A comparison of growth and photosynthetic characteristics of two improved rice cultivars on methane emission from rainfed agroecosystem of northeast India

Wetland rice fields serve as an important anthropogenic source of atmospheric methane, a greenhouse gas implicated in global warming. An experiment was conducted at the North Bank Plain Zone of Assam, India, during summer rice growing season (April-July 2006) in order to elucidate the effects of morpho-physiological characteristics of rice (Oryza sativa L.) plants on methane emission from paddy fields. Two improved rice cultivars viz. Disang and Luit were grown in light textured loamy soil (Sand 28.50%, Clay 30.10%, Silt 41.40%, electrical conductivity (EC) 0.43 mmhos/100 g, cation exchange capacity (CEC) 10.20 meq 100 g(-1)) under rainfed condition. Higher seasonal integrated methane flux was recorded in cultivar Disang (E-sif = 1.38 g m(-2)) compared to Luit (E-sif = 0.96 g m(-2)). Both the cultivars exhibited two emission peaks; one at active vegetative growth stage and the other at panicle initiation stage of the crop. Methane emission from the cultivars was significantly regulated by crop phenology and growth. Vegetative growth in terms of leaf number and area, root volume and length and tiller number was higher in Disang. Statistical analysis of these parameters showed a positive correlation with methane emission. On the other hand, yield and all yield-attributing parameters were found to be superior in cultivar Luit. Cultivar Luit recorded higher photosynthetic rate after panicle initiation. On the other hand, Disang recorded higher rate of photosynthesis during active vegetative growth period. In Luit, maximum partitioning of photosynthates was found towards the developing panicle, whereas in cultivar Disang, photosynthates could not be allocated sufficiently towards the panicle. In Disang, maximum partitioning of photosynthates was recorded towards the vegetative parts (including root) of the rice plant. Variation in organic carbon content of soil was observed in the field planted with two cultivars. Higher soil organic carbon content was recorded in the field planted with cultivar Disang. From this, we hypothesize that in Disang, photosynthetic carbon products were utilized as substrate by methanogens in the rhizosphere leading to more production of methane. Additionally, higher vegetative growth with high methane transport capacity (MTC) may positively contribute to higher methane emission from cultivar Disang. (c) 2007 Elsevier B.V. All rights reserved.