Soil pollution under orchards in central-eastern Portugal: sources, risks and potential solutions

Pesticides and plastics play major roles in most modern agricultural systems resulting in pesticides and microplastics being important soil contaminants, especially given their persistence, mobility, and toxicity. The research behind this thesis focused on the distribution and main effects of pesticide residues and microplastics in soils under peach orchards from central-eastern Portugal. It was divided into 4 main parts. The first part covered 18 orchards. It included their pesticide application records, used for predicting pesticide dissipation, and the quantification of a total of 37 pesticide residues in 270 soil samples collected throughout a complete growing season. Glyphosate was found to be highly relevant considering its application rates, the concentration of this active substance and of its main metabolite, the aminomethylphosponic acid (AMPA), and their potential for persistence in soil. Furthermore, pesticide dissipation in soil was slower than predicted, highlighting the importance of regular soil pesticide monitoring programs. The second part was focused on microplastic levels and their potential sources to soil. Furthermore, it evaluated the microplastic distribution across 3 different soil layers (0-5 cm, 5-15 cm, and 15-25 cm). Microplastic levels were generally lower, and their particle sizes higher, in the shallower layer. Irrigation water was the largest microplastic source to the soil. The third part was based on a mesocosm study which evaluated the single and combined effects of pesticide residues and microplastics, with and without biochar, on soil nutrients, pesticide residues, and in the mycorrhizal colonisation of peach tree roots. The combined effects of pesticides, microplastics, and biochar on soil nutrients were found to be more important than their single effects. Additionally, microplastics were related to higher levels of difenoconazole and biochar was related to higher levels of most quantified pesticide residues, except for glyphosate whose levels were lower in the presence of biochar. The mycorrhizal colonisation of peach tree roots was not substantially influenced by pesticide residues, microplastics or biochar. The fourth part was related to a field study, installed in a commercial peach orchard, which intended to evaluate cover crops as alternatives to herbicides. When compared to the standard practice of herbicide applications, the cover crop treatments did not significantly impact fruit yield or quality. While cover crops did not have a strong effect on mycorrhizal colonisation of peach tree roots or on soil nutrient levels, they were related to lower levels of plant available copper, which is a promising result considering its widespread use as a fungicide and its potential to reach phytotoxic levels in soil. This thesis proposed not only to characterise the soil contamination by pesticides and microplastics, but also to evaluate its consequences and to study the viability of cover crops as a strategy with the potential to decrease the levels of pesticide residues in soil, particularly, herbicides. While interesting and promising results were obtained, further studies are needed to increase our understanding of the distribution and levels of those soil contaminants, of their consequences, and to develop strategies that could reduce their negative impact in soil.