Levine, U. 2009. Comparisons of methanotroph communities in soils that consume atmospheric methane. Ph.D Dissertation, Michigan State University, East Lansing, Michigan, USA.
Methane is a potent greenhouse gas that is 21-25 times more efficient at trapping heat (infrared radiation) than carbon dioxide. Methane oxidation is mediated by methane-consuming microbes (methanotrophs), but only in upland soils does the activity of aerobic methanotrophs account for a net uptake of atmospheric methane. The conversion of native lands to row-crop agriculture diminishes the strength of the soil methane sink, typically dropping the rate of methane consumption by 70%.
To determine the relationship between the rates of methane consumption in soils and the diversity of microbes that catalyze them, we conducted molecular surveys of methanotroph communities across a range of land uses at The Kellogg Biological Station Long Term Ecological Research Site (KBS LTER) and correlated our findings to measurements of the in situ fluxes of methane. Rates of methane consumption and methanotroph diversity were positively correlated, as conversion of lands to row-crop agriculture led to a 7-fold reduction from maximal rates of consumption in the native deciduous forests. In fields abandoned from agriculture both methanotroph richness and the consumption of methane were estimated to require approximately 75 years to return to the present diversity and consumption rate of the native deciduous forests. The linear trajectory for recovery of both measures suggested that managing lands to conserve or restore methanotroph diversity would yield increases in the rate of consumption of atmospheric methane in KBS LTER soils.
Long-term fertilization is one aspect of row-crop agriculture that is likely to be a significant disturbance to the methanotroph community. We hypothesized that a consequence of this disturbance would be its association with decreases in methane consumption and methanotroph richness in fertilized forest sub-plots at KBS LTER, but neither rate nor methanotroph richness declined due to long-term fertilization alone at KBS LTER. A meta-analysis examined the effect of long-term fertilization in other sites, and revealed no consistent decline in methanotroph richness in fertilized soils. The methanotroph communities did display a distinct biogeography with communities clustering together based on geographic location. As a consequence, the composition of the unique soil methanotroph community probably plays a role in dictating the response of the methanotroph community to changing land use and its disturbances.
The causes behind the change in methanotroph richness and the correlated decrease in methane consumption associated with row-crop agriculture at KBS LTER remains unclear as it is not caused by fertilization alone. The quantification of the effect of other variables associated with agricultural management is necessary to determine which management strategies at KBS LTER could be utilized to enhance methanotroph richness. However, a management strategy determined at KBS LTER to be beneficial to the methanotroph community may not be applicable to other paired sites, as the unique methanotroph community and environmental characteristics from each geographic location will probably yield a dissimilar response to the management practice.
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