Masaphy, S., Soils and Water Institute, Agricultural Research Organization, Volcani Center, Bet Dagan, 50-250, Israel
Mandelbaum, R.T., Soils and Water Institute, Agricultural Research Organization, Volcani Center, Bet Dagan, 50-250, Israel

Irrigation with treated waste water (TWW) for an extended period of time modified the conduciveness of soils to microbial degradation of atrazine (2-chloro,4-ethylamino,6-isopropylamino,s-triazine). Only one of the tested soils exhibited intrinsic ability to mineralize atrazine within a period of 35 days of incubation (Tzora soil, not irrigated with TWW). In the corresponding soil irrigated with TWW no atrazine mineralization could be detected. In soils augmented with Pseudomonas sp. strain 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 irrigated with TWW and inoculated with Pse. ADP one day after atrazine application, rapid mineralization occurred and more than 80% of the atrazine was mineralized in 10 days in all soils (inoculated with 106 cells g-1 dry soil). However, in the corresponding soils not irrigated with TWW, a longer lag phase was observed and only 40-60% mineralization was recorded after equivalent incubation times. A different degradation pattern was observed when atrazine was pre-incubated for 12 days in the soils before inoculation with Pse. ADP. In soils irrigated with TWW, atrazine mineralization was retarded and less than 20% of it was mineralized within 20 days, while in the corresponding soils not irrigated 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 positively correlated with organic matter content in the TWW-irrigated soils and may be partially responsible for the inhibition of mineralization after long pre-incubation of atrazine in soil. This observation suggests that if atrazine-degrading bacteria are present in the soil, long-term TWW irrigation could enhance mineralization of newly applied atrazine in the first days after application, but would adversely affect mineralization of atrazine which had been aged in the soil for several 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 atrazine degradation would be toward retardation of mineralization rates. © 1997 Elsevier Science B.V.