Variation in concentrations of organochlorine pesticides in crop rhizosphere soils

Variations in concentrations of organochlorine pesticide (OCP) residues in the rhizosphere soils were evaluated using rhizoboxes. A sequential extraction method was developed to study the fractionation and extractability of OCPs in rhizosphere soils. The key findings are as follows:; In soils spiked with gamma-HCH & DDT, and transplanted with wheat, the differences of gamma-HCH between the rhizosphere and non-rhizosphere soils increased with time, reached the peak on 30th sampling day, and then decreased with time. In the rhizosphere and non-rhizosphere soils, pp'-DDE/SigmaDDTs, op'-DDD/SigmaDDTs and pp'-DDD/SigmaDDTs increased with time; whilst op'-DDT/SigmaDDT and pp'-DDT/SigmaDDT decreased with time. The wheat, corn and soybean rhizosphere soils differed greatly in soil properties, but it was hard to conclude the effect of crop roots on the variation in concentration of gamma-HCH, p,p'-DDE, p,p'-DDD and p,p'-DDT in the rhizosphere soils except for root accumulation and translocation.; Organic carbon (OC), dissolved organic carbon (DOC) and cultivation period were tested to explore their potential effects on target OCPs in the rhizosphere soils. The concentration of the target OCPs in the wheat rhizosphere soils increased proportionally to soil OC, whilst the uptake of OCPs by wheat roots and further translocation to the aboveground part were inversely proportional to soil OC. DOC only showed a negative correlation with concentration of p,p'-DDE and p,p'-DDT in the corn rhizosphere soils. After a longer root-soil interaction, roots had a more significant effect on the concentration of OCPs in the rhizosphere soils closer to the root surface.; The variation of different forms of OCPs in rhizosphere soils and their relationships with nitrogen nutrients and organic carbon were studied.; In the control, wheat and corn rhizosphere soils, the n-hexane extracted fraction of gamma-HCH, DDE, DDD and DDT decreased with time whereas the hexane/acetone extracted fraction increased with time after the 20th sampling day. The n-hexane extracted forms were higher in the rhizosphere soils than those in the non-rhizosphere soils, while the hexane/acetone extracted forms were lower in the rhizosphere soils than in the non-rhizosphere soils.; In the wheat, corn rhizosphere soils and the control, the concentration of NO3-N showed a significant negative correlation with n-hexane extracted DDE, DDD and DDT residues and a significant positive correlation with hexane/acetone extracted residues. The concentration of ammonium nitrogen (NH4-N) showed a significant negative correlation with hexane extracted gamma-HCH, DDE, DDD and DDT residues in the control, corn and wheat rhizosphere soils: but only had significant positive correlation with the n-hexane/acetone extracted fraction in the corn rhizosphere soil. The positive correlations between the n-hexane extracted residues of the target pesticides and soil OC were seldom significant in the control, sometimes significant in the wheat rhizosphere soils, and always strong and significant in the corn rhizosphere soils. The correlation of the n-hexane/acetone extracted residues with soil OC was positive and sometimes significant in the wheat rhizosphere soils, and significant and negative in the corn rhizosphere soils. The results indicated that the concentrations of different OCPs extracted from were strongly influenced by nutritional conditions and soil organic carbon.