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Drained peatlands are significant sources of the greenhouse gas (GHG) carbon dioxide.Rewetting is a proven strategy used to protect carbon stocks; however, it can lead to increasedemissions of the potent GHG methane. The response to rewetting of soil microbiomes as drivers ofthese processes is poorly understood, as are the biotic and abiotic factors that control communitycomposition. We analyzed the pro- and eukaryotic microbiomes of three contrasting pairs ofminerotrophic fens subject to decade-long drainage and subsequent long-term rewetting. Abiotic soilproperties including moisture, dissolved organic matter, methane fluxes, and ecosystem respirationrates were also determined. The composition of the microbiomes was fen-type-specific, but allrewetted sites showed higher abundances of anaerobic taxa compared to drained sites. Based onmulti-variate statistics and network analyses, we identified soil moisture as a major driver ofcommunity composition. Furthermore, salinity drove the separation between coastal and freshwaterfen communities. Methanogens were more than 10-fold more abundant in rewetted than in drainedsites, while their abundance was lowest in the coastal fen, likely due to competition with sulfatereducers. The microbiome compositions were reflected in methane fluxes from the sites. Our resultsshed light on the factors that structure fen microbiomes via environmental filtering.
Abstract
Root phenology influences the timing of plant resource acquisition and carbon fluxes into the soil. This is particularly important in fen peatlands, in which peat is primarily formed by roots and rhizomes of vascular plants. However, most fens in Central Europe are drained for agriculture, leading to large carbon losses, and further threatened by increasing frequency and intensity of droughts. Rewetting fens aims to restore the original carbon sink, but how root phenology is affected by drainage and rewetting is largely unknown.
We monitored root phenology with minirhizotrons in drained and rewetted fens (alder forest, percolation fen and coastal fen) as well as its soil temperature and water table depth during the 2018 drought. For each fen type, we studied a drained site and a site that was rewetted ~25 years ago, while all the sites studied had been drained for almost a century.
Overall, the growing season was longer with rewetting, allowing roots to grow over a longer period in the year and have a higher root production than under drainage. With increasing depth, the growing season shifted to later in time but remained a similar length, and the relative importance of soil temperature for root length changes increased with soil depth.
Synthesis and applications. Rewetting extended the growing season of roots, highlighting the importance of phenology in explaining root productivity in peatlands. A longer growing season allows a longer period of carbon sequestration in form of root biomass and promotes the peatlands' carbon sink function, especially through longer growth in deep soil layers. Thus, management practices that focus on rewetting peatland ecosystems are necessary to maintain their function as carbon sinks, particularly under drought conditions, and are a top priority to reduce carbon emissions and address climate change.