Gauci, Vincent. 2013 Deep instability of deforested tropical peatlands revealed by fluvial organic carbon fluxes. Nature, 493. 660-663. 10.1038/nature11818Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. 39Unlike boreal and temperate forests 5,6 , and higher latitude wetlands 7 , however, the loss of fluvial 40 organic carbon from tropical peats has yet to be fully quantified. 41To quantify the effect of peatland degradation on fluvial organic C loss, we monitored DOC and , and experienced similar annual rainfall (Table 1). 9TOC ( ; Fig. 1). This represents a 55% increase in 12TOC export from the disturbed sites (DPSF1 and 2) over IPSF. Of the annual TOC flux from each land-13 cover class, 94% was lost during the wet season (October-June), the result of higher measured 14 discharge rates (3.9 m 3 s -1 cf. 1.0 m 3 s -1 in the dry season). This was associated with high rainfall 15 rather than changes in C concentration, which remained relatively constant over the study period. 16As with seasonal flux variability, differences in discharge between land-cover classes determined TOC 17flux with higher discharge rates causing larger fluxes in DPSF1 and DPSF2 (1744 mm and 1724 mm, 18respectively) than in IPSF (907 mm). These higher discharge rates in disturbed land-cover classes 19were not counterbalanced by lower TOC concentrations, and occurred despite uniform rainfall across 20 sites (Table 1). This likely reflects a decline in evapotranspiration and increased runoff as a 21consequence of large scale biomass loss and drainage in both disturbed land-cover classes. DOC 22accounted for between 91-98 % of the TOC lost, with lower DOC:POC ratios for disturbed sites ( Table 23 1) suggesting that the drained and exposed peat is vulnerable to mechanical breakdown associated 24 with the increased runoff. 25Surface water DOC can derive from multiple sources, ranging from recent photosynthates to 26 decomposition or dissolution products from deep within the peat column. We used radiocarbon ( 3These data indicate that the increased DOC fluxes from disturbed peatlands are derived from 4 previously stable C stored within the peat column, and suggest that this loss of C from depth is 5 occurring throughout the seasonal hydrologic cycle. Application of an age attribution model (Fig. 2d) 6suggests that two-thirds of DOC in runoff from the DPSF2 site derives from peat carbon of 500-5000 7 years age. 35To quantify the impact peatland disturbance has had on regional long-term fluvial C loss, we applied 36 our TOC flux estimates to land areas of intact and deforested PSF prior to and after peatland 37 disturbance. We omitted industrial plantations from our calculations as, to our knowledge, there are 38 no quantitative data on fluvial C flux from this land cover class, although our DO 14 C data suggest that 39 these ecosystems may also ...

Wetlands are the largest global source of atmospheric methane (CH), a potent greenhouse gas. However, methane emission inventories from the Amazon floodplain, the largest natural geographic source of CH in the tropics, consistently underestimate the atmospheric burden of CH determined via remote sensing and inversion modelling, pointing to a major gap in our understanding of the contribution of these ecosystems to CH emissions. Here we report CH fluxes from the stems of 2,357 individual Amazonian floodplain trees from 13 locations across the central Amazon basin. We find that escape of soil gas through wetland trees is the dominant source of regional CH emissions. Methane fluxes from Amazon tree stems were up to 200 times larger than emissions reported for temperate wet forests and tropical peat swamp forests, representing the largest non-ebullitive wetland fluxes observed. Emissions from trees had an average stable carbon isotope value (δC) of -66.2 ± 6.4 per mil, consistent with a soil biogenic origin. We estimate that floodplain trees emit 15.1 ± 1.8 to 21.2 ± 2.5 teragrams of CH a year, in addition to the 20.5 ± 5.3 teragrams a year emitted regionally from other sources. Furthermore, we provide a 'top-down' regional estimate of CH emissions of 42.7 ± 5.6 teragrams of CH a year for the Amazon basin, based on regular vertical lower-troposphere CH profiles covering the period 2010-2013. We find close agreement between our 'top-down' and combined 'bottom-up' estimates, indicating that large CH emissions from trees adapted to permanent or seasonal inundation can account for the emission source that is required to close the Amazon CH budget. Our findings demonstrate the importance of tree stem surfaces in mediating approximately half of all wetland CH emissions in the Amazon floodplain, a region that represents up to one-third of the global wetland CH source when trees are combined with other emission sources.

SummaryWetlands are the largest source of methane to the atmosphere, with tropical wetlands comprising the most significant global wetland source component. The stems of some wetlandadapted tree species are known to facilitate egress of methane from anoxic soil, but current ground-based flux chamber methods for determining methane inventories in forested wetlands neglect this emission pathway, and consequently, the contribution of tree-mediated emissions to total ecosystem methane flux remains unknown.In this study, we quantify in situ methane emissions from tree stems, peatland surfaces (ponded hollows and hummocks) and root-aerating pneumatophores in a tropical forested peatland in Southeast Asia.We show that tree stems emit substantially more methane than peat surfaces, accounting for 62-87% of total ecosystem methane flux. Tree stem flux strength was controlled by the stem diameter, wood specific density and the amount of methane dissolved in pore water.Our findings highlight the need to integrate this emission pathway in both field studies and models if wetland methane fluxes are to be characterized accurately in global methane budgets, and the discrepancies that exist between field-based flux inventories and top-down estimates of methane emissions from tropical areas are to be reconciled.

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Natural wetlands form the largest source of methane (CH4) to the atmosphere. Emission of this powerful greenhouse gas from wetlands is known to depend on climate, with increasing temperature and rainfall both expected to increase methane emissions. This study, combining our field and controlled environment manipulation studies in Europe and North America, reveals an additional control: an emergent pattern of increasing suppression of methane (CH 4) A tmospheric methane (CH 4 ) is a powerful greenhouse gas (GHG) that is responsible for an estimated 22% of the present anthropogenically enhanced greenhouse effect (1). Natural (nonrice agriculture) wetlands are the world's largest single CH 4 source and are estimated to currently contribute between 110 and 260 Tg (Tg ϭ 10 12 g) to the global methane budget (2), of which one-third is derived from temperate and boreal northern wetlands (3). CH 4 emissions from wetlands are climatesensitive, responding positively to increases in temperature and rainfall as microbial activity and anaerobic conditions increase and negatively to cool temperatures and drought (4, 5). Like many other ecosystems, wetlands are also subject to the effects of aerial pollution and increasing CO 2 levels. The stimulatory effects of increased atmospheric CO 2 concentrations on CH 4 emission (by enhancement of net primary productivity) is well reported (6-8), although a similar stimulatory effect of nitrogen pollution on wetland CH 4 emission has not always been identified (8-10) because of differing effects nitrogen has on the ecosystem, e.g., plant species composition is an important factor in determining the effect of experimental N additions on CH 4 fluxes (10).CH 4 is produced by two different groups of methanogenic archaea (MA); one group obtains energy by the fermentation of simple organic compounds, such as acetate to CO 2 and CH 4 , and the other obtains energy by oxidizing molecular hydrogen to H 2 O by using CO 2 , which is reduced to CH 4 . Acetate-fermenting MA tend to dominate in more nutrient-rich peatlands and in summer, when the supply of labile organic carbon is relatively high. However, it has been recently demonstrated that climate, depth of the acrotelm, and acetate concentrations add a fair degree of plasticity over controls on acetate-fermenting MA (11). Both groups of microorganisms are strictly anaerobic, and both are suppressed by another group of anaerobic microorganisms, sulfate-reducing bacteria (SRB) (12).SRB have a higher affinity for both hydrogen and acetate than MA, which, under ideal conditions, enables them to maintain the pool of these substrates at concentrations too low for MA to use (13,14). In wetlands, however, the balance between sulfate reduction and methanogenesis is affected by factors such as the temperature [warmer temperatures favor methanogenesis (15)], the rate of SO 4 2Ϫ and acetate supply [lower concentrations of sulfate or higher concentrations of acetate reduce the intensity of competition (13)], and the availability of noncompetitive substra...