Isolation and characterization of mesotrione-degrading Bacillus sp. from soil

Dissipation kinetics of mesotrione, a new triketone herbicide, sprayed on soil from Limagne (Puy-de-Dôme, France) showed that the soil microflora were able to biotransform it.Bacteria from this soil were cultured in mineral salt solution supplemented with mesotrione as sole source of carbon for the isolation of mesotrione-degrading bacteria. The bacterial community structure of the enrichment cultures was analyzed by temporal temperature gradient gel electrophoresis (TTGE). The TTGE fingerprints revealed that mesotrione had an impact on bacterial community structure only at its highest concentrations and showed mesotrione-sensitive and mesotrione-adapted strains. Two adapted strains, identified as Bacillus sp. and Arthrobacter sp., were isolated by colony hybridization methods.Biodegradation assays showed that only the Bacillus sp. strain was able to completely and rapidly biotransform mesotrione. Among several metabolites formed, 2-amino-4-methylsulfonylbenzoic acid (AMBA) accumulated in the medium. Although sulcotrione has a chemical structure closely resembling that of mesotrione, the isolates were unable to degrade it.     

Dissipation of the triketone mesotrione herbicide in the soil of corn crops grown on different soil types

The new triketone herbicide mesotrione corresponds to the older sulcotrione in which the 2‐chloro benzoyl substituent is replaced by a nitro group, generating an herbicide of greater efficiency and a broader spectrum of activity. Mesotrione has been applied within the same 15 days period pre‐emergence at the rate of 150gha^‐1 to four corn crops made at different sites located 40 km apart in Belgium and of soils of different textures, but similar pH and organic matter (old humus) contents. The mesotrione soil half‐life in the 0–10 cm surface soil layer (which contained more than 90% of the residue) was 50 days in loam soil (at Zarlardinge), 41 days in sandy loam soil (at Melle) and in clay soil (at Koksijde), and 34 days in sandy soil (at Zingem). The cumulative effects of the recent organic fertilizer treatments and of the soil texture could explain the differences between the soil half‐lives. The time for the 90% dissipation of mesotrione was between 3.6 (in the sandy soil) to 4.7 months (in the sandy loam, loam and clay soils). The low mesotrione soil residues remaining after the corn harvest should disappear with the usual heavy rains in autumn, and the tilling which precedes the following crop and dilutes the mesotrione soil residue. These low mesotrione soil residues thus should have no phytotoxicity toward the following crop, especially at the lower application dose of 100 g mesotrione ha^‐1 used in practice.     

Mobility and adsorption of the triketone herbicide mesotrione in the soil of corn crops

The triketone herbicide mesotrione has been applied pre‐emergence at the dose of 150 g a.i. ha^?1 on corn fields grown within the same period of time at different sites on clay, loam, sandy loam or sandy soils. During the crops and one month after the corn harvest, the mobility of mesotrione has been measured in the 0–20 cm surface soil layer, soil samples being taken in the 0–2,2–4,4–6,6–8, 8–10, 10–15 and 15–20 cm surface soil layers. During the first month after the treatment, mesotrione remained in the 0–2 cm surface soil layer. Thereafter, mesotrione was at a higher concentration in a 2 cm‐thick soil layer which progressively moved down in the clay, loam and sandy loam soils. In the sand soil, mesotrione moved down as a uniform diffusion, its concentration being similar in all the 2 cm‐thick layers of the 0–10 cm soil surface layer. Low mesotrione residues (6–10 ug kg^?1 dry soil) attained the 10–15 cm layer faster in the clay and sand soils than in the loam and sandy loam soils. Mesotrione was not detected in the 15–20 cm soil layer of the loam and sandy loam soils, but well in the sand and clay soils. In the soils of the corn crops mesotrione thus mainly remained in the 0–10 cm surface soil layer. This low mobility and depth of penetration conjugated to the rate of mesotrione soil degradation explain why there was no movement of mesotrione toward the deeper soil layers. The adsorption of mesotrione was greatest on the soils recently treated with organic fertilizers, and having a loam or sandy loam texture. The adsorption coefficients explained the lower mobility and the greatest persistence of mesotrione in these soils.     

Effect of vegetative filter strips on herbicide runoff under various types of rainfall

Narrow vegetative filter strips proved to effectively reduce herbicide runoff from cultivated fields mainly due to the ability of vegetation to delay surface runoff, promote infiltration and adsorb herbicides. A field trial was conducted from 2007 to 2009 in north-east Italy in order to evaluate the effectiveness of various types of vegetative filter strips to reduce spring-summer runoff of the herbicides mesotrione, metolachlor and terbuthylazine, widely used in maize, and to evaluate the effect of the rainfall characteristics on the runoff volume and concentration. Results show that without vegetative filter strip the herbicide load that reaches the surface water is about 5-6 g ha⁻¹ year⁻¹ for metolachlor and terbuthylazine (i.e. 0.5-0.9% of the applied rate), confirming that runoff from flat fields as in the Po Valley can have a minor effect on the water quality, and that most of the risk is posed by a few, or even just one extreme rainfall event with a return period of about 25-27 years, causing runoff with a maximum concentration of 64-77 μg L⁻¹. Mesotrione instead showed rapid soil disappearance and was observed at a concentration of 1.0-3.8 μg L⁻¹ only after one extreme (artificial) rainfall. Vegetative filter strips of any type are generally effective and can reduce herbicide runoff by 80-88%. Their effectiveness is steady even under severe rainfall conditions, and this supports their implementation in an environmental regulatory scheme at a catchment or regional scale.