Soil surface colonization by phototrophic indigenous organisms, in two contrasted soils treated by formulated maize herbicide mixtures

Abstract

Soil phototrophic microorganisms, contributors to soil health and food webs, share their particular metabolism with plants. Current agricultural practices employ mixtures of pesticides to ensure the crops yields and can potentially impair these non-target organisms. However despite this environmental reality, studies dealing the susceptibility of phototrophic microorganisms to pesticide mixtures are scarce. We designed a 3 months microcosm study to assess the ecotoxicity of realistic herbicide mixtures of formulated S-metolachlor (Dual Gold Safeneur(®)), mesotrione (Callisto(®)) and nicosulfuron (Milagro(®)) on phototrophic communities of two soils (Limagne vertisol and Versailles luvisol). The soils presented different colonizing communities, with diatoms and chlorophyceae dominating communities in Limagne soil and cyanobacteria and bryophyta communities in Versailles soil. The results highlighted the strong impairment of Dual Gold Safeneur(®) treated microcosms on the biomass and the composition of both soil phototrophic communities, with no resilience after a delay of 3 months. This study also excluded any significant mixture effect on these organisms for Callisto(®) and Milagro(®) herbicides. We strongly recommend carrying on extensive soil studies on S-metolachlor and its commercial formulations, in order to reconsider its use from an ecotoxicological point of view.

Soil Photosynthetic Microbial Communities Mediate Aggregate Stability: Influence of Cropping Systems and Herbicide Use in an Agricultural Soil

“…This microcosm experiment showed that there was a strong development (4–6 μg chl a g −1 dw ) of indigenous algal and cyanobacterial crusts on soil surfaces under optimal and stable laboratory conditions. Chlorophyll a concentrations, which are a proxy for photosynthetic microbial biomass, fell in the same range as those seen in previous laboratory incubation experiments (Crouzet et al, 2013; Joly et al, 2015) and in some field studies (Tsujimura et al, 2000; Lin et al, 2013). The low values observed in the reference samples were typical for field soil samples at the end of winter (early March).…”

“…Microalgae species have different pigments, and the pigment composition of a given species may vary according to its physiological state, especially if herbicides induce conditions of stress (Bérard and Pelte, 1999). That said, pigment profiling of soil or water samples remains a suitable and widely used approach for describing phytoplankton community structure (Zapata et al, 2000) and for examining how microalgal community composition responds to herbicides (Dorigo et al, 2004; Joly et al, 2015). There are few studies that have looked at the effects of phenyl-urea herbicides on soil microalgal biomass and community composition (Pipe, 1992).…”

“…The extracted chl a were then spectrophotometrically quantified at different wavelengths, and the chl a concentrations were calculated using the method and equation described by SCOR-UNESCO Working Group 17 (1966). Other photosynthetic pigments (chlorophyll b, chlorophyll c, fucoxanthin, lutein, diadinoxanthin, neoxanthin, zeaxanthin, and pheophytin a) were quantified using the same acetone extracts and HPLC (in accordance with Zapata et al, 2000 and Joly et al, 2015). This information helped clarify the biochemical structure of the photosynthetic microbial communities.…”

Mesotrione Herbicide: Efficiency, Effects, and Fate in the Environment after 15 Years of Agricultural Use

“…These results were confirmed in other studies made on close soils where, however, transient effects on microbial communities structures and N‐cycling communities were observed for mesotrione mixtures with formulated S‐metolachlor and nicosulfuron, at environmental doses. No strong effects of Callisto ® used alone or in mixture at the recommended field rate were recorded in a microcosm study, on the phototrophic communities of two different soils in term of fractions and microbial diversity , which confirmed results observed in bioassays . In a study using increasing doses of mesotrione ( i.e .…”

Responses of Limagne “Clay/Organic Matter-Rich” Soil Microbial Communities to Realistic Formulated Herbicide Mixtures, Including S-Metolachlor, Mesotrione, and Nicosulfuron

“…Moreover, an indirect stimulation of saprophytic microbial communities promoted by dead biomass from sensitive organisms (a readily available source of organic matter) may have occurred alongside a concomitant stimulation of resistant microbial population (Thirup et al 2000). A decrease in the abundance of nitrifying archaea occurred in microcosms treated by herbicide mixtures, and we also reported effects on phototrophic communities, mainly in Dual Gold Safeneur ® (alone or in mixture) treated microcosms (Joly et al 2014). These particular communities could then explain this transient increase of respiration activity without modifying the general abundance and diversity data.…”