Addition of electron acceptors, such as ferrihydrite, to paddy fields can stimulate microbial populations that compete with and slow the activity of methanogens, thereby reducing emissions of methane. Rice cultivation is responsible for 10% of GHG emissions from agriculture. In developing countries, the share of rice in GHG emissions from agriculture is even higher, e.g., it was 16% in 1994.
See 'Rice production technologies' for an overview of all climate change mitigation technologies related to rice cultivation.
According to Lueders and Friedrich (2002), methane emissions from paddy fields can be reduced by the addition of electron acceptors to stimulate microbial populations that compete with methanogens. Under ferrihydrite amendment, acetate was consumed efficiently (<60 μM), and a rapid but incomplete inhibition of methanogenesis occurred after three days.
Methanogenesis can be suppressed by the supplementation of alternative electron acceptors such as Fe (III) or sulfate, when electron donors for respiratory processes become limiting (Achtnich et al., 1995). This mitigation strategy is based on the thermodynamic theory which predicts that the energetically more favorable electron acceptor will be utilised first under substrate limiting conditions (Zehnder and Stumm, 1988). Microorganisms which can reduce the energetically more favorable electron acceptor (e.g., nitrate, Fe (III), sulfate) will outcompete those using a less favorable electron acceptor (e.g., CO2).
Functional shifts can occur within a rice field soil microbial community by supplementing alternative electron acceptors in the form of ferrihydrite and gypsum, and thereby respiratory processes other than methanogenesis are promoted. Under gypsum addition, hydrogen was rapidly consumed to low levels (~0.4 Pa), indicating the presence of a competitive population of hydrogenotrophic sulfate-reducing bacteria (SRB). This was paralleled by a suppressed activity of the hydrogenotrophic RC-I methanogens as indicated by the lowest SSU rRNA quantities. Full inhibition of methanogenesis only became apparent when acetate was depleted to non-permissive thresholds (<5 μM) after 10 days.
The enhanced activity of FRB (Ferric iron reducing bacteria) and SRB (sulfate reducing bacteria) resulted in almost complete inhibition of methanogenesis under conditions of limiting substrate and non-limiting electron acceptor availability. Considering the electron uptake potential of eight electrons per CO2 and SO42, and one electron per Fe3+, only the amount of sulfate reduced perfectly matched the quantity of methane which was not produced under inhibition. FRB also participate in the oxidation of electron donors other than acetate and H2, thus limits its properties of reduction in methanogenesis. This may be another reason for the lower efficiency of inhibition of methanogenesis under ferrihydrite amendment. It was also demonstrated by Lueders & Friedrich (2002) that although the mitigating agent such as gypsum is added in the soil about one-tenth that of the ferrihydrite amendment, but still the mitigation effects were comparable: 69% and 85% methane reduction, respectively.
Addition of alternative electron acceptors is experimental and not yet a field practice. More research needs to be done to determine the cost effectiveness of this approach, as well as its consequences on yield and the environment.
- Methane emissions can be reduced.
- The approach is still at the experimental stage.
The economics and mitigation potential have not yet been established.
More research needs to be done to determine the cost effectiveness of this approach.
Achtnich C., Bak F., and Conrad R., (2005): Competition for electron donors among nitrate reducers, ferric iron reducers, sulfate reducers, and methanogens in anoxic paddy soil. Biology and Fertility of Soils 19, 65-72.
Lueders T. and Friedrich M W. (2002): Effects of Amendment with Ferrihydrite and Gypsum on the Structure and Activity of Methanogenic Populations in Rice Field Soil. APPLIED AND ENVIRONMENTAL MICROBIOLOGY, 68(5):2484-2494.
Zehnder AJB. and Stumm W. (1988): Geochemistry and biochemistry of anaerobic habitats, p 1-38. In A.J.B. Zehnder (ed.), Biology of anaerobic microorganisms. Wiley Interscience, New York, N.Y.