Impact of Long-term and Intensive Rice Cultivation on Heavy Metal Accumulation in Soil: An Observation from Mae La River Basin, Central Thailand

Abstract

Intensive rice cultivation relies greatly on chemical fertilizers and pesticides, some of which contain heavy metals as impurities. Application of these agrochemicals can result in heavy metal accumulation in soil. This research discusses the current concentrations and migration pathways of heavy metals (Cu, Pb and Zn) in paddy soil subjected to long-term use of chemical fertilizers and pesticides. The study of soil pH and metal concentrations in 60 surface and subsurface samples from a relatively small river basin revealed that although highly varied, pH values and metal concentrations in topsoil were significantly different from values in subsurface soil. The soil pH as low as 5.4 was observed in topsoil. Although the observed levels in topsoil were in within safe limits according to the Dutch standard, they were significantly elevated above the subsurface concentrations. Evidence on the differences between their concentrations in paddy soil and river sediments suggests that considerable amount of metals in paddy fields have been transported into the river. A detailed study of input inventory, fluxes and migration pathways of these metals in the river basin can provide a better understanding of their mobility and measures that might needed to safeguard aquatic habitats, especially the well-known striped snakehead fish in Mae La River.
 HIGHLIGHTS
 
 Agrochemicals such as chemical fertilizers and pesticides contain heavy metals as impurities
 Long-term and continuous use of agrochemicals in rice production has resulted in lower pH and higher heavy metal contents in topsoil
 Low pH in topsoil can enhance metal mobility, suggesting that some of the heavy metals from agrochemical inputs are likely to be transported from paddy fields to their surrounding environments
 This study demonstrates that the effects of agrochemical use extend beyond agricultural soil. Impact assessment of their application, thus, should cover all major compartments/systems in the basin
 
 GRAPHICAL ABSTRACT