ATRAZINE MINERALIZATION IN SLURRIES FROM SOILS IRRIGATED WITH TREATEDWASTE-WATER

Irrigation with treated waste water (TWW) for an extended period of ti me modified the conduciveness of soils to microbial degradation of atr azine -chloro,4-ethylamino,6-isopropylamino,s-triazine). Only one of t he tested soils exhibited intrinsic ability to mineralize atrazine wit hin a period of 35 days of incubation (Tzora soil, not irrigated with TWW). In the corresponding soil irrigated with TWW no atrazine mineral ization could be detected, In soils augmented with Pseudomonas sp. str ain ADP (an atrazine-degrading bacterium), the atrazine mineralization pattern depended on TWW irrigation history, and on pre-incubation of atrazine in the soil before inoculation with Pse. ADP. In soils irriga ted with TWW and inoculated with Pse. ADP one day after atrazine appli cation, rapid mineralization occurred and more than 80% of the atrazin e was mineralized in 10 days in all soils (inoculated with 10(6) cells g(-) dry soil). However, in the corresponding soils nor irrigated wit h TWW, a longer lag phase was observed and only 40-60% mineralization was recorded after equivalent incubation times. A different degradatio n pattern was observed when atrazine was pre-incubated for 12 days in the soils before inoculation with Pse. ADP. In soils irrigated with TW W, atrazine mineralization was retarded and less than 20% of it was mi neralized within 20 days, while in the corresponding soils not irrigat ed with TWW, atrazine mineralization reached 60-80%. From our results it is evident that aging of the herbicide in the soil has a pronounced effect on the mineralization results. Adsorption of atrazine was posi tively correlated with organic matter content in the TWW-irrigated soi ls and may be partially responsible for the inhibition of mineralizati on after long pre-incubation of atrazine in soil. This observation sug gests that if atrazine-degrading bacteria are present in the soil, lon g-term TWW irrigation could enhance mineralization of newly applied at razine in the first days after application, but would adversely affect mineralization of atrazine which had been aged in the soil for severa l days. Since the atrazine half-life in soils is in the order of weeks , it is to be expected that the overall effect of TWW irrigation on at razine degradation would be toward retardation of mineralization rates . (C) 1997 Elsevier Science B.V.