The disappearance kinetics of fenhexamid in sterile and non-sterile conditions, as revealed by a mixed-effects model analysis

A kinetic model was used to evaluate rate and capacity of both sorption and degradation of fenhexamid in four soils under laboratory conditions. This molecule was chosen for its stability to chemical hydrolysis at environmental pH values and its very fast disappearance in soil. The biotic contribution to disappearance was determined in experiments conducted on four soils that were either unsterilised or sterilised by: chloroform fumigation (CHCl3), autoclaving or phenylmercuric acetate (PMA) addition. Sterilisation with CHCl3 was the least efficient method as proven by microbial counts. The clayey soils sterilised by autoclave showed a sudden down-step around 50–75 h in disappearance kinetics. Biotic contribution should be minimal because soils were sterilised as shown by plate counts and the disappearance might be due to new sorption surfaces, available after 50–75 h as a consequence of the disruption of some aggregates. Sterilisation of the soil with the highest organic matter content by PMA increased the instantaneous sorption, and in some cases, the fast sorption of fenhexamid. The treatment of soil A with PMA did not completely inhibit microbial activity. A non-linear regression analysis with mixed-effects models was a valuable tool in describing the disappearance kinetics in soils because it showed small differences among treatments. The analysis showed that three processes were important in the disappearance of fenhexamid: instantaneous sorption, fast sorption and slow sorption/degradation. The biotic contribution probably depended on soil type, being predominant only in one soil, whereas sorption (instantaneous and fast) was prevailing in the other soils.