Abstract
Forest replacement by exotic plantations drive important changes at the level of the overstory, understory and forest floor. In the Atlantic Forest of northern Argentina, large areas have been replaced by loblolly pine (Pinus taeda L.) monocultures. Plant and litter transformation, together with harvesting operations, change microclimatic conditions and edaphic properties. Management practices such as thinning promote the development of native understory vegetation and could counterbalance negative effects of forest replacement on soil. Here, the effects of pine plantations and thinning on physical, chemical and microbiological soil properties were assessed. Bacterial, archaeal, and fungal community structure were analyzed using a metabarcoding approach targeting ribosomal markers. Forest replacement and, to a lesser extent, thinning practices in the pine plantations induced significant changes in soil physico-chemical properties and associated shifts in bacterial and fungal communities. Most measured physical and chemical properties were altered due to forest replacement, but a few of these properties reached values similar to natural forests under the thinning operation. Fungal alpha diversity decreased in pine plantations, whereas bacterial alpha diversity tended to increase but with little statistical support. Shifts in community composition were observed for both fungal and bacterial domains, and were mostly related to changes in plant understory composition, soil carbon, organic matter, water content, pH and bulk density. Among several other changes, highly abundant phyla such as Proteobacteria (driven by many genera) and Mortierellomycota (mainly driven by Mortierella) decreased in relative abundance in the plantations, whereas Acidobacteria (mainly driven by Acidothermus and Candidatus Koribacter) and Basidiomycota (mainly driven by the ectomycorrhiza Russula) showed the opposite response. Taken together, these results provide insights into the effects of forest replacement on belowground properties and elucidate the potentially beneficial effect of thinning practices in intensive plantation systems through promoting the understory development. Although thinning did not entirely counterbalance the effects of forest replacement on physical, chemical and biological soil properties, the strategy helped mitigating the effects and might promote resilience of these properties by the end of the rotation cycle, if subsequent management practices compatible with the development of a native understory vegetation are applied.
Highlights
The economy-driven replacement of natural forests with exotic monospecific plantations has visible effects on plant structure and aboveground biodiversity, but the effects on belowground biodiversity and the ecosystem functions it provides are less clear
Magnesium, nitrogen, organic matter, organic carbon and water content was lower in the soils of both plantation treatments compared to natural forests
The current study highlights the impact of replacing a botanically diverse subtropical native forest by exotic monospecific pine plantations on various soil properties including a detailed assessment of the soil microbiota
Summary
The economy-driven replacement of natural forests with exotic monospecific plantations has visible effects on plant structure and aboveground biodiversity, but the effects on belowground biodiversity and the ecosystem functions it provides are less clear. Changes in vegetation can alter soil microbial community diversity and function directly through new plant-microbe interactions or indirectly through changes in physico-chemical soil properties and litter layer composition (Zak et al, 2003; Moukoumi et al, 2006; Broeckling et al, 2008). Forest management strategies in pine plantations such as thinning promote the development of a native understory vegetation due to the increase in available solar radiation (e.g., Cheng et al, 2017; Trentini et al, 2017; Dang et al, 2018). Reducing soil moisture can decrease microbial activity and change the trophic composition of both bacterial and fungal communities, with consequences in carbon mineralization in the system (Hartmann et al, 2017). The direct and indirect effects of thinning will impact different groups of microorganisms and potentially alter associated soil processes such as nitrogen fixation (e.g., diazotrophic bacteria), nutrient uptake (mycorrhizae), or carbon utilization (fungal and bacterial saprotrophs) (Dang et al, 2018)
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