Fate of Organic and Inorganic Pollutants in Paddy Soils

Abstract

Paddy soils have a heterogenous nature, with complex physico-chemical interactions and varying soil characteristics. Paddy soils remain flooded and are considered as rich sources of nutrients for plant growth. The nutrient levels mostly depend on different management practices, such as fertilizer application, irrigation, and tillage, and the movement of nutrients in the soils. These paddy soils normally show less movement of applied nutrients out of the medium than other soils, because of stagnant water that reduces the mobility rate. Paddy soils can become polluted by anthropogenic practices such as the use of sewage wastewater; industrial wastewater containing heavy metals; fertilizers; and pesticides, and the leakage of petrochemicals. Some natural pollutants can be oxidized by microbial activity, but most pollutants do not undergo biotic and chemical degradation. Inorganic (heavy metals) and organic pollutants (polychlorinated biphenyls, polychlorinated dibenzodioxins, and polychlorinated dibenzofurans) are the major types of pollutants in paddy soils. The numerous organic and inorganic pollutants resulting from anthropogenic activities can remain for long periods in nature and can be transported over long distances. In particular, organic pollutants can be bioaccumulated and biomagnified, thus reaching high levels that can be dangerous for human wellbeing and biological communities. Inorganic pollutants such as the heavy metals Pb, Cr, As, Zn, Cd, Cu, Hg, and Ni cause hazards for human health, for plants, for animals, and for the fertility status of the soil. These heavy metals are common pollutants in paddy soil and they bioaccumulate; in this way the concentrations of these pollutants increase in living systems, owing to their retention rates being higher than their discharge rates in these systems. The fate of these pollutants depends on their bioavailability, degradation by microorganisms, adsorption, desorption, leaching, and runoff. The transport and degradation of these pollutants in paddy soils and groundwater results in contamination. The physico-chemical characteristics of the paddy soil framework; for example, the water content, soil organic matter, presence of clay, and pH, influence the sorption or desorption and degradation of pollutants and also influence leaching to the groundwater and runoff to surface waters. The translocation of natural pesticides in paddy soils depends upon the ionic or neutral behavior of the soil constituents, on the pesticides’ solubility in water, extremity on the substance, and the colloidal nature of the paddy soils.