Microbial degradation of trifluralin and atrazine residues in soil

Pre-emergent herbicides play a crucial role in farming systems by providing early weed control for better crop establishment. However, their persistence in soil for extended period may lead to soil and groundwater contamination. Long term persistence of herbicide residues near the surface zone may affect rotational crops. Several factors namely, soil conditions, chemical structure of herbicides as well as application method and frequency regulate their breakdown in the soil. A recent Grains Research and Development Corporation (GRDC) survey around Australia detected residues of 23 pesticides persisting in soil; with atrazine and trifluralin being frequently detected in New South Wales (NSW) soils. As soil microorganisms are known to be highly adaptable in extreme environmental conditions, certain microorganisms can be used to degrade a range of pesticides in soil.
The first objective of this study was to determine critical concentration levels of trifluralin and atrazine in soil at which crops are susceptible. The second objective was to determine the effect of temperature and moisture conditions on the persistence of atrazine and trifluralin residues in a clay loam soil. The third objective was to determine the changes in microbial community structure and functions associated with various concentrations of trifluralin and atrazine application in clay loam soil.
To fulfil the first objective, critical concentrations of trifluralin and atrazine causing phytotoxic effects to crops were investigated using a bioassay technique with five test crop species including cereal and legume species. Sandy soil was used in this purpose to minimize the interference of organic matter binding herbicide particles so that the actual phytotoxic effects of trifluralin and atrazine can be investigated. Shoot and root parameters of the tested crop species were fitted in logistic equations against herbicide concentrations to calculate effective doses (ED50) for 50% growth inhibition. Both herbicides affected shoot and root parameters of all the test crop species significantly. Trifluralin delayed crop emergence at lower concentrations (0.075 mg/kg) while it completely inhibited the growth of test crops at higher concentrations (2.40 mg/kg). However, atrazine had no significant effect on the crop emergence but drastically reduced overall crop performance. Legumes were comparatively more sensitive than cereals to both herbicides, while lucerne was the most sensitive crop species to both herbicides, with ED50 ranging from 0.01 to 0.07 mg/kg soil for trifluralin, and from 0.004 to 0.01 mg/kg for atrazine. Based on the crop sensitivity, lucerne can be used in soil bioassay techniques to quickly determine trifluralin and atrazine residues in soil before sowing so that a suitable rotational crop can be chosen to minimize the negative impacts of herbicide on crops.
The environmental impact on the dissipation of trifluralin and atrazine in clay loam soil was further investigated in laboratory incubation experiments, under various temperature and moisture conditions. A stochastic gamma model was used to predict the dissipation of both herbicides from the clay loam soil by incorporating environmental factors as covariates to determine the half-life and the days to complete dissipation. Temperature played the crucial role on atrazine persistence while the combined effect of temperature and moisture was critical on trifluralin persistence in the clay loam soil. Rapid loss of atrazine was observed under 30 °C than 10 and 20 °C; with a half-life of 7.50 days and 326.23 days to reach complete dissipation. Trifluralin dissipation was maximum under 30 °C with a moisture content of 70% field capacity (FC); with an estimated half-life of 5.80 days and 182.01 days to reach complete dissipation. Half-life of both herbicides tended to double with every 10 °C drop of temperatures over the range tested. Gamma distribution model estimated that both trifluralin and atrazine could persist in the clay loam soil for several years at ≤20 °C temperature. Moreover, high temperature and moisture conditions reduced their persistence in the clay loam soil which was likely due to the changes in soil microbial community compositions.
The changes in soil microbial community compositions and their functions were evaluated under various concentrations of atrazine and trifluralin in soil using DNA extraction and 16s RNA sequencing. In addition, soil microbial respiration was also investigated under different concentrations of both herbicides. Both herbicides stimulated soil microbial respiration as maximum CO2-C evolved was recorded under RD5x of the atrazine and RD2x of the trifluralin. The relative bacterial abundance was highest under these concentrations due to promotion of dominant bacterial population. The relative abundance of phylum Firmicutes clearly elevated with increasing atrazine concentrations at 20 and 40 DAT, whereas relative abundance of phylum Actinobacteria was maximum under RD2x at 40 DAT. On the other hand, trifluralin promoted relative bacterial abundance at later stages (40 DAT) while no change was observed until 20 DAT. The relative abundance of phylum Actinobacteria was maximum under RD5x whereas, relative abundance of Firmicutes was increased under RD2x at 40 DAT. At genus level, atrazine and trifluralin stimulated and supported the growth of genus Bacillus. Changes in microbial composition upon exposure to atrazine and trifluralin inhibited some bacterial groups due to toxic effects. Atrazine and trifluralin suppressed relative abundance of genus Kaistobacter from Proteobacteria phylum with increasing concentrations upon exposure. Atrazine had significant effect on overall bacterial diversity whereas effect of trifluralin was nonsignificant. These results conflicted with BIOLOG results as overall soil microbial diversity functions were suppressed by both herbicides studied. This is most likely due to the limitation of BIOLOG technique primarily depending on the fast-growing microorganisms for the colour development while ignoring catabolically inactive and other non-culturable microorganisms. Considering other environmental, chemical and soil properties, the changes in microbial compositions in soil under higher temperature and moisture conditions could promote herbicide degradation by increasing the abundance of some bacterial groups, even though some other groups were inhibited. However, lower temperature and moisture conditions could increase persistence of trifluralin and atrazine in clay loam soil long enough possibly due to reduced microbial activity. Farmers should remain cautious about the carryover potential of trifluralin and atrazine before selecting crops in rotation. Soil should be tested for persistence of suspected herbicides through bioassay or laboratory detection techniques, especially under prolonged drought conditions.