Differential inhibition of CH4 oxidation in bare, bulk and rhizosphere soils of dryland rice field by nitrogen fertilizers

Laboratory experiments were conducted to elucidate the variation of CH4 oxidation pattern in three different soils (rhizosphere, bulk and bare) of a dryland rice (Oryza sativa L. cultivar Narendra-118) field. The rhizosphere soil exhibited strongest CH, oxidation activity and the bare soil the weakest. Control soils (no N-fertilization) exhibited higher capacity for CH4 oxidation than N-fertilized soils. Above conclusions were supported by measurements of MOB (methane oxidizing bacteria) population size, NH4+ -N concentration (as a potential inhibitor of CH4 oxidation), soil moisture content and kinetic parameters (K-m and V-max) for the same conditions. MOB population size was significantly higher in the rhizosphere (548.6 x 10(5) cells g(-1)) than bulk (459.1 x 10(5) cells g(-1)) and bare (38.3 x 10(5) cells g(-1)) soils. The MOB population size was highest in control (418.8 x 10(5) cells g(-1)) followed by NH4Cl (359.2 x 10(5) cells g(-1)) NH4NO3 (323 x 10,5 cells g(-1)), and urea (293.7 x 105 cells g(-1)) treated soils. NH4+-N concentration was the highest in bare soil (10.5 mug g(-1)) followed by bulk (8.8 mug g(-1)) and rhizosphere (6.8 mug g(-1)) soil. Urea treated soil had maximum NH4+-N (10.1 mug g(-1)) and control soil the minimum (6.3 mug g(-1)). Apparent half saturation constant (K-m) and maximum oxidation rate (V-max) increased significantly from bare to bulk to rhizosphere in control as well as fertilized soil. K-m ranged from 4.79 to 139.50 mug g(-1) dry soil, and V from 0.04 to 0.60 mug h(-1) g(-1) dry soil. These differences in kinetic parameters might be due to differential species composition of CH oxidizing community and/or conditioning of MOB to different soil microenvironments. This study has demonstrated competitive inhibition effect of NH4+-N on CH4 oxidation, which is relatively less strong for rhizospheric MOB due to root associated processes.