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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 - 1) and higher CH 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.)

River systems connect the terrestrial biosphere, the atmosphere and the ocean in the global carbon cycle. A recent estimate suggests that up to 3 petagrams of carbon per year could be emitted as carbon dioxide (CO2) from global inland waters, offsetting the carbon uptake by terrestrial ecosystems. It is generally assumed that inland waters emit carbon that has been previously fixed upstream by land plant photosynthesis, then transferred to soils, and subsequently transported downstream in run-off. But at the scale of entire drainage basins, the lateral carbon fluxes carried by small rivers upstream do not account for all of the CO2 emitted from inundated areas downstream. Three-quarters of the world's flooded land consists of temporary wetlands, but the contribution of these productive ecosystems to the inland water carbon budget has been largely overlooked. Here we show that wetlands pump large amounts of atmospheric CO2 into river waters in the floodplains of the central Amazon. Flooded forests and floating vegetation export large amounts of carbon to river waters and the dissolved CO2 can be transported dozens to hundreds of kilometres downstream before being emitted. We estimate that Amazonian wetlands export half of their gross primary production to river waters as dissolved CO2 and organic carbon, compared with only a few per cent of gross primary production exported in upland (not flooded) ecosystems. Moreover, we suggest that wetland carbon export is potentially large enough to account for at least the 0.21 petagrams of carbon emitted per year as CO2 from the central Amazon River and its floodplains. Global carbon budgets should explicitly address temporary or vegetated flooded areas, because these ecosystems combine high aerial primary production with large, fast carbon export, potentially supporting a substantial fraction of CO2 evasion from inland waters.

One of the greatest challenges in understanding the Amazon basin functioning is to ascertain the role played by floodplains in the organic matter (OM) cycle, crucial for a large spectrum of ecological mechanisms. Fatty acids (FAs) were combined with environmental descriptors and analyzed through multivariate and spatial tools (asymmetric eigenvector maps, AEM and principal coordinates of neighbor matrices, PCNM). This challenge allowed investigating the distribution of suspended particulate organic matter (SPOM), in order to trace its seasonal origin and quality, along a 800 km section of the Amazon river-floodplain system. Statistical analysis confirmed that large amounts of saturated FAs (15:0, 18:0, 24:0, 25:0 and 26:0), an indication of refractory OM, were concomitantly recorded with high pCO(2) in rivers, during the high water season (HW). Contrastingly, FAs marker which may be attributed in this ecosystem to aquatic plants (18:2ω6 and 18:3ω3) and cyanobacteria (16:1ω7), were correlated with higher O(2), chlorophyll a and pheopigments in floodplains, due to a high primary production during low waters (LW). Decreasing concentrations of unsaturated FAs, that characterize labile OM, were recorded during HW, from upstream to downstream. Furthermore, using PCNM and AEM spatial methods, FAs compositions of SPOM displayed an upstream-downstream gradient during HW, which was attributed to OM retention and the extent of flooded forest in floodplains. Discrimination of OM quality between the Amazon River and floodplains corroborate higher autotrophic production in the latter and transfer of OM to rivers at LW season. Together, these gradients demonstrate the validity of FAs as predictors of spatial and temporal changes in OM quality. These spatial and temporal trends are explained by 1) downstream change in landscape morphology as predicted by the River Continuum Concept; 2) enhanced primary production during LW when the water level decreased and its residence time increased as predicted by the Flood Pulse Concept.

Abstract: Degassing of terrestrially-respired CO2 from streams and small rivers appears to be a significant component in watershed carbon budgets. Here we propose an original approach to quantify CO2 degassing in small headwater bodies using pCO2, DIC (or total alkalinity, TA) and δ13C-DIC data in stream waters that avoids the difficulty of measuring or choosing a gas transfer velocity. Our inversed model applies to acidic, non-buffered (humic-type) waters and relies on two main assumptions, i.e., the stable isotopic composition of DIC in groundwater seeping to surface water (CO2 from respired soil organic carbon and from weathering) and on kinetic fractionation at the water–air interface (12CO2 degases to the atmosphere more rapidly than 13CO2). We first consider both the soil organic matter isotopic composition and the isotopic fractionation of CO2 in the soil, to derive the δ13C–CO2 in that soil and groundwater. From the concentrations in streams, we estimate the relative contribution of silicate and carbonate weathering (the latter being minor in these waters) to the and its associated isotopic composition. Model calculations start from the δ13C-DIC value computed by the aforementioned method and consist of two interlocked iterative procedures. The first procedure simulates the decrease in pCO2 and the increase in δ13C-DIC that occur along the stream watercourse during degassing, starting from an assumed initial soil pCO2 and ending at the in situ pCO2 or δ13C-DIC. The second iteration procedure consists of adjusting the initial soil pCO2 until pCO2 and δ13C-DIC simultaneously reach the in situ measured values. After convergence is obtained, the model computes a theoretical concentration of DIC, [DIC]ex., that has been lost as CO2 to the atmosphere from the headwater to the sampling point in the river. [DIC]ex. can be multiplied by the river discharge to derive the quantity of carbon degassed from the river surface. The model was tested on seasonal field datasets from three small rivers draining sandy podsols in southern France and gave annual areal degassing rates comparable to those reported in other studies, though somewhat larger (upper half range in two rivers, ∼10 times the average in one stream). Part of this discrepancy might have been caused by an intense degassing in the vicinity of groundwater seeps, which was accounted for our integrative method but not by classical methods based on stream water pCO2 and gas transfer velocity. The sensitivity of the model results on the assumption of the importance of carbonate weathering might also explain part of this high degassing rate. The model reproduced consistent values and seasonal trends of soil pCO2 (maximal in summer) and gas transfer velocity (maximal at high water flow). We discuss the sensitivity of the model to the different parameters and assumptions and propose some improvements including groundwater sampling, for better constraining the computed degassing rates. [Copyright &y& Elsevier]

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]