Amazon River carbon dioxide outgassing fuelled by wetlands

Abstract

River systems connect the terrestrial biosphere, the atmosphere and the ocean in the global carbon cycle. A recent estimate suggests that up to 3 petagrams of carbon per year could be emitted as carbon dioxide (CO2) from global inland waters, offsetting the carbon uptake by terrestrial ecosystems. It is generally assumed that inland waters emit carbon that has been previously fixed upstream by land plant photosynthesis, then transferred to soils, and subsequently transported downstream in run-off. But at the scale of entire drainage basins, the lateral carbon fluxes carried by small rivers upstream do not account for all of the CO2 emitted from inundated areas downstream. Three-quarters of the world's flooded land consists of temporary wetlands, but the contribution of these productive ecosystems to the inland water carbon budget has been largely overlooked. Here we show that wetlands pump large amounts of atmospheric CO2 into river waters in the floodplains of the central Amazon. Flooded forests and floating vegetation export large amounts of carbon to river waters and the dissolved CO2 can be transported dozens to hundreds of kilometres downstream before being emitted. We estimate that Amazonian wetlands export half of their gross primary production to river waters as dissolved CO2 and organic carbon, compared with only a few per cent of gross primary production exported in upland (not flooded) ecosystems. Moreover, we suggest that wetland carbon export is potentially large enough to account for at least the 0.21 petagrams of carbon emitted per year as CO2 from the central Amazon River and its floodplains. Global carbon budgets should explicitly address temporary or vegetated flooded areas, because these ecosystems combine high aerial primary production with large, fast carbon export, potentially supporting a substantial fraction of CO2 evasion from inland waters.

Regional-scale lateral carbon transport and CO<sub>2</sub> evasion in temperate stream catchments

“…In the Amazon, the fraction of terrestrial production that is exported by the fluvial network is more than 2-fold higher (nearly 7 % of NPP; Richey et al, 2002). However, it must be noted that a large fraction of the regional NPP in the Amazon is supported by aquatic primary production by macrophytes and C export is predominantly controlled by wetland connectivity, with wetlands covering up to 14 % of the land surface area (Abril et al, 2013). An additional peculiarity of the Amazon is that, in contrast to the remaining systems, the vast majority (87 %) of the total C-export is governed by CO 2 evasion (Table 3), whereas lateral export constitutes a much smaller component.…”

Effects of agricultural land use on fluvial carbon dioxide, methane and nitrous oxide concentrations in a large European river, the Meuse (Belgium)

“…In pristine river networks, CO 2 and CH 4 emissions are driven by instream production related to the degradation of terrestrial organic matter (Cole and Caraco, 2001;Richey et al, 2002), as well as lateral inputs from groundwater and/or wetlands (Abril et al, 2014;Borges et al, 2015aBorges et al, , 2015b. Pristine rivers are usually nitrogen poor and seem to be low sources or even sinks of N 2 O, related to sediment denitrification that removes N 2 O from the water column (Richey et al, 1988;Baulch et al, 2011;Borges et al, 2015a).…”

“…Enhanced nutrient inputs will fuel primary production leading to low CO 2 and high CH 4 concentrations, the latter related to enhanced organic matter delivery to sediments . Other human impacts that affect carbon and nitrogen cycling in river networks that can potentially influence cycling of GHGs are river bank stabilization and floodplain drainage that disrupt the river-wetland connectivity that is important for CO 2 and CH 4 dynamics in rivers (Abril et al, 2014;Teodoru et al, 2015;Borges et al, 2015aBorges et al, , 2015bSieczko et al, 2016). The introduction of invasive animal species such as the zebra mussel (Dreissena polymorpha) in US rivers and lakes (Caraco et al, 1997;Evans et al, 2011) (Hussner, 2012), some with high production and biomass (Hussner, 2009); invasive floating macrophytes such as the water hyacinth (Eichhornia crassipes) have been documented to increase CO 2 and CH 4 levels in tropical rivers (Koné et al, 2009(Koné et al, , 2010, but this remains undocumented in temperate rivers.…”

Globally significant greenhouse-gas emissions from African inland waters

“…Alternatively, if most of the CO 2 emissions from SSA river channels are derived from wetland C, the net balance would be nearly neutral as it is balanced by the atmospheric CO 2 fixation by the emerged vegetation, as shown in the Amazon lowland areas [11][12][13] . Untangling the relative contributions of wetland and upland C in sustaining CO 2 and CH 4 emissions from inland waters is essential to better understand the role of tropical inland waters in the global C cycle and related potential feedbacks on a warming climate.…”

“…In particular, there is a need to further understand the link between inland water GHG fluxes and catchment characteristics, in particular regarding their connectivity with upland terrestrial 9,10 and wetland [11][12][13] C production and stocks. The CO 2 emissions from inland waters have been traditionally interpreted as fuelled by organic C from upland terrestrial biomass 1,14 .…”