Spatial variation in the capacity of soil for CH4 uptake and population size of methane oxidizing bacteria in dryland rice agriculture

The pattern of methane (CH4) oxidation and population size of methane oxidi zing bacteria (MOB) were studied in three different soils (rhizosphere, bul k and bare) of a dryland rice (Oryza sativa L. cultivar Narendra-118) field . The rhizosphere soil exhibited the strongest CH4 oxidation activity and b are soil the weakest. MOB population size was significantly higher in the r hizosphere (671.0 x 10(5) cells g(-1) soil) than in the bulk (569.0 x 10(5) cells g(-1) soil) or the bare soil (49.2 x 10(5) cells g(-1) soil), and NH 4+-N concentration was highest in the bare soil (6.74 mu g g(-1) soil) foll owed by the bulk (5.58 mu g g(-1) soil) and rhizosphere soil (4.02 mu g g(- 1) soil). Half saturation constant (K-m) and maximum oxidation rate (V-max) decreased significantly from the rhizosphere to bulk to bare soil and rang ed from 84.01 to 5.81 mu g g(-1) dry soil and 0.62 to 0.05 mu g h(-1) g(-1) dry soil, respectively. The rice rhizosphere not only supports a larger po pulation of MOB but also contributes substantially to the capacity of soil for CH4 uptake, leading to a predictable spatial pattern in CH4 sink streng th within the dryland rice ecosystem.