Abstract
The degradation of plastics results in the formation of smaller particles called microplastics (MP). MP is defined as plastic particles of less than 5 mm. Although MP is the most common pollutant in aquatic and terrestrial systems, there needs to be more information on MP in agricultural soil. Thus, the first study presented in this thesis is a review that discusses the current research regarding MP's potential impact on soil and rhizosphere. The review summarises different extraction and identification technics of MP from the rhizosphere, soil and plant tissue. The data collected revealed that MP could affect the physicochemical properties of the soil, leading to a reduction in water retention and nutrient availability. They can also interfere with plant growth, reproduction, and functionality and can be taken up by animals and plants. MP in soil can also lead to other pollutants accumulation, such as heavy metals and persistent organic pollutants. These can have a range of negative impacts on plant health, soil fauna, and fungal and bacterial communities. The second study investigated the species-specific effects of MP on three economically essential crops at morphological and metabolomic levels. Tomato and cucumber (dicotyledons) and barley (monocotyledon) were chosen. The results showed no toxic effect on seedling growth, but there was a reprogramming of the metabolomic profile of both roots and shoots of all three plants. Also, the result showed that the impact levels of MP on seedling growth and metabolome depend on plant species, plant section, and MP concentrations. The third study of the dissertation investigated the effect of the aerial exposition of MP on tomato plant growth, mineral profile, root metabolome, and rhizosphere microbial communities. The results showed no impact of MP on tomato growth. However, there was an alteration of mineral and metabolomic profile and the bacterial and fungal diversity. Studies published until now investigated the effect of MP on rhizosphere microbial communities after direct soil contamination. However, no study went near describing the mechanism of action of MP on microorganisms. In the fourth study, we investigated how MP affects the soilborne fungi and the tripartite interaction between crops-MP-soilborne IV fungi. The results showed that MP affects the spore germination and germ tube elongation, fungal hypha growth, and sporulation differently between Fusarium solani and Trichoderma viride. Besides, MP altered the interaction fungal-fungal to favour T. viride, which may indicate the enhancement of the T. viride biocontrol effect. MP also enhanced the performance of F. solani and T. viride toward tomato seedlings, where MP ameliorated the plant growth-promoting effect of T. viride and increased the pathogenicity of F. solani. In conclusion, this Ph.D. thesis summarises the potential impacts of MP in agroecosystems, including soil chemistry, biodiversity, and plants.