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

Abstract

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 CH₄ 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 × 10⁵ cells g⁻¹) than bulk (459.1 × 10⁵ cells g⁻¹) and bare (38.3 × 10⁵ cells g⁻¹) soils. The MOB population size was highest in control (418.8 × 10⁵ cells g⁻¹) followed by NH₄Cl (359.2 × 10⁵ cells g⁻¹) NH₄NO₃ (323 × 10⁵ cells g⁻¹), and urea (293.7 × 10⁵ cells g⁻¹) treated soils. NH₄⁺-N concentration was the highest in bare soil (10.5 μg g⁻¹) followed by bulk (8.8 μ g g⁻¹) and rhizosphere (6.8 μ g g⁻¹) soil. Urea treated soil had maximum NH₄⁺-N (10.1 μ g g⁻¹) and control soil the minimum (6.3 μ g 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 μ g g⁻¹ dry soil, and V_max from 0.04 to 0.60μ g h⁻¹ g⁻¹ dry soil. These differences in kinetic parameters might be due to differential species composition of CH₄ 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.