Impact of buried debris from agricultural biodegradable plastic mulches on two horticultural crop plants: Tomato and lettuce

Biodegradable mulches (BDM) are increasingly valued and used for substituting non-biodegradable plastic mulches polluting agricultural soils. They are tilled into soil, where they fragment and release compounds throughout their biodegradation. The consequences of BDM use on the plant-soil environment have been partially studied with pristine and with artificially-weathered BDM fragments. However, to guarantee safety use of BDM, studies on the BDM debris effects are required. For this, to determine potential effects of the field-weathering BDM on plants, a mesocosm experiment was performed by sowing seeds from two major plant species commonly cultivated with BDM, tomato (Lycopersicon esculentum) and lettuce (Lactuca sativa), in plant pots containing pristine and field-weathered pieces from seven different BDM formulations, one paper mulch, a polyethylene (PE) mulch, and no-mulch control pots. Germination of both plant species was unaffected by any of the mulch treatments; however, PHB (polyhydroxybutyrate)-based BDM fragments severely inhibited tomato and lettuce plant growth, 90 and 95 %, respectively. Moreover, all pristine and field-weathered BDM significantly delayed lettuce plant development. Tomato plant growth progressed better, but growth retardation was also evidenced with most field-weathered BDM treatments. Overall, field-weathered fragments caused stronger effects on plants than the pristine unused ones. No effects were found for PE mulch, either pristine or field-weathered. The obtained results highlight that BDM debris may alter plant development depending on their nature and on their weathering, rather than on their physical presence, and evidence the need to conduct further experiments on the impact of field-weathered BDM on the plant-soil environment.