Methane Consumption and Production in Desert Ecosystems Experiencing Mesquite Invasion and Control

We are studying the influences of vegetation change on soil fluxes of methane (CH₄) in semi-arid ecosystems. Soils under the natural grass vegetation are a strong sink of atmospheric CH₄ year-round, consuming -35.1 +/- 8.3 μ g CH₄ m^-2 h^-1 during the monsoon summer and -24.3 +/- 11.2 μ g CH₄ m^-2 h^-1 during winter, but consumption falls to near zero in June. The depth of maximum CH₄ oxidation varies during the year, corresponding to the presence or absence of surface soil moisture. Invasion of mesquite (Prosopis spp.) significantly (p < 0.001) reduces the monsoon CH₄ oxidation (-24.4 +/- 8.3 μ g CH₄ m^-2 h^-1), possibly due to the formation of a moist litter layer impeding CH₄ diffusion into the soil surface or from ammonium inhibition from the N-rich mesquite litter. CH₄ consumption is equally high in mesquite and grassland during the winter. Historically, ranchers have attempted to control invasion of mesquite onto grazing lands. Our work indicates that mesquite eradication may result in pockets of strong CH₄ production (42.3 +/- 17.6 μ g CH₄ m^-2 h^-1) as the mesquite trunk and roots decay. Pool dilution experiments with ¹³CH₄ are being performed to ascertain if CH₄ production is from soil methanogens or from termites. This work, when coupled with ecosystem C and N inventories and quantification of net N₂O and CO₂ respired, will help determine the potential for vegetation management in semi-arid ecosystems to mitigate or force potential climate change.