Effect of Soluble Sugar and Organic Acid on the Active Organic Carbon Content and Methane Emission in Paddy Soil

PurposeThe exudates of rice roots are an important factor in regulating methane emissions from rice fields. Through anaerobic cultivation of paddy field soil, the influence of rice root exudates on methane emission from paddy field was explored.

MethodsSoluble sugar (Sr), oxalic acid (O), succinic acid (S), malic acid (M) and their combinations were added to rice root exudates, and then the soil enzyme activity, soil active organic carbon content and methane emissions were measured.

Results(1) The additional Sr and organic acids significantly affected soil enzyme activities, but the effects of different components on enzyme activities were varied. Among them, the activities of polyphenol oxidase and dehydrogenase increased significantly under the treatments of S, OSM, SrO, SrS and SrOSM compared with the blank (P<0.05); the activities of sucrase and cellulase treated with S, M, Sr and SrOSM were significantly reduced (P<0.05); the catalase activity of all treatments significantly reduced (P<0.05). (2) Soil microbial biomass carbon and soluble organic carbon content were significantly affected by the addition of Sr and organic acids (P<0.05). The contents of microbial biomass carbon and soluble carbon for treatments of O, S, M, OSM, Sr, SrO, and SrM, as well as easily oxidized organic carbon under the SrO treatment increased significantly (P<0.05); but the contents of easily oxidized organic carbon under the O and OSM treatments decreased significantly (P<0.05). (3) The cumulative emissions and fluxes of CH₄ under the treatments of SrM, M, SrO, O, SrS, and SrOSM significantly increased (P<0.05). (4) The soluble organic carbon content and catalase activity showed a very significant negative correlation (P<0.01), and it has a very significant positive correlation with the activities of dehydrogenase and polyphenol oxidase (P<0.01). Soil microbial biomass carbon was significantly positively correlated with methane emission flux (P<0.05).

ConclusionThe addition of soluble sugars and organic acids can change the active organic carbon content of the paddy soil and the activity of carbon-converting enzymes, and increase methane emissions. When the total carbon input is 0.3 mg/(g·d), the treatment of adding soluble sugar and malic acid maximizes methane emissions.