Open Access

Ana Carolina Cugler Moreira

• Agricultural plastics (e.g., greenhouses, tunnels, and mulch films) are indispensable in many parts of modern agriculture. They enhance crop yields by improving growing conditions and enabling year-round production. Mostly, they are made of resistant materials, such as polyethylene. Due to their persistence, plastic residues accumulate in soil, increasing agricultural soil pollution and negatively affecting soil and plant health. Additionally, once accumulated in soil, these residues can be transported to surrounding ecosystems (e.g., aquatic environments) via, for instance, surface runoff. One way to avoid the plastic residue accumulation in soils is to use soil-biodegradable mulch films. However, as the full biodegradation in soil takes years, soil-biodegradable residues can also be transported to surrounding ecosystems. Due to the many variables and effects that agricultural plastics can have on the environment, this thesis is divided into three main sections, aiming to elucidateAgricultural plastics (e.g., greenhouses, tunnels, and mulch films) are indispensable in many parts of modern agriculture. They enhance crop yields by improving growing conditions and enabling year-round production. Mostly, they are made of resistant materials, such as polyethylene. Due to their persistence, plastic residues accumulate in soil, increasing agricultural soil pollution and negatively affecting soil and plant health. Additionally, once accumulated in soil, these residues can be transported to surrounding ecosystems (e.g., aquatic environments) via, for instance, surface runoff. One way to avoid the plastic residue accumulation in soils is to use soil-biodegradable mulch films. However, as the full biodegradation in soil takes years, soil-biodegradable residues can also be transported to surrounding ecosystems. Due to the many variables and effects that agricultural plastics can have on the environment, this thesis is divided into three main sections, aiming to elucidate main research gaps regarding (1) the degradation and (2) influencing factors on the accumulation of conventional agricultural plastics, as well as (3) the transport and soil effects of soil-biodegradable alternatives. Understanding the degradation and accumulation factors of agricultural plastics is essential to elucidate the environmental abundance of plastic residues and to develop effective mitigation strategies to avoid and control soil pollution. Hence, the first main section of the thesis is based on a controlled laboratory experiment, where photodegradation via ultraviolet radiation and mechanical abrasion of different types of low-density polyethylene agricultural plastics over different time periods was analysed. The analysis underlined that photodegradation and the removal of microplastic surface particles through mechanical abrasion already occurred after short ultraviolet radiation exposure. Abrasion has been identified as an important modulator in photodegradation, as it removes photodegraded surface particles, thereby exposing undegraded film surfaces to ultraviolet radiation. The second main section of the thesis focuses on analysing field samples to understand the influence of different regional climates (Germany vs. Spain) and management practices on the concentrations of macroplastics (> 1 cm) and microplastics (1–5 mm) in agricultural soil systems. Regional differences were evident, with Spanish fields generally presenting higher plastic concentrations than German fields, likely due to differences in climate and/or management practices (i.e., compost and plastic characteristics). The results of this study highlight that plastic contamination in agricultural soils cannot be attributed only to the use of mulch films, but rather to a combination of factors (i.e., intensity of plastic use, persistence of different plastic products, soil management, as well as other environmental factors like site-specific ultraviolet radiation). Also, the diversity of methodological approaches across studies often hinders comparability. More standardised methodologies are necessary to increase comparability and isolate and identify influences of different factors (e.g., input source, management, or climate) in a holistic way. Finally, the third section of the thesis focuses on field rainfall experiments to elucidate the effects of soil-biodegradable residues on surface runoff, soil erosion, and residue loss during heavy rainfall events. In this experiment, a significant increase in the runoff coefficient was caused by the soil-biodegradable residues, followed by a small transport of small residues, even though the sediment delivery didn’t present a significant change between treatments. However, it is expected that surface runoff will diminish as larger residues continue to biodegrade, while smaller residues may be transported during erosion events, as long as they remain in the topsoil. In summary, the knowledge gained from this PhD thesis provides a better understanding of the mechanisms and factors influencing plastic concentration in agricultural soil, as well as the impact of soil-biodegradable mulch films on residue transport and soil erosion processes. It has been demonstrated that the degradation and fragmentation of agricultural films are complex processes influenced by various factors. Additionally, it has been shown that climatic influences and different agricultural management practices affect the accumulation and distribution of plastic residues in agricultural soil. Furthermore, it has been demonstrated that the use of soil-biodegradable mulch films affects soil erosion and hydrological processes, and their residues are transported to surrounding ecosystems. This shows that the use of soil-biodegradable mulch films still presents concerns that must be considered, as it impacts not only the agricultural ecosystems but possibly other ecosystems as well (e.g., aquatic ecosystems). The insights of this thesis emphasise the importance of considering and understanding degradation processes, management practices, climate, and transport mechanisms, and their influence on soil erosion, when evaluating environmental impacts and mitigation policies.…