Plastic mulch and pesticide residues in intensive agriculture

Abstract

In the 1960s, the agricultural “Green Revolution” brough new technologies and heavy mechanisation to the fields. These technologies brought rapid answers to increase the crop yields and improve food security. Among these technologies we found pesticide and plastic mulch applications. Since then the scientific community and the general public became worried of the long term consequences of these short term solutions. Indeed plastic mulch and pesticides leave debris which accumulate in the soil and are potentially harmful to divers organisms. With this PhD thesis we try to encompass the threat posed by plastic mulch and pesticide residues in agriculture. The research was organised in seven different chapters described below.

Chapter 1: In introduction we presented the diverse benefices of pesticide and plastic use in agriculture. The many services they provide explained why they are use abundantly worldwide. However both plastic and pesticides have the potential to accumulate in the soil. We narrowed down the focus to plastic mulch as it is used abundantly worldwide and is recognize as an important source of plastic debris. We presented three different plastic types: Low density Polyethylene (LDPE), Pro-oxidant Additive Containing (PAC) and biodegradable (BIO) mulches. Finally, we presented the threats posed by plastic and pesticide residues and the need for more research. We elaborated six main questions that are addressed in the following five research chapters.

Chapter 2: The objective of this field assessment was to measure plastic and pesticide residues in agricultural soils and their effects on the soil microbiome. For this, we sampled soil from 18 parcels from 6 vegetable farms in Southeast Spain. The farms were under either organic or conventional management, where plastic mulch had been used for > 25 years. We measured the macro and micro light density plastic debris content, the pesticide residue levels, and a range of physiochemical properties. We also carried out DNA sequencing on the soil fungal and bacterial communities. Plastic debris was found in all samples and 4-10 different pesticide residues were also found in all conventional soils. Overall, pesticide content was ~100 times lower in organic farms, whereas no significant difference in plastic content was observed between organic and conventional farms. The fungal and bacterial communities were farm-specific and related to different soil physicochemical parameters as well as plastic and pesticides contaminants.

Chapter 3: The aim of this research was to measure the sorption pattern of active substances from pesticides on LDPE, PAC and BIO plastic mulches and to compare the decay of the active substances in the presence and absence of plastic debris. For this purpose, 38 active substances commonly applied with plastic mulching in South-east Spain were incubated with a piece of plastic mulch. The sorption behaviour depended on both the pesticide and the plastic mulch type. On average, the sorption percentage was ~23% on LDPE and PAC and ~50% on BIO. The decay of active substances in the presence of plastic was ~30% lesser than the decay of active substances in solution alone.

Chapter 4: We investigated whether the plastic debris found in the soil in chapter 2 could modify the soil physicochemical properties. We tested the impact of macro (around 5 mm) and micro (< 1 mm) sized plastic debris from LDPE and one type of starch-based BIO mulch film on soil physicochemical and hydrological properties. The bulk density, porosity, saturated hydraulic conductivity, field capacity and soil water repellency were altered significantly in the presence of the four kinds of plastic debris. The type, size and content of plastic debris as well as the interactions between these three factors played complex roles in the variations of the measured soil parameters.

Chapter 5: We expanded the focus of our work from the soil to living being walking on it by investigating the question: Do sheep ingest the plastic debris when they are grazing in contaminated fields? To give an answer, we collected sheep faeces from 5 different herds and analysed the light density microplastic content. We found ~103 particles∙kg-1 in the faeces. The data showed that livestock ingested plastic, in the form of microplastics and/or macroplastics. Further studies should focus on: assessing how much of the plastic found in faeces is coming directly from plastic mulching, estimating the plastic degradation in the guts of sheep and understanding the potential effects of these plastic residues on the health of livestock.

Chapter 6: In a final mesocosm experiment, we replicated field conditions to test if plastic mulch and pesticide residues could affect the plant production. We tested three plastic mulches and three pesticides commonly used by farmers. LDPE, PAC and BIO mulches were laid in a field for four months, shredded into micro- and macro- plastics and added to the soil with pesticides. Plastic and pesticides were left in the mesocosm to incubate for a year before lettuces seedlings, Lactuca sativa, were planted. After 14 weeks growing periode, we observed a decreased leaf area, fresh shoot biomass and dry shoot biomass in plants growing in soil where BIO plastic was present compared to the control.

Chapter 7: The main findings of the thesis were summarized in the last chapter. We compared our results with existing literature and discussed the links between the chapters. We explored the limitations of current plastic and pesticide residues detection. We suggested that a diversity of methods is required to describe the diversity of plastic types, sizes and shapes and the methods need to be adapted to the specific objective of the study. Finally we explored different options to control the impacts of plastic and pesticide residues in agriculture. No ‘one-fits-all’ approach can be suggested as agricultural systems are very diverse and face many challenges. Nevertheless best practices are recommended to limit the accumulation of residues in the environment. We conclude that plastics and pesticides use are artificial inputs which affect natural processes in some detrimental manner. Natural processes and artificial inputs are two pillars of agriculture. Science carries both to help farmers getting the most of their fields.