Soil fungal and prokaryotic community structure exhibits differential short-term responses to timber harvest in the Pacific Northwest

Conventional clear-cut timber harvest is a widespread industrial practice across the Pacific Northwest; however, information regarding how these practices impact soil microbial community structure at the regional scale is limited. With evidence of consistent and substantial impact of harvest on soil microbial functional profiles across the region (despite a range of environmental conditions), the objective of this study was to determine the extent to which harvest also influences the structure of prokaryotic and fungal soil microbial communities, and how generalized these trends are throughout the geographic region. Paired soil samples were collected one year before and after harvest across nine second-growth Douglas-fir forests in the Pacific Northwest. Total community DNA was extracted from the soils, and high-throughput targeted gene sequencing of the 16S rRNA gene for prokaryotes and the internal transcribed spacer (ITS) gene for fungi was performed. Alpha diversity was consistently and significantly higher after harvest; it was moderately so for fungal communities (+14.6%), but only marginally so for prokaryotic communities (+2.0%). Similarly, on average, a greater proportion of the variation in the community structure of fungi (20.1%) at each site was associated with forest harvest compared to that of prokaryotes (13.2%). Overall, the greatest influence of timber harvest on soil microbial communities appeared to be a relative depletion of ectomycorrhizal fungi, with a concomitant enrichment of saprotrophic fungi. Understanding the short-term responses of soil microbial communities across the region, particularly those of tree root-associated symbionts, may aid our understanding of the role soil microbial communities play in ecological succession.