ABSTRACT
More than a third of all CH4 emissions come from soils, as a result of the microbial breakdown of organic compounds in strictly anaerobic conditions. This process occurs in natural wetlands (Chapter 3), in flooded rice fields and in landfill sites rich in organic matter (Chapter 11), as well as in the gut of some species of soil-dwelling termites (Chapter 5). Rice fields make a significant contribution to anthropogenic CH4 emissions, which are responsible for the concentration in the atmosphere more than doubling since the pre-industrial era, when, on the basis of evidence from ice core analysis, it was only about 0.7μmol mol–1 (Prather et al, 1995). Early estimates suggested that rice production contributes about a quarter of the total anthropogenic CH4 source and is of similar strength to the ruminant source or the energy sector (Fung et al, 1991; Hein et al, 1997). However, estimates have declined substantially with time, to values mostly between 25 and 50Tg CH4 year–1. Higher values generally originate from inverse modelling of observed fluctuations in atmospheric CH4 mixing ratios (top-down method) (for example Hein et al, 1997; Chen and Prinn, 2006), whereas lower values are obtained by scaling up field observations (bottom-up method) (Figure 8.1).
