Effect of plastic film and hemp canvas mulching on soil properties, microbial diversity and lettuce yield

Plastic films efficiently control weed development in agriculture but may have environmental impacts, including alterations of the soil functioning and its microbiota. Canvases made of plant fibres are promising biodegradable alternatives showing uniform soil covering like plastic films, unlike straw mulching which is often laid unevenly on the ground. Hemp is particularly interesting for its resistance and possible effects on the soil microbiota. We tested the effect of several mulches differing in their biodegradability and homogeneity (uniform/uneven soil covering) on soil functioning and crop yield. In greenhouse, we assessed the effects of different mulching on lettuce yields, soil properties (temperature, moisture, enzymatic activities) and the soil microbiota. We cropped lettuces either on bare soil (control), a homogeneous non-biodegradable mulch (plastic film), a biodegradable heterogeneous mulch (hemp straw) and a biodegradable and homogenous mulch (hemp canvas). Plastic film increased soil temperature, decreased most enzymatic activities, and altered the soil microbiota composition. The hemp canvas decreased fungal diversity, while increasing soil moisture, laccase activity, and the abundance of specific Ascomycota, Proteobacteria and Actinobacteria taxa. Plastic and hemp canvas gave similar lettuce yields. Mulching with plastic films and hemp canvases changed soil functioning (C cycle enzymatic activities) and the soil microbiota. Although similar lettuce yields were obtained, effects of the plastic film were likely mediated by the increased soil temperature and accelerated organic matter degradation, while effects of the hemp canvas resulted from increased soil moisture and recalcitrant matter degradation, combined with the stimulation of potentially beneficial soil microorganisms.