Effects of microarthropod density on soil fungal community composition in nutrient-poor ecosystems

Microarthropods such as mites (Acari) and springtails (Collembola) play a key role in the functioning of nutrient-poor ecosystems. They contribute to the comminution of organic matter and they are likely to affect the structure and function of soil microbial communities through grazing and by changing substrate availability. However, the precise effects of their interactions with microbiota remain poorly understood. In this study, we conducted a mesocosm field experiment to determine the effects of soil microarthropod density on community composition of soil fungi. We manipulated microarthropod densities in 100 defaunated soil-filled meshed mesocosms by inoculating microarthropods in half of the units i.e. high density and comparing it to the other half containing few specimens surviving the defaunation i.e. low density. All mesocosms were then inserted in the top soil of a range of grasslands and heathlands in Belgium for the duration of three months. Mesocosm mesh size (30 um) allowed for a rapid (re-)colonization by microbes from the surrounding soils, but it excluded migration of microarthropods. Litter bags (30 um mesh size) containing two types of organic matter were also buried within each mesocosm, and organic matter mass loss over time was used as a proxy to assess effects of microarthropod density on microbe-regulated decomposition. We found that fungal community composition differed strongly between the two microarthropod density treatments, regardless of habitat type. The mesocosms with higher microarthropod densities were characterized by a higher fungal species richness and a higher number of fungal indicator species already known to science, while fungal indicators of the low microarthropod density treatment primarily consisted of unknown taxa. Microarthropod density, however, did not affect microbe-regulated organic matter decomposition rates, nor did it affect relative abundances of putative fungal functional groups, thus suggesting some level of fungal functional redundancy. Our study suggests that effects of microarthropods on ecosystem functions via fungal communities may be lower than originally though, thus encouraging further study of belowground interactions at an ecosystem level.