Transport Of Organic Carbon Research Articles

Temporal variability of soil organic carbon transport in the Enxoé agricultural watershed

The temporal variability of particulate (POC) and dissolved (DOC) organic carbon concentrations was analyzed in the Enxoe temporary river, southern Portugal, between September, 2010 and August, 2013. The overall aim was to study the variability of those elements during storm events, and determine their origin and the main transfer mechanisms to the river. Twenty-one flood events were observed. An empirical model was used to describe changes in solute concentrations, and the magnitude and rotational patterns of the hysteretic loops during flood events. POC and DOC concentrations varied between 0.49–88.93 and 0.25–25.75 mg L−1, respectively. POC and DOC annual yields varied between 0.06–2.15 and 0.03–1.47 t km−2, respectively. Flood events had greater effect in POC than in DOC variability. POC had mostly a terrestrial origin, with exports being related to soil erosion and runoff. POC revealed a flushing behavior during the entire monitored period, and clockwise or anticlockwise trajectory loops whenever the predominant origin of the exports was in river bed deposits or arable lands, respectively. DOC had also a terrestrial origin, but it revealed a contrasting dilution behavior and, in general, anticlockwise hysteresis loops. DOC showed a delay in the arrival of solutes to the river, consistent with mass flow through subsurface flow. DOC exports were thus associated with soil weathering and crop mineralization. This work highlights the main processes involved in POC and DOC loads in a temporary river during flood events, with a precise quantification of those elements.

Late Holocene changes in the humic state of a boreal lake and their associations with organic matter transport and climate dynamics

An important factor in the ontogeny of boreal lakes is the development of their humic state through terrigenous input of organic matter (carbon) that affects strongly the functioning and structure of these ecosystems. The long-term dynamics and role of humic substances for these systems in relation to climate are not clear. In this study, a boreal lake from southern Finland was investigated using paleolimnological methods, including diatom, chironomid and geochemical analyses, for Late Holocene changes in the humic state. The aim was to examine the relationship between sediment biogeochemistry and climate variation. Consistent trends were found in diatom-inferred total lake-water organic carbon (TOC) and in the ratio of humic/oligohumic chironomids. Sediment geochemistry provided further evidence for the limnological development of the lake and related long-term climate trends in the region. The results indicated three distinct phases with differing humic state; the beginning of the record at ca. 4,500 cal year BP was characterized by extremely humic conditions coinciding with warm and dry climatic conditions, a meso-oligohumic period between ca. 3,000–500 cal year BP with increasing allochthonous organic matter transport and cooler and wetter climate, and recent period with polyhumic (TOC >10 mg L−1) lake status and warming climate. This study shows that instead of straightforward linear development, boreal lakes evolve through dynamic humic stages related to climate and lake-catchment coupling processes. As the changes in the humic state are ultimately climate-driven, the ongoing climate change probably has a major influence on boreal lakes through direct and indirect effects on organic carbon transport, utilization and accumulation.

Organic carbon cycling in sediments of the Changjiang Estuary and adjacent shelf: Implication for the influence of Three Gorges Dam

Surface sediments collected from the Changjiang Estuary and adjacent shelf were analyzed for elemental and stable carbon isotopic composition, and lignin–phenols to investigate spatial variability of the sources, transport and decay of sedimentary organic carbon (OC). Bulk and molecular proxy data indicated a mixed marine/terrestrial OC sources in the study area. A three end-member mixing model using Monte-Carlo simulation showed that marine OC was the predominant OC source, accounting for an increasing fraction along the coast and seaward, while soil-derived OC and C3 vascular plant detrital OC decreased seaward and southward. Large fragments of lignin-rich C3 vascular plant OC were deposited mainly near the river mouth, whereas fine-grained lignin-poor soil-derived OC was delivered further south alongshore. Higher values of lignin decay indices, seaward and southward, were attributed to selective transport of terrestrial OC on fine-grained particles and efficient remineralization in mobile muds. Λ8 of OC in Changjiang Estuary sediments has slightly decreased in recent years, which could in part be due to the trapping of terrestrial coarse particles by the Three Gorges Dam (TGD). Also, we propose that there has been an increasing input of phytodetritus derived from freshwater phytoplankton to coastal sediments after the construction of the TGD.

Dynamics of CO2 in a karst catchment in the southwestern plateau, China

Inland waters, including rivers and lakes, are increasingly recognized as playing significant roles in the transport, mineralization and burial of organic carbon exported from land. However, in many areas, dissolved inorganic carbon (DIC) dominates the carbon export from catchments. Owing to different production processes and turnover times of organic versus inorganic carbon, CO2 emitted from rivers and lakes may have different impacts on global carbon cycling depending on its origin. Here, pCO2 and dissolved oxygen concentrations were determined, and the ratios of excess CO2 to O2 depletion (ΔCO2/ΔO2) were compared in spring water, river water and lake water in a carbonate catchment located in the southwestern plateau region of China. Results show that groundwater CO2 evasion, at 2.0 g C m−2year−1, is insignificant in terms of terrestrial carbon loss compared with soil CO2 emission. In the rivers, calcite precipitation due to oversaturation is an important mechanism for CO2 production in some seasons. In the lake, HCO3− contributed approximately 75 % of the total carbon supply to organic matter production and calcite deposition during seasons favoring photosynthesis. The seasons which had high ΔCO2/ΔO2 are the main periods of CO2 emission from the lake, and the extra CO2 may be produced from HCO3− titration by H+. Thus, lake CO2 evasion was controlled primarily by pH, not respiration. The spring, river, and lake waters mainly process DIC exported from the catchment, of which HCO3− is primarily derived from carbonate weathering by soil CO2 that, with extraordinarily high ΔCO2/ΔO2, may originate from sources including organic matter decomposition, root respiration (autotrophic), and acid dissolution. Therefore, freshwater CO2 emission is a return pathway of catchment soil CO2 to the atmosphere more than that of net primary production and net ecosystem production.

Phytoplankton Dynamics and Its Further Implication for Particulate Organic Carbon in Surface Waters of a Tropical/Subtropical Estuary

Phytoplankton succession along with salinity in estuaries not only influences the riverine ecosystem but also interferes our understanding of riverine materials (e.g., organic carbon) transport to the sea. Four cruises were conducted in a mid-size river and its estuary (i.e., the Wanquan River) to elucidate the riverine phytoplankton decease along increasing salinity and to quantify the amount of algal particulate organic carbon (POC) present in the system at different seasons. CHEMTAX calculations suggested that chlorophytes were the main chlorophyll a (CHLa) contributor in the dissolved inorganic phosphorus-limited river, which contributed over 60 % of the total CHLa. Microscopy further revealed that the dominant species was Scenedesmus sp. In the estuary, phytoplankton succession along with salinity in the estuary was observed. Chlorophyte contribution to total CHLa dramatically decreased from over 60 % in the river (S = 0) segment to 20, suggesting the removal of riverine algae in the estuary. The mean algal POC concentration ranged from 80 μg/L (summer) to 140 μg/L (winter), and the riverine algal POC accounted for 6–56 % of the bulk riverine POC. The annual flux of riverine algal POC was estimated to be 660 tons.

Benthic control on the statistical distribution of transported sediment carbon in a low-gradient stream

Summary Results from a numerical model that simulates particulate organic carbon source, fate and transport were used to generate the statistical distribution of transported sediment carbon in a low-gradient, agriculturally-impacted stream over a five-year model simulation. Results suggest that the statistical distribution of transported sediment carbon is Gamma distributed (RMSEA = 0.066) for the low-gradient stream. The distributional form of transported sediment carbon is governed by seasonal variability of temporarily stored benthic carbon and the relative contributions of benthic, bank and upland carbon sources. Results of the study suggest that shape and skew of the Gamma distribution are governed by biological activity (i.e., autochthonous production and decomposition) of the streambed. Analysis was performed to examine how field sampling factors, including flow conditions during sampling, sampling frequency, and the sampling temporal domain including event, seasonal and annual variability, capture the statistical distribution of transported sediment carbon. Contrary to conventional wisdom, sampling flow conditions and sampling frequency showed little impact on the sampled distribution of transported sediment carbon, which reflects the amalgamation of streambank and upland carbon sources on the stream bed in this low-gradient stream. Annual variability, i.e., wet and dry years, and seasonal variability were needed to adequately capture the statistical distribution of transported sediment carbon, which reflects the stochastic nature of the hydrologic regime annually and the seasonal variability of biological processes. The results provide a testable hypothesis, and a sampling design approach, for the statistical distribution of transported sediment carbon in low-gradient systems where benthic biological processes are prominent.

Stochastic modeling of fine particulate organic carbon dynamics in rivers

The majority of particulate organic matter standing stock in streams is < 1 mm in diameter, and the mobile phase is primarily very fine particles. Such fine particles transport downstream in a series of deposition and resuspension events mediated by interactions with coarser bed sediment, yielding fine particle retention over a wide range of time scales. This retention controls the opportunity for biogeochemical processing of particulate organic carbon in streams. We present a conceptual model of particulate organic carbon transport in rivers categorized in three cyclic processes: (i) migration of fine particles from the water column to the underlying and surrounding sediments, (ii) fine particle transport and retention within the bed sediments, and (iii) resuspension of fine particles back to the water column. We developed a stochastic model to describe the transport and retention of fine suspended particles in rivers, including advective delivery of particles to the streambed, transport through pore waters, and reversible filtration within the streambed. We then apply this model to observations of fine particle transport in two small streams, and show that the stochastic mobile-immobile model supports improved interpretation of particulate organic carbon dynamics under base flow conditions. Analysis of in-stream solute and particle data shows that particles engage in multiple deposition and resuspension events during downstream transport, and that long-term retention in the streambed produces extended slow releases to the stream even during base flow conditions. We also show how multiscale stochastic modeling can be used to incorporate local observations of particle retention in predictions of whole-stream particle dynamics.

Late Holocene elemental and isotopic carbon and nitrogen records from the East China Sea inner shelf: Implications for monsoon and upwelling

The East China Sea (ECS) is characterized by a relatively high riverine martial input that is associated with East Asian monsoon systems. In this study, we investigated a 272cm-long sediment core (THB-2) from the ECS inner shelf for the AMS 14C dating, grain size, total organic carbon (TOC), total nitrogen (TN), and stable carbon and nitrogen (δ13C and δ15N) isotopes to provide insights into previous changes in the monsoon-climate and the intensity of the coastal upwelling. A three end-member (riverine, deltaic, and marine) mixing model was applied to determine the temporal variations in sedimentary organic matter sources. The transport and burial of the Changjiang riverine organic carbon in the ECS inner shelf is strongly controlled by the combined effects of the monsoon-climate and human activities during the late Holocene. The sedimentary δ15N records of the THB-2 core are more complex and may be best explained by changes in the nutrient utilization in the coastal zone surface waters. The downcore variations of the marine organic matter-δ15N are closely related to the intensity of East Asian summer monsoon during the last 3.6ka. The stronger East Asian summer monsoon likely favored the intensive coastal upwelling and additional Changjiang discharge during the 3.6–2.7ka and 0.4–0ka periods, which triggered the greater relative nitrate utilization in the coastal surface water.

Source, transport and fluxes of Amazon River particulate organic carbon: Insights from river sediment depth-profiles

In order to reveal particulate organic carbon (POC) source and mode of transport in the largest river basin on Earth, we sampled the main sediment-laden tributaries of the Amazon system (Solimões, Madeira and Amazon) during two sampling campaigns, following vertical depth-profiles. This sampling technique takes advantage of hydrodynamic sorting to access the full range of solid erosion products transported by the river. Using the Al/Si ratio of the river sediments as a proxy for grain size, we find a general increase in POC content with Al/Si, as sediments become finer. However, the sample set shows marked variability in the POC content for a given Al/Si ratio, with the Madeira River having lower POC content across the measured range in Al/Si. The POC content is not strongly related to the specific surface area (SSA) of the suspended load, and bed sediments have a much lower POC/SSA ratio. These data suggest that SSA exerts a significant, yet partial, control on POC transport in Amazon River suspended sediment. We suggest that the role of clay mineralogy, discrete POC particles and rock-derived POC warrant further attention in order to fully understand POC transport in large rivers.To examine the source of POC in the Amazon Basin in more detail, we use radiocarbon (14C) content and the stable isotope composition (δ13C) of POC. 14C activity demonstrates that rock-derived POC is a significant component of river bed sediments and contributes to POC across the river depth-profiles of suspended sediments. We estimate that the flux of rock-derived POC may reach ∼10% of the total POC export by the Amazon River. After correcting for rock-derived POC input, we find that POC from the terrestrial biosphere (biospheric POC) is mostly sourced from C3-plants. Higher biospheric POC δ13C values in the Madeira River (by ∼0.5–1‰) are best explained by a small (<5%) contribution of C4-grasses from Bolivian savannas. Finally, we use 14C to estimate the mean age of biospheric POC exported from the Amazon Basin. The data show that biospheric POC is younger in the Solimões River (1120years) than in the Madeira River (2850 years). These ages are much younger than the corresponding estimates of sediment residence time in the basin, suggesting that lowland areas and/or young POC from above-ground biomass may contribute disproportionately to the biomarker signals in terrigenous sediments of the Amazon delta.

Effects of riverbank restoration on the removal of dissolved organic carbon by soil passage during floods – A scenario analysis

Summary River restoration typically aims at improving and preserving the ecological integrity of rivers and their floodplains. Restoration projects may, however, decrease the ability of the riparian zone to remove contaminants as the river water moves into the aquifer, especially during high river discharges. The purpose of this paper is to analyze several factors involved during riverbank restoration (i.e. changes in riverbank topography and hydraulic conductivity of the upper sediments of the riverbank), with respect to their effect on enhancing dissolved organic carbon (DOC) transport from rivers into the groundwater. 3-D groundwater flow and transport with first-order decay was simulated for a typical setting of a porous groundwater aquifer near a large river. The simulations indicate that, during a 5 m flooding event, DOC concentrations in the groundwater can be 1.7–9 times higher at a restored riverbank (i.e. 250 m wide, no clogging within one meter of riverbank sediments) compared to a steep riverbank (i.e. 8 m wide, clogging within 1 m of sediments), in coarse to fine sandy gravel. 51–84 % of this increase in DOC concentration levels in the groundwater were due to an increase in submerged area of the riverbank, depending on the type of soil of the aquifer. The remaining part was caused by a change in riverbank hydraulic conductivity. The simulations further showed that the arrival times of DOC concentration peaks at 400–500 m distance from the river axis can be 18–27 days shorter at restored than at steep riverbanks. 77–100 % of the earlier arrival times of DOC concentration peaks at 400–500 m from the river axis were due to an increase in submerged area of the riverbank. The remaining part was due to a change in riverbank hydraulic conductivity. The effects of riverbank restoration on DOC concentrations and arrival times were bigger if river DOC concentrations increased than if they were assumed constant during the flood, the more the river water level increased and the closer the distance was to the river. The findings suggest that riverbank restoration projects as conducted as part of the implementation of the European Water Framework Directive, potentially, may have adverse effects on the groundwater quality near rivers. Additional monitoring strategies will therefore be needed in the future in such projects to protect alluvial ground water resources for public drinking water supply.

Mobilization of colloidal carbon during iron reduction in basaltic soils

The transport of organic carbon (C) to deep mineral horizons in soils can lead to long-term C stabilization. In basaltic soils, C associations with short-range-ordered (SRO) minerals often lead to colloid-sized aggregates that can be dispersed and mobilized by changes in soil solution chemistry. In the montane forest region of Hawaii, basaltic soils are exposed to high rainfall and anoxic conditions that facilitate ferric (FeIII) (oxyhydr)oxide reduction. We explored the potential of iron (Fe)-reducing conditions to mobilize C by exposing the surface mineral horizons of three soils from the Island of Hawai'i (aged 0.3, 20, and 350ky) to 21days of anoxic incubation in 1:10 soil slurries. Mobilized C was quantified by fractionating the slurries into three particle-size classes (<430nm,<60nm,<2.3nm≈10kDa). In all three soils, we found Fe reduction (maximum Fe2+ (aq) concentration≈17.7±1.9mmolkg−1 soil) resulted in ~500% and ~700% increase of C in the 2.3–430nm, and <2.3nm size fractions, respectively. In addition, Fe reduction increased solution ionic strength by 127μScm−1 and generated hydroxyl ions sufficient to increase the slurry pH by one unit. We compared this to C mobilized from the slurries during a 2-h oxic incubation across a similar range of pH and ionic strength and found smaller amounts of dissolved (<2.3nm) and colloidal (2.3–430nm) C were mobilized relative to the Fe reduction treatments (p<0.05). In particular, C associated with the largest particles (60–430nm) was dispersed almost exclusively during the Fe reduction experiments, suggesting that it had been bound to Fe-oxide phases. Our experiments suggest that colloidal dispersion during Fe-reducing conditions mobilizes high concentrations of C, which may explain how C migrates to deep mineral horizons in redox dynamic soils.

Modeling the transport of freshwater and dissolved organic carbon in the Neuse River Estuary, NC, USA following Hurricane Irene (2011)

Numerical models are useful tools that aid in understanding complex flows and the distribution of suspended material over large geographic areas and during extreme weather events. Here we describe the use of a three-dimensional numerical model (Delft3D) to simulate freshwater and dissolved organic carbon (DOC) transport over a 3-week period, following intense precipitation that led to high river discharge into the brackish Neuse River Estuary (NRE), NC, from Hurricane Irene (Aug. 2011). The model was calibrated and validated using field measurements of water level elevations, vertical salinity profiles, and surface DOC concentrations in the estuary. DOC was simulated as a conservative tracer over the study period. Model results indicate differences in the intensity of the freshwater and DOC-laden plumes as they propagated along estuary due to a one week time lag between the maximum discharge of 540 m3 s−1 and maximum DOC concentration of 29.85 mg L−1 entering the NRE from the river. In the upper estuary, the surface DOC concentration increased by 18 mg L−1 above the pre-storm value of 7 mg L−1; the maximum concentration occurred 10 days after the passage of the storm. In the lower estuary, the outer edge of the DOC plume reached Pamlico Sound after 3 weeks with a surface DOC concentration that was 3 mg L−1 above the pre-storm value. Results also indicate cross-channel salinity differences up to 10 ppt and DOC concentration differences up to 15 mg L−1 in the upper NRE to due to wind-driven motion of the estuary. The methods described here could be applied to other coastal plain estuarine systems to simulate and characterize flow rates and DOC transport during and succeeding storm events where field measurements are often limited.

Sources and transport of organic carbon from the Dongjiang River to the Humen outlet of the Pearl River, southern China

Transport of organic carbon via rivers to estuary is a significant geochemical process in the global carbon cycle. This paper presents bulk total organic carbon (TOC) from the Dongjiang catchment to the adjacent Humen outlet, and discusses the applicability of δ 13C and ratio of carbon to nitrogen (C/N) as indicators for sources of organic matter in the surface sediments. Survey results showed that organic carbon concentration in summer were higher than in the winter. An elevated trend of TOC occurred along the river to the Humen outlet in both surveys, and the highest mean values of dissolved and particular organic carbon (DOC∼279 μmol L−1 and POC∼163 μmol L−1) were observed in the urban deltaic region in summer flood flow. Winter samples had a wide range of δ 13C and C/N (δ13C −24.6‰ to −30.0‰, C/N 4–13), and summer ones varied slightly (δ13C −24.2‰ to −27.6‰, C/N 6–18). As results suggest that POC in the three zones of upstream-delta-outlet dominantly came from riverbank soil, phytoplankton and agricultural C3 plants in winter, whereas main sources were from the riverbank and mangrove soil in summer. Moreover, anthropogenic sewage inputs had 11% and 7% contribution to POC in the deltaic and outlet. Transport accompanied with seasonal freshwater variation, phytoplankton production and degradation, and removal behavior caused variation of organic carbon concentration. The results also discovered that TOC export bulk in Dongjiang was approximately one quarter of Humen flux in the dry flow, and anthropogenic activity significantly impacted the river export contribution.

Microbial biomarkers support organic carbon transport from methane-rich Amazon wetlands to the shelf and deep sea fan during recent and glacial climate conditions

We have investigated the delivery of terrestrial organic carbon (OC) to the Amazon shelf and deep sea fan based on soil marker bacteriohopanepolyols (BHPs; adenosylhopane and related compounds) and branched glycerol dialkyl glycerol tetraethers (GDGTs), as well as on 14C dating of bulk organic matter. The microbial biomarker records show persistent burial of terrestrial OC, evidenced by almost constant and high BIT values (0.6) and soil marker BHP concentration [80–230μg/g TOC (total OC)] on the late Holocene shelf and even higher BIT values (0.8–0.9), but lower and more variable soil-marker BHP concentration (40–100μg/g TOC), on the past glacial deep sea fan. Radiocarbon data show that OC on the shelf is 3–4kyr older than corresponding bivalve shells, emphasizing the presence of old carbon in this setting.We observe comparable and unexpectedly invariant BHP composition in both marine sediment records, with a remarkably high relative abundance of C-35 amino BHPs including compounds specific for aerobic methane oxidation on the shelf (avg. 50% of all BHPs) and the fan (avg. 40%). Notably, these marine BHP signatures are strikingly similar to those of a methane-producing floodplain area in one of the Amazonian wetland (várzea) regions. The observation indicates that BHPs in the marine sediments may have initially been produced within wetland regions of the Amazon basin and may therefore document persistent export from terrestrial wetland regions, with subsequent re-working in the marine environment, both during recent and past glacial climate conditions.

A framework for assessing the role of karst conduit morphology, hydrology, and evolution in the transport and storage of carbon and associated sediments

Karst aquifers and conduits form by dissolution of carbonate minerals and the slow release of inorganic carbon to the surface environment. As conduits evolve in size, morphology, and position within the aquifer, their function and capacity change relative to the storage and transport of inorganic and organic carbon as sediment. Conduits serve mostly as transport mechanisms in relation to sediments. Quantified data are sparse, but for conduits to function effectively there must be at least equilibrium in the amount of sediment entering and exiting the aquifer. If sediment discharge exceeds input, little sediment will remain underground. When natural declines in base level cease removing sediments and only deposit calcite speleothems, these materials are stored until the rock mass is denuded. While sediment storage is mostly transient in hydrologically active conduits, relative differences occur. Aquifers with conduits developed at multiple levels or as floodwater mazes store proportionately greater volumes of sediment. Hypogenic systems should store greater volumes of sediment than epigenic aquifers because they mostly discharge a dissolved load as opposed to both dissolved and suspended clastic loads. However, some hypogenic aquifers are diffusely recharged and receive and store little sediment from the surface. The global volume of sediment and organic carbon stored in karst aquifers is estimated in this study to be on the order of 2x104 km3and 2x102 km3, respectively. The amount of organic carbon stored in paleokarst is not estimated, but available data indicate it is substantially greater than that stored in modern karst aquifers. Development of such data may suggest that paleokarst petroleum reservoirs might serve as efficient carbon sinks for global carbon sequestration. Hydrocarbon-depleted paleokarst reservoirs should provide substantially more storage per injection well than sequestration in non-paleokarstic rocks.Keywords: Sediment, carbon, paleokarst, conduit morphology, karst hydrology, sequestration.DOI: 10.3986/ac.v42i2-3.662

Root and dissolved organic carbon controls on subsurface soil carbon dynamics: A model approach

Although subsurface horizons store more than half the total soil organic carbon (SOC) globally, the sources and dynamics of subsurface SOC are still unknown. Root litter input and dissolved organic carbon (DOC) transport are possible sources. Using a vertically extended soil C model, we explore the role of root C input and DOC transport in controlling subsurface SOC dynamics. The model involves organic matter decomposition and DOC leaching in the aboveground litter layer, the belowground input of C from roots, and SOC turnover and DOC transport along water flows throughout the soil profile for three SOC pools (active, slow, and passive — characterized by a turnover time of years, decades, and millennia, respectively). Model simulations with a range of rooting profiles demonstrate that a large proportion (36% – 78% — greater in deeper rooting profiles) of SOC is apportioned to the subsurface horizons (below the top 30 cm). A significant part (39% – 73%) of subsurface SOC was found to be associated with C pools that turn over on time scales of decades or less. DOC transport appeared to be dominant in distributing the added C to the deeper soil layers, making the SOC content profile deeper than that of the root litter (C) input. The results suggest that current soil C studies focusing on the surface alone, significantly underestimate the stock of decadally cycling C, especially for deeply rooted ecosystems. Studies ignoring subsurface C dynamics therefore underpredict the responses of soil C to changes in climate, land use, and vegetation.

Impact of Hydropower Regulation on River Water Composition in Northern Sweden

Using hydrogeochemical analysis of two large boreal rivers (pristine Kalix and hydropower regulated Lule) discharging into the Gulf of Bothnia, the major impacts of regulation on water discharge, element transport and their seasonal redistribution have been assessed. The pre-regulation hydrogeochemical features were assumed to be similar for the two rivers. For the Lule River, the average maximum runoff was almost halved, while the average minimum was tripled as a result of the regulation. The fraction of winter transport of total organic carbon (TOC), Fe, Si, suspended Mn and P in the Lule River was, according to a conservative estimate, two to three times higher than in the pristine river. Longer residence time in the Lule River delayed arrival of the suspended Mn peak and dissolved Si decline to the river mouth. During summer, the suspended C/N ratio in the regulated river was 10–20 compared to <10 for the pristine, suggesting presence of predominantly old organic material. This was supported by a virtually constant suspended P/Fe ratio throughout the year in the Lule River, indicating low abundance of phytoplankton. TOC varied irregularly in the Lule River suggesting temporal disconnection between the river and the upper riparian zone. The disappearance of the spring flow maximum, a shift of element transport from spring to winter and supply of mainly old organic material during the vegetation growth season may have a pronounced impact on the ecosystem of the Gulf of Bothnia and the river itself.

Comparison of vertical transport of 137Cs and organic carbon in agricultural cracking soils

The movement of soil organic carbon (SOC) down the soil profile has been compared to that of the environmental tracer 137Cs at two neighbouring field sites sharing the same land management history but comprising clay soils with different cracking characteristics (cracking black Vertisol and a red Luvisol). A finite element model (FEM) simulation of the vertical transport of SOC and 137Cs was developed for each site which accommodates the differing spatial and temporal trends of input and decay of the two species. From these models the diffusion and convection coefficients which best describe the movement of 137Cs at each site were determined. Both convection and diffusion coefficients were found to be substantially higher in cracking Vertisol soils (DCs=721mm2/yr, VCs=−0.84mm/yr) than in the neighbouring Luvisol soils (DCs=94mm2/yr, VCs=0mm/yr). Finally the 137Cs transport coefficients determined for each site were used in modelling the SOC profile. The excellent match between predicted and observed SOC profiles suggests that transport of the SOC and 137Cs down the soil column at the Luvisol site follows the same pathways. While the match between predicted and observed SOC profiles at the Vertisol site was weaker this was concluded to be more likely due to the impact of extensive soil cracking which is not explicitly accounted for in the SOC FEM rather than the result of the use of 137Cs transport coefficients.

Spatial and seasonal variability of organic carbon transport in the Yellow River, China

Summary In this study, we examined the spatial and seasonal variability in the concentrations of dissolved and particulate organic carbon (DOC and POC) of the Yellow River. Weekly samples of water and suspended solids were collected along the main stem channel between July 2011 and July 2012 for the upstream Toudaoguai and Tongguan stations, and between August 2008 and July 2012 for the downstream Lijin station near the river mouth. The DOC export at the upstream two stations was primarily controlled by hydrological events such as melting of ice and snow with high DOC concentrations occurring in spring. In contrast, it was more affected by human activities, mainly reservoir regulation, at the lowermost Lijin station. Lower DOC concentration in the wet season indicates that most of the leachable DOC in surface soils may have largely been flushed away by spring floods. In addition, it is also likely due to dilution effect of the rapidly increased water discharge. As a result of low organic carbon content in the parent soils, the Yellow River sediments were characterized by low POC content (POC%). The averaged POC% at Toudaoguai, Tongguan, and Lijin was 0.48%, 0.47%, and 0.37%, respectively, which is significantly lower than the global mean of around 0.95%. The POC% decreased exponentially with total suspended solids (TSS) concentration. This is likely due to the dilution of riverine POC, because high TSS generally means a higher proportion of coarse sediments that have more mineral matter. During the study period, the total DOC and POC fluxes into the ocean were estimated at 0.06 × 10 12 g/yr and 0.41 × 10 12 g/yr, respectively. Combining our previous estimate of dissolved inorganic carbon (DIC) export shows that the Yellow River transports a carbon flux of 1.52 × 10 12 g/yr into the Bohai Sea, accounting for about 0.19% of the global total riverine carbon flux (DOC + POC + DIC). The extremely low DOC/POC ratio represents the lowest level among major world rivers, which is consistent with its intense soil erosion and highlights the effect of soil erosion on organic carbon export.

Improvement of soil carbon sink by cover crops in olive orchards under semiarid conditions. Influence of the type of soil and weed

The olive tree is one of the most important crops in Spain, and the main one in the region of Andalusia. Most orchards are rain-fed, with high slopes where conventional tillage (CT) is the primary soil management system used. These conditions lead to high erosion and a significant transport of organic carbon (OC). Moreover, soil tillage accelerates the oxidation of the OC. Cover crops (CC) are the conservation agriculture (CA) approach for woody crops. They are grown in-between tree rows to protect the soil against water erosion and their organic residues also help to increase the soil carbon (C) sink. Soil and OC losses associated to the sediment were measured over four seasons (2003-07) using micro-plots for the collection of runoff and sediment in five experimental fields located in rain-fed olive orchards in Andalusia. Two soil management systems were followed, CC and CT. Furthermore, the changes in soil C in both systems were analyzed at a depth of 0-25 cm. CC reduced erosion by 80.5%, and also OC transport by 67.7%. In addition, Cover crops increased soil C sink by 12.3 Mg ha-1 year-1 of carbon dioxide (CO2) equivalent, with respect to CT. CC in rainfed olive orchards in a Mediterranean climate could be an environmental friendly and profitable system for reducing erosion and increasing the soil C sink. However, C fixing rate is not regular, being very high for the initial years after shifting from CT to CC and gradually decreasing over time.