Gaseous and fluvial carbon export from an Amazon forest watershed

Abstract

The transfer of carbon (C) from Amazon forests to aquatic ecosystems as CO 2 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 (CH 4 ) flux, direct CO 2 and CH 4 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 pCO 2 concentrations ranged from 6,491 to 14,976 latm and directly-measured stream CO 2 outgassing flux was 5,994 ± 677 g C m -2 y -1 of stream surface. Stream pCH 4 concentrations ranged from 291 to 438 latm and measured stream CH 4 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 m 2 , or 0.007% of watershed area) led to small directly-measured annual fluxes of CO 2 (0.44 ± 0.05 g C m 2 y -1 ) and CH 4 (0.07 ± 0.02 g C m 2 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 CO 2 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.

Old carbon contributes to aquatic emissions of carbon dioxide in the Amazon

“…In the Tambopata River the δ 13 C and 14 C content of evasion, and δ 13 C DIC remained consistent across different water levels targeted in the sampling. The CO 2 evasion rates measured here were comparable to the range (0.18-24.3 µmol m −2 s −1 ) determined in previous studies within the basin (Alin et al, 2011;Salimon et al, 2013;Neu et al, 2011;Rasera et al, 2008), apart from New Colpita, which had higher evasion rates probably resulting from greater turbulent flow, since evasion rate is strongly controlled by flow conditions (Alin et al, 2011).…”

The Volta Grande do Xingu: reconstruction of past environments and forecasting of future scenarios of a unique Amazonian fluvial landscape

“…Dissolved gas concentration was calculated using Henry's law adjusted for temperature (Wiesenburg and Guinasso, 1979). Fluxes from soils were measured using static chambers and procedures similar to the used by Neu et al (2011). Gas samples were analyzed on a Picarro Cavity Ring Down Spectroscopy Analyzer (model G2201-i).…”

CO<sub>2</sub> Flux and its Relationship with Water Parameters and Biological Activity in the Ji-Paraná River (Rondônia State – Western Amazon)

“…All periods showed positive flux values and several studies have found the same tendency Aufdenkampe 170 et al, 2011;Borges et al, 2004;Butman and Raymond, 2011;Cole and Caraco, 2001;Davidson et al, 2010;Dawson et al, 2004;Dubois et al, 2010;Gupta et al, 2008;Ho et al, 2007;Johnson et al, 2008;Jones et al, 2003;Neu et al, 2011;Prasad et al, 2013;Rasera et al, 2013;Richey et al, 2002;Salimon et al, 2013;Sand-Jensen and Staehr, 2012). In loworder systems, the higher pCO2 concentration and CO2 flux are sustained by groundwater and high-order rivers and estuaries show a lower air-water concentration gradient, but a larger contribution of carbon emission to the atmosphere 175 due to bigger superficial areas.…”

“…Several studies conducted in the Amazon have detected the same pattern, with differences in the carbon evasion between the periods (wet and dry stages) ranging from 1.1 to 44 times (Gomes, 2009;Neu et al, 2011;Rasera et al, 2008Rasera et al, , 2013Richey et al, 2002;Salimon et al, 2013;Sousa, 2013).…”

“…Recent studies have focused on the strong influence of rivers on the regional and global carbon balance, since they can behave as sinks or sources of atmospheric carbon (e.g. Richey et al, 2002;Ciais et al, 2008;Johnson et al, 2008;Brunet et al, 2009;Davidson et al, 2010;Dubois et al, 2010;Alim et al, 2011;Aufdenkampe et al, 2011;Butman & Raymond, 2011;Neu et al, 2011;Huang et al, 2012;de Fátima F. L. Rasera et al, 2013;Salimon et al, 2013).…”