|Olk, Daniel - Dan|
Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/6/2004
Publication Date: 3/1/2005
Citation: Mitra, S., Aulakh, M.S., Wassmann, R., Olk, D.C. 2005. Triggering of Methane Production in Rice Soils by Root Exudates: Effects of Soil Properties and Crop Management. Soil Science Society of America Journal. 69:556-570. Interpretive Summary: When methane gas enters the atmosphere, it can efficiently absorb heat and thereby contribute to global warming. A significant proportion of global methane emissions into the atmosphere comes from irrigated lowland rice fields. The methane that is produced at later crop growth stages during a rice season may be formed by soil microorganisms from simple organic compounds that enter the soil as exudates from living roots of the rice crop. Little is known about the factors that control this conversion. This study investigated the effects of soil properties and crop management practices on methane production from root exudates. During laboratory incubations, root exudates and glucose were added to nine rice soils that varied either in their soil properties or in previous crop management. Trends in the amounts of methane produced could not be explained by any of the soil properties. In one set of soils, methane production was enhanced in those soils that during previous field management had been better aerated, compared to the soils that had been less aerated. This finding opposes the conventional association of anaerobic soil conditions with methane formation from other materials at earlier crop growth stages. The results suggest that crop management practices can affect methane production in multiple manners. The results enable scientists to better understand the complexities of methane production in rice soils.
Technical Abstract: Elevated methane (CH4) production in flooded soils during the reproductive growth stages of rice (Oryza sativa L.) might result from decomposition of root exudates and root tissue. Little is known about the factors of this CH4 production, which may be fully independent of CH4 produced at earlier growth stages from decomposing crop residues. This study investigates the influences of soil properties and crop management practices on CH4 production from root exudates. In two incubation experiments of 20-d duration, CH4 production was measured following addition of root exudates and glucose to (I) rice soils from five farmers' fields in the Philippines, and (II) one soil sampled from four long-term management treatments that varied in degree of soil aeration through crop rotation and timing of crop residue incorporation. The conversion of glucose-C to CH4 was 1.6 to 3.6 times greater than for root exudate C. In Experiment I, rates of CH4 production clearly differed among the five rice soils. The sole soil property that was correlated with cumulative CH4 production was cation exchange capacity. In Experiment II, however, no soil property was correlated with CH4 production. Methane production from glucose and root exudates was substantially greater in soil from the more aerated treatments, which opposes the common association of CH4 production with anaerobic conditions. Results demonstrate the production of CH4 from root exudates. We speculate that soil organic matter in the more aerated treatments interacted less chemically with the amended substances, enabling their faster conversion into CH4. Soil properties alone are inadequate to explain CH4 production from root exudates; crop management practices might also need to be considered.