Hydrolysis and biological degradation of atrazine in soils

Detoxification of atrazine in soils results from both chemical hydrolysis and microbial degradation. Infrared analysis was used to study the hydrolysis of atrazine upon interaction with soil colloids and to ascertain the existence of enol, keto, and protonated-keto forms of hydroxyatrazine. Evolution of ¹⁴CO₂ from ¹⁴C-atrazine and ¹⁴C-hydroxyatrazine was indicative of microbial degradation in soils. Objectives of this investigation were: 1) to determine the transitional forms of hydroxyatrazine in different pH environments; 2) to establish the interactions of atrazine on H⁺, Al³⁺, Cu²⁺- or saturated surfaces of "allophane, " montmorillonite, or "natural montmorillonitic clay"; and 3) to ascertain the contribution of microbial degradation and chemical hydrolysis to atrazine detoxification in three Oregon soils. Infrared spectra provide evidence for the existence of enol, keto, and protonated-keto forms of hydroxy-s-triazines. The following transition is correlated with changes in pH: 1) an anionic species at pH > 11.5, 2) an enol form between pH 11.5 and 3.3, 3) a keto form at pH < 3.3, and 4) a protonated-keto species at pH < 0. Asymmetrical side chains (ethyl and isopropyl) of hydroxyatrazine apparently induced a doublet at 3400 and 3520 cm⁻¹ (protonated-ring vNH), whereas the symmetrical side chains (ethyl) of hydroxysimazine yielded a single band at 3330 cm⁻¹ in the protonated-keto forms. Hydroxypropazine was not protonated in these experiments. Acidic cations (H⁺ and Al³⁺) on the exchange complex of montmorillonite and Coker soil clay promoted the hydrolysis of atrazine as evidenced by a strong hydroxyatrazine carbonyl band at 1745 cm⁻¹ in infrared spectra. Reaction of atrazine with Ca- or Cu-montmorillonite did not produce a 1745 cm⁻¹ band, whereas a small degree of hydrolysis of atrazine was indicated in Cu-Coker clay by a weak band at 1745 cm⁻¹. Dehydration increased the hydrolysis of atrazine as evidenced by a more intense band at 1745 cm⁻¹ in the reaction product of Ca- or Cu-Coker soil clay plus atrazine, whereas the infrared spectra of Ca- or Cu-montmorillonite plus atrazine were not affected by dehydration. An "allophanic" colloid did not catalyze the hydrolysis of atrazine when the exchange complex was saturated with H⁺, Al³⁺, Ca²⁺, or Cu²⁺. "Al-allophane" was not sufficiently acidic to protonate added hydroxyatrazine as a carbonyl band was not observed in the reaction product. Thus under acidic field conditions, one might expect the smectites to enhance the chemical hydrolysis of atrazine while "allophanic" colloids and perhaps other amorphous materials would be relatively inert. Respired ¹⁴CO₂ from the ¹⁴C-ethyl side-chain component of atrazine represented approximately 10% of the input ¹⁴C-activity in Parkdale-A and Woodburn soils and 4.5% in Parkdale-C and Coker soils after 28 days of incubation. The isopropyl side-chain and the ring constituent of atrazine were subject to minimal attack by soil microorganisms. The hydroxyatrazine ring was attacked more readily than the atrazine ring. Hydroxyatrazine accounted for approximately 10% of the extracted ¹⁴C-activity from ¹⁴C-atrazine-treated Parkdale-A, Parkdale-C, and Coker soils, and 40% from the Woodburn soil. Hydrolysis is considered the dominant pathway of detoxification in the Woodburn soil, whereas detoxification of atrazine in Parkdale-A, Parkdale-C, and Coker soils is a combination of microbial and chemical activity

Microbial degradation of atrazine in soils

Atrazine is an asymmetrical s-triazine herbicide used pre- and post-emergence for the control of weeds in many crops. Under conditions considered unfavorable for microbial activity, atrazine may persist in soils for extended periods of time. However, the significance of chemical versus microbial degradation is not known. This study was conducted to determine the significance of microbial degradation of atrazine by pure cultures and the native soil population in non-sterile soils. Isolated bacterial cultures were used to inoculate seeds in an attempt to provide protection against atrazine residues. Atrazine-treated soil was incubated at 30°C for varying periods and the subsequent loss of activity was correlated to evolution of ¹⁴CO₂ from labeled-atrazine in a radiorespirometric system. Microorganisms, mostly bacteria, were isolated from a soil solution; pour plates of atrazine-treated and non-treated soil; and the rhizosphere of corn, oats, tomatoes, and soybeans. Viable cell counts were used as an index to test for the utilization of atrazine as the sole source of carbon. Eight bacterial isolates did not show an appreciable difference in cell counts with or without atrazine as the sole source of carbon. Seed inoculation with a mixture of three bacterial isolates did not increase the growth of oats grown in atrazine-treated soil as an indication of crop protection. In synthetic media bacterial cultures evolved a small amount of ¹⁴CO₂ from chain-labeled atrazine during the first 24 hours and none thereafter. In sterile soil the same isolates evolved 0.4-0.7 percent of the input activity in two weeks. A mold respired 4.0 percent. No ring breakage was observed. In non-sterile soils, 1.4-1.6 percent of chain and 0.6-1.0 percent of ring-labeled atrazine was evolved in two weeks and 1.1-1.6 percent of ring-labeled hydroxyatrazine. The latter rate was 2-3 fold greater than from ring -labeled atrazine and indicated the formation of hydroxyatrazine as the rate limiting step in the dissipation of atrazine from soils. Data from the incubation experiment showed a 73 percent loss of the initial atrazine after 3-4 weeks. In a similar time period, only 2.2-2.6 percent of chain and 1.0-1.2 percent of ring-labeled atrazine was respired. Thus, the ¹⁴CO₂ data did not account for the loss of atrazine and further supports the formation of hydroxyatrazine as the rate limiting step. Extraction of the soils containing labeled-atrazine showed the presence of hydroxyatrazine in non-sterile and sterile soils after two weeks. The radiorespirometric system designed for these studies is proposed as a means to obtain a relative index of the residual life of herbicides or pesticides. The ¹⁴CO₂ data may be extrapolated to give an index based on microbial participation. Extraction of the soils would provide a test for possible non-toxic metabolites that may be formed via chemical reactions. Such data would be most beneficial in selecting and recommending new herbicides

Comparative Effects of Propylene Oxide, Sodium and Autoclaving on Selected Soil Properties

Samples of soil (25 g) were treated with 1 or 2 ml of propylene oxide, 400 or 800 parts/10? of sodium azide, or autoclaved for 1.5 or 3.0 h. Soil sterilization was achieved by the propylene oxide and autoclaving treatments. Sodium azide inhibited the bacteria and actinomycetes and drastically reduced the fungal population. The autoclaving treatment decreased the soil pH 0.2 unit, while propylene oxide and sodium azide treatments increased it 0.5-1.1 units. Extractable manganous—Mn was increased 2- to 3-fold by all treatments except for a 90- to 120-fold increase in an autoclaved soil; extractable Ca was not affected; and the extractable K changes were slight. Total extractable N was increased 10-20 parts/10?, and available P was generally increased by the treatments. Propylene oxide induced the least chemical alterations upon sterilization and is considered an appropriate sterilant to study chemical transformations in soils; but, germination and growth of wheat and alfalfa were retarded in propylene oxide treated soil

Survival of a Rifampicin-Resistant Pseudomonas fluorescens

Pseudomonas fluorescens strain D7 (P.f. D7) is a naturally occurring soil bacterium that shows promise as a biological herbicide to inhibit growth of annual grass weeds, including downy brome (Bromus tectorum L.), in crop- and rangelands. Pseudomonas fluorescens strain D7rif (P.f. D7rif) is a rifampicin-resistant strain of P.f. D7. One of the greatest obstacles to successful biological weed control is survival of the organism under field conditions. Nine soils in the taxonomic order of Mollisols, collected from downy brome-infested areas of the Western and Central United States, were inoculated with P.f. D7rif and incubated in the laboratory to determine the effects of soil type, soil properties, incubation temperature, and soil water potential on survival of P.f. D7rif over 63 days. Silt loam soils from Lind, Washington, and Moro, Oregon, sustained the highest P.f. D7rif populations, and recovery was the lowest from Pendleton, Oregon soil. Survival and recovery of P.f. D7rif varied with soil type and temperature but not with the two soil water potentials tested. After 63 days, P.f. D7rif was recovered at levels greater than log 5.5 colony forming units (CFU) g−1 soil from five of the nine test soils, a level adequate to suppress downy brome under field or range conditions

Comparative Effects of Propylene Oxide, Sodium and Autoclaving on Selected Soil Properties

Samples of soil (25 g) were treated with 1 or 2 ml of propylene oxide, 400 or 800 parts/10? of sodium azide, or autoclaved for 1.5 or 3.0 h. Soil sterilization was achieved by the propylene oxide and autoclaving treatments. Sodium azide inhibited the bacteria and actinomycetes and drastically reduced the fungal population. The autoclaving treatment decreased the soil pH 0.2 unit, while propylene oxide and sodium azide treatments increased it 0.5-1.1 units. Extractable manganous—Mn was increased 2- to 3-fold by all treatments except for a 90- to 120-fold increase in an autoclaved soil; extractable Ca was not affected; and the extractable K changes were slight. Total extractable N was increased 10-20 parts/10?, and available P was generally increased by the treatments. Propylene oxide induced the least chemical alterations upon sterilization and is considered an appropriate sterilant to study chemical transformations in soils; but, germination and growth of wheat and alfalfa were retarded in propylene oxide treated soil