Tropical wetlands are not included in Earth system models, despite being an important source of methane (CH4) and contributing a large fraction of carbon dioxide (CO2) emissions from land use, land use change, and forestry in the tropics. This review identifies a remarkable lack of data on the carbon balance and gas fluxes from undisturbed tropical wetlands, which limits the ability of global change models to make accurate predictions about future climate. We show that the available data on in situ carbon gas fluxes in undisturbed forested tropical wetlands indicate marked spatial and temporal variability in CO2 and CH4 emissions, with exceptionally large fluxes in Southeast Asia and the Neotropics. By upscaling short-term measurements, we calculate that approximately 90 ± 77 Tg CH4 year−1 and 4540 ± 1480 Tg CO2 year−1 are released from tropical wetlands globally. CH4 fluxes are greater from mineral than organic soils, whereas CO2 fluxes do not differ between soil types. The high CO2 and CH4 emissions are mirrored by high rates of net primary productivity and litter decay. Net ecosystem productivity was estimated to be greater in peat-forming wetlands than on mineral soils, but the available data are insufficient to construct reliable carbon balances or estimate gas fluxes at regional scales. We conclude that there is an urgent need for systematic data on carbon dynamics in tropical wetlands to provide a robust understanding of how they differ from well-studied northern wetlands and allow incorporation of tropical wetlands into global climate change models.
Tropical peatlands play an important role in the global carbon cycling but little is known about factors regulating carbon dioxide (CO 2 ) and methane (CH 4 ) fluxes from these ecosystems. Here, we test the hypotheses that (i) CO 2 and CH 4 are produced mainly from surface peat and (ii) that the contribution of subsurface peat to net C emissions is governed by substrate availability. To achieve this, in situ and ex situ CO 2 and CH 4 fluxes were determined throughout the peat profiles under three vegetation types along a nutrient gradient in a tropical ombrotrophic peatland in Panama. The peat was also characterized with respect to its organic composition using 13 C solid state crosspolarization magic-angle spinning nuclear magnetic resonance spectroscopy. Deep peat contributed substantially to CO 2 effluxes both with respect to actual in situ and potential ex situ fluxes. CH 4 was produced throughout the peat profile with distinct subsurface peaks, but net emission was limited by oxidation in the surface layers. CO 2 and CH 4 production were strongly substrate-limited and a large proportion of the variance in their production (30% and 63%, respectively) was related to the quantity of carbohydrates in the peat. Furthermore, CO 2 and CH 4 production differed between vegetation types, suggesting that the quality of plant-derived carbon inputs is an important driver of trace gas production throughout the peat profile. We conclude that the production of both CO 2 and CH 4 from subsurface peat is a substantial component of the net efflux of these gases, but that gas production through the peat profile is regulated in part by the degree of decomposition of the peat. , 2010). In areas of the tropics which are expected to experience reduced rainfall and more prolonged drought (Meehl et al., 2007), peatlands may become less important sources of atmospheric methane (CH 4 ). However, this would be offset by greatly increased rates of aerobic decomposition and carbon dioxide (CO 2 ) release, with the result that their combined global warming potential would increase (Hirano et al., 2009;Couwenberg et al., 2010;Sjö gersten et al., 2010). It is also plausible that old C stored deep in the peat profile would be metabolized if climatic conditions become more favourable for decomposition, as has been reported for northern peatlands (e.g. Dorrepaal et al., 2009). In tropical environments, a substantial draw down of the water table would be required to impact deep peat layers. Considerable variation in C fluxes occurs between vegetation types in tropical wetland systems (Melling et al., 2005a, b;Sjö gersten et al., 2010), suggesting that C inputs from the vegetation (Sebacher et al., 1985;Joabsson & Christensen, 2001;Konnerup et al., 2010) are strong drivers of C fluxes in these systems. However, it is not known if these differences in C fluxes result mainly from surface processes or are maintained throughout the peat profile. Tropical peatlands may reach depths of up to 15 m (Phillips et al., 1997;Page et al., 1999Page...
Tropical peatlands play an important role in the global storage and cycling of carbon (C) but information on carbon dioxide (CO 2 ) and methane (CH 4 ) fluxes from these systems is sparse, particularly in the Neotropics. We quantified short and long-term temporal and small scale spatial variation in CO 2 and CH 4 fluxes from three contrasting vegetation communities in a domed ombrotrophic peatland in Panama. There was significant variation in CO 2 fluxes among vegetation communities in the order Campnosperma panamensis > Raphia taedigera > Cyperus. ) while very high emissions were found during the 2009 wet period, suggesting that peak CO 2 fluxes may occur following both low and high rainfall. In contrast, only weak relationships between CH 4 flux and rainfall (positive at the C. panamensis site) and solar radiation (negative at the C. panamensis and Cyperus sites) was found. CO 2 fluxes showed a diurnal pattern across sites and at the Cyperus sp. site CO 2 and CH 4 fluxes were positively correlated. The amount of dissolved carbon and nutrients were strong predictors of small scale within-site variability in gas release but the effect was site-specific. We conclude that (i) temporal variability in CO 2 was greater than variation among vegetation communities; (ii) rainfall may be a good predictor of CO 2 emissions from tropical peatlands but temporal variation in CH 4 does not follow seasonal rainfall patterns; and (iii) diurnal variation in CO 2 fluxes across different vegetation communities can be described by a Fourier model.
Although water table depth is commonly regarded as the primary determinant of litter decomposition rate in tropical peatlands, this has rarely been tested experimentally. This study explored the influence of flooding on decomposition of litter from three dominant plant species in a neotropical peatland. The non-flooded treatment reduced the mass remaining after 14 months from 84 to 81 % for Raphia taedigera, 65 to 58 % for Campnosperma panamensis, and 69 to 58 % for Cyperus sp. The proportions of carbon, nitrogen and phosphorus in the labile, semi-labile and recalcitrant carbon pools, did not reliably predict differences among species in the mass loss rate of litter. Phosphorus was rapidly lost from litter, while carbon losses, including soluble carbon, were slower, but significant for all fractions. The nonflooded treatment substantially reduced the quantity of C remaining in the residue fraction of leaf litter after 12 weeks, with 31, 19 and 6 % less remaining in the non-flooded treatment for R. taedigera, C. panamensis and Cyperus sp. This suggests that lower water table depth on litter decay increase degradation of mainly aliphatic and aromatic carbon in the residual fraction. Thus, although lowering the water table increased decomposition, the chemical composition of litter clearly influences peat accumulation.
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