Habitat fragmentation and edges matter for soil microorganisms

Despite their importance, scientists know little about how soil microorganisms are being affected by habitat fragmentation. This includes the conditions at fragment edges, which usually differ from the patch interior in temperature, moisture, light, and other important variables.

Soil microorganisms differ from plants and animals in ways that may mean they respond differently to edges: they are incredibly numerous, can often disperse by animals and wind, and can remain dormant in an environment that shelters them from unfavorable conditions. Given these differences, do edges matter for soil microorganisms?

To explore this question, our recent research looked at soils collected at Savannah River Site in South Carolina, in an experimental landscape consisting of one-hectare open savanna-like patches, a pine-dominated plantation matrix, and the edge habitat where they meet.

We found that both soil fungi and bacteria were affected by edges. How much they were affected depended on the contrast in variables like canopy cover and pH between the patch and matrix. In sites with high contrast, edges produced distinct fungal and bacterial communities; in lower contrast sites, edges were generally less distinct. Interestingly, we observed very few species that were unique to the edge, meaning that edge communities consisted of patch- or matrix-specializing taxa in different abundances that are found in either habitat.

We also detected most species in similar abundances across the patch, edge, and matrix. It may be that these microbes can tolerate the conditions in these three habitats equally, or that we detected them where they are not actively growing.

In contrast, arbuscular mycorrhizal fungi (AMF) and Chloroflexi bacteria associated with the open patch largely avoiding the edge and matrix. AMF form important symbioses with plants, like those in the patch. Chloroflexi are often photosynthetic and are integral members of biological soil crusts, which are common in the patch and support soil moisture and fertility. It’s possible that these groups cannot easily move through the matrix due to lower light and the presence of pine trees (which do not form AMF symbioses), meaning that these important microbes might be disproportionately affected by habitat fragmentation in a negative way.

This work is among the first to demonstrate that habitat edges affect soil microorganisms. Certain groups, such as arbuscular mycorrhizae and some biocrust bacteria, may be particularly affected and warrant future research. However, we show that by maintaining similar vegetation in the matrix as in the patch, land managers may be able to help preserve soil microbial communities and the important ecosystem services they support in fragmented patches of habitat.

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