Ebullition of methane from rice paddies: the importance of furthering understanding

Abstract

Flooded rice fields and other wetlands (such as peatlands) are the most important source of atmospheric methane (CH4) (Rothfuss and Conrad 1998). Whalen (2005) estimated the total annual CH4 emission at approximately 600 Tg CH4 yr −1 (anthropogenic and natural sources), of which 20 % and 24 % were attributable to rice paddies and natural wetlands, respectively. Until recently, CH4 has not been comprehensively included in C inventories in wetlands because it represents a relatively small proportion of the total C budget (e.g., in peatlands less than 10 % in mass terms of the budget (Baird et al. 2009; Thompson 2008)). The implications of such exclusion can be large in terms of Global Warming Potential (GWP) (Baird et al. 2009), although there is still great uncertainty regarding the formation, size and dynamics of key components of the soil CH4 pool. Compiling a soil methane inventory requires data on free-phase CH4, and dissolved CH4; the latter is limited by the low solubility of CH4, and lack of ionic form such as those associated with CO2 dissolution. The total soil pool is generated and regulated via the balance of methanogenesis (CH4 production), methanotrophy (CH4 oxidation) and atmospheric-soil CH4 interactions (steady/episodic ebullition, diffusion, and plant-mediated transport). Ebullition has only really expanded in research terms in the last decade, and the number of cited studies, although building is limited. Ebullition is the formation and release of bubbles (predominately containing CH4, CO2 and N-related gaseous species) from inundated sub-surface soils or sediments to the water-table and, thereafter, the atmosphere. It remains unclear how bubbles are formed, what processes and conditions are required for their formation and the prerequisites for their release and transport towards the surface. It was originally assumed that the subsurface CH4 existed primarily in the dissolved form, but it has been estimated that 33–88 % of the total CH4 sub-surface store is in the gas-phase (Tokida et al. 2005b; Strack and Waddington 2008). Bubbles are thought to form when the combined partial pressures of the dissolved gases exceed the hydrostatic pressure (Chanton and Whiting 1995). The presence of these bubbles has important biogeochemical effects, including the development of localized CH4 Plant Soil (2013) 370:31–34 DOI 10.1007/s11104-013-1790-1