The Impacts of Tilled-In Biodegradable Plastic Mulch on Soil Microbial Communities

Plastics mulch films are used in agriculture to help increase crop production by regulating soil water retention and soil temperature while also preventing weed growth. Despite these benefits, plastic mulches have led to higher levels of microplastic pollution as weathering occurs. In response to this issue, biodegradable plastic mulches (BDMs) have been introduced to help reduce plastic pollution as they are designed to be tilled into soil after each growing season to continue degradation. Degradation of these mulch films is dependent on climate and, in some areas, complete degradation can take years. Since some biodegradable plastic mulches are made of ~50% carbon (Hayes et al., 2017), their addition yearly into soil may cause changes to carbon cycling and microbial communities. The purpose of this study was to understand how the addition and build up biodegradable plastic mulch films in soils affects soil microbial communities by studying their usage and response to the carbon input as well as examining potential toxic effects on soil microbes. These studies were performed in a lab using soils taken from 0-10 cm depths from the East Tennessee Research and Education Center – Plant Sciences Unit in Knoxville, TN. Microplastics derived from BDMs and traditional polyethylene films (PEs) were added to soil samples at varying concentrations and incubated for five weeks to stimulate degradation. The samples were studied to determine changes in respiration, microbial biomass growth, microbial carbon use efficiency, and ecotoxicity. The results showed a slight ecotoxic effect (>20% reduction in bioluminescence from the bacteria V. fischeri) from samples containing BDMs but not from LDPE or control samples. This is likely due to the high concentration (5.0%, w/w) of BDM microplastics that were added to the samples. The results also showed higher microbial respiration as BDM concentrations increased and a reduction in microbial biomass carbon, showing a decrease in carbon use efficiency (CUE). Samples with no plastic and PE microplastics showed no significant differences in respiration but samples containing LDPE decreased in biomass growth over time. Our results show that the addition of microplastics from biodegradable plastics can stimulate the activity of microbes.