Methane flux from irrigated rice fields in relation to crop growth and N-fertilization

The effects of N-fertilization on rice plant growth (number of tillers, shoot and root biomass, root volume and porosity, grain yield) and their relationship with methane flux was investigated in three irrigated varieties of Oryza sativa L. (Sarju-52, Malviya-36 and Pant Dhan-4). The study design consisted of (a) control (unfertilized) vegetated, (b) fertilized vegetated, (c) control (unfertilized) bare, and (d) fertilized bare plots; laid down in a completely randomized block design in triplicate. Urea was applied in (b) and (d) in three split doses at a rate of 40, 30 and 30 kg N ha−1 at the time of transplanting, active tillering and grain filling stages of crop. The field was submerged before transplanting and the water depth ranged from 6.7 to 23.9 cm in response to rainfall. Every 10 d, crop growth and CH4 flux were measured from d 9 to 115 after rice transplanting. Sarju-52 and Pant Dhan-4 were similar in phenological stages but different than Malviya-36. Results showed that there were significant differences in all the growth variables measured for all the rice varieties due to growth period and fertilization. Variety×treatment, variety×growth period and treatment×growth period interactions were significant for all growth variables. Maximum CH4 flux from control (vegetated) plots was observed at the flowering stage (65 d after transplanting in Sarju-52 and Pant Dhan-4 and 76 d after transplanting in Malviya-36) and ranged from 10.79 to 14.20 mg m−2 h−1. In vegetated fertilized plots, maximum CH4 emission was observed 10 d later than in the vegetated control plots and ranged from 14.43 to 20.20 mg m−2 h−1. These values were from 7- to 12.3-fold higher than bare (unfertilized) plots. All growth variables, except mean shoot and root biomass, showed strong positive relationships with seasonal CH4 emission. It was concluded that the CH4 source strength was dependent on the rice variety under cultivation, its phenology, growth variables and soil fertilization.