SPATIAL-DISTRIBUTION OF METHANE-OXIDIZING ACTIVITY IN A FLOODED RICE SOIL

In a study on spatial distribution of methane oxidation in an unplante d flooded field, methane-oxidizing activity, analysed in soil samples under laboratory conditions, decreased with increasing depth (25 cm an d beyond). In a flooded field planted to rice, rates of methane oxidat ion followed the order : rhizosphere (collected from roots at 10-20 cm depth) > surface soil at (0-1 cm) > subsurface soil at 10-20 cm depth , diagonally 10-15 cm away from the centre of hill. Application of amm onium sulfate and, to a lesser extent, urea to surface, rhizosphere an d subsurface soil samples from flooded field planted to rice effected a distinct inhibition of methane oxidation. Nitrification inhibitors ( thiourea, sodium thiosulfate and dicyandiamide) were also effective in inhibiting methane oxidation. Both surface and rhizosphere soil sampl es harbored higher populations of methane-oxidizing bacteria than the subsurface soil. Inhibition of methane oxidation in surface and rhizos phere soil samples concomitant with the suppression of autotrophic amm onium oxidizers by nitrification inhibitors implicates an active invol vement of autotrophic ammonium oxidizers in methane oxidation.