Effect of Sunlight on Decomposition of Soil Humic and Non-Humic Fractions

Keywords: Humic, Non-humic, Global warming, Sunlight, Photodecomposition, Thermal effect


Soils function as huge carbon sinks, storing more than twice the atmospheric C. The contributions of soil organic carbon (SOC) to global warming have mainly focused on the effect of biotic factors that are accelerated by temperature rise. Recently we demonstrated that sunlight causes significant reduction in soil organic matter (SOM). In this study, we recorded the changes in the two main fractions, humic and non-humic components of SOM under the influence of sunlight and UV light. Photodecomposition losses are higher in non-humic compared to the humic fraction. Forest soils have higher contents and proportionately larger amounts of non-humic fraction that are more rapidly decomposed. Therefore, deforestation could be a significant source of atmospheric CO2 adding about 1x104 kg of CO2 annually, for every hectare of land that is deforested. Both thermal (heat) as well as photolytic (light) effects of sunlight cause decomposition losses of humic and non-humic fractions; however, proportionately, more humic fraction is decomposed by light while non-humic fraction is more affected by heat.  Increase in pH causes greater photodecomposition of both humic and non-humic fractions. We conclude that sunlight needs to be considered as an important causative agent of global warming, with maximum contributions from the non-humic component of SOM.


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