Clémence Thiour-Mauprivez

Microplastics (MPs) frequently co-occur with pesticides and veterinary medicines in agricultural soils. However, their interactive effects on soil microbiota remain largely unknown. Therefore, we investigated the effects of three MP types (LDPE-, PBAT-, and starch-based), applied at two concentrations (0.01% and 0.1%), either alone or in combination with the fungicide pyraclostrobin and/or the anthelminthic albendazole (ABZ), on soil microbial functioning. Nitrate levels, nitrification rates, ammonia-oxidizing microorganisms, and denitrifying bacteria served as indicators of perturbations on soil N cycling in soils from France, the Netherlands, and Greece. Microbial responses were soil-dependent, with the Greek soil being the most affected. In contrast, plastic type- and dose-dependent effects were sporadic and limited in the French and Dutch soil. In the Greek soil, all MP types increased the abundance of ammonia-oxidizing bacteria and nitrification rates, accompanied by a compensatory decline in ammonia-oxidizing archaea and commamox bacteria. These effects were reversed by the co-application of MPs with ABZ. On the other hand, denitrifying bacteria remained unaffected in all soils. Our results are alarming, considering the perturbation of nitrification imposed by MPs and other soil pollutants, which could enhance greenhouse gas emissions or adversely affect soil fertility and agricultural production.

Summary: Rhizosphere microbiota conditioning is a promising strategy to enhance plant growth. We conditioned the rhizosphere microbiota of Brassica juncea to water deficit to assess its impact on plant growth. In a glasshouse, plants were first grown under well‐watered conditions, then exposed to moderate (MD, pF = 2.3) or extreme (ED, pF = 3.5) water deficits. We extracted and inoculated the rhizosphere microbiota to new plants and repeated this process 10 times. Control plants were kept well‐watered. We monitored changes in plant phenotypes and in rhizosphere microbial communities (bacteria and eukaryotes). The initial water‐deficit growth inhibition of plants was successfully alleviated by 19.3% in MD and 29.4% in ED after conditioning (MD: from −35.6% to −16.3%; ED: from −56.8% to −27.4%). This beneficial effect on plants was not observed during the well‐watered phases, suggesting an active role of the microbiota when water became scarce. The increase in plant growth correlated with aggregated rhizosphere soil and significantly matched changes in the bacterial community, featuring reduced diversity and increased biofilm production capacity along the conditioning process. We showed that microbiota conditioning was a fast and efficient way to achieve better plant growth under adverse conditions, likely via the adaptation capabilities of the rhizosphere bacterial community.

During crop growth cycle, often several different plant protection products (PPPs) are applied, in combination or sequentially. Such sequential applications result in unintentional mixtures of residues that may affect ecosystem services supported by non-target organisms such as soil microbes and nematodes. This scenario of sequential PPP application is frequent in agricultural practice but rarely addressed experimentally at field scale with regard to environmental impacts. The objective of this study was to evaluate the effect of individual and sequential application of three PPPs (the herbicide clopyralid, the insecticide zeta-cypermethrin, and the fungicide pyraclostrobin) on soil microbial communities, and on the abundance of free-living nematode. Single applications (at 1× or 10× the agronomical dose) were made to triplicated field plots with each one of the PPPs or all three PPPs in sequence, with untreated plots serving as controls. Plots were sampled prior to each application, and 7 and 28 days thereafter. The fungal community’s composition and abundance were found to be more susceptible than the bacterial community to PPP applications, while the bacterial was mainly driven by the in-field heterogeneity of soil properties. Transient effects of PPP applications were detected on nematode abundance. Higher tier ecotoxicological studies offer greater ecological relevance compared to the standard laboratory tests but are challenged by environmental variations that should be accounted for when evaluating the ecotoxicity of pesticides on soil organisms.

Alternative farming systems have developed since the beginning of industrial agriculture. Organic, biodynamic, conservation farming, agroecology and permaculture, all share a grounding in ecological concepts and a belief that farmers should work with nature rather than damage it. As ecology-based agricultures rely greatly on soil organisms to perform the functions necessary for agricultural production, it is thus important to evaluate the performance of these systems through the lens of soil organisms, especially soil microbes. They provide numerous services to plants, including growth promotion, nutrient supply, tolerance to environmental stresses and protection against pathogens. An overwhelming majority of studies confirm that ecology-based agricultures are beneficial for soil microorganisms. However, three practices were identified as posing potential ecotoxicological risks: the recycling of organic waste products, plastic mulching, and pest and disease management with biopesticides. The first two because they can be a source of contaminants; the third because of potential impacts on non-target microorganisms. Consequently, developing strategies to allow a safe recycling of the increasingly growing organic matter stocks produced in cities and factories, and the assessment of the ecotoxicological impact of biopesticides on non-target soil microorganisms, represent two challenges that ecology-based agricultural systems will have to face in the future.