Methane oxidation in an intensively cropped tropical rice field soil under long-term application of organic and mineral fertilizers.

Methane (CH 4 ) oxidation is the only known biological sink process for mitigating atmospheric and terrestrial emissions of CH 4 , a major greenhouse gas. Methane oxidation in an alluvial soil planted to rice (Oryza sativa L.) under long-term application of organic (compost with a C/N ratio of 21.71), and mineral fertilizers was measured in a field-cum-laboratory incubation study. Oxidation rates were quantified in terms of decrease in the concentration of CH 4 in the headspace of incubation vessels and expressed as half-life (t1/2) values. Methane oxidation rates significantly differed among the treatments and growth stages of the rice crop. Methane oxidation rates were high at the maximum tillering and maturity stages, whereas they were low at grain-filling stage. Methane oxidation was low (t 1/2 = 15.76 d) when provided with low concentration of CH 4 . On the contrary, high concentration of CH 4 resulted in faster oxidation (t 1/2 = 6.67 d), suggesting the predominance of "low affinity oxidation" in rice fields. Methane oxidation was stimulated following the application of mineral fertilizers or compost implicating nutrient limitation as one of the factors affecting the process. Combined application of compost and mineral fertilizer, however, inhibited CH 4 oxidation probably due to N immobilization by the added compost. The positive effect of mineral fertilizer on CH 4 oxidation rate was evident only at high CH 4 concentration (t 1/2 =4.80 d), while at low CH 4 concentration their was considerable suppression (t 1/2 = 17.60 d). Further research may reveal that long-term application of fertilizers, organic or inorganic, may not inhibit CH 4 oxidation.