Short-term recovery trends of the soil microbiome and its environment indicate an uncertain future of restored peatland forests

Restoring peatland ecosystems is a complex problem with uncertain outcomes due to multiple ecological and socio-economic feedbacks and, presumably, alternative states involved. A major question is how, and to what extent, can natural functioning of drainage-affected peatland soils be restored, and what is the role of soil microbiota for that purpose. We explored these questions in a large scale BACI experiment in drained peatland forests in Estonia, where the restoration treatments combined ditch closure and partial cutting to raise water levels and restore stand structure. The soil samples and environmental data were collected before and 3 and 4 years after the treatments; the samples were sequenced for fungi and bacteria using Illumina MiSeq. We documented some elements of a shift toward the reference state (natural mixotrophic bog-forests): the spatial heterogeneity in soil fungi and bacteria increased, as well as the relative abundance of saprotrophic fungi; and soil nitrogen content decreased. However, a general stability of physico-chemical properties (notably elevated acidity) and annual fluctuations indicated that these responses may remain partial and patchy. We conclude that hydrologically restored peatland forests should be explicitly managed for uncertain future, including their long term monitoring and a precautionary approach for avoiding further damage both within these systems and to surrounding intact peatlands.