Young organic matter as a source of carbon dioxide outgassing from Amazonian rivers

Abstract

Amazonian river system. We find that respiration of contemporary organic 2 matter (less than 5 years old) originating on land and near rivers is the dominant source of excess carbon dioxide that drives outgassing in mid-size 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.Riverine CO 2 concentrations in Amazonian lowlands are 5-30 times supersaturated with respect to atmospheric equilibrium 1 ; such conditions may be prevalent throughout the humid tropics. In situ respiration is the primary source of CO 2 sustaining supersaturation in rivers, although inputs from groundwater supersaturated by soil respiration can be important in small systems and from submerged riparian root respiration in floodplain influenced systems [1][2][3][6][7][8] . While air-water gas exchange is a bidirectional process, atmospheric CO 2 invasion has a negligible role compared to the large CO 2 evasion fluxes, except at low supersaturation 2,3,6,7 . 13 C and 14 C isotopes can provide constraints on sources and turnover times of organic carbon (OC) fuelling river respiration, yet no previous tropical study has used a dual-isotope approach to address these questions. Studies in temperate eastern USA provide contrasting findings. In the Hudson River, up to 70% of the centuries-old terrestrial OC entering the river is respired in transit, and the average age of riverine OC decreases downstream 2 .However, the youngest components of dissolved OC (DOC) are preferentially respired in the York River 5 , and modern dissolved inorganic carbon (DIC) in the Parker River may be explained by respiration of young DOC produced within the estuary 4 .Documenting key patterns and controls on CO 2 sources in diverse ecosystems is critical to advance our understanding of CO 2 outgassing from rivers and its contribution to regional net carbon budgets. 3To identify dominant sources and turnover times of riverine carbon throughout the Amazon basin, we analysed 14 C and 13 C of DIC, DOC, and suspended fine and coarse particulate OC fractions (FPOC and CPOC), grouping sites topographically (Fig. 1).This survey represents the most extensive dual carbon isotope inventory to date in a large, diverse basin, and the first 14 C analysis of DIC in Amazonian rivers. It complements but greatly exceeds previous carbon isotope surveys 5,7,9 , enabling an integrated assessment of carbon cycling.DIC is composed of dissolved CO 2 and bicarbonate and carbonate ions in pHdependent chemical and isotopic equilibrium 10 . In studies of marine and homogeneous river systems, where pH is nearly uniform, it has been the convention to report the isotopic composition of total DIC, which is directly measured. However, when the turnover of DIC by CO 2 fluxes is as rapid as in many of these tropical rivers, a quasisteady-state co...

Annual variability in the radiocarbon age and source of dissolved CO2 in a peatland stream

“…14 rivers has previously been undertaken using one of two methods: (1) an indirect method 63 whereby water samples are collected in the field, returned to the laboratory and the entire DIC 64 recovered as CO 2 by "gas-stripping" (e.g. Mayorga et al, 2005), or (2) a direct method 65 whereby CO 2 evading from the water surface is captured using a floating chamber and 66 adsorbed onto a molecular sieve, which can be subsequently processed in the laboratory to 67 recover the CO 2 for isotope analysis (Billett et al, 2006;Billett and Garnett, 2010).…”

The fate of buried organic carbon in colluvial soils: a long-term perspective

“…Various studies have been conducted to explore the fate of the SOC exported from the catchment: for instance, labile young organic matter delivered to river systems was found to be rapidly mineralised in the channel (e.g. Raymond and Bauer, 2001;Mayorga et al, 2005), while SOC reaching lakes and oceans has been shown to be well preserved over longer timescales (e.g. Rosen and Hammarlund, 2007;Galy et al, 2007).…”

Do Riparian Plants Fix CO2 Lost by Evasion from Surface Waters? An Investigation Using Carbon Isotopes

“…However, it could be more significant at other sites where the CO 2 degassed from the stream surface is more 14 C-depleted. For example, Mayorga et al (2005) observed 14 CO 2 concentrations in Amazonian rivers as low as ~25% Modern. In addition, at some sites the proportion of plant carbon being derived from evasion might be greater due to higher evasion rates, lower wind speeds, and different stream bank morphology.…”

“…This recently developed method allows direct collection of the evaded CO 2 and is considered preferable to previously used indirect methods (e.g. Mayorga et al 2005). …”

“…Surface waters in wetland or peatland areas also contain high concentrations of dissolved carbon dioxide (CO 2 ), which may form an important vertical flux from the river to the atmosphere (Hope et al 2001;Richey et al 2002). Although relatively little is known about the source and age of CO 2 lost to the atmosphere from surface waters, a recent isotopic study by Mayorga et al (2005) suggested that most dissolved CO 2 from Amazonian rivers was derived from organic matter that had mainly been fixed less than 5 yr earlier. The authors also reported several much older ages for dissolved CO 2 in certain rivers (mainly headwater regions of the Amazon Basin), implying a contribution from the weathering of carbonate minerals.…”