University of New Mexico Albuquerque, New Mexico, United States
Interactions between plants and soil microbial communities are important drivers of plant diversity, productivity, and coexistence. Soil microbes are often assumed to be spatially homogeneous and well mixed, but data on soil fungi and bacteria show that this is not the case. Spatial distributions of soil microbes are frequently heterogeneous even at small, sub-meter scales. Neighboring plants may thus interact with very different sets of microbes. Localized patches of specialized mutualists or pathogens may increase or decrease growth and fitness of certain plant species and encourage plant coexistence via creation of cryptic environmental heterogeneity, but this has not been demonstrated in natural environments. Integrating knowledge of plant-microbe interactions with microbial community spatial structure could provide insight into plant population dynamics and coexistence. We asked if plants respond to small-scale spatial heterogeneity in naturally occurring soil microbial communities. We performed a greenhouse experiment using field soils collected from within a small area in a spatially structured design. The soils were used as inoculum to grow arbuscular mycorrhizal blue grama grass (Bouteloua gracilis) and ectomycorrhizal piñon pine (Pinus edulis) and assess the effects of microbial heterogeneity or plant growth and mycorrhizal colonization.
Naturally occurring soil fungal and bacterial communities differed significantly and demonstrated spatial structure at small scales. Soils collected fewer than four meters apart contained different communities, with soils occurring farther apart tending to have fewer microbes in common. Abundance of ectomycorrhizal fungi also varied at this fine scale. Blue grama growth and pine ectomycorrhizal root colonization responded to these differing microbial communities, and these responses were spatially structured at the same small scales at which the microbes varied. Blue grama growth responses tended to be negative, suggesting the predominance of parasitic and pathogenic microbes impacting these plants. Some soils, however, had a positive impact on grass growth. Ectomycorrhizal root colonization similarly varied, with some soils inducing high colonization rates while others produced little to none. This heterogeneous patchwork of soil microbes and corresponding plant responses may render some locations more amenable to plant growth and establishment, while closely adjacent locations are less hospitable. These locations differ by plant species, with some being good for one species but not the other. We showed that microbial heterogeneity can create spatial variation in environmental suitability, which can promote plant diversity and species coexistence in natural environments.