Soil compaction that follows the clearing of tropical forest for cattle pasture is associated with lower soil hydraulic conductivity and increased frequency and volume of overland flow. We ...investigated the frequency of perched water tables, overland flow and stormflow in an Amazon forest and in an adjacent 25-year-old pasture cleared from the same forest. We compared the results with the frequencies of these phenomena estimated from comparisons of rainfall intensity and soil hydraulic conductivity. The frequency of perched water tables based on rainfall intensity and soil hydraulic conductivity was expected to double in pasture compared with forest. This corresponded closely with an approximate doubling of the frequency of stormflow and overland flow in pasture. In contrast, the stormflow volume in pasture increased 17-fold. This disproportional increase of stormflow resulted from overland flow generation over large areas of pasture, while overland flow generation in the forest was spatially limited and was observed only very near the stream channel. In both catchments, stormflow was generated by saturation excess because of perched water tables and near-surface groundwater levels. Stormflow was occasionally generated in the forest by rapid return flow from macropores, while slow return flow from a continuous perched water table was more common in the pasture. These results suggest that deforestation for pasture alters fundamental mechanisms of stormflow generation and may increase runoff volumes over wide regions of Amazonia.
Rivers are generally supersaturated with respect to carbon dioxide, resulting in large gas evasion fluxes that can be a significant component of regional net carbon budgets. Amazonian rivers were ...recently shown to outgas more than ten times the amount of carbon exported to the ocean in the form of total organic carbon or dissolved inorganic carbon. High carbon dioxide concentrations in rivers originate largely from in situ respiration of organic carbon, but little agreement exists about the sources or turnover times of this carbon. Here we present results of an extensive survey of the carbon isotope composition (13C and 14C) of dissolved inorganic carbon and three size-fractions of organic carbon across the Amazonian river system. We find that respiration of contemporary organic matter (less than five years old) originating on land and near rivers is the dominant source of excess carbon dioxide that drives outgassing in medium to large rivers, although we find that bulk organic carbon fractions transported by these rivers range from tens to thousands of years in age. We therefore suggest that a small, rapidly cycling pool of organic carbon is responsible for the large carbon fluxes from land to water to atmosphere in the humid tropics.
Coastal oceans link terrestrial and marine carbon cycles. Yet, carbon sources and sinks in these biomes remain poorly understood. Here, we explore the dynamics of dissolved organic matter (DOM) along ...the Amazon River-to-ocean continuum from the lower mainstem at Óbidos to the open ocean of the western tropical North Atlantic. We molecularly characterized DOM via ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), determined DOM stable carbon isotopes, and interpreted the data in the context of bacterial abundance and production, phytoplankton biomass and composition. Multivariate analysis revealed that the DOM molecular variability in the plume was mainly influenced by the input of terrigenous DOM. Incubation experiments with water from close to the river mouth showed that photo- and bio-degradation preferentially removed 13C-depleted and 13C-enriched terrigenous DOM, respectively. However, there was no significant quantitative change in the total amount of dissolved organic carbon (DOC) over five days. This result suggests that most of the reactive DOM had already been bio-degraded upstream within the river and that photo-degradation was diminished in the turbid plume close to the river mouth. Terrigenous DOM therefore appeared to be relatively non-reactive nearshore. In the less turbid offshore plume, enhanced light penetration stimulated growth of phytoplankton and increased bacterial production. Although marine DOM compounds became relatively enriched, bulk DOC concentrations were 9 to 30% below levels expected from conservative mixing of river and ocean endmembers suggesting that quantitative removal of terrigenous DOM was not compensated by marine DOM production. We propose that removal of terrigenous DOM in the outer plume may be enhanced by (i) bio-degradation primed by reactive algal DOM, (ii) photo-degradation, which may further break down DOM into more bio-available forms, and possibly (iii) sorption of DOM to sinking particles.
•DOM was characterized using ultrahigh-resolution mass spectrometry from the lower Amazon River to the Caribbean Sea•DOM molecular variability in the plume was primarily influenced by terrigenous river DOM input•In the intermediate plume, phytoplankton biomass and bacterial activity were significantly correlated to DOM composition•9 - 30% of initial DOC was lost along the plume: removal of terrigenous DOM was not compensated by in situ new production•Molecular DOM patterns suggest bio- and photo-degradation as DOM sinks
Constraining the fate of dissolved organic matter (DOM) delivered by rivers is a key to understand the global carbon cycle, since DOM mineralization directly influences air‐sea CO2 exchange and ...multiple biogeochemical processes. The Amazon River exports large amounts of DOM, and yet the fate of this material in the ocean remains unclear. Here we investigate the molecular composition and transformations of DOM in the Amazon River‐ocean continuum using ultrahigh resolution mass spectrometry and geochemical and biological tracers. We show that there is a strong gradient in source and composition of DOM along the continuum, and that dilution of riverine DOM in the ocean is the dominant pattern of variability in the system. Alterations in DOM composition are observed in the plume associated with the addition of new organic compounds by phytoplankton and with bacterial and photochemical transformations. The relative importance of each of these drivers varies spatially and is modulated by seasonal variations in river discharge and ocean circulation. We further show that a large fraction (50–76%) of the Amazon River DOM is surprisingly stable in the coastal ocean. This results in a globally significant river plume with a strong terrigenous signature and in substantial export of terrestrially derived organic carbon from the continental margin, where it can be entrained in the large‐scale circulation and potentially contribute to the long‐term storage of terrigenous production and to the recalcitrant carbon pool found in the deep ocean.
Key Points
There is a strong gradient in source and composition of DOM in the continuum
Phytoplankton inputs and bacterial/photochemical transformations are significant
A large fraction of the Amazon River DOM is exported from the continental margin
River systems play a pivotal role in transporting and transforming organic carbon (OC) fixed by terrestrial primary production. However, there is a fundamental gap in our understanding of the ...connectivity of terrestrial, aquatic, and marine carbon budgets due to a lack of measurements along the lower (i.e. tidally-influenced) reaches of large river systems. For example, all estimates of carbon fluxes from the world's largest river, the Amazon, are based on measurements made at and upstream of obidos, roughly 900km from the mouth. Here we examine the evolution of OC concentrations and composition from obidos to two discreet channels near the mouth of the Amazon River during five cruises from 2010 to 2012. OC characteristics of the Tapajos River, which enters the Amazon River downstream of obidos, and the Tocantins River, which mixes with the Amazon River plume in the Atlantic Ocean, were also assessed. The average concentration of particulate organic carbon (POC) across the two main channels near the mouth was 0.6 plus or minus 0.3mgL-1 during the study period, decreasing from 1.2 plus or minus 1.0mgL-1 at obidos. Average dissolved organic carbon (DOC) concentrations, on the other hand, increased from 3.9 plus or minus 0.6mgL-1 at obidos to 4.2 plus or minus 0.9mgL-1 across the mouth. The discharge of total OC to the ocean was composed of 89 plus or minus 3% dissolved load, compared to 76 plus or minus 13% at obidos. Measurements of bulk OC stable isotopic signatures, chlorophyll a concentrations, and lignin phenol abundance were used to assess unique OC sources along the continuum. For example, the stable isotopic composition of POC and DOC indicated a replacement of highland forest-derived OC with lowland and floodplain-derived OC from obidos to the mouth. Likewise, lignin phenol signatures showed an increase in the degradation state of vascular plant-derived OC from obidos to the mouth. Results from this study illustrate that the abundance and composition of OC continue to evolve along the lower reaches of large tropical rivers, which has significant implications on estimations of geochemical fluxes to the ocean.
Methane (CH₄) fluxes from world rivers are still poorly constrained, with measurements restricted mainly to temperate climates. Additional river flux measurements, including spatio‐temporal studies, ...are important to refine extrapolations. Here we assess the spatio‐temporal variability of CH₄ fluxes from the Amazon and its main tributaries, the Negro, Solimões, Madeira, Tapajós, Xingu, and Pará Rivers, based on direct measurements using floating chambers. Sixteen of 34 sites were measured during low and high water seasons. Significant differences were observed within sites in the same river and among different rivers, types of rivers, and seasons. Ebullition contributed to more than 50% of total emissions for some rivers. Considering only river channels, our data indicate that large rivers in the Amazon Basin release between 0.40 and 0.58 Tg CH₄ yr⁻¹. Thus, our estimates of CH₄ flux from all tropical rivers and rivers globally were, respectively, 19–51% to 31–84% higher than previous estimates, with large rivers of the Amazon accounting for 22–28% of global river CH₄ emissions.
Large Amazonian rivers are known to emit substantial amounts of CO2 to the atmosphere, while the magnitude of CO2 degassing from small streams remains a major unknown in regional carbon budgets. We ...found that 77% of carbon transported by water from the landscape was as terrestrially‐respired CO2 dissolved within soils, over 90% of which evaded to the atmosphere within headwater reaches of streams. Hydrologic transport of dissolved CO2 was equivalent to nearly half the gaseous CO2 contributions from deep soil (>2 m) to respiration at the soil surface. Dissolved CO2 in emergent groundwater was isotopically consistent with soil respiration, and demonstrated strong agreement with deep soil CO2 concentrations and seasonal dynamics. During wet seasons, deep soil (2–8 m) CO2 concentration profiles indicated gaseous diffusion to deeper layers, thereby enhancing CO2 drainage to streams. Groundwater discharge of CO2 and its subsequent evasion is a significant conduit for terrestrially‐respired carbon in tropical headwater catchments.
Spatial and temporal patterns in microbial biodiversity across the Amazon river-ocean continuum were investigated along ∼675 km of the lower Amazon River mainstem, in the Tapajós River tributary, and ...in the plume and coastal ocean during low and high river discharge using amplicon sequencing of 16S rRNA genes in whole water and size-fractionated samples (0.2-2.0 μm and >2.0 μm). River communities varied among tributaries, but mainstem communities were spatially homogeneous and tracked seasonal changes in river discharge and co-varying factors. Co-occurrence network analysis identified strongly interconnected river assemblages during high (May) and low (December) discharge periods, and weakly interconnected transitional assemblages in September, suggesting that this system supports two seasonal microbial communities linked to river discharge. In contrast, plume communities showed little seasonal differences and instead varied spatially tracking salinity. However, salinity explained only a small fraction of community variability, and plume communities in blooms of diatom-diazotroph assemblages were strikingly different than those in other high salinity plume samples. This suggests that while salinity physically structures plumes through buoyancy and mixing, the composition of plume-specific communities is controlled by other factors including nutrients, phytoplankton community composition, and dissolved organic matter chemistry. Co-occurrence networks identified interconnected assemblages associated with the highly productive low salinity near-shore region, diatom-diazotroph blooms, and the plume edge region, and weakly interconnected assemblages in high salinity regions. This suggests that the plume supports a transitional community influenced by immigration of ocean bacteria from the plume edge, and by species sorting as these communities adapt to local environmental conditions. Few studies have explored patterns of microbial diversity in tropical rivers and coastal oceans. Comparison of Amazon continuum microbial communities to those from temperate and arctic systems suggest that river discharge and salinity are master variables structuring a range of environmental conditions that control bacterial communities across the river-ocean continuum.
► Soil hydraulic responses to Amazon land-use changes: forest
>
pasture
>
soybeans. ► Pasture cultivation decreases infiltrability and saturated hydraulic conductivity. ► Soybean cultivation seems to ...enhance subsoil compaction. ► Mainly vertical water movement within the soil is not altered by land-use changes. ► Near-surface hydrology of this landscape is well buffered against land-use changes.
Clearing for large-scale soy production and the displacement of cattle-breeding by soybeans are major features of land-use change in the lowland Amazon that can alter hydrologic properties of soils and the runoff generation over large areas. We measured infiltrability and saturated hydraulic conductivity (Ksat) under natural forest, pasture, and soybeans on Oxisols in a region of rapid soybean expansion in Mato Grosso, Brazil. The forest-pasture conversion reduced infiltrability from 1258 to 100
mm/h and Ksat at all depths. The pasture-soy conversion increased infiltrability from 100 to 469
mm/h (attributed to shallow disking), did not affect Ksat at 12.5
cm, but decreased Ksat at 30
cm from 122 to 80
mm/h, suggesting that soybean cultivation enhances subsoil compaction. Permeability decreased markedly with depth under forest, did not change under pasture, and averaged out at one fourth the forest value under soybeans with a similar pattern of anisotropy. Comparisons of permeability with rainfall intensities indicated that land-use change did not alter the predominantly vertical water movement within the soil. We conclude that this landscape is well buffered against land-use changes regarding near-surface hydrology, even though short-lived ponding and perched water tables may occur locally during high-intensity rainfall on pastures and under soybeans.
Outgassing of carbon dioxide (CO2) from rivers and streams to the atmosphere is a major loss term in the coupled terrestrial‐aquatic carbon cycle of major low‐gradient river systems (the term “river ...system” encompasses the rivers and streams of all sizes that compose the drainage network in a river basin). However, the magnitude and controls on this important carbon flux are not well quantified. We measured carbon dioxide flux rates (FCO2), gas transfer velocity (k), and partial pressures (pCO2) in rivers and streams of the Amazon and Mekong river systems in South America and Southeast Asia, respectively. FCO2 and k values were significantly higher in small rivers and streams (channels <100 m wide) than in large rivers (channels >100 m wide). Small rivers and streams also had substantially higher variability in k values than large rivers. Observed FCO2 and k values suggest that previous estimates of basinwide CO2 evasion from tropical rivers and wetlands have been conservative and are likely to be revised upward substantially in the future. Data from the present study combined with data compiled from the literature collectively suggest that the physical control of gas exchange velocities and fluxes in low‐gradient river systems makes a transition from the dominance of wind control at the largest spatial scales (in estuaries and river mainstems) toward increasing importance of water current velocity and depth at progressively smaller channel dimensions upstream. These results highlight the importance of incorporating scale‐appropriate k values into basinwide models of whole ecosystem carbon balance.
PDF
