Spatio-temporal variation and effect of urea fertilization on methanotrophs in a tropical dryland rice field

Population size of methanotrophs in a dryland field planted to Oryza sativa L. variety Narendra-118 was quantified over a period of 13 weeks. Methanotroph numbers were higher in control plots (52.9–736.6×105 cells g−1 dry soil) than in plots treated with urea (43.8–676.0×105 cells g−1 dry soil), and were highest in the rhizosphere soil (499.8–736.6×105 cells g−1 dry soil) followed by bulk (451.4–684.1×105 cells g−1 dry soil) and bare (43.8–67.5×105 cells g−1 dry soil) soil. The concentrations of NH4+-N were significantly (P<0.001) lower in the rhizosphere (3.1–6.4 μg g−1 soil) than in bulk (4.1–8.3 μg g−1 soil) and bare soils (5.1–10.7 μg g−1 soil). The study suggests that the development of the rice rhizosphere brings about a spatial pattern in the distribution of methanotrophic bacteria which increases in size, over time, within the rhizosphere and adjoining bulk soil, and that the rhizosphere is a potential microsite of intense CH4 oxidation activity.