Determination of Methane Oxidation in the Rhizosphere of Sagittaria landfolia Using Methyl Fluoride

Methane oxidation in the rhizosphere of wetland plants may significantly attenuate methane losses from wetland soils to the atmosphere. Our objective was to measure the extent of methane production and oxidation in the rhizosphere of a common wetland plant (Sagittaria landfolia L. Per.). Methyl fluoride (CH3F), a water-soluble gas and a specific inhibitor of methane oxidation, was used in conjunction with a closed chamber technique to determine rhizospheric methane oxidation in a greenhouse study. Rhizospheric methane oxidation was also estimated using a mass balance approach. Measurements of soil methane production were made using short-term anaerobic incubations of soil. Soil methane production and plant emissions of methane were inversely related to plant biomass, presumably because larger plants transported more O2 into the rhizosphere and inhibited methanogenic activity. Methane oxidation averaged 65% (SD = 24%, n = 14) as estimated by the CHjF technique and 79% (SD = 20%, n = 14) using the mass balance approach. Methane oxidation percentage calculated by either method was not correlated to plant biomass. Results suggest that rhizospheric methane oxidation is an important attenuator of methane emissions from vegetated wetland soils. M IS A KEY RADIATIVE GAS thought tO Contribute to global warming (Bolle et al., 1986), and wetland soils have been shown to be an important source of atmospheric methane at the global scale (Bartlett and Harriss, 1993). Studies of methane emissions have identified a number of environmental factors that determine the magnitude of methane flux from wetlands including temperature (Crill et al., 1988; Lansdown et al., 1992), water depth (Sebacher et al., 1986: Harriss et al., 1988), soil salinity (Bartlett et al., 1987), and vegetation presence and productivity (Whiting et al., 1991; Whiting and Chanton, 1993). Another potentially dominant attenuator of methane emissions from wetlands is oxidation of methane by methanotrophs. To date, the isolated methanotrophs are obligate aerobes and oxidize methane to CC>2 for energy production (King, 1992). Methane oxidation in the surface of wetland soils has been shown to reduce methane emissions from saturated soils by up to 90% (e.g., King et al., 1990). However, less is known about the extent of methane oxidation in the rhizosphere of wetland plants (Frenzel et al., 1992; Epp and Chanton, 1993). Wetland soils are often water saturated, organic rich, and depleted of C>2. Many wetland plants transport O2 L.A. Schipper, Landcare Research NZ Ltd, Private Bag 3127, Hamilton, New Zealand; and K.R. Reddy, 106 Newell Hall, Soil and Water Science Dep., Univ. of Florida, Gainesville, FL 32611-0510. Florida Agric. Exp. Stn. Journal Series no. R-04031. Received 7 July 1994. Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 60:611-616 (1996). internally to their roots for respiration. Oxygen in excess of root respiration requirements may be lost from roots to the surrounding rhizosphere and used by methanotrophs (King et al., 1990). In vegetated wetlands, internal gas transport by plants is responsible for up to 90% of the methane emissions from the wetland soil to the atmosphere (Chanton and Dacey, 1991). Methane produced in the soil passes through the rhizosphere prior to being lost from the plant and methane oxidation in the rhizosphere may significantly attenuate methane emissions from wetland soils. There have been only few values reported of the magnitude of methane oxidation in the plant rhizosphere because of difficulties associated with making in situ measurements. Indirect methods using whole roots, soil slurries, and mass balances have shown widely different methane oxidation percentages (Sass et al., 1990; King et al., 1990; Holzapfel-Pschorn et al., 1986). In some cases, it has been suggested that methane oxidation may be insignificant compared with methane production (Happell et al., 1993; King et al., 1990). Recently, CH3F was described as a specific inhibitor of methyl monooxygenase (MMO), a key enzyme in methane oxidation (Oremland and Culbertson, 1992a,b). Because CH3F acts as a specific inhibitor, it has considerable advantages over other inhibitors of methane oxidation such as nitrapyrine, acetylene, and picolinic acid, which inhibit the activity of nonmethanotrophs (Oremland and Capone, 1988). Additionally, CH3F is a watersoluble gas that can be easily transported by the plant to sites in the rhizosphere and the locus of methane oxidation. Oremland and Culbertson (1992a) demonstrated the usefulness of CH3F in determination of methane oxidation in soils and compost in laboratory studies. They also made preliminary measurements of methane oxidation in field studies using closed chamber techniques with CH3F added to the headspace (Oremland and Culbertson, 1992b). This study found methane emissions from surface soils to be three to 400 times higher when CH3F was added to the chamber, demonstrating the potential importance of methane oxidation in global methane cycling. Epp and Chanton (1993) also used closed chamber techniques in conjunction with CH3F and found that methane oxidation in the rhizosphere accounted for between 23 and 90% of potential methane emissions from a number of wetland plants. Further studies in the rhizosphere are needed to refine estimates of methane oxidation and to determine the dominant regulators of the extent of oxidation. The primary objective in this greenhouse study was to compare methane oxidation in the rhizosphere of Sagittaria landfolia determined by two methods: (i)