diel POC measurements to estimate NCP in lower-productivity waters with limited vertical carbon export and the potential utility of coupled O2 and optical measurements to estimate the fate of POC in high-productivity regions with significant POC export.

Blue carbon sequestration dynamics within tropical seagrass sediments: long-term incubations for changes over climatic scales

Determination of blue carbon sequestration in seagrass sediments over climatic time scales (&amp;gt;100 years) relies on several assumptions, including no loss of particulate organic carbon (POC) after 1–2 years, tight coupling between POC loss and CO2 emissions, no dissolution of carbonates, and removal of the recalcitrant black carbon (BC) contribution. We tested these assumptions via 500-day anoxic decomposition and mineralisation experiments to capture centennial parameter decay dynamics from two sediment horizons robustly dated as 2 and 18 years old. No loss of BC was detected, and decay of POC was best described for both horizons by near-identical reactivity continuum models. The models predicted average losses of 49 and 51% after 100 years of burial for the surface and 20–22-cm horizons respectively. However, the loss rate of POC was far greater than the release rate of CO2, even after accounting for CO2 from particulate inorganic carbon (PIC) production, possibly as siderite. The deficit could not be attributed to dissolved organic carbon or dark CO2 fixation. Instead, evidence based on δ13CO2, acidity and lack of sulfate reduction suggested methanogenesis. The results indicated the importance of centennial losses of POC and PIC precipitation and possibly methanogenesis in estimating carbon sequestration rates.

Preservation of organic carbon during active fluvial transport and particle abrasion

Abstract Oxidation of particulate organic carbon (POC) during fluvial transit releases CO2 to the atmosphere and can influence global climate. Field data show large POC oxidation fluxes in lowland rivers; however, it is unclear if POC losses occur predominantly during in-river transport, where POC is in continual motion within an aerated environment, or during transient storage in floodplains, which may be anoxic. Determination of the locus of POC oxidation in lowland rivers is needed to develop process-based models to predict POC losses, constrain carbon budgets, and unravel links between climate and erosion. However, sediment exchange between rivers and floodplains makes differentiating POC oxidation during in-river transport from oxidation during floodplain storage difficult. Here, we isolated in-river POC oxidation using flume experiments transporting petrogenic and biospheric POC without floodplain storage. Our experiments showed solid phase POC losses of 0%–10% over ∼103 km of fluvial transport, compared to ∼7% to &gt;50% losses observed in rivers over similar distances. The production of dissolved organic carbon (DOC) and dissolved rhenium (a proxy for petrogenic POC oxidation) was consistent with small POC losses, and replicate experiments in static water tanks gave similar results. Our results show that fluvial sediment transport, particle abrasion, and turbulent mixing have a minimal role on POC oxidation, and they suggest that POC losses may accrue primarily in floodplain storage.

Aragonite dissolution kinetics and calcite/aragonite ratios in sinking and suspended particles in the North Pacific

The lack of consensus on CaCO3 dissolution rates and calcite to aragonite production and export ratios in the ocean poses a significant barrier for the construction of global carbon budgets. We present here a comparison of aragonite dissolution rates measured in the lab vs. in situ along a transect between Hawaii and Alaska using a 13C labeling technique. Our results show a general agreement of aragonite dissolution rates in the lab versus in the field, and demonstrate that aragonite, like calcite, shows a non-linear response of dissolution rate as a function of saturation state (Ω). Total carbon fluxes along the N. Pacific transect in August 2017, as determined using sediment traps, account for 11∼23 weight % of total mass fluxes in the upper 200 m, with a PIC (particulate inorganic carbon) /POC (particulate organic carbon) mole ratio of 0.2∼0.6. A comparison of fluxes at depths of 100 m and 200 m indicates that 30∼60% PIC dissolves between these depths with 20∼70% attenuation in POC fluxes. The molar ratio of PIC to POC loss is 0.29. The simultaneous loss of PIC and POC in the upper 200 m potentially indicates PIC dissolution driven by organic matter respiration, or metazoan/zooplankton consumption. The calcite/aragonite ratio in trap material is significantly lower in the subtropical gyre than in the subarctic gyre. Aragonite fluxes vary from 0.07 to 0.38 mmolm−2day−1 at 100 m, and 0.06 to 0.24 mmolm−2day−1 at 200 m along the North Pacific transect, with no specific trend over latitude. The identification of suspended PIC mineral phases by Raman spectroscopy shows the presence of aragonite below 3000 m in the subtropical gyre, but none in the subpolar gyre. These multiple lines of evidence suggest that predictions based on a strictly thermodynamic view of aragonite dissolution, combined with measured aragonite fluxes, underestimate observed alkalinity excess and measured PIC attenuation in sinking particles. Our measured aragonite flux combined with our inorganic dissolution rate only account for 9% and 0.2% of the excess alkalinity observed in the North Pacific (Feely et al., 2004), assuming aragonite sinking rates of 1 mday−1 and 100 mday−1, respectively. However, respiration-driven dissolution or metazoan/zooplankton consumption, indicated by the simultaneous attenuation of PIC and POC in sediment traps, is able to generate the magnitude of dissolution suggested by observed excess alkalinity.

Depth-resolved particle-associated microbial respiration in the northeast Atlantic

Abstract. Atmospheric levels of carbon dioxide are tightly linked to the depth at which sinking particulate organic carbon (POC) is remineralised in the ocean. Rapid attenuation of downward POC flux typically occurs in the upper mesopelagic (top few hundred metres of the water column), with much slower loss rates deeper in the ocean. Currently, we lack understanding of the processes that drive POC attenuation, resulting in large uncertainties in the mesopelagic carbon budget. Attempts to balance the POC supply to the mesopelagic with respiration by zooplankton and microbes rarely succeed. Where a balance has been found, depth-resolved estimates reveal large compensating imbalances in the upper and lower mesopelagic. In particular, it has been suggested that respiration by free-living microbes and zooplankton in the upper mesopelagic are too low to explain the observed flux attenuation of POC within this layer. We test the hypothesis that particle-associated microbes contribute significantly to community respiration in the mesopelagic, measuring particle-associated microbial respiration of POC in the northeast Atlantic through shipboard measurements on individual marine snow aggregates collected at depth (36–500 m). We find very low rates of both absolute and carbon-specific particle-associated microbial respiration (&lt; 3 % d−1), suggesting that this term cannot solve imbalances in the upper mesopelagic POC budget. The relative importance of particle-associated microbial respiration increases with depth, accounting for up to 33 % of POC loss in the mid-mesopelagic (128–500 m). We suggest that POC attenuation in the upper mesopelagic (36–128 m) is driven by the transformation of large, fast-sinking particles to smaller, slow-sinking and suspended particles via processes such as zooplankton fragmentation and solubilisation, and that this shift to non-sinking POC may help to explain imbalances in the mesopelagic carbon budget.

The effect of hydrostatic pressure on the decomposition of inundated terrestrial plant detritus of different quality in simulated reservoir formation

AbstractThe formation of reservoirs usually incorporates the inundation of terrestrial vegetation in the basin. The decomposition of organic matter from the flooded vegetation may have several implications for reservoir functioning, including eutrophication and dissolved oxygen depletion. The hydrostatic pressure increases with depth in a reservoir, and its influence on the decomposition process has not previously been evaluated. This study was undertaken to evaluate the decomposition of terrestrial plant detritus of different qualities (leaves and branches) under different hydrostatic pressure conditions. Detritus were placed separately in glass bottles in the laboratory and incubated in tight stainless steel pressure vessels, simulating three different depths (surface, 30 and 100 m). The masses (mg) of particulate organic carbon (POC), dissolved organic carbon (DOC) and inorganic carbon (IC) were determined for the 4 months of the detritus decomposition simulated in this study. The mass values were transformed in percentages of the initial detritus carbon. The results of temporal variations of the compounds studied were fitted to a first‐order biphasic decay model. The hydrostatic pressure exhibited no significant effects on litter decomposition. On the other hand, the detritus chemical composition (i.e. the presence of labile and refractory compounds) was the determining factor for the decomposition curve shape and for the differences observed between the leaves and branches. The greatest POC loss from leaves, and resulting larger DOC mass, indicated the leaves were more labile than the branches. The results also indicated the branches are the main detritus remaining in a reservoir over time.

Forms and Fluxes of Soil Organic Carbon Transport via Overland Flow, Interflow, and Soil Erosion

Core Ideas Interflow is another crucial route of soil organic carbon lateral transport. Overland flow, interflow, and sediment contributed 5, 24, and 71%, respectively, of annual loss loads of soil organic carbon lateral transport. Dissolved organic carbon is another important component for SOC lateral transport. The contributions of hydrological pathways (including overland flow, interflow, and soil erosion) to lateral soil organic carbon (SOC) transport have remained unclear until now. Hillslopes were monitored using free‐draining lysimeters (8 m by 4 m) to quantify dissolved organic carbon (DOC) losses due to overland flow and interflow and the total organic carbon (TOC), water extractable organic carbon (WEOC), particulate organic carbon (POC), and mineral‐associated organic carbon (MOC) fractions in sediments from sloping croplands containing Regosols in Southwest China. The average annual DOC losses due to overland flow and interflow were 158.8 ± 33.0 and 750.4 ± 79.3 mg C m–2, respectively, and the TOC lost with sediment was 2201.0 ± 429.2 mg C m‒2. Overland flow, interflow, and sediment accounted for 5, 24, and 71%, respectively, of the annual SOC losses. The average annual DOC, POC, and MOC loss fluxes were 918.6 ± 115.3, 375.2 ± 94.4, and 1816.4 ± 331.8 mg C m–2, respectively. The MOC contents in the sediments were positively correlated with rainfall, and the DOC concentrations in the interflow water were negatively correlated with rainfall. In conclusion, soil erosion is the dominant hydrological route for lateral SOC transport, and interflow is another crucial route that is usually underestimated. Soil organic C is mainly lost in the forms of MOC and DOC, which is an important component of water ecosystems. Therefore, the mitigation of SOC losses would be more effective if soil erosion and interflow conservation practices were adopted together, particularly for Regosols on hillslopes.

Scale-dependent lateral exchanges of organic carbon in a dryland river during a high-flow experiment

We estimated the magnitude and direction of exchanges of particulate organic carbon (POC) and dissolved organic carbon (DOC) between the river and four floodplain wetlands (billabongs) and a 140-km reach of riverbank and floodplain of the Murrumbidgee River during a managed high-flow experiment. There was a net transport of organic carbon from the river to billabongs during connection, ranging from 87 to 525kg POC per billabong or from 1.4 to 5.7g POC m–2 of billabong sediment surface area and from 36 to 4357kg DOC, or from 0.4 to 29.8g DOC m–2. At the whole-reach scale, there was a net loss of 754Mg POC from the river channel to riverbank and floodplain and a net input of 821Mg DOC to the river channel. This DOC input, which was small relative to the total organic carbon in transit, was likely to have contributed significantly to oxidative processes in the river. The DOC entering the river was derived from litter and soils in riverbank habitats or from abraded biofilms in the river channel. The results support an extended flood-pulse concept that includes in-channel flow pulses as important elements in the biogeochemistry of dryland rivers. Piggybacking dam releases on tributary flows to deliver in-channel flows delivers significant benefit for riverine organic-matter cycles.

Soil carbon losses by sheet erosion: a potentially critical contribution to the global carbon cycle

AbstractDespite soil erosion through water being a ubiquitous process and its environmental consequences being well understood, its effects upon the global carbon cycle still remain largely uncertain. How much soil organic carbon (SOC) is removed each year from soils by sheet wash, an important if not the most efficient mechanism of detachment and transport of surficial soil material? What are the main environnemental controls worldwide? These are important questions which largely remain unanswered. Empirical data from 240 runoff plots studied over entire rainy seasons from different regions of the world were analysed to estimate particulate organic carbon (POC) losses (POCL), and POC enrichment in the sediments compared to the bulk soil (ER), which can be used as a proxy of the fate of the eroded POC. The median POCL was 9.9 g C m‐2 y‐1 with highest values observed for semi‐arid soils (POCL = 10.8 g C m‐2 y‐1), followed by tropical soils (POCL = 6.4 g C m‐2 y‐1) and temperate soils (POCL = 1.7 g C m‐2 y‐1). Considering the mean POCL of 27.2 g C m‐2 y‐1, the total amount of SOC displaced annually by sheet erosion from its source would be 1.32 ± 0.20 Gt C, i.e. 14.6% of the net annual fossil fuel induced C emissions of 9 Gt C. Because of low sediment enrichment in POC, erosion‐induced CO2 emissions are likely to be limited in clayey environments while POC burial within hillslopes is likely to constitute an important carbon sink. In contrast, most of the POC displaced from sandy soils is likely to be emitted to the atmosphere. These results underpin the major role sheet wash plays in the displacement of SOC from its source and in the fate of the eroded SOC, with large variations across the different pedo‐climatic regions of the world. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Particulate organic carbon in the surface waters of the North Atlantic: spatial and temporal variability based on satellite ocean colour

We have examined the 16-year time series of particulate organic carbon (POC) concentration in the surface waters of the North Atlantic derived from SeaWiFS and MODIS-Aqua data. The annual mean POC concentrations are the highest in the northern North Atlantic, reaching 120 mg m−3. Moving south, the mean annual POC concentrations decrease to minimum values of about 30 mg m−3 at around 30° N and increase in the equatorial region to about 70 mg m−3. The seasonal amplitude of POC concentration in the northern North Atlantic region is larger when compared to other regions. The annual mean surface POC concentrations in the entire North Atlantic basin show a statistically significant trend with an average decrease of 0.79 mg m−3 year−1. Regionally averaged 16-year mean POC biomass integrated over the optical depth, euphotic depth, and mixed-layer depth is estimated at about 1.27, 4.34, and 4.59 g m−2, respectively. Even larger biomass of 6.26 g m−2 is estimated if one chooses to use in the calculations the greatest from the daily values of the estimates listed above at each pixel of the satellite data. Comparisons of POC biomass with primary productivity allowed us to assess temporal and spatial patterns of POC losses.

Soil aggregates and organic carbon affected by the land use change from rice paddy to vegetable field

Land use changes can influence soil organic carbon (SOC) and other properties significantly. The aim of this study was to investigate the effect of paddy conversion to vegetable field on organic carbon and soil structure. Samples of soil and plant residue were collected from paddy fields and vegetable fields converted from paddy for over 20 years. Results showed that the SOC content and density decreased 19.68% and 0.4kg/m2 after the land use change. The decrease of soil carbon was mainly attributable to the loss of particulate organic carbon (0.35kg/m2). Compared to the paddy field, 14.01% of water-stable macro-aggregates were broken into micro-aggregates in the vegetable field, resulting in the mean weight diameter of aggregates and the percentage of aggregate destruction were about 71.5% and two times, respectively. The organic carbon content associated with water-stable macro-aggregates was higher than that with micro-aggregates (12.2–16.4 vs. 12.0–12.4g/kg). The organic carbon content associated with water-stable macro-aggregates in the paddy soil was higher than that in the vegetable field soil (16.4–15.0 vs. 13.1–12.2g/kg). Therefore, the total soil organic carbon density decreased due to the land use conversion. The decrease of root biomass and plant residue returning and the excessive inorganic nitrogenous fertilizer application resulted in the microbial carbon content decrease of 37.5%. The information from this study should be useful to understand changes of soil properties and soil carbon sequestration with the land use conversion.

Deposition, burial and sequestration of carbon in an oligotrophic, tropical lake

The amount of biogenic carbon that may be deposited, buried and eventually preserved (sequestered) in the sediments of a tropical, oligotrophic lake, was evaluated based on i) the temporal variation of the particulate organic carbon (POC) concentration in the superficial sediments in the deep zone of lake Alchichica, Puebla, Mexico; and ii) the POC accumulation and preservation in a 210Pb-dated sediment core from the lake. In lake Alchichica the POC concentration in the surficial sediments ranged between 12 and 60 mg POC g-1 (25 ± 12 mg POC g-1 dry weight). The magnitude of the sedimented POC in Alchichica was high and mostly of autochthonous origin. The POC concentrations recorded in the sediment core (16.6 to 31.6 mg g-1 dry weight) were comparable to the concentration range observed in the surface sediment samples collected during the study period, which signaled a high POC preservation capacity in the sedimentary column of lake Alchichica. The POC fluxes, estimated from the 210Pb-dated sediment core, varied between 14.9 and 35.3 g m-2 year-1 within the past century; and the maximum POC losses through diagenesis during this period were estimated to be lower than 25%. This study concludes that deep tropical lakes, exemplified by lake Alchichica, accumulate and preserve most of the POC deposited, playing an important role in regional carbon balances.

Origin and composition of particulate organic matter in a macrotidal turbid estuary: The Gironde Estuary, France

At the interface between continent and ocean, estuaries receive particles, and especially particulate organic matter (POM) originating from these two reservoirs, but also produce POM, through autochthonous primary production. The origin and composition of surface POM in the Gironde Estuary (SW France) and the environmental forcing of its variability was investigated using the data set produced by the French Coastal Monitoring Network SOMLIT (Service d'Observation en Milieu LITtoral; monthly like sampling during years 2007–2009). This estuary is considered as a model of macrotidal turbid estuaries. Using elemental and isotopic composition of the POM, we estimated that, at the inner estuary space scale and inter-annual time scale, surface particulate organic carbon (POC) was composed of terrestrial POM originated from the turbidity maximum (96.4%; refractory POC) and flood events (1.6%; labile and refractory POC), and of riverine (0.1%), estuarine (0.8%) and marine (1.1%) phytoplankton, i.e. that POC was 98% and 2% of terrestrial and phytoplankton origin, respectively. However, there was a clear spatial gradient: the phytoplankton contribution increases from ca. 1% in the upper and middle estuary to 8.5% in the lower estuary, where light condition is more favourable to plankton growth. The low contribution of phytoplankton to the POC is a characteristic of the Gironde estuary and contrast with other large temperate estuaries. Statistical analysis indicates that salinity, river flow and SPM concentration, and thus associated hydro-dynamic and sedimentary processes, were the only environmental forcings to the composition of surface POC in this system, at intra- and inter-annual time scale. In contrast, temperature and nutrient concentrations, and thus associated processes, do not force this composition of POC.By combining POC fluxes entering the inner estuary (literature data), POC loss as dissolved organic carbon and CO2 and as sediment trapping within the inner estuary (literature data), and our estimate of the composition of POC flux at the mouth of the estuary (96% and 4% of terrestrial and phytoplankton origin), a first-order net export of POC originating from the Gironde to the continental shelf was estimated: it amounts 48,150tCyr−1, and is composed of 46,200tCyr−1 of terrestrial material and of 1950tCyr−1 of estuarine phytoplankton. POC exported by the Gironde Estuary is thus poorly bioavailable for shelf pelagic and benthic food webs.

Impact of gully erosion on carbon sequestration in blanket peatlands

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 45:31-41 (2010) - DOI: https://doi.org/10.3354/cr00887 Impact of gully erosion on carbon sequestration in blanket peatlands Martin Evans1,*, John Lindsay2 1Upland Environments Research Unit, School of Environment and Development, University of Manchester, Manchester M19 3PL, UK 2Department of Geography, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1, Canada *Email: martin.evans@manchester.ac.uk ABSTRACT: Over 50% of UK soil carbon is stored in peatland systems and 75% of these peatlands are upland blanket bog. The upland blanket bogs of the UK have suffered severe erosion over the last millennium so that they are widely dissected by gully systems. Gully erosion entails primary removal of particulate carbon from the peatland system but also has secondary effects in that it enhances drainage and lowers water tables, potentially enhancing decomposition of surface peats. This paper exploits recent high resolution mapping of the gully erosion on the Bleaklow Plateau in the southern Pennines and existing data on peat growth rates in the area to assess the impact of gully erosion on peatland carbon balance and the relative importance of primary and secondary impacts of gullying on carbon sequestration. The results indicate that gully erosion during the last millennium has shifted the Bleaklow Plateau from being a net sink of carbon (–20.3 gC m–2 yr–1) to a net source (29.4 gC m–2 yr–1). The relative importance of gullying impacts can be expressed as follows: particulate organic carbon (POC) flux > change in gully net ecosystem exchange >> loss of gully margin carbon sequestration. The implication of these findings is that the magnitude of the potentially reversible impacts of gully erosion (reduced carbon fixation due to vegetation loss and ongoing erosional loss of POC) far exceed the effects associated with irreversible morphological change (enhanced peat decomposition in gully margin locations). KEY WORDS: Blanket peat · Gully erosion · Carbon balance · Carbon sequestration Full text in pdf format PreviousNextCite this article as: Evans M, Lindsay J (2010) Impact of gully erosion on carbon sequestration in blanket peatlands. Clim Res 45:31-41. https://doi.org/10.3354/cr00887 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 45. Online publication date: December 30, 2010 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2010 Inter-Research.

Peatland Geomorphology and Carbon Cycling

AbstractPeatlands in the northern hemisphere are important stores of soil carbon. The stability of peatlands is therefore important for the preservation of a major terrestrial carbon store as well as for the maintenance of the carbon storage function of peatlands. Peatland erosion has the potential to significantly impact carbon storage. This article reviews the evidence from eroding peatland systems, largely in the United Kingdom, to develop the thesis that as peatlands begin to erode, geomorphological processes and form become the dominant controls on peatland carbon cycling. The impacts of geomorphological change on carbon cycling include direct erosional losses of particulate organic carbon (POC) and indirect effects largely resulting from the effect of gully erosion on peatland water tables, and consequently on dissolved and gaseous carbon losses. During erosional phases, POC loss from eroding systems can be sufficient to shift peatlands from net carbon sinks to net carbon sources. Re‐vegetation of eroding peatlands dramatically reduces POC loss but morphological changes resulting from erosion are likely to cause long‐term changes in gaseous and dissolved carbon flux from re‐vegetated peatlands. The potential for enhanced peatland erosion resulting from climatic changes in peatland areas requires a fuller understanding of geomorphological controls on carbon losses. In particular the fate of POC losses from peatlands is an urgent research need.

Particulate organic carbon–234Th relationships in particles separated by settling velocity in the northwest Mediterranean Sea

Abstract In order to better understand the relationship between the natural radionuclide 234 Th and particulate organic carbon (POC), marine particles were collected in the northwestern Mediterranean Sea (spring/summer, 2003 and 2005) by sediment traps that separated them according to their in situ settling velocities. Particles also were collected in time-series sediment traps. Particles settling at rates of >100 m d −1 carried 50% and 60% of the POC and 234 Th fluxes, respectively, in both sampling years. The POC flux decreased with depth for all particle settling velocity intervals, with the greatest decrease (factor of ∼2.3) in the slowly settling intervals (0.68–49 m d −1 ) over trap depths of 524–1918 m, likely due to dissolution and decomposition of material. In contrast the flux of 234 Th associated with the slowly settling particles remained constant with depth, while 234 Th fluxes on the rapidly settling particles increased. Taking into account decay of 234 Th on the settling particles, the patterns of 234 Th flux with depth suggest that either both slow and fast settling particles scavenge additional 234 Th during their descent or there is significant exchange between the particle classes. The observed changes in POC and 234 Th flux produce a general decrease in POC/ 234 Th of the settling particles with depth. There is no consistent trend in POC/ 234 Th with settling velocity, such as might be expected from surface area and volume considerations. Good correlations are observed between 234 Th and POC, lithogenic material and CaCO 3 for all settling velocity intervals. Pseudo- K d s calculated for 234 Th in the shallow traps (2005) are ranked as lithogenic material ⩽opal K d , and for lithogenic material, opal and CaCO 3 , the fraction is ∼22% each. Decreases in POC/ 234 Th with depth are accompanied by increases in the ratio of 234 Th to lithogenic material and opal. No change in the relationship between 234 Th and CaCO 3 was evident with depth. These patterns are consistent with loss of POC through decomposition, opal through dissolution and additional scavenging of 234 Th onto lithogenic material as the particles sink.