Photochemical Oxidation of Soil Organic Matter by Sunlight

Although soils are considered as major carbon stores, rapid oxidation of soil organic carbon can contribute to CO2 evolution and global warming on a large scale. Generally, microbial and climatic factors are thought to be mainly responsible for such oxidative losses. Another probable factor is photodegradation by sunlight. Cultivated soils of the tropics are left barren for a greater part of the year particularly during summer when sunlight is at its peak intensity. This could cause photodecomposition of soil organic constituents and account for rapid losses of soil organic carbon. Studies showed that 2-14% of organic carbon in soils could be oxidized within 3 years by sunlight. This amounts to 5000-47,000 kg of CO2 for every hectare of soil. Oxidation is not only due to the effect of light itself but also due to the heating caused by sunlight. Oxidative losses are higher in soils with higher levels of organic carbon and in soils of higher pH. It is suggested that covering soils with mulches or green cover during the fallow period in summer may be adopted to reduce photodegradative CO2 losses from soils.