Model substrate particles uncover dynamics of microbial communities associated with particulate organic matter decomposition in soil

Soil organic matter is the largest terrestrial reservoir of organic carbon. Its particulate fraction, particulate organic matter (POM), serves as a resource and surface for microbial colonization. Degradation of complex biopolymers like cellulose and chitin requires extracellular enzymes produced by phylogenetically diverse microbes. Despite their importance for carbon cycling, the structure and spatio-temporal dynamics of particle-associated microbial communities in soil and how specific substrates influence them remain poorly understood. This study investigated whether microbial communities associated with POM change in composition and richness over time and whether chitin and cellulose select for distinct fungal and bacterial taxa. We incubated self-manufactured millimeter-sized model POM substrate particles containing chitin or cellulose in soil under laboratory and field conditions. Over 50 days, we assessed particle-associated bacterial and fungal communities at multiple time points in the lab and after 47 days in the field. Our results show that community structure and temporal dynamics of particle-associated microbial communities were substrate-specific. While microbial biomass increased on both particle types, chitin-associated microbial exhibited stronger temporal changes. Communities on chitin and cellulose particles were enriched in specific bacterial and fungal genera compared to communities in the surrounding soil. In conclusion, our study demonstrates that microbial communities associated with model chitin particles underwent notable temporal changes. More specifically, microbial richness decreased, and community composition shifted over the incubation period. Both chitin and cellulose particles favored certain fungal, and especially bacterial genera compared to surrounding soil.