The rhizosphere responds: rich fen peat and root microbial ecology after long-term water table manipulation

Hydrologic shifts due to climate change will affect the cycling of carbon (C) stored in boreal peatlands. Carbon cycling in these systems is carried out by microorganisms and plants in close association. This study investigated the effects of experimentally manipulated water tables (lowered, raised) and plant functional groups on the peat and root microbiomes in a boreal rich fen. All samples were sequenced and processed for bacterial, archaeal (16S rDNA—V4), and fungal (ITS2) DNA. Depth had a strong effect on microbial and fungal communities across all water table treatments. Bacterial and archaeal communities were most sensitive to the water table treatments, particularly at the 10-20 cm depth—this area coincides with the rhizosphere or rooting zone. Iron cyclers, particularly members of the family Geobacteraceae, were enriched around the roots of sedges, horsetails, and grasses. The fungal community was affected largely by plant functional group, especially cinquefoils. Fungal endophytes (particularly Acephala spp.) were enriched in sedge and grass roots, which may have underappreciated implications for organic matter breakdown and cycling. Fungal lignocellulose degraders were enriched in the lowered water table treatment. Our results were indicative of two main methanogen communities: a rooting zone community dominated by the archaeal family Methanobacteriaceae and a deep peat community dominated by family Methanomicrobiaceae. Importance This study demonstrated that roots and the rooting zone in boreal fens support organisms likely capable of methanogenesis, iron cycling, and fungal endophytic association, and are directly or indirectly affecting carbon cycling in these ecosystems. These taxa, which react to changes in water table and associate with roots and particularly graminoids, may gain greater biogeochemical influence as projected higher precipitation rates could lead to an increased abundance of sedges and grasses in boreal fens.

Consequences of Increased Variation in Peatland Hydrology for Carbon Storage: Legacy Effects of Drought and Flood in a Boreal Fen Ecosystem

Globally important carbon (C) stores in boreal peatlands are vulnerable to altered hydrology through changes in precipitation and runoff patterns, groundwater inputs, and a changing cryosphere. These changes can affect the extent of boreal wetlands and their ability to sequester and transform C and other nutrients. Variation in precipitation patterns has also been increasing, with greater occurrences of both flooding and drought periods. Recent work has pointed to the increasing role of algal production in regulating C cycling during flooded periods in fen peatlands, but exactly how this affects the C sink-strength of these ecosystems is poorly understood. We evaluated temporal trends in algal biomass, ecosystem C uptake and respiration (using static and floating chamber techniques), and spectroscopic indicators of DOM quality (absorbance and fluorescence) in a boreal rich-fen peatland in which water table position had been experimentally manipulated for 13 years. Superimposed on the water table treatments were natural variations in hydrology, including periods of flooding, which allowed us to examine the legacy effects of flooding on C cycling dynamics. We had a particular focus on understanding the role of algae in regulating C cycling, as the relative contribution of algal production was observed to significantly increase with flooding. Ecosystem measures of gross primary production (GPP) increased with algal biomass, with higher algal biomass and GPP measured in the lowered water table treatment two years after persistent flooding. Prior to flooding the lowered treatment was the weakest C sink (as CO2), but this treatment became the strongest sink after flooding. The lower degree of humification (lower humification index, HIX) and yet lower bioavailability (higher spectral slope ratio, Sr) of DOM observed in the raised treatment prior to flooding persisted after two years of flooding. An index of free or bound proteins (tyrosine index, TI) increased with algal biomass across all plots during flooding, and was lowest in the raised treatment. As such, antecedent drainage conditions determined the sink-strength of this rich fen—which was also reflected in DOM characteristics. These findings indicate that monitoring flooding history and its effects on algal production could become important to estimates of C balance in northern wetlands.

Greenhouse Gas Emissions Dynamics in Restored Fens After In-Situ Oil Sands Well Pad Disturbances of Canadian Boreal Peatlands

In-situ oil extraction activities impact the vast mosaic of boreal wetlands and uplands. Peatland restoration in these regions aims at reestablishing crucial peatland functions, such as peat accumulation and carbon (C) sequestration. In order to assess the success of fen restoration, we evaluated the biogeochemical conditions, the seasonal carbon balances via carbon dioxide (CO2) fluxes and methane (CH4) emissions, and addressed the global warming potential following different restoration techniques at two restored in-situ oil sands well pads, during two consecutive growing seasons. Restoration work involved: 1) the partial or complete removal of introduced well pad construction materials, and spontaneous revegetation, or 2) the partial removal of foreign clay, in addition to reintroduction of typical fen plant species such as Larix laricina, Salix lutea and Carex aquatilis. Comparisons were done with regional reference ecosystems (REF) consisting of three peatlands: a wooded bog, a wooded rich fen and a wooded extreme-rich fen. While the average electric conductivity of restored sectors (946 μS cm−1) was higher compared to REF (360 μS cm−1), the pH was quite similar (pH 5.8 REF, pH 6 restored). Dissolved organic carbon concentration was lower in all restored sectors (5–11 mg L-1 restored sectors, 15–35 mg L-1 REF), presumably due to the still incomplete recovery of vegetation and lower organic matter content associated with remnant well pad material. Re-establishment of shrub and brown moss species improved significantly the C uptake. However, the active introduction of plant species was no crucial restoration step, in order to return species beneficial for C uptake. Restoration treatments that were leveled closest to the surrounding REF showed the most similar seasonal C balance to REF. In shallow open water areas resulting from the complete removal of all construction materials, we measured the highest methane emissions making these flooded sites net C sources to the atmosphere with elevated global warming potential. The partial removal of the well pad’s mineral soil to near the water table level and the surface elevation of the surrounding ecosystem seems to be the most effective site management method to sequester carbon efficiently. However, further research is needed to evaluate the suitability of this restoration method for the recovery of biodiversity and possible impacts of residual foreign materials on fen ecosystems.

Impact of local- and regional-scale restoration measures on a vulnerable rich fen in the Naardermeer nature reserve (the Netherlands)

Restoration of rich fens is commonly attempted through local-scale measures, such as removal of sod or blockage of ditches. However, regional-scale restoration measures, that aim to re-establish the original hydrology in which rich fens developed, might have a more long-lasting effect. We investigated the effect of local- and regional-scale restoration measures on a vulnerable rich fen in the Naardermeer nature reserve in the Netherlands. We compared water quality and vegetation composition of the fen before and after the restoration measures, almost 30 years apart. Overall rich fen species increased and although this indicates the desired increased supply of fresh mineral-rich groundwater to the fen, continued succession towards poor fen vegetation has not been prevented in the entire fen. Despite sod layer removal, we observed an increase in a Polytrichum-dominated vegetation in patches that are primarily fed by rainwater. Our findings confirm results from a previous study which showed that brackish palaeo-groundwater is still contributing substantially to the water balance of the fen, especially in periods of precipitation deficit. We conclude that the local- and regional-scale restoration measures have been successful in increasing the abundance of rich fen species in parts of the fen. However, considering the pressures of climate change and high atmospheric N-deposition on the fen, it is uncertain whether rich fen species can be sustained in quite nutrient-poor conditions in the future. Therefore, there is a need for continued management that keeps the nutrient-poor and mineral-rich conditions of the fen intact.

Alleviation of Plant Stress Precedes Termination of Rich Fen Stages in Peat Profiles of Lowland Mires

Mesotrophic rich fens, that is, groundwater-fed mires, may be long-lasting, as well as transient ecosystems, displaced in time by poor fens, bogs, forests or eutrophic reeds. We hypothesized that fen stability is controlled by plant stress caused by waterlogging with calcium-rich and nutrient-poor groundwater, which limits expansion of hummock mosses, tussock sedges and trees. We analysed 32 European Holocene macrofossil profiles of rich fens using plant functional traits (PFTs) which indicate the level of plant stress in the environment: canopy height, clonal spread, diaspore mass, specific leaf area, leaf dry matter content, Ellenberg moisture value, hummock-forming ability, mycorrhizal status and plant functional groups. Six PFTs, which formed long-term significant trends during mire development, were compiled as rich fen stress indicator (RFSI). We found that RFSI values at the start of fen development were correlated with the thickness of subsequently accumulated rich fen peat. RFSI declined in fens approaching change into another mire type, regardless whether it was shifting into bog, forest or eutrophic reeds. RFSI remained comparatively high and stable in three rich fens, which have not terminated naturally until present times. By applying PFT analysis to macrofossil data, we demonstrated that fens may undergo a gradual autogenic process, which lowers the ecosystem’s resistance and enhances shifts to other mire types. Long-lasting rich fens, documented by deep peat deposits, are rare. Because autogenic processes tend to alleviate stress in fens, high levels of stress are needed at initial stages of rich fen development to enable its long persistence and continuous peat accumulation.