Effects of environmental chemicals on soil and plant health

The world’s soils are threatened by intensive farming, industrial pollution, mining, and poor waste management that are poisoning them. All soils are under a lot of pressure, as 95% is used towards the production of food for human and animal consumption; consequently, and unsurprisingly this soil has extensive damage. Soil pollution is described as soils that contain toxic chemicals or elements, such as pollutants, can be present in high enough concentrations to constitute a risk for the environmental and human health. To understand environmental health more holistically – e.g., in the context of a food chain – we urgently need to understand decode the interactions of these environmental chemicals with soils, their intrinsic microbiomes as well as with their crops. Such understanding will allow to find possible solutions to reduce the negative impacts of contaminants in our food chain. Plants are extremely adaptable, despite their limited ability to move, and have developed multiple physiological strategies to tolerate and defend themselves against a vast array of external pressures and stresses. In recent years microbiomes are more and more recognized for their importance on environmental health. One hypothesis is that plants modulate their associated microbiomes to cope with and survive under soil contamination, for instance by enhancing degradation, transformation, immobilization, or safe storage. However, evidence for this phenomenon remains fragmented. The aims of this thesis were to understand, taking a holistic One Health perspective, the microbiome-mediated chemical feedbacks on plant growth due to soil pollution. As contaminants we studied herbicides (Chapter I), arsenic (Chapter II) and the interaction between arsenic and benzoxazinoids (Chapter III). Soil herbicide contaminations were studied with glyphosate and terbuthylazine as chemical stresses of a healthy food chain. We found small effects on soil enzymes activities and that soil bacteria were more susceptible to herbicide contamination than fungi. However, investigation the food chain perspective, we did not find herbicide- or microbiome-mediated effects on the performance of maize plants (Chapter I). These results revealed that herbicides have a reduced impact on the surrounding environment without affecting maize growth, suggesting that herbicides present overall well-designed chemicals. Taking arsenic contamination of soil as a second stress of a healthy food chain, we found a shift in soil bacterial microbial community, as a response to arsenic toxicity in soil (Chapter II). We did not detect any changes in leaves, root, and kernel microbiomes, as well as in enzyme activities when the soil was challenged with arsenic. These results illustrated the importance of microbiomes in a One Health concept. Interestingly, we discovered a clear positive impact on plant performance by root-secreted benzoxazinoids that help to cope with arsenic stress: plants exuding benzoxazinoids tolerated better soil arsenic toxicity and growth of benzoxazinoid-deficient mutant plants could be rescued by exogenous applied benzoxazinoids (Chapter III). Hence, we confirmed the multifunctional and beneficial nature of benzoxazinoids, root exudates of grasses, to enhance plant resilience against arsenic in contaminated soils. This finding has a great agronomic potential in crop rotation systems, as it presents a tool to alleviate toxic effects due to soil arsenic contamination and therefore, ensuring better yield of crops in contaminated sites. Taken together the results of this thesis showed the importance of tackling the impact of chemicals introduced or present in the environment that potentially cause health problems to food chains

The effect of root‐associated microbes on plant growth and chemical defence traits across two contrasted elevations

1. Ecotypic differences in plant growth and anti‐herbivore defence phenotypes are determined by the complex interactions between the abiotic and the biotic environment. 2. Root‐associated microbes (RAMs) are pervasive in nature, vary over climatic gradients and have been shown to influence the expression of multiple plant functional traits related to biomass accumulation and biotic interactions. We addressed how variation in climatic conditions between lowland and subalpine habitats in the Alps and RAMs can independently or interactively affect plant growth and anti‐herbivore defence trait expression. 3. To address the contribution of climate and RAMs on growth and chemical defences of high‐ and low‐elevation Plantago major ecotypes, we performed a full‐factorial reciprocal transplant field experiment at two elevations. We coupled it with plant functional trait measurements and metabolomics analyses. 4. We found that local growing climatic conditions mostly influenced how the ecotypes grew, but we also found that the high‐ and low‐elevation ecotypes improved biomass accumulation if in the presence of their own‐elevation RAMs. We also found that while chemical defence expression was affected by climate, they were also more highly expressed when plants were inoculated with low‐elevation RAMs. 5. Synthesis: Our research demonstrated that root‐associated microbes (RAMs) from contrasted elevations impact how plants grow or synthesize toxic secondary metabolites. At low elevation, where biotic interactions are stronger, RAMs enhance plant biomass accumulation and the production of toxic secondary metabolites

Growth-, resistance-, and chemical-related trait measurement of C. pratensis and P major plant

The data file consists into an excel document divided in three worksheets; the first two named "C_pratensis" and P_major" contain data about the treatments, measurements of plant growth and plant secondary metabolites and the herbivore performance (one file for each species used in the study); the third worksheet called "Unit and Info About Variables" contains info and measuring units about treatments and dependent variables used in the study

Root-exuded specialized metabolites reduce arsenic toxicity in maize.

By releasing specialized metabolites, plants modify their environment. Whether and how specialized metabolites protect plants against toxic levels of trace elements is not well understood. We evaluated whether benzoxazinoids, which are released into the soil by major cereals, can confer protection against arsenic toxicity. Benzoxazinoid-producing maize plants performed better in arsenic-contaminated soils than benzoxazinoid-deficient mutants in the greenhouse and the field. Adding benzoxazinoids to the soil restored the protective effect, and the effect persisted to the next crop generation via positive plant-soil feedback. Arsenate levels in the soil and total arsenic levels in the roots were lower in the presence of benzoxazinoids. Thus, the protective effect of benzoxazinoids is likely soil-mediated and includes changes in soil arsenic speciation and root accumulation. We conclude that exuded specialized metabolites can enhance protection against toxic trace elements via soil-mediated processes and may thereby stabilize crop productivity in polluted agroecosystems

Soil (microbial) disturbance affect the zinc isotope biogeochemistry but has little effect on plant zinc uptake.

Zinc (Zn) is an important micronutrient but can be toxic at elevated concentrations. We conducted an experiment to test the effect of plant growth and soil microbial disturbance on Zn in soil and plants. Pots were prepared with and without maize and in an undisturbed soil, a soil that was disturbed by X-ray sterilization and a soil that was sterilized but reconditioned with the original microbiome. The Zn concentration and isotope fractionation between the soil and the soil pore water increased with time, which is probably due to physical disturbance and fertilization. The presence of maize increased the Zn concentration and isotope fractionation in pore water. This was likely related to the uptake of light isotopes by plants and root exudates that solubilized heavy Zn from the soil. The sterilization disturbance increased the concentration of Zn in the pore water, because of abiotic and biotic changes. Despite a threefold increase in Zn concentration and changes in the Zn isotope composition in the pore water, the Zn content and isotope fractionation in the plant did not change. These results have implications for Zn mobility and uptake in crop plants and are relevant in terms of Zn nutrition

Glyphosate and terbuthylazine effects on soil functions, microbiome composition and crop performance

Herbicides are widely used for weed control in agriculture, though their fate and impact on non-target organisms like soil microbes and their function remain relatively unknown. A further complication is that herbicide effects vary depending on how they are applied and due to varying soil moisture conditions. In this study we tested the hypothesis that spraying glyphosate or terbuthylazine directly onto bare soil and when soil moisture is high would impact the soil microbial communities and their function most strongly. We measured similar amounts of glyphosate and terbuthylazine in soil whether the herbicides were directly applied to soil or first sprayed on the weed Chenopodium album and we found evidence for more rapid metabolization at high soil moisture. We found that the soil bacterial rather than the fungal community was mainly affected by a single application of the two tested herbicides. The identified shifts in community composition were independent of the modes of herbicide application but strongly dependent on soil moisture. We further found that herbicide applications only had a small impact on soil microbial function, which was approximated with analyses of the activities of N-β-acetylglucosaminidase, acid phosphatase and β-glucosidase enzymes in soil. Finally, we also assessed the postapplication performance of the subsequent crop and found that the herbicides did not affect maize height, chlorophyll content and biomass. Overall, our study revealed that a single application of herbicides in recommended doses had minor effects on the soil microbiome with a temporal and soil moisture dependency. The latter finding points out that to avoid repercussions on non-target organisms and soil function, key research needs to solve the context-dependency of rapid herbicide degradation in soil

The Effects of Soil Microbial Disturbance and Plants on Arsenic Concentrations and Speciation in Soil Water and Soils

Arsenic (As) in soils harms soil organisms and plants, and it can enter the human food chain via the dietary consumption of crops. The mobility, bioavailability and toxicity of As are determined by its concentration and speciation. A greenhouse pot experiment was conducted to study the effects of soil microbial disturbance and maize plants on arsenic concentration and speciation in soil (pore) water and soils. Three soil treatments with varying microbial disturbance were designed for this experiment: native soil, sterilized soil and sterilized soil reconditioned with soil indigenous microbes. The three soil treatments were intersected with three levels of As in soils (0, 100 and 200 mg kg−1 spiked As). Ten pots of each treatment were planted with maize, while three pots were filled with soil without maize. The difference between native and reconditioned soil indicated the abiotic sterilization effect (artifact of the sterilization process), while the difference between sterilized and reconditioned soil showed the microbial disturbance effect. Both effects increased As release into soil water. The microbial disturbance effect was more pronounced for organic As species, showing the influence of soil microbes involved in As methylation. The abiotic sterilization effect was more evident in unplanted pots than planted pots and the microbial disturbance effect was observed only in unplanted pots, suggesting that both effects were mitigated by the presence of maize