Influence of incorporation or dual cropping of Azolla on methane emission from a flooded alluvial soil planted to rice in eastern India

Green manures are widely used in rice production and may influence methane efflux (CH4). Influence of application of Azolla (A. caroliniana Wild.), a widely used biofertilizer for rice (Oryza sativa L.), on CH4 efflux from a flooded alluvial soil planted to rice, and select soil and plant variables were investigated in a field experiment at Cuttack, India. Azolla was either incorporated as green manure at the beginning of the experiment or grown as dual crop in the standing water along with the rice crop. Dual cropping of Azolla (equivalent to 30 kg N ha−1) in conjunction with urea (30 kg N ha−1) effected lowest CH4 flux (89.29 kg CH4 ha−1). Cumulative CH4 flux followed the order of urea > Azolla (incorporated) + urea > Azolla (incorporated + dual crop) > no N control > urea + Azolla (dual crop). Growing Azolla had a moderating effect on CH4 efflux from flooded soil through an increase in the dissolved oxygen concentration at the soil–floodwater interface. Among the different soil and plant variables studied, soil redox potential, dissolved oxygen concentration at the soil–floodwater interface and a-naphthylamine oxidase activity of root base exhibited significant negative relationship with CH4 flux. In addition, Fe2+ and ninhydrin reactive nitrogen (NRN) contents of the flooded soil exhibited significant positive relationship with CH4 flux. Results indicated that, dual cropping of Azolla in conjunction with urea considerably reduced CH4 efflux without affecting the rice yields and can be used as a practical mitigation option for minimizing CH4 flux from flooded paddy. © 2000 Elsevier Science B.V. All rights reserved.