A large fraction of the organic carbon derived from land that is transported through inland waters is decomposed along river systems and emitted to the atmosphere as carbon dioxide (CO2). The Amazon ...River outgasses nearly as much CO2 as the rainforest sequesters on an annual basis, representing ~25% of global CO2 emissions from inland waters. However, current estimates of CO2 outgassing from the Amazon basin are based on a conservative upscaling of measurements made in the central Amazon, meaning both basin and global scale budgets are likely underestimated. The lower Amazon River, from Óbidos to the river mouth, represents ~13% of the total drainage basin area, and is not included in current basin-scale estimates. Here, we assessed the concentration and evasion rate of CO2 along the lower Amazon River corridor and its major tributaries, the Tapajós and Xingu Rivers. Evasive CO2 fluxes were directly measured using floating chambers and gas transfer coefficients (k600) were calculated for different hydrological seasons. Temporal variations in pCO2 and CO2 emissions were similar to previous observations throughout the Amazon (e.g. peak concentrations at high water) and CO2 outgassing was lower in the clearwater tributaries compared to the mainstem. However, k600 values were higher than previously reported upstream likely due to the generally windier conditions, turbulence caused by tidal forces, and an amplification of these factors in the wider channels with a longer fetch. We estimate that the lower Amazon River mainstem emits 0.2 Pg C yr-1 within our study boundaries, or as much as 0.48 Pg C yr-1 if the entire spatial extent to the geographical mouth is considered. Including these values with updated basin scale estimates and estimates of CO2 outgassing from small streams we estimate that the Amazon running waters outgasses as much as 1.39 Pg C yr-1, increasing the global emissions from inland waters by 43% for a total of 2.9 Pg C yr-1. These results highlight a large missing gap in basin-scale carbon budgets along the complete continuum of the Amazon River, and likely most other large river systems, that could drastically alter global scale carbon budgets.
Large‐scale soy agriculture in the southern Brazilian Amazon now rivals deforestation for pasture as the region's predominant form of land use change. Such landscape‐level change can have substantial ...consequences for local and regional hydrology, but these effects remain relatively unstudied in this ecologically and economically important region. We examined how the conversion to soy agriculture influences water balances and stormflows using stream discharge (water yields) and the timing of discharge (stream hydrographs) in small (2.5–13.5 km2) forested and soy headwater watersheds in the Upper Xingu Watershed in the state of Mato Grosso, Brazil. We monitored water yield for 1 year in three forested and four soy watersheds. Mean daily water yields were approximately four times higher in soy than forested watersheds, and soy watersheds showed greater seasonal variability in discharge. The contribution of stormflows to annual streamflow in all streams was low (<13% of annual streamflow), and the contribution of stormflow to streamflow did not differ between land uses. If the increases in water yield observed in this study are typical, landscape‐scale conversion to soy substantially alters water‐balance, potentially altering the regional hydrology over large areas of the southern Amazon.
The expansion and intensification of soya bean agriculture in southeastern Amazonia can alter watershed hydrology and biogeochemistry by changing the land cover, water balance and nutrient inputs. ...Several new insights on the responses of watershed hydrology and biogeochemistry to deforestation in Mato Grosso have emerged from recent intensive field campaigns in this region. Because of reduced evapotranspiration, total water export increases threefold to fourfold in soya bean watersheds compared with forest. However, the deep and highly permeable soils on the broad plateaus on which much of the soya bean cultivation has expanded buffer small soya bean watersheds against increased stormflows. Concentrations of nitrate and phosphate do not differ between forest or soya bean watersheds because fixation of phosphorus fertilizer by iron and aluminium oxides and anion exchange of nitrate in deep soils restrict nutrient movement. Despite resistance to biogeochemical change, streams in soya bean watersheds have higher temperatures caused by impoundments and reduction of bordering riparian forest. In larger rivers, increased water flow, current velocities and sediment flux following deforestation can reshape stream morphology, suggesting that cumulative impacts of deforestation in small watersheds will occur at larger scales.
The transfer of carbon (C) from Amazon forests to aquatic ecosystems as CO2 supersaturated in groundwater that outgases to the atmosphere after it reaches small streams has been postulated to be an ...important component of terrestrial ecosystem C budgets. We measured C losses as soil respiration and methane (CH4) flux, direct CO2 and CH4 fluxes from the stream surface and fluvial export of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate C over an annual hydrologic cycle from a 1,319-ha forested Amazon perennial first-order headwater watershed at Tanguro Ranch in the southern Amazon state of Mato Grosso. Stream pCO2 concentrations ranged from 6,491 to 14,976 μatm and directly-measured stream CO2 outgassing flux was 5,994 ± 677 g C m−2 y−1 of stream surface. Stream pCH4 concentrations ranged from 291 to 438 μatm and measured stream CH4 outgassing flux was 987 ± 221 g C m−2 y−1. Despite high flux rates from the stream surface, the small area of stream itself (970 m2, or 0.007% of watershed area) led to small directly-measured annual fluxes of CO2 (0.44 ± 0.05 g C m2 y−1) and CH4 (0.07 ± 0.02 g C m2 y−1) per unit watershed land area. Measured fluvial export of DIC (0.78 ± 0.04 g C m−2 y−1), DOC (0.16 ± 0.03 g C m−2 y−1) and coarse plus fine particulate C (0.001 ± 0.001 g C m−2 y−1) per unit watershed land area were also small. However, stream discharge accounted for only 12% of the modeled annual watershed water output because deep groundwater flows dominated total runoff from the watershed. When C in this bypassing groundwater was included, total watershed export was 10.83 g C m−2 y−1 as CO2 outgassing, 11.29 g C m−2 y−1 as fluvial DIC and 0.64 g C m−2 y−1 as fluvial DOC. Outgassing fluxes were somewhat lower than the 40–50 g C m−2 y−1 reported from other Amazon watersheds and may result in part from lower annual rainfall at Tanguro. Total stream-associated gaseous C losses were two orders of magnitude less than soil respiration (696 ± 147 g C m−2 y−1), but total losses of C transported by water comprised up to about 20% of the ± 150 g C m−2 (±1.5 Mg C ha−1) that is exchanged annually across Amazon tropical forest canopies.
The replacement of undisturbed tropical forest with cattle pasture has the potential to greatly modify the hydrology of small watersheds and the fluxes of solutes. We examined the fluxes of water, Cl
...−,
NO
3
-
–
N,
SO
4
2
-
–
S,
NH
4
+
–
N
, Na
+, K
+, Mg
2+ and Ca
2+ in different flow paths in ∼1
ha catchments of undisturbed open tropical rainforest and a 20 year-old pasture established from forest in the southwestern Brazilian Amazon state of Rondônia. Storm flow discharge was 18% of incident rainfall in pasture, but only 1% in forest. Quickflow predominated over baseflow in both catchments and in both wet and dry seasons. In the pasture, groundwater and quickflow were important flow paths for the export of all solutes. In the forest, quickflow was important for
NO
3
-
–
N
export, but all other solutes were exported primarily by groundwater outflow. Both catchments were sinks for
SO
4
2
-
–
S
and Ca
2+, and sources of Na
+. The pasture catchment also lost K
+ and Mg
2+ because of higher overland flow frequency and volume and to cattle excrement. These results show that forest clearing dramatically influences small watershed hydrology by increasing quickflow and water export to streams. They also indicate that tropical forest watersheds are highly conservative for most solutes but that pastures continue to lose important cations even decades after deforestation and pasture establishment.
The flux of methane (CH₄) from inland waters to the atmosphere has a profound impact on global atmospheric greenhouse gas (GHG) levels, and yet, strikingly little is known about the dynamics ...controlling sources and sinks of CH₄ in the aquatic setting. Here, we examine the cycling and flux of CH₄ in six large rivers in the Amazon basin, including the Amazon River. Based on stable isotopic mass balances of CH₄, inputs and outputs to the water column were estimated. We determined that ecosystem methane oxidation (MOX) reduced the diffusive flux of CH₄ by approximately 28–96% and varied depending on hydrologic regime and general geochemical characteristics of tributaries of the Amazon River. For example, the relative amount of MOX was maximal during high water in black and white water rivers and minimal in clear water rivers during low water. The abundance of genetic markers for methane‐oxidizing bacteria (pmoA) was positively correlated with enhanced signals of oxidation, providing independent support for the detected MOX patterns. The results indicate that MOX in large Amazonian rivers can consume from 0.45 to 2.07 Tg CH₄ yr⁻¹, representing up to 7% of the estimated global soil sink. Nevertheless, climate change and changes in hydrology, for example, due to construction of dams, can alter this balance, influencing CH₄ emissions to atmosphere.
We analyzed the molecular composition of dissolved organic matter (DOM) in the lower Amazon River (ca. 850 km from Óbidos to the mouth) using ultrahigh-resolution mass spectrometry and geochemical ...tracers. Changes in DOM composition along this lower reach suggest a transition from higher plant-derived DOM to more algal/microbial-derived DOM. This result was likely due to a combination of autochthonous production, alteration of terrigenous DOM as it transits down the river, and increased algal inputs from floodplain lakes and clearwater tributaries during high discharge conditions. Spatial gradients in dissolved organic carbon (DOC) concentrations varied with discharge. Maximal DOC concentrations were observed near the mouth during high water, highlighting the importance of lateral inputs of DOM along the lower river. The majority of DOM molecular formulae did not change within the time it takes the water in the mainstem to be transported through the lower reach. This is indicative of molecules representing a mixture of compounds that are resistant to rapid alteration and reactive compounds that are continuously replenished by the lateral input of terrestrial organic matter from the landscape, tributaries, and floodplains. River water incubations revealed that photo- and bio-transformation alter at most 30% of the DOM molecular formulae. River discharge at the mouth differed from the sum of discharge measurements made at Óbidos and the main gauged tributaries in the lower Amazon. This indicates that changes in hydrology and associated variations in the source waters along the lower reach affected the molecular composition of the DOM that is being transported from the Amazon River to the coastal ocean.
Intensive cropland agriculture commonly increases streamwater solute concentrations and export from small watersheds. In recent decades, the lowland tropics have become the world's largest and most ...important region of cropland expansion. Although the effects of intensive cropland agriculture on streamwater chemistry and watershed export have been widely studied in temperate regions, their effects in tropical regions are poorly understood. We sampled seven headwater streams draining watersheds in forest (n = 3) or soybeans (n = 4) to examine the effects of soybean cropping on stream solute concentrations and watershed export in a region of rapid soybean expansion in the Brazilian state of Mato Grosso. We measured stream flows and concentrations of NO₃⁻, PO₄³⁻, SO₄²⁻, Cl⁻, NH₄, Ca²⁺, Mg²⁺, Na⁺, K⁺, Al³⁺, Fe³⁺ and dissolved organic carbon (DOC) biweekly to monthly to determine solute export. We also measured stormflows and stormflow solute concentrations in a subset of watersheds (two forest, two soybean) during two/three storms, and solutes and δ¹⁸O in groundwater, rainwater, and throughfall to characterize watershed flowpaths. Concentrations of all solutes except K⁺ varied seasonally in streamwater, but only Fe³⁺ concentrations differed between land uses. The highest streamwater and rainwater solute concentrations occurred during the peak season of wildfires in Mato Grosso, suggesting that regional changes in atmospheric composition and deposition influence seasonal stream solute concentrations. Despite no concentration differences between forest and soybean land uses, annual export of NH₄⁺, PO₄³⁻, Ca²⁺, Fe³⁺, Na⁺, SO₄²⁻, DOC, and TSS were significantly higher from soybean than forest watersheds (5.6-fold mean increase). This increase largely reflected a 4.3-fold increase in water export from soybean watersheds. Despite this increase, total solute export per unit watershed area (i.e., yield) remained low for all watersheds (< 1 kg NO₃⁻ N·ha⁻¹·yr⁻¹, <2.1 kg NH₄⁺-N·ha⁻¹·yr⁻¹, <0.2 kg PO₄³⁻-P·ha-yr⁻¹, <1.5 kg Ca²⁺·ha⁻¹·yr⁻¹). Responses of both streamflows and solute concentrations to crop agriculture appear to be controlled by high soil hydraulic conductivity, groundwater-dominated hydrologie flowpaths on deep soils, and the absence of nitrogen fertilization. To date, these factors have buffered streams from the large increases in solute concentrations that often accompany intensive croplands in other locations.
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► Stemflow (SF) solute fluxes can equal or exceed those by throughfall (TF). ► Nitrate SF fluxes delivered 51% of total nitrogen to the forest floor (SF+TF). ► A single species ...contributed nearly all SF flux of chloride, nitrate and hydrogen. ► Smaller trees should not be excluded from nutrient cycling studies.
Rainfall, throughfall and stemflow were monitored on an event basis in an undisturbed open tropical rainforest with a large number of palm trees located in the southwestern Amazon basin of Brazil. Stemflow samples were collected from 24 trees with a diameter at breast height (DBH)>5cm, as well as eight young and four full-grown babassu palms (Attalea speciosa Mart.) for 5weeks during the peak of the wet season. We calculated rainfall, throughfall and stemflow concentrations and fluxes of Na+, K+, Ca2+, Mg2+, NH4+, Cl-, SO42-, NO3- and H+ and stemflow volume-weighted mean concentrations and fluxes for three size classes of broadleaf trees and three size classes of palms. The concentrations of most solutes were higher in stemflow than in rainfall and increased with increasing tree and palm size. Concentration enrichments from rainfall to stemflow and throughfall were particularly high (81-fold) for NO3-. Stemflow fluxes of NO3- and H+ exceeded throughfall fluxes but stemflow fluxes of other solutes were less than throughfall fluxes. Stemflow solute fluxes to the forest soil were dominated by fluxes on babassu palms, which represented only 4% of total stem number and 10% of total basal area. For NO3-, stemflow contributed 51% of the total mass of nitrogen delivered to the forest floor (stemflow+throughfall) and represented more than a 2000-fold increase in NO3- flux compared what would have been delivered by rainfall alone on the equivalent area. Because these highly localized fluxes of both water and NO3- persist in time and space, they have the potential to affect patterns of soil moisture, microbial populations and other features of soil biogeochemistry conducive to the creation of hotspots for nitrogen leaching and denitrification, which could amount to an important fraction of total ecosystem fluxes. Because these hotspots occur over very small areas, they have likely gone undetected in previous studies and need to be considered as an important feature of the biogeochemistry of palm-rich tropical forest.
The sustainability of fast-growing tropical Eucalyptus plantations is of concern in a context of rising fertilizer costs, since large amounts of nutrients are removed with biomass every 6-7 years ...from highly weathered soils. A better understanding of the dynamics of tree requirements is required to match fertilization regimes to the availability of each nutrient in the soil. The nutrition of Eucalyptus plantations has been intensively investigated and many studies have focused on specific fluxes in the biogeochemical cycles of nutrients. However, studies dealing with complete cycles are scarce for the Tropics. The objective of this paper was to compare these cycles for Eucalyptus plantations in Congo and Brazil, with contrasting climates, soil properties, and management practices. The main features were similar in the two situations. Most nutrient fluxes were driven by crown establishment the two first years after planting and total biomass production thereafter. These forests were characterized by huge nutrient requirements: 155, 10, 52, 55 and 23kgha⁻¹ of N, P, K, Ca and Mg the first year after planting at the Brazilian study site, respectively. High growth rates the first months after planting were essential to take advantage of the large amounts of nutrients released into the soil solutions by organic matter mineralization after harvesting. This study highlighted the predominant role of biological and biochemical cycles over the geochemical cycle of nutrients in tropical Eucalyptus plantations and indicated the prime importance of carefully managing organic matter in these soils. Limited nutrient losses through deep drainage after clear-cutting in the sandy soils of the two study sites showed the remarkable efficiency of Eucalyptus trees in keeping limited nutrient pools within the ecosystem, even after major disturbances. Nutrient input-output budgets suggested that Eucalyptus plantations take advantage of soil fertility inherited from previous land uses and that long-term sustainability will require an increase in the inputs of certain nutrients.
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