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

Abstract 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 the present 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 and soil‐filled meshed mesocosms by inoculating half of the units with microarthropods (i.e. high‐density treatment), while the other half of the units only contained few specimens possibly hatching from recalcitrant eggs that survived defaunation (i.e. low‐density treatment). All mesocosms were then inserted in the top soil of a range of nutrient‐poor grasslands and heathlands in Belgium for the duration of three months. The entire mesocosms were composed of mesh (30 μm mesh size) to allow for a rapid (re‐)colonization by microbes from the surrounding soils, while migration of microarthropods in or out of the mesocosms was precluded. Litter bags (30 μm 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 rates. 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 number of fungal OTUs and a higher number of fungal indicator species associated to this treatment. 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 considerable levels of fungal functional overlap. Synthesis. The lack of measurable changes in organic matter decomposition or shifts in fungal functional group representation suggest that effects of microarthropods on the functioning of fungal communities may be lower than originally thought. Read the free Plain Language Summary for this article on the Journal blog.