Effect of compounds from agricultural biodegradable plastics on the environment and on plant development

Resumo

Agricultural plastic mulches are an essential part of the agricultural system, contributing to face the food demand for the growing world population. Its use increases crop production, earliness and quality, reduces water consumption and pesticide delivery and prevents weed development. Mulches are mostly made of polyethylene (PE), non-biodegradable. Although they must be removed after harvesting, many fragments remain and accumulate in the field, reducing soil and crop quality. Biodegradable plastic mulches (BDM) have been fostered as a sustainable alternative preventing this accumulation. After harvest they will be biodegraded by the soil microorganisms in which they are integrated. However, this entails the various compounds (polymers and additives) present in the fragments are supplied to the soil, but their effects on cultivated plants and on soil organisms have hardly been studied. The objective of this PhD thesis is to evaluate the effect of eight BDM of different formulation, and their components, on the agricultural soil microbiome and on plants. For this purpose two plant species commonly cultivated with mulches which are among the main horticultural products were targeted, lettuce (Lactuca sativa L.) and tomato (Lycopersicon esculetum Mill.). One PE mulch was included as control mulch. Firstly, it was evaluated whether BDM can release compounds by contact with an aqueous environment before the onset of their biodegradation, and whether the released compounds can affect plant development. It was found that all the BDM tested released a diversity of compounds, which in several cases (Bioplast SP4 and SP6, Mirel and Biofilm) inhibited germination, root morphology or the development and physiology of both plant species, while those from other BDM caused minor (Ecovio, Mater-Bi) or non-significant (Bioflex) effects. Next, the released compounds were identified, which were eventually diverse, both components of its polymeric structure (1,4-butanediol, lactic acid, terephthalic acid, etc.) and additives (fatty acids, glycerol, etc.). Among those identified, the ones having previously shown to affect tomato and lettuce plant development (1,4-butanediol, lactic acid and adipic acid) were quantified. They were found to be in substantially lower concentrations than the ones responsible for causing effects on plants, which does not allow establishing a direct relationship between their release from BDM and the effects they may have on plants. Thirdly, the effect of the accumulation of BDM fragments in the soil on tomato and lettuce germination and plant development was studied. For most BDM, the presence of their fragments did not affect germination but it reduced plant growth and chlorophyll content in tomato and especially in lettuce. In general, the identified effects were consistent with those of compounds released from BDM previously found, and PE fragments caused no effects. Altogether, results suggest that the BDM chemical composition plays a relevant role in its interaction with the plant root system, and that the consequences of the presence of BDM fragments in the soil is related to this composition, likely due to the release of components, rather than to their physical presence. Finally, the impact of the BDM fragments’ accumulation in the soil on the structure and functions of the agricultural soil microbial communities was studied. After incubation for three months, this accumulation had a low impact on the soil microbial communities’ diversity and structure. However, some materials caused significant changes in the abundance and diversity of selected bacterial (Mater-Bi), fungi (MIMGreen paper) and protists (Ecovio) groups. Although the total microbial activity was not altered, the chitinase activity, involved in the nitrogen cycle, was significantly decreased by both BDM and PE presence. The results obtained in this doctoral thesis provide new knowledge on the potential effects of BDMs on cultivated plants and soil microorganisms. They mainly show that BDM (1) easily release several compounds soon before their biodegradation starts, after contact with water, (2) the solution containing the released compounds, depending on its composition, may have effects on plants and (3) the accumulation of BDM fragments in the soil has the capacity to affect plant development and to modify the abundance and diversity of soil microorganisms depending on the composition of the BDM. The results will contribute to the design and development of biodegradable plastic mulches that have a low impact on cultivated plants and the environment.