Man-made landscaping materials form uppermost soil layers in urban green parks and lawns. To optimize effects of landscaping materials on biodiversity, plant growth and human health, it is necessary to understand microbial community dynamics and physicochemical characteristics of the landscaping materials during storage. In the current three-year study, the consequences of long-term storage on biotic and abiotic characteristics of eight commercial landscaping materials were evaluated. We hypothesized that long-term storage results in changes in microbial utilization of various energy sources and the diversity and relative abundance of bacteria with pathogenic or immunomodulatory characteristics. Three-year storage led to remarkable changes in bacterial community composition. Diversity and richness of taxa associated with immune modulation, particularly phylum Proteobacteria and class Gammaproteobacteria, decreased over time. Bacteroidetes decreased while Actinobacteria increased in relative abundance. Functional orthologs associated with biosynthesis of antibiotics and degradation of complex carbon sources increased during storage. Relative abundance of genera containing potential pathogens were mostly constant or decreased with time. Major changes can be explained by tightening competition over lessening resources. Bacterial communities in landscaping materials adjust to absent inflow of carbon and nutrients during storage. The increased signalling of functional orthologs related to degradation of complex carbon sources hints that bacteria dependent on labile carbon and readily available nutrients were outcompeted. This suggests storage reduces plant seedling growth. Long-term storage seems to decrease immunomodulatory potential of landscaping materials, but storage did not enrich pathogens or functional orthologs associated with pathogenicity. We recommend short storage and shelf life of organic landscaping materials.
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