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During land-aquatic transfer, carbon (C) and inorganic nutrients (IN) are transformed in soils, groundwater, and at the groundwater-surface water interface as well as in stream channels and stream sediments. However, processes and factors controlling these transfers and transformations are not well constrained, particularly with respect to land use effect. We compared C and IN concentrations in shallow groundwater and first-order streams of a sandy lowland catchment dominated by two types of land use: pine forest and maize cropland. Contrary to forest groundwater, crop groundwater exhibited oxic conditions all-year round as a result of higher evapotranspiration and better lateral drainage that decreased the water table below the organic-rich soil horizon, prevented the leaching of soil-generated dissolved organic carbon (DOC) in groundwater, and thus limited consumption of dissolved oxygen (O 2 ). In crop groundwater, oxic conditions inhibited denitrification and methanogenesis resulting in high nitrate (NO 3 - ; on average 1140 ± 485 μmol L -1 ) and low methane (CH 4 ; 40 ± 25 nmol L -1 ) concentrations. Conversely, anoxic conditions in forest groundwater led to lower NO 3 - (25 ± 40 μmol L - ) concentrations. The partial pressure of carbon dioxide (pCO 4 (1770 ± 1830 nmol L -1 ) concentrations. The partial pressure of carbon dioxide (pCO 2 ; 30,650 ± 11,590 ppmv) in crop groundwater was significantly lower than in forest groundwater (50,630 ± 26,070 ppmv), and was apparently caused by the deeper water table delaying downward diffusion of soil CO 2 to the water table. In contrast, pCO 2 was not significantly different in crop (4480 ± 2680 ppmv) and forest (4900 ± 4500 ppmv) streams, suggesting faster degassing in forest streams resulting from greater water turbulence. Although NO 3 - concentrations indicated that denitrification occurred in riparian-forest groundwater, crop streams nevertheless exhibited important signs of spring and summer eutrophication such as the development of macrophytes. Stream eutrophication favored development of anaerobic conditions in crop stream sediments, as evidenced by increased ammonia (NH 4 + ) and CH 4 in stream waters and concomitant decreased in NO 3 - concentrations as a result of sediment denitrification. In crop streams, dredging and erosion of streambed sediments during winter sustained high concentration of particulate organic C, NH 4 + and CH 4 . In forest streams, dissolved iron (Fe 2+ ), NH 4 + and CH 4 were negatively correlated with O 2 reflecting the gradual oxygenation of stream water and associated oxidations of Fe 2+ , NH 4 + and CH 4 . The results overall showed that forest groundwater behaved as source of CO 2 and CH 4 to streams, the intensity depending on the hydrological connectivity among soils, groundwater, and streams. CH 4 production was prevented in cropland in soils and groundwater, however crop groundwater acted as a source of CO 2 to streams (but less so than forest groundwater). Conversely, in streams, pCO 2 was not significantly affected by land use while CH 4 production was enhanced by cropland. At the catchment scale, this study found substantial biogeochemical heterogeneity in C and IN concentrations between forest and crop waters, demonstrating the importance of including the full vegetation-groundwater-stream continuum when estimating land-water fluxes of C (and nitrogen) and attempting to understand their spatial and temporal dynamics., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Abstract: Tidal wetlands play a significant role in the coastal carbon cycle and exchange material with the atmosphere and coastal ocean. Here, we report on changes in dissolved inorganic carbon speciation and isotopic composition throughout a 24 h cycle (2 tidal cycles) in Feb. 2007 in a channel connecting the Amazon estuary to the basin of a tidal forest. At this site, tropical forest soils are inundated at high tide by estuarine freshwater, and temporal concentration changes in the channel reflect exchanges between the forest and estuary. Our data show an export of dissolved inorganic carbon (DIC) in the form of excess CO2 and, to a much lesser extent, CH4. However, the tidal forest traps suspended sediments. Mixing plots of DIC versus conductivity showed that the DIC originated from the tidal forest soil, with a negligible contribution from the local watershed. Evolution of the isotopic signature of DIC reveals a 13C-depleted source (−56.9 ± 3.3‰), presumably originating from a dominant methanogenic pathway of carbon mineralization followed by almost complete CH4 oxidation in the organic clay-rich freshwater soil. [Copyright &y& Elsevier]

We measured spatial and temporal variations in carbon concentrations, isotopic compositions and exports during a complete hydrological cycle in nine watercourses draining a lowland forested podzolized catchment, flowing into the Arcachon lagoon (France). In addition, integrated fluxes of CO across the water-atmosphere interface were estimated to assess the relative importance of CO evasion versus lateral carbon transport at the catchment scale. Watercourse similarities and specificities linked to the local catchment characteristics are discussed and compared with other riverine systems. Low concentrations of suspended particulate matter and particulate organic carbon (POC) were generally measured in all the watercourses (8.4 ± 3.4 and 1.6 ± 0.6 mg L, respectively), reflecting limited mechanical soil erosion. The generally high POC content in the suspended matter (20 %), low Chl a concentrations (1.3 ± 1.4 μg L) and the relatively constant δC-POC value (near −28 ‰) throughout the year reveal this POC originates from terrestrial C plant and soil detritus. The presence of podzols leads to high levels of dissolved organic carbon (DOC; 6.6 ± 2.2 mg L). Similarly, high dissolved inorganic carbon (DIC) concentrations were measured in the Arcachon lagoon catchment (5.9 ± 2.2 mg L). The δC-DIC value around −20 ‰ throughout the year in many small watercourses reveals the predominance of terrestrial carbon mineralisation and silicate rock weathering in soils as the major DIC source. With pCO between 1,000 and 10,000 ppmv, all watercourses were a source of CO to the atmosphere, particularly during the low river stage. Organic carbon parameters remained relatively stable throughout the year, whereas DIC parameters showed strong seasonal contrasts closely linked to the hydrological regime and hyporheic flows. In total, the carbon export from the Arcachon watershed was estimated at 15,870 t C year or 6 t C km year, mostly exported to the lagoon as DOC (35 %), DIC (24 %) and lost as CO degassing to the atmosphere (34 %). [ABSTRACT FROM AUTHOR]

A study of the particulate organic carbon (POC) in the estuarine turbidity maxima (ETMs) of the three major French macrotidal estuaries shows that the average contents are 1.5, 3.3 and 3.1% (expressed in % of dry suspended sediment) in the Gironde, Loire and Seine Estuaries, respectively. There is no seasonal variation of POC contents in the Gironde Estuary, whereas, they often increase in the Loire and the Seine Estuaries in spring and summer. The lability of the estuarine particulate organic matter was estimated by two analyses: 1/labile organic matter was measured as the organic carbon loss during incubation tests over one month; 2/ the hydrolysable organic fraction was determined after 6N HCl digestion. The organic fractions of the ETMs are mainly refractory. Any increase in the amount of POC as compared to the background levels (cited above) is always correlated to an increase of organic matter lability. The yearly average fluvial contributions by various particulate organic pools (soil and litter organic matter; organic matter of phytoplanktonic and human origin) that enter the three estuaries were quantified. In the Garonne River, soil and litter are the major (90%) POC sources. In the Loire system, due to the eutrophication of the river water, phytoplankton contributes up to 50% of the total POC load. In the Seine river, soil and litter contribute 70% of the total POC input; POC of human origin is also significant (10%), due to the impact of the City of Paris (10 million inhabitants). The lability of the different types of organic matter ranks as follows: phytoplankton ∼litter > human-origin organic matter >> soil. By combining the POC budgets and the lability of each type of organic fraction, it was possible to explain why the POC of the three ETMs is different and characterizes its refractory vs. labile nature. [ABSTRACT FROM AUTHOR]

The emissions of carbon dioxide (CO2) and methane (CH4) from the Petit Saut hydroelectric reservoir (Sinnamary River, French Guiana) to the atmosphere were quantified for 10 years since impounding in 1994. Diffusive emissions from the reservoir surface were computed from direct flux measurements in 1994, 1995, and 2003 and from surface concentrations monitoring. Bubbling emissions, which occur only at water depths lower than 10 m, were interpolated from funnel measurements in 1994, 1997, and 2003. Degassing at the outlet of the dam downstream of the turbines was calculated from the difference in gas concentrations upstream and downstream of the dam and the turbined discharge. Diffusive emissions from the Sinnamary tidal river and estuary were quantified from direct flux measurements in 2003 and concentrations monitoring. Total carbon emissions were 0.37 ± 0.01 Mt yr-1 C (CO2 emissions, 0.30 ± 0.02; CH4 emissions, 0.07 ± 0.01) the first 3 years after impounding (1994-1996) and then decreased to 0.12 ± 0.01 Mt yr-1 C (CO2, 0.10 ± 0.01; CH4, 0.016 ± 0.006) since 2000. On average over the 10 years, 61% of the CO2 emissions occurred by diffusion from the reservoir surface, 31% from the estuary, 7% by degassing at the outlet of the dam, and a negligible fraction by bubbling. CH4 diffusion and bubbling from the reservoir surface were predominant (40% and 44%, respectively) only the first year after impounding. Since 1995, degassing at an aerating weir downstream of the turbines has become the major pathway for CH4 emissions, reaching 70% of the total CH4 flux. In 2003, river carbon inputs were balanced by carbon outputs to the ocean and were about 3 times lower than the atmospheric flux, which suggests that 10 years after impounding, the flooded terrestrial carbon is still the predominant contributor to the gaseous emissions. In 10 years, about 22% of the 10 Mt C flooded was lost to the atmosphere. Our results confirm the significance of greenhouse gas emissions from tropical reservoir but stress the importance of: (1) considering all the gas pathways upstream and downstream of the dams and (2) taking into account the reservoir age when upscaling emissions rates at the global scale. [ABSTRACT FROM AUTHOR]

The Nivelle River, a typical Pyrenean mountainous watershed reaching the Bay of Biscay (Atlantic Ocean), was sampled with high resolution during 1996. The particulate organic carbon (POC) contents during successive floods shows that there is a graduated impoverishment of the organic fraction of suspended particulate matter (SPM) from the first flood to the next ones, reaching a threshold value (3%) attributed to allochtonous fraction (soil). On the basis of the high frequency data of water discharge and POC concentration, an annual POC flux was established: 845 tons, corresponding to a specific POC flux of 5.3 tC km−2 yr−1. This value was obtained during a dry period and must be considered as a minimum value for longer time scale. The POC originated mostly from soil (55%) and riparian/litter (~40%) with a very minor (<5%) contribution of autochthonous POC. Thirty-two percent of the annual POC flux was carried in 1% of time and 66% in 10% of time. The specific POC yield, 5.3 tC km−2 yr−1, if extended to the whole mountainous area of the southern coast of the Bay of Biscay (19,000 km2), leads to an estimated POC flux around 100,000 t yr−1. Although small Cantabrian mountainous rivers contributed to only 28% of the freshwater discharge in the Bay of Biscay, their POC load was estimated to account for 70% of the total POC inputs in the Bay. [ABSTRACT FROM AUTHOR]