One of the developments in agriculture that has had the greatest impact on the greenhouse effect is the dramatic increase in rice paddy agriculture that has occured over the past 50 years. Productive and easy to grow in sub-tropical climates, rice cultivation has proved to be one of the most important ways to provide an adequate food supply in the developing world. Unfortunately, rice cultivation has several negative environmental impacts. Rice cultivation, like most other agriculture, leaches significant amounts of fertilizer to the environment. Beyond this, however, rice cultivation poses unique environmental challenges. The anoxic milieu of a flooded rice paddy is home to a large number of species of both aeorobic and anaeorbic bacteria. Their complex chemical interplay can releases a number of harmful chemicals including methane (CH4), nitrous oxide, methyl halides, volatile organic compounds, methyl iodide and dimethyl sulfide. (Redeker
The most significant global problem posed by rice paddies are the greenhouse effects caused by the release of methane and nitrous oxide. Methane is formed by methanogenic, anaerobic bacteria buried in the soil of a rice paddy. These organisms obtain energy by the disproportionation of organic carbon. The overall reaction is labelled 1 below, and the oxidation half reaction is labelled 2.
Important reactions in anaerobic methane production
The Eh value of -.244 volts is the electrical potential of the reaction at a pH of 7 with all other species at unit activity. This oxidation potential is much lower than aerobic oxidation, which has an Eh of .812 volts. This difference means that anaerobic disproportionation of organic carbon is much less efficient energetically than aerobic processes, and will only predominate in areas without any significant oxygen concentration. These anaerobic conditions do not necessarily predominate within a rice paddy. There are some aerobic bacteria present, some of which consume methane that is a by-product of the anaerobic bacteria. These methanothopic bacteria are found predominately at the surface of the submerged soil and and in the rice rhizosphere, where aerobic conditions are present. The balance of these two types of bacteria depends mainly on the moisture content of the soil, and consequently so does the amount of methane produced. The longer fields are allowed to be flooded with stagnant water, the more the balance shifts towards the methanogenic bacteria. The methane is not consumed but rather bubbles out of the field into the atmosphere.
The strategies that have been divised for reducing methane emissions are relatively simple and involve changing soil moisture levels. These solutions are relatively easy to implement, require no new technologies and reduce water consumption which is also a key environmental issue in the developing world. These factors make cutting or stabilizing greenhouse emissions from rice agriculture seem likely. (Xu)
