Abstract: Soil is the main sink of hydrophobic organic pollutants (PAHs). Desorption and bioavailability of PAHs are mainly affected by the interaction strength between PAHs and soil, as well as their physical and chemical properties. The effects of natural freezing-thawing cycle and long-term placement on desorption of PAHs in soil of Southwest China karst area are still unclear. In this study, we examined the effects of two environment processes of freezing-thawing cycle and long-time placement on the desorption behavior of PAHs (phenanthrene and pyrene) in two typical soils (yellow soil and limestone soil) in Southwest China karst area. The results showed that the freezingthawing cycle and long-term placement significantly affected the desorption behavior of PAHs in soil, probably because due to the changes of soil particle structure and pores. Such changes may alter the interaction mode and intensity between soil particles and pollutants, change the isolation of adsorbed PAHs molecules from aqueous solution, and lead to the decrease of desorption ability and even being locked. Under the freezing-thawing cycle and long-term placement, the desorption rate of PAHs in Southwest China karst soil had a negative correlation with its hydrophobicity.

Key words: substrate enzyme activity, quality., microbial agent, tomato, yield