Agricultural land‐use history and restoration impact soil microbial biodiversity

Human land uses, such as agriculture, can leave long‐lasting legacies as ecosystems recover. As a consequence, active restoration may be necessary to overcome land‐use legacies; however, few studies have evaluated the joint effects of agricultural history and restoration on ecological communities. Those that have studied this joint effect have largely focused on plants and ignored other communities, such as soil microbes. We conducted a large‐scale experiment to understand how agricultural history and restoration tree thinning affect soil bacterial and fungal communities within longleaf pine savannas of the southern United States. This experiment contained 64 pairs of remnant (no history of tillage agriculture) and post‐agricultural (reforested following abandonment from tillage agriculture >60 years prior) longleaf pine savanna plots. Plots were each 1 ha and arranged into 27 blocks to minimize land‐use decision‐making biases. We experimentally restored half of the remnant and post‐agricultural plots by thinning trees to reinstate open‐canopy savanna conditions and collected soils from all plots five growing seasons after tree thinning. We then evaluated soil bacterial and fungal communities using metabarcoding. Agricultural history increased bacterial diversity but decreased fungal diversity, while restoration increased both bacterial and fungal diversity. Both bacterial and fungal richness were correlated with a range of environmental variables including above‐ground variables like leaf litter and plant diversity, and below‐ground variables such as soil nutrients, pH and organic matter, many of which were also impacted by agricultural history and restoration. Fungal and bacterial community compositions were shaped by restoration and agricultural history resulting in four distinct communities across the four treatment combinations. Synthesis and applications . Past agricultural land use has left persistent legacies on soil microbial biodiversity, even over half a century after agricultural abandonment and after intensive restoration activities. The impacts of these changes on soil microbe biodiversity could influence native plant establishment, plant productivity and other aspects of ecosystem functioning following agricultural abandonment and during restoration.