Factors influencing rates of degradation of an arylamide and a benzoic acid in subsoils

The kinetics of fundamental reactions (hydrolytic, oxidative and reductive) involved in the degradation of organic compounds such as pesticides in subsoils were investigated using the model compounds N-(4-nitrophenyl.)propanamide and 4-nitrobenzoic acid. The rates of hydrolysis of N-(4-nitrophenyl)propanamide were also measured in aqueous buffers, hydrolysis being extremely slow at neutral pH; its degradation in three soils was by microbially mediated hydrolysis, being very much fas ter than aqueous hydrolysis at the same pH. Rates of degradation of N-(4-nitrophenyl) propanamide in subsoils were initially up to thirty times slower than those in topsoil, and in some subsoils degradation showed a marked lag-phase of between 72-144h. For 4-nitrobenzoic acid, a similar lag-phase of slow degradation, followed by a phase of rapid degradation, was observed in both topsoils and subsoils. Remarkably, the rapid phases of degradation in subsoils often approached rates occurring in the corresponding topsoil. No reduction of the nitro group on either compound was observed, even in a water-saturated subsoil. Sometimes there were differences in the length of the lag-phases measured for replicate samples of subsoils; also, application of lower concentrations of 4-nitrobenzoic acid generally gave rise to shorter lag-phases. Partial sterilization of soils by azide greatly slowed breakdown of both compounds, confirming the important role of microbial degradation. Such behaviour is consistent with the variable build-up of populations of micro-organisms able to degrade the compound, smaller populations being able to deal rapidly with the lower concentrations. After applying a second dose of 4-nitrobenzoic acid to soil, degradation was rapid but initially not as fast as the final rates during breakdown of the first treatment. Hence, soil may only partially retain the ability to degrade previously applied xenobiotics. Nonetheless it is noteworthy that, even in deep subsoils, indigenous microbial populations can rapidly adapt to degrade certain small organic molecules. (C) 2000 Society of Chemical industry.