Consistent centennial-scale change in European sub-Arctic peatland vegetation toward Sphagnum dominance-Implications for carbon sink capacity.

Climate warming is leading to permafrost thaw in northern peatlands, and current predictions suggest that thawing will drive greater surface wetness and an increase in methane emissions. Hydrology largely drives peatland vegetation composition, which is a key element in peatland functioning and thus in carbon dynamics. These processes are expected to change. Peatland carbon accumulation is determined by the balance between plant production and peat decomposition. But both processes are expected to accelerate in northern peatlands due to warming, leading to uncertainty in future peatland carbon budgets. Here, we compile a dataset of vegetation changes and apparent carbon accumulation data reconstructed from 33 peat cores collected from 16 sub-arctic peatlands in Fennoscandia and European Russia. The data cover the past two millennia that has undergone prominent changes in climate and a notable increase in annual temperatures toward present times. We show a pattern where European sub-Arctic peatland microhabitats have undergone a habitat change where currently drier habitats dominated by Sphagnum mosses replaced wetter sedge-dominated vegetation and these new habitats have remained relatively stable over the recent decades. Our results suggest an alternative future pathway where sub-arctic peatlands may at least partly sustain dry vegetation and enhance the carbon sink capacity of northern peatlands.

Methane in West Siberia terrestrial seeps: Origin, transport, and metabolic pathways of production.

The expansive plains of West Siberia contain globally significant carbon stocks, with Earth's most extensive peatland complex overlying the world's largest-known hydrocarbon basin. Numerous terrestrial methane seeps have recently been discovered on this landscape, located along the floodplains of the Ob and Irtysh Rivers in hotspots covering more than 2500 km2 . We articulated three hypotheses to explain the origin and migration pathways of methane within these seeps: (H1) uplift of Cretaceous-aged methane from deep petroleum reservoirs along faults and fractures, (H2) release of Oligocene-aged methane capped or trapped by degrading permafrost, and (H3) horizontal migration of Holocene-aged methane from surrounding peatlands. We tested these hypotheses using a range of geochemical tools on gas and water samples extracted from seeps, peatlands, and aquifers across the 120,000 km2 study area. Seep-gas composition, radiocarbon age, and stable isotope fingerprints favor the peatland hypothesis of seep-methane origin (H3). Organic matter in raised bogs is the primary source of seep methane, but observed variability in stable isotope composition and concentration suggest production in two divergent biogeochemical settings that support distinct metabolic pathways of methanogenesis. Comparison of these parameters in raised bogs and seeps indicates that the first is bogs, via CO2 reduction methanogenesis. The second setting is likely groundwater, where dissolved organic carbon from bogs is degraded via chemolithotrophic acetogenesis followed by acetate fermentation methanogenesis. Our findings highlight the importance of methane lateral migration in West Siberia's bog-dominated landscapes via intimate groundwater connections. The same phenomenon could occur in similar landscapes across the boreal-taiga biome, thereby making groundwater-fed rivers and springs potent methane sources.

Isotopic evidence for increased carbon and nitrogen exchanges between peatland plants and their symbiotic microbes with rising atmospheric CO2 concentrations since 15,000 cal. year BP.

Whether nitrogen (N) availability will limit plant growth and removal of atmospheric CO2 by the terrestrial biosphere this century is controversial. Studies have suggested that N could progressively limit plant growth, as trees and soils accumulate N in slowly cycling biomass pools in response to increases in carbon sequestration. However, a question remains over whether longer-term (decadal to century) feedbacks between climate, CO2 and plant N uptake could emerge to reduce ecosystem-level N limitations. The symbioses between plants and microbes can help plants to acquire N from the soil or from the atmosphere via biological N2 fixation-the pathway through which N can be rapidly brought into ecosystems and thereby partially or completely alleviate N limitation on plant productivity. Here we present measurements of plant N isotope composition (δ15 N) in a peat core that dates to 15,000 cal. year BP to ascertain ecosystem-level N cycling responses to rising atmospheric CO2 concentrations. We find that pre-industrial increases in global atmospheric CO2 concentrations corresponded with a decrease in the δ15 N of both Sphagnum moss and Ericaceae when constrained for climatic factors. A modern experiment demonstrates that the δ15 N of Sphagnum decreases with increasing N2 -fixation rates. These findings suggest that plant-microbe symbioses that facilitate N acquisition are, over the long term, enhanced under rising atmospheric CO2 concentrations, highlighting an ecosystem-level feedback mechanism whereby N constraints on terrestrial carbon storage can be overcome.

Large stocks of peatland carbon and nitrogen are vulnerable to permafrost thaw.

Northern peatlands have accumulated large stocks of organic carbon (C) and nitrogen (N), but their spatial distribution and vulnerability to climate warming remain uncertain. Here, we used machine-learning techniques with extensive peat core data (n > 7,000) to create observation-based maps of northern peatland C and N stocks, and to assess their response to warming and permafrost thaw. We estimate that northern peatlands cover 3.7 ± 0.5 million km2 and store 415 ± 150 Pg C and 10 ± 7 Pg N. Nearly half of the peatland area and peat C stocks are permafrost affected. Using modeled global warming stabilization scenarios (from 1.5 to 6 °C warming), we project that the current sink of atmospheric C (0.10 ± 0.02 Pg C⋅y-1) in northern peatlands will shift to a C source as 0.8 to 1.9 million km2 of permafrost-affected peatlands thaw. The projected thaw would cause peatland greenhouse gas emissions equal to ∼1% of anthropogenic radiative forcing in this century. The main forcing is from methane emissions (0.7 to 3 Pg cumulative CH4-C) with smaller carbon dioxide forcing (1 to 2 Pg CO2-C) and minor nitrous oxide losses. We project that initial CO2-C losses reverse after ∼200 y, as warming strengthens peatland C-sinks. We project substantial, but highly uncertain, additional losses of peat into fluvial systems of 10 to 30 Pg C and 0.4 to 0.9 Pg N. The combined gaseous and fluvial peatland C loss estimated here adds 30 to 50% onto previous estimates of permafrost-thaw C losses, with southern permafrost regions being the most vulnerable.

Aquatic export of young dissolved and gaseous carbon from a pristine boreal fen: Implications for peat carbon stock stability.

The stability of northern peatland's carbon (C) store under changing climate is of major concern for the global C cycle. The aquatic export of C from boreal peatlands is recognized as both a critical pathway for the remobilization of peat C stocks as well as a major component of the net ecosystem C balance (NECB). Here, we present a full year characterization of radiocarbon content (14 C) of dissolved organic carbon (DOC), carbon dioxide (CO2 ), and methane (CH4 ) exported from a boreal peatland catchment coupled with 14 C characterization of the catchment's peat profile of the same C species. The age of aquatic C in runoff varied little throughout the year and appeared to be sustained by recently fixed C from the atmosphere (<60 years), despite stream DOC, CO2 , and CH4 primarily being sourced from deep peat horizons (2-4 m) near the mire's outlet. In fact, the 14 C content of DOC, CO2 , and CH4 across the entire peat profile was considerably enriched with postbomb C compared with the solid peat material. Overall, our results demonstrate little to no mobilization of ancient C stocks from this boreal peatland and a relatively large resilience of the source of aquatic C export to forecasted hydroclimatic changes.

Divergent evolution and niche differentiation within the common peatmoss Sphagnum magellanicum.

PREMISE OF THE STUDY: Populations with phenotypic polymorphism in discrete characters may be good models for investigating genome evolution and speciation. Sphagnum magellanicum Brid. is found throughout the northern hemisphere, and despite considerable variation in morphological characters, it is considered one of the least taxonomically controversial peatmoss species. We have observed two main morphs of the species associated with different microhabitats. Here we investigated the genomic and environmental basis of this intraspecific morphological variation. METHODS: We conducted transplant and common garden experiments to test whether the two morphs are genetically differentiated. We then used RAD-sequencing to quantify the genomic divergence between the morphs and approximate Bayesian computation (ABC) to infer the most likely demographic scenario explaining the genome-wide differentiation of the two morphs. KEY RESULTS: We found that genomic differentiation between the two morphs is unexpectedly high and that several of the differentiated morphological characters have a genetic basis. Using simulation approaches, we found support for a scenario of ancient divergence followed by recent secondary contact. CONCLUSIONS: We show that the two morphs represent the two main genetic clusters previously found worldwide. Our results demonstrate that relatively minor morphological differentiation in a presumed phenotypically plastic peatmoss may be associated with massive divergence across the genome.

Integrating McGill Wetland Model (MWM) with peat cohort tracking and microbial controls.

Peatlands store a large amount of organic carbon and are vulnerable to climate change and human disturbances. However, ecosystem-scale peatland models often do not explicitly simulate the decrease in peat substrate quality, i.e., decomposability or the dynamics of decomposers during peat decomposition, which are key controls in determining peat carbon's response to a changing environment. In this paper, we incorporated the tracking of each year's litter input (a cohort) and controls of microbial processes into the McGill Wetland Model (MWMmic) to address this discrepancy. Three major modifications were made: (1) the simple acrotelm-catotelm decomposition model in MWM was changed into a time-aggregated cohort model, to track the decrease in peat quality with decomposition age; (2) microbial dynamics: growth, respiration and death were incorporated into the model and decomposition rates are regulated by microbial biomass; and (3) vertical and horizontal transport of the dissolved organic carbon (DOC) were added and used to regulate the growth of microbial biomass. MWMmic was evaluated against measurements from the Mer Bleue peatland, a raised ombrotrophic bog located in southern Ontario, Canada. The model was able to replicate microbial and DOC dynamics, while at the same time reproduce the ecosystem-level CO2 and DOC fluxes. Sensitivity analysis with MWMmic showed increased peatland resilience to perturbations compared to the original MWM, because of the tracking of peat substrate quality. The analysis revealed the most important parameters in the model to be microbial carbon use efficiency (CUE) and turnover rate. Simulated microbial adaptation with those two physiological parameters less sensitive to disturbances leads to a significantly larger peat C loss in response to warming and water table drawdown. Thus, the rarely explored peatland microbial physiological traits merit further research. This work paves the way for further model development to examine important microbial controls on peatland's biogeochemical cycling.

Including hydrological self-regulating processes in peatland models: Effects on peatmoss drought projections.

The water content of the topsoil is one of the key factors controlling biogeochemical processes, greenhouse gas emissions and biosphere - atmosphere interactions in many ecosystems, particularly in northern peatlands. In these wetland ecosystems, the water content of the photosynthetic active peatmoss layer is crucial for ecosystem functioning and carbon sequestration, and is sensitive to future shifts in rainfall and drought characteristics. Current peatland models differ in the degree in which hydrological feedbacks are included, but how this affects peatmoss drought projections is unknown. The aim of this paper was to systematically test whether the level of hydrological detail in models could bias projections of water content and drought stress for peatmoss in northern peatlands using downscaled projections for rainfall and potential evapotranspiration in the current (1991-2020) and future climate (2061-2090). We considered four model variants that either include or exclude moss (rain)water storage and peat volume change, as these are two central processes in the hydrological self-regulation of peatmoss carpets. Model performance was validated using field data of a peatland in northern Sweden. Including moss water storage as well as peat volume change resulted in a significant improvement of model performance, despite the extra parameters added. The best performance was achieved if both processes were included. Including moss water storage and peat volume change consistently reduced projected peatmoss drought frequency with >50%, relative to the model excluding both processes. Projected peatmoss drought frequency in the growing season was 17% smaller under future climate than current climate, but was unaffected by including the hydrological self-regulating processes. Our results suggest that ignoring these two fine-scale processes important in hydrological self-regulation of northern peatlands will have large consequences for projected climate change impact on ecosystem processes related to topsoil water content, such as greenhouse gas emissions.