The sources and seasonal fluxes of particulate organic carbon in the Yellow River

The Yellow River transports a large amount of sediment and particulate organic carbon (POC), which is thought to mainly derive from erosion of the Chinese Loess Plateau (CLP). However, the compositions, sources and erosional fluxes of POC in the Yellow River remain poorly constrained. Here we combined measurements of mineralogy, total organic carbon content (OC_total), stable organic carbon isotopes (δ¹³C_org), radiocarbon (¹⁴C) activity of organic matter in bulk suspended sediments collected seasonally from the upper and middle Yellow River, to quantify the compositions and fluxes of the POC and to assess its sources (biospheric and petrogenic POC, i.e. POC_bio and POC_petro, respectively). The results showed that the POC loading of sediments was controlled by mineralogy, grain size and specific surface area of loess particles. The F_mod of POC (0.71 to 0.31) can be explained by mixing of POC_petro with modern and aged POC_bio. A binary mixing model based on the hyperbolic relationship of the F_mod and OC_total revealed a wide range of ages of POC_bio from 1300 to 11100 ¹⁴C years. Relative to the upstream station, the annual POC_bio and POC_petro fluxes in the Yellow River are more than doubled after it flows crossing the CLP within 35% drainage area gain, resulting in POC_bio and POC_petro yields of the CLP at 3.50 ± 0.59 and 0.48 ± 0.49 tC/km²/yr, respectively. POC flux seasonal variation revealed that monsoon rainfall exerts a first-order control on the export of both POC_bio and POC_petro from the CLP to the Yellow River, resulting in more than 90% of the annual POC exported during the monsoon season. Around one third of annual POC erosional flux was transported during a storm event period, highlighting the important role of extreme events in POC export in this large river. © 2020 John Wiley & Sons, Ltd.     

Organic carbon fluxes from the upper Yangtze basin: an example of the Longchuanjiang River,China

To investigate the effects of anthropogenic activity, namely, land use change and reservoir construction, on particulate organic carbon (POC) transport, we collected monthly water samples during September 2007 to August 2009 from the Longchuanjiang River to understand seasonal variations in the concentrations of organic carbon species and their sources and the yield of organic and inorganic carbon from the catchment in the Upper Yangtze basin. The contents of riverine POC, total organic carbon and total suspended sediment (TSS) changed synchronously with water discharge, whereas the contents of dissolved organic carbon had a small variation. The POC concentration in the suspended sediment decreased non‐linearly with increasing TSS concentration. Higher molar C/N ratio of particulate organic matter (average 77) revealed that POC was dominated by terrestrially derived organic matter in the high flows and urban wastewaters in the low flows. The TSS transported by this river was 2.7 × 10⁵ t/yr in 2008. The specific fluxes of total organic carbon and dissolved inorganic carbon (DIC) were 5.6 and 6 t/km²/yr, respectively, with more than 90% in the high flow period. A high carbon yield in the catchment of the upper Yangtze was due to human‐induced land use alterations and urban wastes. Consistent with most rivers in the monsoon climate regions, the dissolved organic carbon–POC ratio of the export flux was low (0.41). Twenty‐two percent (0.9 t/km²/yr) of POC out of 4 t/km²/yr was from autochthonous production and 78% (3.1 t/km²/yr) from allochthonous production. The annual sediment load and hence the organic carbon flux have been affected by environmental alterations of physical, chemical and hydrological conditions in the past 50 years, demonstrating the impacts of human disturbances on the global and local carbon cycling. Finally, we addressed that organic carbon flux should be reassessed using adequate samples (i.e. at least two times in low‐flow month, four times in high‐flow month and one time per day during the flood period), daily water discharge and sediment loads and appropriate estimate method. Copyright © 2011 John Wiley & Sons, Ltd.     

Towards constraining the magnitude of global agricultural sediment and soil organic carbon fluxes

Reliable quantitative data on the extent and rates of soil erosion are needed to understand the global significance of soil‐erosion induced carbon exchange and to underpin the development of science‐based mitigation strategies, but large uncertainties remain. Existing estimates of agricultural soil and soil organic carbon (SOC) erosion are very divergent and span two orders of magnitude. The main objective of this study was to test the assumptions underlying existing assessments and to reduce the uncertainty associated with global estimates of agricultural soil and SOC erosion. We parameterized a simplified erosion model driven by coarse global databases using an empirical database that covers the conterminous USA. The good agreement between our model results and empirical estimates indicate that the approach presented here captures the essence of agricultural erosion at the scales of continents and that it may be used to predict the significance of erosion for the global carbon cycle and its impact on soil functions. We obtained a global soil erosion rate of 10.5 Mg ha^‐1 y^‐1 for cropland and 1.7 Mg ha^‐1 y^‐1 for pastures. This corresponds to SOC erosion rates of 193 kg C ha^‐1 y^‐1 for cropland and 40.4 kg C ha^‐1 y^‐1 for eroding pastures and results in a global flux of 20.5 (±10.3) Pg y^‐1 of soil and 403.5 (±201.8) Tg C y^‐1. Although it is difficult to accurately assess the uncertainty associated with our estimates of global agricultural erosion, mainly due to the lack of model testing in (sub‐)tropical regions, our estimates are significantly lower than former assessments based on the extrapolation of plot experiments or global application of erosion models. Our approach has the potential to quantify the rate and spatial signature of the erosion‐induced disturbance at continental and global scales: by linking our model with a global soil profile database, we estimated soil profile modifications induced by agriculture. This showed that erosion‐induced changes in topsoil SOC content are significant at a global scale (an average SOC loss of 22% in 50 years) and agricultural soils should therefore be considered as dynamic systems that can change rapidly. Copyright © 2011 John Wiley & Sons, Ltd.     

The fluvial flux of particulate organic matter from the UK: the emission factor of soil erosion

Soil erosion has been identified as a potential global carbon sink since eroded organic matter is replaced at source and eroded material is readily buried. However, this argument has relied on poor estimates of the total fate of in‐transit particulates and could erroneously imply soil erosion could be encouraged to generate carbon stores. These previous estimates have not considered that organic matter can also be released to the atmosphere as a range of greenhouse gases, not only carbon dioxide (CO₂), but also the more powerful greenhouse gases methane (CH₄) and nitrous oxide (N₂O). As soil carbon lost by erosion is only replaced by uptake of CO₂, this could represent a considerable imbalance in greenhouse gas warming potential, even if it is not significant in terms of overall carbon flux. This work therefore considers the flux of particulate organic matter through UK rivers with respect to both carbon fluxes and greenhouse gas emissions. The results show that, although emissions to the atmosphere are dominated by CO₂, there are also considerable fluxes of CH₄ and N₂O. The results suggest that soil erosion is a net source of greenhouse gases with median emission factors of 5.5, 4.4 and 0.3 tonnes CO_2eq/yr for one tonne of fluvial carbon, gross carbon erosion and gross soil erosion, respectively. This study concludes that gross soil erosion would therefore only be a net sink of both carbon and greenhouse gases if all the following criteria are met: the gross soil erosion rate were very low (<91 tonnes/km²/yr); the eroded carbon were completely replaced by new soil organic matter; and if less than half of the gross erosion made it into the stream network. By establishing the emission factor for soil erosion, it becomes possible to properly account for the benefits of good soil management in minimizing losses of greenhouse gases to the atmosphere as a by‐product of soil erosion. Copyright © 2015 John Wiley & Sons, Ltd.     

The relationship between dissolved organic carbon in stream water and soil organic carbon pools at different spatial scales

The relationship between stream water DOC concentrations and soil organic C pools was investigated at a range of spatial scales in subcatchments of the River Dee system in north‐east Scotland. Catchment percentage peat cover and soil C pools, calculated using local, national and international soils databases, were related to mean DOC concentrations in streams draining small‐ (<5 km²), medium‐ (12–38 km²) and large‐scale (56–150 km²) catchments. The results show that, whilst soil C pool is a good predictor of stream water DOC concentration at all three scales, the strongest relationships were found in the small‐scale catchments. In addition, in both the small‐ and large‐scale catchments, percentage peat cover was as a good predictor of stream water DOC concentration as catchment soil C pool. The data also showed that, for a given soil C pool, streams draining lowland (<700 m) catchments had higher DOC concentrations than those draining upland (>700 m) catchments, suggesting that disturbance and land use may have a small effect on DOC concentration. Our results therefore suggest that the relationship between stream water DOC concentration and catchment soil C pools exists at a range of spatial scales and this relationship appears to be sufficiently robust to be used to predict the effects of changes in catchment soil C storage on stream water DOC concentration. Copyright © 1999 John Wiley & Sons, Ltd.     

Proposed methods for potential evapotranspiration calculation of the Red River basin (North Vietnam)

Potential evapotranspiration (ETP) is an important part of a climatic water balance and a crucial variable in many kinds of models in computing actual evapotranspiration. The objective of this study was to find a reasonable approach of ETP calculation for a height‐differentiated landscape in subtropical climate. From the pool of diverse approaches, six common methods [Hamon, Priestley–Taylor, Thornthwaite, Blaney–Criddle, Turc and Food and Agricultural Organization Irrigation and Drainage Paper No. 56 (FAO‐56)] were selected. With the meteorological data for long‐term period (1964–2008), the calculation of ETP values was performed for 12 different meteorological stations in the Red River basin. Among the applied ETP calculation methods, the Turc and the FAO‐56 methods agreed well at most stations and represent best the expectations for the ETP values of the Thao and Da subbasins. The findings of our investigations indicate that in highly structured (land use and elevation) regions, not all methods provide satisfying results. Copyright © 2011 John Wiley & Sons, Ltd.     

The freshwater dissolved organic matter fluorescence–total organic carbon relationship

The fluorescent properties of dissolved organic matter (DOM) enable comparisons of humic‐like (H‐L) and fulvic‐like (F‐L) fluorescence intensities with dissolved organic carbon (DOC) in aquatic systems. The fluorescence‐DOC relationship differed in gradient, i.e. the fluorescence per gram of carbon, and in the strength of the correlation coefficient. We compare the fluorescence intensity of the F‐L and H‐L fractions and DOC of freshwater DOM in north Shropshire, England, featuring a river, wetland, spring, pond and sewage DOM sources. Correlations between fluorescence and DOC varied between sample sites. Wetland water samples for the F‐L peak gave the best correlation, r = 0·756; the lowest correlation was from final treated sewage effluent, r = 0·167. The relationship between fluorescence and DOC of commercially available International Humic Substances Society standards were also examined and they generally showed a lower fluorescence per gram of carbon for the F‐L peak than the natural samples, whereas peat wetland DOM gave a greater fluorescence per gram of carbon than river DOM. Here, we propose the strength of the fluorescence–DOC correlation to be a useful tool when discriminating sources of DOM in fresh water. Copyright © 2007 John Wiley & Sons, Ltd.     

Effect of human‐controlled hydrological regime on the source,transport, and flux of particulate organic carbon from the lower Huanghe (Yellow River)

Evaluating the role of fluvial transfer of terrestrial organic carbon (OC) and subsequent burial in the global carbon cycle requires the sources and fluxes of fluvial OC to be assessed, which remains poorly constrained in the Huanghe (Yellow River). Here, we report the elemental, stable isotopic, and radiocarbon activity of particulate organic carbon (POC) sampled at the outlet of Huanghe in 2012–2013. We show that the Huanghe riverine POC can be explained by binary mixing of fossil (POC_fossil) and non‐fossil (POC_non‐fossil) components, the former may reach ~40% of the total POC. The Huanghe POC_non‐fossil is mostly sourced from C3 plants, with a mean residence time of c. 2200 years. The current human‐controlled hydrological regime strongly influenced the POC sources, transport modes, and fluxes. In 2012–2013, the Huanghe delivered 0.73 Tg (1 Tg = 10¹² g) of POC to the sea, and about 28% of the annual POC flux occurred within a short human induced flood event. Globally, the Huanghe should be one of the largest rivers in the transfer and re‐burial of fossil OC. However, the fate of Huanghe fossil OC is still unconstrained and needs to be further investigated. Copyright © 2015 John Wiley & Sons, Ltd.     

Elemental and isotopic signatures of particulate organic carbon in the Zengjiang River,southern China

Riverine total suspended sediment (TSS) at the lower reach section of the Zengjiang River, a low‐turbidity river in the southern China, was sampled on a 4‐week basis from March 2002 to February 2003. The gross TSS was divided into sedimentary and suspended fractions (SED and SUS) by the sedimentation method. Organic carbon and nitrogen, ¹⁴C and ¹³C were analysed using an elemental analyser and accelerator mass spectrometer respectively. The results show that particulate organic carbon (POC) yield is 0·8 × 10⁶ g km^?2 year^?1 in the Zengjiang River drainage basin, which is about one‐tenth of that in the Zhujiang (Pearl) River drainage basin. The C/N ratio demonstrates that aquatic biomass is the major contributor to POC in the Zengjiang River. The average share of aquatic biomass in the SUS‐fraction POC and SED‐fraction POC is about 88·89% and 62·76% respectively, with a substantial seasonal variation. δ¹³C values of SUS‐fraction POC (?26·56 to ? 22·89‰) is slightly lighter than that of SED‐fraction POC (?25·05 to ? 22·20‰), indicating that the contribution of aquatic biomass to δ¹³C values is more pronounced in the SUS‐fraction POC than in the SED‐fraction POC. The ‘bomb’–¹⁴C signature is not detected in the POC of Zengjiang River, and the contribution from geological organic carbon is very little. Δ¹⁴C values of the SED‐fraction POC vary from ? 44 to ? 223‰, and the Δ¹⁴C values of the SUS‐fraction POC vary from ? 33 to ? 165‰. For most paired samples, the SED‐fraction POC is generally more depleted in ¹⁴C than that of its counterpart SUS‐fraction POC. Compared with other small mountainous rivers, the ¹⁴C enrichment of POC in the Zengjiang River indicates slight drainage basin erosion. Copyright © 2006 John Wiley & Sons, Ltd.     

Fluvial transport of suspended sediment and organic carbon during flood events in a large agricultural catchment in southwest France

Water draining from a large agricultural catchment of 1 110 km² in southwest France was sampled over an 18‐month period to determine the temporal variability in suspended sediment (SS) and dissolved (DOC) and particulate organic carbon (POC) transport during flood events, with quantification of fluxes and controlling factors, and to analyze the relationships between discharge and SS, DOC and POC. A total of 15 flood events were analyzed, providing extensive data on SS, POC and DOC during floods. There was high variability in SS, POC and DOC transport during different seasonal floods, with SS varying by event from 513 to 41 750 t; POC from 12 to 748 t and DOC from 9 to 218 t. Overall, 76 and 62% of total fluxes of POC and DOC occurred within 22% of the study period. POC and DOC export from the Save catchment amounted to 3090 t and 1240 t, equivalent to 1·8 t km^?2 y^?1 and 0·7 t km^?2 y^?1, respectively. Statistical analyses showed that total precipitation, flood discharge and total water yield were the major factors controlling SS, POC and DOC transport from the catchment. The relationships between SS, POC and DOC and discharge over temporal flood events resulted in different hysteresis patterns, which were used to deduce dissolved and particulate origins. In both clockwise and anticlockwise hysteresis, POC mainly followed the same patterns as discharge and SS. The DOC‐discharge relationship was mainly characterized by alternating clockwise and anticlockwise hysteresis due to dilution effects of water originating from different sources in the whole catchment. Copyright © 2011 John Wiley & Sons, Ltd.     

Controls on fluvial carbon efflux from eroding peatland catchments

Global peatlands store an unparalleled proportion of total global organic carbon but it is vulnerable to erosion into fluvial systems. Fluvial networks are being recognized as areas of carbon transformation, with eroded particulate organic carbon processed to dissolved organic carbon and CO₂. Existing studies indicate biodegradation and photodegradation as key processes controlling the transformation of organic carbon in fluvial systems, with initial concentrations of dissolved organic carbon (DOC) identified as a control on the rate of carbon mineralization. This study manipulates temperature and incident light intensity to investigate carbon mineralization rates in laboratory simulations of peatland sediment transport into fluvial systems. By directly measuring gaseous CO₂ emissions from sampled stream water, the relationship of temperature and light intensity with carbon efflux is identified. In simulations where sediment (as particulate organic matter, POM) is absent, temperature is consistently the dominant factor influencing carbon efflux rates. This influence is independent of the initial DOC concentration of the water sample. In simulations where POM was added, representing a peatland river receiving eroded terrestrial sediment, initial DOC concentration predicts 79% of the variation in total gaseous carbon efflux whereas temperature and light intensity predict 12% and 3%, respectively. When sampled stream water's mineralization rates in the presence of added POM are analysed independently, removing DOC as a model variable, the dominant variable affecting CO₂ efflux is opposite for each sample. This study presents novel data suggesting peatland erosion introduces further complexity to dynamic stream systems where rates of carbon transformation processes and the influence of specific environmental variables are interdependent. Anthropogenic climate change is identified as a leading risk factor perpetuating peatland erosion; therefore, understanding the fate of terrestrial sediment in rivers and further quantifying the benefits of protecting peatland soils will be of increasing importance to carbon budgeting and ecosystem function studies.     

Factors structuring phytoplankton community in a large tropical river: Case study in the Red River (Vietnam)

Algal assemblages have been widely used as an ecological indicator of aquatic ecosystem health conditions because of their specific sensitivity to a wide variety of environmental conditions. In turbid rivers, as in other aquatic systems, phytoplankton structure plays an important role in structuring aquatic food webs. Worldwide, phytoplankton is less studied in turbid, large tropical rivers compared to temperate river systems. The present study aimed to describe the phytoplankton diversity and abundance in a turbid tropical river (the Red River, northern part of Vietnam from 20°00 to 25°30 North; from 100°00 to 107°10 East) and to determine the importance of a series of environmental variables in controlling the phytoplankton community composition. Phytoplankton community was composed of 169 phytoplankton taxa from six algal groups including Bacillariophyceae, Chlorophyceae, Cryptophyceae, Euglenophyceae, Dinophyceae and Cyanobacteria. Community composition varied both spatially and with season. Sixteen measurement environmental variables were used as input variables for a three-layer backpropagation neural network that was developed to predict the phytoplankton abundance. Phytoplankton abundance was successfully predicted using the tagsig transfer function and the Levenberg-Marquardt backpropagation algorithm. The network was trained to provide a good overall linear fit to the total data set with a slope (R) and mean square error (MSE) of 0.808 and 0.0107, respectively. The sensitivity analysis and neutral interpretation diagram revealed that total phosphorus (TP), flow discharge, water temperature and P-PO₄³⁻ were the significant variables. The results showed that the developed ANN model was able to simulate phytoplankton abundance in the Red River. These findings can help for gaining insight into and the relationship between phytoplankton and environmental factors in this complex, turbid, tropical river.     

Dissolved organic carbon export from a cutover and restored peatland

High demand for horticultural peat has increased peatland drainage and peat extraction in Canada. The hydrology and carbon cycling of these cutover peatlands is greatly altered, necessitating active restoration efforts to permit the regeneration of Sphagnum mosses and the re‐establishment of natural peatland function. The effect of peatland extraction and restoration on the export of dissolved organic carbon (DOC) was examined for three successive seasons (May to October, 1999 to 2001) at two different sites (cutover and restored) in eastern Québec. A shift towards higher DOC concentrations was observed following peatland extraction (maximum: 182·6 mg L^?1) and concentrations remained high post‐restoration (maximum: 191·0 mg L^?1). The cutover site exported more DOC than the restored site in all three study seasons. The highest exports occurred during the wettest year (1999), with cutover and restored site export of 10·3 and 4·8 g m^?2, respectively. In 2000, 8·5 g C m^?2 was released from the cutover site, while the restored site released less than half that amount (3·4 g C m^?2). In 2001, the restored site released about the same amount of DOC as in the previous year (3·5 g C m^?2), while the cutover site load dropped to 6·2 g C m^?2. Both sites were net exporters of DOC in all years. Copyright © 2007 John Wiley & Sons, Ltd.     

Response of soil aggregate disintegration to the different content of organic carbon and its fractions during splash erosion

Aggregate disintegration is a critical process in soil splash erosion. However, the effect of soil organic carbon (SOC) and its fractions on soil aggregates disintegration is still not clear. In this study, five soils with similar clay contents and different contents of SOC have been used. The effects of slaking and mechanical striking on splash erosion were distinguished by using deionized water and 95% ethanol as raindrops. The simulated rainfall experiments were carried out in four heights (0.5, 1.0, 1.5 and 2.0 m). The result indicated that the soil aggregate stability increased with the increases of SOC and light fraction organic carbon (LFOC). The relative slaking and the mechanical striking index increased with the decreases of SOC and LFOC. The reduction of macroaggregates in eroded soil gradually decreased with the increase of SOC and LFOC, especially in alcohol test. The amount of macroaggregates (>0.25 mm) in deionized water tests were significantly less than that in alcohol tests under the same rainfall heights. The contribution of slaking to splash erosion increased with the decrease of heavy fractions organic carbon. The contribution of mechanical striking was dominant when the rainfall kinetic energy increased to a range of threshold between 9 J m⁻² mm⁻¹ and 12 m⁻² mm⁻¹. This study could provide the scientific basis for deeply understanding the mechanism of soil aggregates disintegration and splash erosion.     

Carbon dioxide fluxes and DOC concentrations of eroding blanket peat gullies

UK peatlands are affected by severe gully erosion with consequent impacts on ecosystem services from these areas. Incision into the peat can damage the vegetation and hydrology and lead to increases in carbon loss and sediment transfer downstream. Gullies represent then a conduit for and a hotspot of carbon loss but the relatively high water tables of gullies have meant that they have been identified as areas with a high restoration potential because of easily restored peat‐forming conditions. This study uses a series of gully sites, subject to different restoration interventions, to investigate differences in carbon pathways (DOC, CO₂) and hydrology between restoration strategies and gully position. The results show that the position within the gully (interfluve, gully side, or gully floor) does not significantly affect water quality but that it plays a significant role in CO₂ exchange. Gully floors are areas of high photosynthesis and ecosystem respiration, though net ecosystem exchange is not significantly different across the gully. While gully position plays a role in the cycling of some carbon species, this study highlights the importance of vegetation as a key control on carbon cycling. Copyright © 2012 John Wiley & Sons, Ltd.     

Evaluating the impact of soil redistribution on the in situ mineralization of soil organic carbon

Soil erosion, transport and deposition by water drastically affect the distribution of soil organic carbon (SOC) within a landscape. Moreover, soil redistribution may have a large impact on the exchange of carbon (C) between the pedosphere and the atmosphere. One of the large information gaps within this research domain, concerns the fate of SOC after erosion by water. According to different (mainly laboratory) studies, soil redistribution leads to aggregate breakdown, thereby exposing the contained SOC to mineralization. Our study aims to quantify the extent to which such increased mineralization occurs in a real field situation. Carbon dioxide (CO₂)‐efflux was measured in the field after an important erosion event for a continuous period of 112 days. The specific situation on the field ensured that almost none of eroded SOC was exported from the field. Measurements of CO₂‐efflux were done in areas with sediment deposition, as well as in comparable areas without sedimentation. Comparison of these measurements allowed the net effect of soil deposition on CO₂‐efflux to be assessed. Field data were complemented by measurements on incubated, undisturbed soil core samples, in order to disentangle the contribution of environmental factors (moisture, temperature) from any erosional effect on CO₂‐efflux. Results of these measurements on the field showed that CO₂‐efflux was regulated by a complex interplay of different factors (mostly soil porosity, soil moisture and soil temperature). In combination with the incubation measurements, it could be concluded that the processes of erosion and transport indeed led to an increased mineralization of SOC, as a result of aggregate breakdown and exposure of previously encapsulated SOC. This effect was, however, much smaller than observed in previous laboratory studies. Moreover, it was only important in the first weeks, immediately after the erosion event. The calculated net erosional effect on CO₂‐efflux represented a mere 1·6% of total SOC, originally present in the soil. Copyright © 2010 John Wiley & Sons, Ltd.     

Seasonal and spatial variation in suspended matter,organic carbon,nitrogen, and nutrient concentrations of the Senegal River in West Africa

The Senegal River is of intermediate size accommodating at present about 3.5 million inhabitants in its catchment. Its upstream tributaries flow through different climatic zones from the wet tropics in the source area in Guinea to the dry Sahel region at the border between Senegal and Mauritania. Total suspended matter, particulate and dissolved organic carbon and nitrogen as well as nutrient concentrations were determined during the dry and wet seasons at 19 locations from the up- to downstream river basin. The aims of the study were to evaluate the degree of human interference, to determine the dissolved and particulate river discharges into the coastal sea and to supply data to validate model results. Statistical analyses showed that samples from the wet and dry season are significantly different in composition and that the upstream tributaries differ mainly in their silicate and suspended matter contents. Nutrient concentrations are relatively low in the river basin, indicating low human impact. Increasing nitrate concentrations, however, show the growing agriculture in the irrigated downstream areas. Particulate organic matter is dominated by C4 plants during the wet season and by aquatic plankton during the dry season. The total suspended matter (TSM) discharge at the main gauging station Bakel was about 1.93 Tg yr⁻¹ which is in the range of the only available literature data from the 1980s. The calculated annual discharges of particulate organic carbon (POC), dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) are 55.8 Gg yr⁻¹, 54.1 Gg yr⁻¹, and 5.3 Gg yr⁻¹, respectively. These first estimates from the Senegal River need to be verified by further studies.     

Soil particle size distribution and induced soil carbon transport by ephemeral gully erosion in Mediterranean mountain arable land

In Mediterranean mountain agroecosystems, soil erosion associated with the development of ephemeral gullies is a common environmental problem that contributes to a loss of nutrient-rich topsoil. Little is known about the influence of ephemeral gully erosion on particle size distribution and its effect on soil organic (SOC) and inorganic (SIC) carbon among different sized soil particles in agricultural soils. In this study, laboratory tests were conducted using velocity settling tube experiments to examine the effects of erosion on sediment particle size distributions from an incised ephemeral gully, associated with an extreme event (235 mm). We also consider subsequent deposition on an alluvial fan in order to assess the distribution of SOC and SIC concentrations and dissolved carbon before and after the extreme event. Soil fractionation was carried out on topsoil samples (5 cm) collected along an ephemeral gully in a cultivated field, located in the lower part of a Mediterranean mountain catchment. The results of this study showed that the sediment transported downstream by runoff plays a key role in the particle size distribution and transportability of soil particles and associated carbon distribution in carbonate rich soils. The eroding sediment is enriched in clay and silt-sized particles at upslope positions with higher SOC contents and gradually becomes coarser and enriched in SIC at the end of the ephemeral gully because the finest particles are washed-out of the study field. The extreme event was associated with an accumulation of dissolved organic carbon at the distal part of the depositional fan. Assessment of soil particle distribution using settling velocity experiments provides basic information for a better understanding of soil carbon dynamics in carbonate rich soils. Processes of soil and carbon transport and relationships between soil properties, erodibility and aggregate stability can be helpful in the development of more accurate soil erosion models. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

Seasonal variations in the inorganic carbon system in the Pearl River (Zhujiang) estuary

Seasonal variations in the inorganic carbon system in the Pearl River estuary are examined based on data from five surveys during the spring, summer, fall, and winter seasons. Both total dissolved inorganic carbon (DIC) and total alkalinity (TAlk) values in the freshwater end-members are high in the dry season (>2700 μmol kg⁻¹ for DIC and >2400 μmol kg⁻¹ for TAlk) and substantially lower in the wet season (DIC and TAlk were ∼1000 and 700 μmol kg⁻¹, respectively). Riverine DIC flux and drainage basin weathering rates, however, are significantly higher in the wet season (611×10⁹ mol yr⁻¹ and 13.6×10⁵ mol km⁻² yr⁻¹) than in the dry season (237×10⁹ mol yr⁻¹ and 5.3×10⁵ mol km⁻² yr⁻¹).     

Dissolved organic carbon transport in the Qilian mountainous areas of China

The Dissolved Organic Carbon (DOC) content of rivers is the most significant part of the carbon cycle migration in the basin under consideration, and it is the basis for a comprehensive understanding of the regional carbon cycle. In this study, we periodically collected samples from four monitoring stations in the Xiying River Basin of the Qilian Mountains in the northern Qinghai-Tibet Plateau. We calculated the fluxes of organic carbon in the rivers within the study area and have discussed the influencing factors of DOC concentration in these rivers. The results showed that: (a) The DOC concentration and transport flux of the Xiying river showed significant seasonal changes. The DOC concentration during summer and autumn was higher than that in winter and spring, and the output flux in summer and autumn accounted for approximately 88.3% of the total annual output. (b) Precipitation runoff has a higher DOC concentration than meltwater runoff. Climate factors, river-water chemical characteristics, and seasonal frozen-soil changes in the river basin have significant effects on the river DOC concentration and transport flux. (c) Larger runoff causes higher DOC concentrations in rivers. Runoff is the primary means of carbon migration in the inland river basin. Carbon migration is significant from the upstream to the middle and downstream sections of the inland river basin.