INVESTIGATION OF THE METHYL-FLUORIDE TECHNIQUE FOR DETERMINING RHIZOSPHERIC METHANE OXIDATION

Methane oxidation rates in the rhizosphere of Pontederia cordata, Sagi ttaria lancifolia, and Typha latifolia were quantified in field studie s using the methyl fluoride inhibition technique. An average oxidation of 22.9 +/- 17.7% (sd, n = 44) was found for all field experiments (o xidation is expressed as a % of total potential emission in the presum ed absence of oxidation). Greenhouse experiments using: the same techn ique gave an average rhizospheric oxidation of 64.9 +/- 17.0% (sd, n = 44). Comparison of a subset of greenhouse plants with the methyl fluo ride (MF) and a light oxic/dark anoxic (LOIDA) technique for suppressi ng CH4 oxidation yielded similar percentages (57.7 +/- 15.0% for MF an d 58.5 +/- 13.9% for LO/DA, n = 11). Rhizospheric oxidation displayed a seasonal trend in Typha latifolia with decreasing oxidation percenta ges during warmer months as the importance of rhizospheric CH4 oxidati on declined relative to CH4 emission (46.5 +/- 13.8% in December and 1 3.5 +/- 1.7% in July). However, the absolute rate of methane oxidation was highest during the warmer months (44.2 +/- 3.4 mg m(-2) d(-1) in December and 318.7 +/- 151.4 mg m(-2) d(-1) in July). As methane emiss ion rates increased, the sensitivity of the methyl fluoride technique decreased due to the larger error between replicate flux measurements.