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 sub soils were investigated using the model compounds N-(4-nitrophenyl.)propana mide and 4-nitrobenzoic acid. The rates of hydrolysis of N-(4-nitrophenyl)p ropanamide were also measured in aqueous buffers, hydrolysis being extremel y slow at neutral pH; its degradation in three soils was by microbially med iated hydrolysis, being very much fas ter than aqueous hydrolysis at the sa me pH. Rates of degradation of N-(4-nitrophenyl) propanamide in subsoils we re initially up to thirty times slower than those in topsoil, and in some s ubsoils degradation showed a marked lag-phase of between 72-144h. For 4-nit robenzoic acid, a similar lag-phase of slow degradation, followed by a phas e of rapid degradation, was observed in both topsoils and subsoils. Remarka bly, the rapid phases of degradation in subsoils often approached rates occ urring in the corresponding topsoil. No reduction of the nitro group on eit her compound was observed, even in a water-saturated subsoil. Sometimes the re were differences in the length of the lag-phases measured for replicate samples of subsoils; also, application of lower concentrations of 4-nitrobe nzoic acid generally gave rise to shorter lag-phases. Partial sterilization of soils by azide greatly slowed breakdown of both compounds, confirming t he important role of microbial degradation. Such behaviour is consistent wi th 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 soi l, degradation was rapid but initially not as fast as the final rates durin g breakdown of the first treatment. Hence, soil may only partially retain t he ability to degrade previously applied xenobiotics. Nonetheless it is not eworthy that, even in deep subsoils, indigenous microbial populations can r apidly adapt to degrade certain small organic molecules. (C) 2000 Society o f Chemical industry.