Here we present direct measurements of the biological breakdown of 13C‐labeled substrates to CO2 at seven locations along the lower Amazon River, from Óbidos to the mouth. Dark incubation experiments ...were performed at high and low water periods using vanillin, a lignin phenol derived from vascular plants, and at the high water period using four different 13C‐labeled plant litter leachates. Leachates derived from oak wood were degraded most slowly with vanillin monomers, macrophyte leaves, macrophyte stems, and whole grass leachates being converted to CO2 1.2, 1.3, 1.7, and 2.3 times faster, respectively, at the upstream boundary, Óbidos. Relative to Óbidos, the sum degradation rate of all four leachates was 3.3 and 2.6 times faster in the algae‐rich Tapajós and Xingu Rivers, respectively. Likewise, the leachates were broken down 3.2 times more quickly at Óbidos when algal biomass from the Tapajós River was simultaneously added. Leachate reactivity similarly increased from Óbidos to the mouth with leachates breaking down 1.7 times more quickly at Almeirim (midway to the mouth) and 2.8 times more quickly across the river mouth. There was no discernible correlation between in situ nutrient levels and remineralization rates, suggesting that priming effects were an important factor controlling reactivity along the continuum. Further, continuous measurements of CO2, O2, and conductivity along the confluence of the Tapajós and Amazon Rivers and the Xingu and Jarauçu Rivers revealed in situ evidence for enhanced O2 drawdown and CO2 production along the mixing zone of these confluences.
Key Points
Biological conversion of 13C‐labeled substrates to CO2 was tracked in the Amazon River
Grasses were the most reactive followed by macrophyte stems, macrophyte leaves, and oak wood
The presence of priming substrates enhanced leachate degradation rates by 1.5 to 6.0 times
Current estimates of CO₂ outgassing from Amazonian rivers and streams have considerable uncertainty since they are based on limited-time surveys of pCO₂ measurements along the Amazon mainstem and ...mouths of major tributaries, using conservative estimates of gas exchange velocities. In order to refine basin-scale CO₂ efflux estimates from Amazonian rivers, we present a long time (5-year) dataset of direct measurements of CO₂ fluxes, gas transfer velocities and pCO₂ measurements in seven representative rivers of the lowland Amazon basin fluvial network, six non-tidal (Negro, Solimões, Teles Pires, Cristalino, Araguaia and Javaés) and one tidal river (Caxiuanã), with sizes ranging from 4th to 9th order. Surveys were conducted from January 2006 to December 2010, in a total of 389 campaigns covering all stages of their hydrographs. CO₂ fluxes and gas transfer velocities (k) were measured using floating chambers and pCO₂ was measured simultaneously by headspace extraction followed by gas chromatography analysis. Results show high CO₂ flux rate variability among rivers and hydrograph stages, ranging from −0.8 to 15.3 μmol CO₂ m⁻² s⁻¹, with unexpected negative fluxes in clear-water rivers during low waters. Non-tidal rivers showed marked seasonal CO₂ flux patterns, with significantly higher exchange during high waters. Seasonality was modulated by pCO₂, which was positive and strongly correlated with discharge. In these rivers k was well correlated with wind speed, which allowed the use of wind data to model k. We estimate a release of 360 ± 60 Tg C year⁻¹ from Amazonian rivers and streams within a 1.47 million km² quadrant in the central lowland Amazon. Extrapolating these values to the basin upstream of Óbidos, results in an outgassing of 0.8 Pg C to the atmosphere each year. Our results are a step forward in achieving more accurate gas emission values for Amazonian rivers and their role in the annual carbon budget of the Amazon basin.
Landscape characteristics and access type may exert a strong influence on groundwater quality, thereby adversely affecting human health. The aim of this study was to evaluate groundwater quality in ...springs and shallow wells of rural areas in terms of hydrochemical properties and different water quality indexes by comparing distinct microregions, groundwater sources (springs and shallow wells), and surrounding properties (e.g., presence of livestock, presence of fences, other protection structures, and restrictions on human access). Physical–chemical, chemical, and microbiological parameters were analyzed over 12 months between 2013 and 2014 in the Marombas River basin located in Santa Catarina State, Southern Brazil. Land use and landforms played an important role in controlling groundwater hydrochemistry in rural areas. The type of groundwater source (springs or shallow wells) did not influence water quality, although springs tended to be more susceptible to bacterial contamination, especially in areas with livestock in the surroundings. Chemical relationships allowed classifying these waters as predominantly calcium–magnesium bicarbonate or calcium–magnesium chloride, the latter being the most common in the study region. Groundwater was acidic, with low dissolved salt content, large range in dissolved oxygen concentrations, low turbidity, and presence of fecal coliforms in most studied months. Results indicate that water might be affected by septic tank leakage associated with wastewater and output from agricultural fields, given precarious installation and conservation conditions of springs and shallow wells in this region. When comparing the results with drinking water standards established by Brazilian Health Ministry, 70% of springs and shallow wells were found not suitable for consumption, especially due to organoleptic properties, high aluminum concentrations, and presence of fecal coliforms. Water quality indexes demonstrated that groundwater is suitable for agricultural uses (irrigation, livestock, and fish farming) and drinking, if treated via disinfection, filtering, or boiling before consumption. Nevertheless, use of this water resource, especially without any treatment—as is currently common among users—raises concerns related to its susceptibility to spread waterborne diseases, and lack of information among water users regarding procedures to improve water quality.
► Interception of soybean is higher than forest interception. ► Differences in interception are due to plant structure and stem density. ► Stemflow in soybean contributes a high proportion to net ...precipitation. ► Stemflow in forest depends on bark roughness and the inclination of branches. ► Multi-cropping in Mato Grosso reduces water availability in the long term.
The expansion of soybean cultivation into the Amazon in Brazil has potential hydrological effects at local to regional scales. To determine the impacts of soybean agriculture on hydrology, a comparison of net precipitation (throughfall, stemflow) in undisturbed tropical forest and soybean fields on the southern edge of the Amazon Basin in the state of Mato Grosso is needed. This study measured throughfall with troughs and stemflow with collar collectors during two rainy seasons. The results showed that in forest 91.6% of rainfall was collected as throughfall and 0.3% as stemflow, while in soybean fields with two-month old plants, 46.2% of rainfall was collected as throughfall and 9.0% as stemflow. Hence, interception of precipitation in soybean fields was far greater than in intact forests. Differences in throughfall, stemflow and net precipitation were found to be mainly associated with differences in plant structure and stem density in transitional forest and soybean cropland. Because rainfall interception in soybean fields is higher than previously believed and because both the area of cropland and the frequency of crop cycles (double cropping) are increasing rapidly, interception needs to be reconsidered in regional water balance models when consequences of land cover changes are analyzed in the Amazon soybean frontier region. Based on the continued expansion of soybean fields across the landscape and the finding that net precipitation is lower in soy agriculture, a reduction in water availability in the long term can be assumed.
The brief interaction of precipitation with a forest canopy can create a high spatial variability of both throughfall and solute deposition. We hypothesized that (i) the variability in natural forest ...systems is high but depends on system-inherent stability, (ii) the spatial variability of solute deposition shows seasonal dynamics depending on the increase in rainfall frequency, and (iii) spatial patterns persist only in the short-term. The study area in the north-western Brazilian state of Rondônia is subject to a climate with a distinct wet and dry season. We collected rain and throughfall on an event basis during the early wet season (
n
=
14) and peak of the wet season (
n
=
14) and analyzed the samples for pH and concentrations of
NH
4
+
, Na
+, K
+, Ca
2+, Mg
2+, Cl
−,
NO
3
-
,
SO
4
2
-
and DOC. The coefficient of variation for throughfall based on both sampling intervals was 29%, which is at the lower end of values reported from other tropical forest sites, but which is higher than in most temperate forests. Coefficients of variation of solute deposition ranged from 29% to 52%. This heterogeneity of solute deposition is neither particularly high nor particularly low compared with a range of tropical and temperate forest ecosystems. We observed an increase in solute deposition variability with the progressing wet season, which was explained by a negative correlation between heterogeneity of solute deposition and antecedent dry period. The temporal stability of throughfall patterns was low during the early wet season, but gained in stability as the wet season progressed. We suggest that rapid plant growth at the beginning of the rainy season is responsible for the lower stability, whereas less vegetative activity during the later rainy season might favor the higher persistence of “hot” and “cold” spots of throughfall quantities. The relatively high stability of throughfall patterns during later stages of the wet season may influence processes at the forest floor and in the soil. Solute deposition patterns showed less clear trends but all patterns displayed a short-term stability only. The weak stability of those patterns is apt to impede the formation of solute deposition-induced biochemical microhabitats in the soil.
GHG emissions from the Belo Monte reservoirs in Brazilian Amazon are up to three times higher than preimpoundment fluxes.
The current resurgence of hydropower expansion toward tropical areas has been ...largely based on run-of-the-river (ROR) dams, which are claimed to have lower environmental impacts due to their smaller reservoirs. The Belo Monte dam was built in Eastern Amazonia and holds the largest installed capacity among ROR power plants worldwide. Here, we show that postdamming greenhouse gas (GHG) emissions in the Belo Monte area are up to three times higher than preimpoundment fluxes and equivalent to about 15 to 55 kg CO
2
eq MWh
−1
. Since per-area emissions in Amazonian reservoirs are significantly higher than global averages, reducing flooded areas and prioritizing the power density of hydropower plants seem to effectively reduce their carbon footprints. Nevertheless, total GHG emissions are substantial even from this leading-edge ROR power plant. This argues in favor of avoiding hydropower expansion in Amazonia regardless of the reservoir type.
Most measurements of respiration rates in large tropical rivers do not account for the influence of river flow conditions on microbial activity. We developed a ship‐board spinning incubation system ...for measuring O2 drawdown under different rotation velocities and deployed the system along the lower Amazon River during four hydrologic periods. Average respiration rates in incubation chambers rotated at 0.22 m s−1 and 0.66 m s−1 were 1.4 and 2.4 times higher than stationary chambers, respectively. On average, depth‐integrated respiration rates in chambers spun at 0.22 m s−1 and 0.66 m s−1 accounted for 64% ± 22% and 104% ± 36% of CO2 outgassing rates, respectively, in mainstem sites. Continuous measurements of in situ pCO2 were also made along with cross‐channel profiles of river velocity. A positive correlation between river velocity and pCO2 was observed along the lower river (r2 = 0.67–0.96) and throughout a tidal cycle.
As a raindrop falls from the atmosphere, over vegetation and forest canopies, and enters soils and streams it experiences a dynamic exchange of carbon constituents with the surrounding environment. ...Understanding the magnitude and dynamics of carbon export in above and below ground flow paths is critical for constraining the influence of terrestrial and aquatic ecosystems on global carbon cycling. Here we examine the concentration and flux of dissolved organic and inorganic carbon (DOC and DIC) in rainfall, throughfall, stemflow, overland flow, soil solution, groundwater, and stream water in an Amazonian transitional forest located near the arc of deforestation. Rainfall was enriched in DOC by interactions with atmospheric particles derived from both biogenic and anthropogenic emissions, resulting in a flux of 82.3 kg C ha-1 yr-1, which was the largest flux from each respective flow path. Forest throughfall, stemflow, and direct overland flow mobilized DOC from products of terrestrial primary production and decomposition. Net throughfall represented the second largest DOC flux (68.4 kg C ha-1 yr-1), whereas stemflow and overland flow only had a flux of 1.5 and 3.9 kg C ha-1 yr-1, respectively. Much of the DOC in above ground flow paths was removed from solution as the rain percolated through soil layers due to both biological decomposition and sorption/desorption to mineral surfaces, resulting in low concentrations in stream and groundwater (2.6 ± 2.4 mg L-1 and 1.45 ± 0.3 mg L-1, respectively) relative to throughfall (43.9 ± 5.2 mg L-1) and stemflow (30.6 ± 2.7 mg L-1). The flux of DIC in each respective flow path was generally lower than for DOC, and likely driven by atmospheric gas exchange and inputs from respiration and decomposition. DOC concentrations in above ground flow paths were highest during the first rainfall after a dry period, whereas DIC concentrations generally increased throughout the rainy season as soil moisture increased.
Human activities that modify land cover can alter the structure and biogeochemistry of small streams but these effects are poorly known over large regions of the humid tropics where rates of forest ...clearing are high. We examined how conversion of Amazon lowland tropical forest to cattle pasture influenced the physical and chemical structure, organic matter stocks and N cycling of small streams. We combined a regional ground survey of small streams with an intensive study of nutrient cycling using 15 N additions in three representative streams: a second-order forest stream, a second-order pasture stream and a third-order pasture stream. These three streams were within several km of each other and on similar soils. Replacement of forest with pasture decreased stream habitat complexity by changing streams from run and pool channels with forest leaf detritus (50% cover) to grass-filled (63% cover) channel with runs of slow-moving water. In the survey, pasture streams consistently had lower concentrations of dissolved oxygen and nitrate (NO₃⁻) compared with similar-sized forest streams. Stable isotope additions revealed that second-order pasture stream had a shorter NH₄⁺ uptake length, higher uptake rates into organic matter components and a shorter ¹⁵NH₄⁺ residence time than the second-order forest stream or the third-order pasture stream. Nitrification was significant in the forest stream (19% of the added ¹⁵NH₄⁺) but not in the secondorder pasture (0%) or third-order (6%) pasture stream. The forest stream retained 7% of added 15 N in organic matter compartments and exported 53% (¹⁵NH₄⁺ = 34%; ¹⁵NO₃⁻ = 19%).). In contrast, the second-order pasture stream retained 75% of added ¹⁵N, predominantly in grasses (69%) and exported only 4% as ¹⁵NH₄⁺. The fate of tracer ¹⁵N in the third-order pasture stream more closely resembled that in the forest stream, with 5% of added N retained and 26% exported (¹⁵NH₄⁺ = 9%; ¹⁵NO₃⁻ = 6%). These findings indicate that the widespread infilling by grass in small streams in areas deforested for pasture greatly increases the retention of inorganic N in the first-and second-order streams, which make up roughly three-fourths of total stream channel length in Amazon basin watersheds. The importance of this phenomenon and its effect on N transport to larger rivers across the larger areas of the Amazon Basin will depend on better evaluation of both the extent and the scale at which stream infilling by grass occurs, but our analysis suggests the phenomenon is widespread.
A consistent observation of river waters in the Amazon Basin and elsewhere is that suspended fine particulate organic matter (FPOM) is compositionally distinct from coexisting dissolved organic ...matter (DOM). The present article presents experimental results that show that at least some of these compositional patterns are the outcome of selective partitioning of nitrogen-rich DOM components onto mineral surfaces. Nine laboratory experiments were conducted in which natural DOM from two rivers, one wetland, and two leachates from the Peruvian Amazon were mixed with natural suspended riverine minerals or organic-free kaolinite. Concentrations of organic carbon, organic nitrogen, and hydrolyzable amino acids were measured in both dissolved and particulate phases before and after mixing. In each of these trials, nitrogen was preferentially taken into the particulate fraction relative to the "parent" DOM, as were total hydrolyzable amino acids with respect to total organic carbon and nitrogen. Amino acid compositional patterns also indicated preferential sorption of basic amino acids, with positively charged nitrogen side chains, to the negatively charged aluminosilicate clay minerals. In short, sorption of natural DOM to minerals reproduced all contrasting organic nitrogen compositional patterns observed in the Amazon Basin. Although previously conjectured from FPOM-DOM compositional trends from fiver samples, this is the first direct evidence for preferential uptake of naturally occurring nitrogenous DOM by suspended riverine minerals. Last, nonprotein amino acids, which are commonly used as diagenetic indicators in sediments, preferentially remained dissolved, which suggests that sorptive fractionation may significantly complicate comparisons of FPOM and DOM diagenesis on the basis of interpretation of organic composition.
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