Methane is produced abiotically from combustion of organic carbon during biomass burning and by thermal alteration of sedimentary organic carbon.Despite generally inhospitable conditions, there is abundant evidence of methanogenic activity in upland soils. Andersen et al. used a 14CH4-labeling technique to infer that two forest soils produced CH4 even though the soils as a whole were net CH4 sinks. von Fischer and Hedin used a stable isotope technique to make direct measurements of gross CH4 production in 130 soil cores from 17 sites and found that even dry, oxic soils produced CH4. Aerobic forest and agricultural soils have been reported to switch from net CH4 uptake to CH4 emission in the presence of a compound that blocks CH4 oxidation . Finally, upland soils incubated anaerobically begin producing CH4 within days or weeks . Collectively, these studies suggest that upland soils harbor populations of methanogens and are capable of becoming net sources of CH4 when sufficiently wet. The possibility of CH4 production in upland soil microsites is consistent with the occurrence of denitrification and Fe reduction in upland soils, and observations that acetate, a CH4 precursor, is found in upland soils . Although stud ies of methanogen isolates suggest they are extremely O2 sensitive, other evidence suggests that they can tolerate a certain amount of O2 . Methanogens have been reported to survive long periods in dry and oxic soils ,arandanos en maceta perhaps protected from O2 by reactive soil minerals . The evidence that upland soils can support low rates of methanogenesis suggests that CH4 oxidizing bacteria consume CH4 from two sources, the atmosphere and the soil itself . The juxtaposition of these sources may explain a puzzling observation about the response of CH4 fluxes to changes in soil water content. Andersen et al. reported that an intact upland forest soil core left uncovered at room temperature changed from a net sink for atmospheric CH4 to a net source.

Isotopic data showed that CH4 oxidation fell to almost zero over this period, suggesting that CH4 oxidizing bacteria attached to soil surfaces were more sensitive to soil drying than methanogens buried in the anaerobic center of soil aggregates. The cessation of CH4 oxidation could have been caused by a physiological drought response among methanotrophic bacteria, more rapid CH4 diffusion from the soil to the atmosphere due to low tortuosity , or both. In other circumstances, decreases in soil water content can enhance CH4 oxidation in upland soils by increasing CH4 diffusion from the atmosphere into soil pore spaces . In addition to microsites, anaerobic conditions occur in saturated zones that coincide with the water table surface. Soils with a deep source of CH4 have a soil CH4 concentration pro file characterized by two maxima—one at the soil surface and the other near the water table—separated by a minimum. Such profiles have been observed in a variety of upland ecosystems, including desert , temperate hardwood for est and temperate coniferous forest . It is possible that plants transport CH4 from a deep groundwater source through the transpiration stream, effectively bypassing the zone of CH4 oxidation . The most direct evidence of methanogenesis in upland soils is that they occasionally emit CH4 to the atmosphere. There are numerous reports of upland forests and savannas that switched for periods of time to CH4 sources , and wet-land forests that switched to CH4 sinks . In most cases the proxi mate cause for the shift was a change in soil water content, but the ultimate cause varied from seasonal shifts in precipitation and evapotranspiration , to plant community successional stage , to experimentally imposed warming . Because transpiration helps regulate soil water content, these studies suggest that tree physiology influences CH4 fluxes between upland forests and the atmosphere.Tree physiology influences both the production and oxidation of CH4, and can play an important role in determining whether a particular forest is a net source or sink of CH4. In the near absence of studies on plant regulation of CH4 cycling in upland forests, it is instructive to consider studies in wetland systems.

Plants are the ultimate source of organic carbon—in the form of root exudates or detritus—that microorganisms metabolize to CH4, and several isotope tracer studies have demonstrated a tight coupling between plant photosynthesis and methano genesis . A full cycle of CO2 assimilation by plants, release of photosynthate into soils and emission as CH4 requires as little as 2 hours, and up to 6% of the assimilated CO2 is emitted as CH4 in wetland ecosystems. Elevated CO2 concentration stimulates CH4 emissions from wetland soils , an effect that is di rectly proportional to the stimulation of photosynthesis by elevated [CO2] . Although most stud ies relating the effects of elevated [CO2] to CH4 emissions from wetland soils have been with herbaceous plants, a single study confirmed a linear relationship between CH4 emissions and photosynthesis in the wetland tree Taxodium distichum Rich. . It is reasonable to hypothesize that similar relationships between plant productivity and methane production occur in upland forests. For example, increasing inputs of labile carbon to upland soils may promote CH4 production both by enhancing the electron donor supply to methanogens, and expanding anaerobic microsites via in creased microbial O2 demand. Trees exert indirect regulation of CH4 production and oxidation through their influence on soil water content, which deter mines the proportion of the soil profile that is anaerobic and producing CH4 versus aerobic and oxidizing CH4. An example of tree physiology influencing CH4 cycling in upland forests is provided by the Duke FACE experiment. McLain et al. found that elevated [CO2] increased soil water content,planta de arándanos en maceta which simultaneously increased CH4 production and decreased CH4 oxidation. The increase in soil water content was caused by reduced transpiration in the elevated [CO2] treatment, and the net effect was a positive feedback on radiative forcing by CH4. A possible mechanism for CH4 emissions from upland veg etation is transport from the saturated zone below the water table through the transpiration stream.

In most ecosystems, the deepest 5% of roots occur at depths greater than 1 m and maximum rooting depths can exceed 4 m . The deepest root systems are found in tropical areas where high concentrations of atmospheric methane have been observed . Specifically, they occur in tropical semiarid to humid savanna, and tropical seasonally dry semideciduous to evergreen forests . Deep roots that access the water table may contribute disproportionately to transpiration fluxes . In such cases, CH4 dissolved in groundwater would presumably be entrained in the transpiration stream in a man nersimilar to CO2 from root respiration . We are unaware of any published measurements of CH4 concentrations in xylem sap.There are several recent reports suggesting that tropical forests may be larger sources of CH4 than previously believed. The most comprehensive analysis used a satellite-mounted instrument to show that atmospheric CH4 concentrations are far greater than expected from ground-based emissions inventories of tropical rain forests . The deviation between modeled and observed column-averaged at mospheric CH4 concentrations was especially large over the Amazon Basin and was correlated with the distribution of broad leaf evergreen forest. Frankenberg et al. noted that the discrepancies in measured and modeled CH4 concentrations could be explained by underestimates of known emissions sources such as wetlands, biomass burning, termites and cattle. The measurements were taken during the dry season when wetland emissions should be lowest and bio mass burning emissions should be highest, suggesting the bio mass burning was the more important source. However, localized measurements of atmospheric CH4 concentrations show that there can be significant biogenic CH4 sources in tropical upland forests. Methane concentration profiles in three upland forests of the Brazilian Amazon showed a CH4 source within the lower 10 m of the forest canopy , and nighttime pooling of CH4 at 2 m above the soil surface was observed in a mixture of forest and savanna in Venezuela . In both cases, when extrapolated to large areas, the estimated CH4 emission rates were potentially significant on a global scale . Scharffe et al. concluded that soil emissions were a relatively small contribution to CH4 sources at the Venezuelan site and suggested that termite mounds and waterlogged pools were unmeasured CH4 emission hotspots. Crutzen et al. reinterpreted these data as evidence of an aerobic plant CH4 source. Regardless of whether the source of the CH4 in these systems was vegetation or a combination of several known sources, none of which can be distinguished by these studies, it is clear that CH4 exchange between tropical upland ecosystems and the atmosphere has not been adequately characterized.Frankenberg et al. recognized that the discrepancies in measured and modeled CH4 concentrations could be explained by a “… hitherto unknown methane source that might be directly related to the broad leaf evergreen forest.” Just 7 months later, Keppler et al. published the first observations supporting one possible unknown CH4 source—direct emissions from aerobic vegetation. They reported that CH4 was emitted from every plant tissue tested, including detached leaves from 30 species, leaf litter and intact plants. The data of Keppler et al. suggested that sunlight, temperature and physiological activity were key variables regulating aerobic CH4 emissions. The sunlit rates for intact plants were significantly higher than those for detached leaves , dark emission rates for intact plants and detached leaves were significantly lower than sunlit leaves , and the temperature coefficient was about 2 over the range 30–70 °C. The process appeared to be non-enzymatic because emissions in creased monotonically up to 70 °C and CH4 was emitted from commercially available apple pectin. More recently, Dueck et al. used an isotope-labeling technique in an attempt to verify emissions of CH4 from aerobic plant tissue. This approach indicated rates that were not significantly different from zero, and at best, an order of magnitude lower than those of Keppler et al. . Increasing the amount of plant bio mass in the experimental chambers improved the detection limit of their technique and suggested that little or no CH4 is emitted by plant tissue. These data suggest that the fluxes re ported by Keppler et al. were an artifact of their methods. The experiments performed by Dueck et al. were more controlled and physiologically relevant than those by Keppler et al. , but it is unclear whether the hydroponic system they used effectively excluded CH4 oxidizing bacteria, which are aerobic and capable of consuming CH4 produced by plant tissue. The negative rates of CH4 production reported by Dueck et al. were reasonably interpreted as experimental error, but they could also be interpreted as net consumption of CH4 and it is unclear whether the leak tests they performed were long enough to allow for this possibility. Given the absence of in situ measurements of aerobic plant CH4 emissions, it is instructive to compare the Keppler et al. rates to other volatile organic carbon compounds such as methanol, which are relatively well under stood. There are many different VOCs, but the total flux from foliage is dominated by a few compounds such as isoprene and methanol. The initial studies of methanol emissions from plants reported rates from mature leaves that typically ranged from about 0.8 to 44 µg g–1 h–1 , which is at least an order of magnitude higher than the CH4 emission rates observed by Keppler et al. under similar conditions of light and of temperature. Methanol emission rates from young leaves are even higher than rates observed for mature leaves . Lower methanol emission rates have since been reported for most plants, but average methanol emission rates for mature sunlit leaves are at least 1.5 µg g–1 h–1, which is four times the CH4 emission rate reported by Keppler et al. . These figures suggest that the global contribution of CH4 from aerobic plant biomass, if it occurs at all, are considerably less than global emissions of methanol, which are estimated to be between 100 and 260 Tg year –1 .Keppler et al. offered a provocative global extrapolation of their intact plant CH4 emission rates that suggested up to 243 Tg year –1 of CH4 was emitted from this new source. This figure was derived by scaling leaf-mass-based emission rates to the globe with day length, growing season length and total net primary productivity as driving variables, all stratified by the major biomes.