Microbes are important regulators of ecosystem function, and as such, much research has been directed at understanding how microbial composition and diversity shift along environmental gradients. However, little is known about how interactions among microbial taxa change across the landscape and how many taxa may share environmental niches. Furthermore, how multitrophic relationships, such as decomposition pathways and predation or grazing foodwebs, are structured is poorly understood. Here, we test how a fundamental ecological gradient, plant density, influences the composition and network structure of soil microbes. We do this first for combined bacterial (16S), fungal (ITS), and eukaryotic (18S) communities, and second we focus on one abundant consumer clade and look at microbial interactions with nematodes. Our study system is a high-elevation subnival ecosystem that exhibits considerable variability in plant density across the landscape due to topography and harsh environmental conditions. We hypothesize that as plant density increases, richness of the microbial community will increase, species composition will change, and soil microbial networks will become more complex.
We find that phylogenetic diversity of bacteria, eukaryotes, and nematodes and OTU richness of fungi increase with plant density. Microbial community composition changes across the gradient: Verrucomicrobia, Glomeromycota, Amoebozoa, and plant parasitic nematodes increase with increasing plant density, while Cyanobacteria, Archaeplastida, and omnivorous nematodes decrease. Despite increases in richness and changes in community composition, network structure of total microbial communities actually becomes less complex across the plant density gradient, and the networks are dominated by relationships among bacteria. However, network structure of nematode interactions becomes much more complex as plant density increases. Overall, results show a surprising complexity of bacterial communities in sparsely vegetated soil indicating syntrophic relationships and shared niches, but suggest that networks with higher trophic levels (i.e. nematodes) require more plants to develop.