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Plastics are used widely as agricultural mulches to suppress weeds and retain soil moisture. Disposal of conventional plastic mulches requires physical removal for disposal in a landfill or incineration. Biodegradable plastic mulches that could be tilled into the soil at the end of a growing season represent an attractive alternative to conventional plastic mulches. In this study, three commercially available mulches labeled as 'biodegradable' and one experimental, potentially biodegradable mulch were used during a tomato growing season, and then buried in field soil at three locations for approximately 6 months, as would occur typically in an agricultural setting. Degradation after 6 months in soil was minimal for all but the cellulosic mulch. After removal of mulches from soil, fungi were isolated from the mulch surfaces and tested for their ability to colonize and degrade the same mulches in pure culture. The majority of culturable soil fungi that colonized biodegradable mulches were within the family Trichocomaceae (which includes beneficial, pathogenic, and mycotoxigenic species of Aspergillus and Penicillium). These isolates were phylogenetically similar to fungi previously reported to degrade both conventional and biodegradable plastics. Under pure culture conditions, only a subset of fungal isolates achieved detectable mulch degradation. No isolate substantially degraded any mulch. Additionally, DNA was extracted from bulk soil surrounding buried mulches and ribosomal DNA was used to assess the soil microbial community. Soil microbial community structure was significantly affected by geographical location, but not by mulch treatments. [ABSTRACT FROM AUTHOR]