Impacts of warming with open-top chambers on microbial communities in wet conditions and in conditions resulting from moderate water-level drawdown (WLD) were studied across 0-50 cm depth in northern and southern boreal sedge fens. Warming alone decreased microbial biomass especially in the northern fen. Impact of warming on microbial PLFA and fungal ITS composition was more obvious in the northern fen and linked to moisture regime and sample depth. Fungal-specific PLFA increased in the surface peat in the drier regime and decreased in layers below 10 cm in the wet regime after warming. OTUs representing Tomentella and Lactarius were observed in drier regime and Mortierella in wet regime after warming in the northern fen. The ectomycorrhizal fungi responded only to WLD. Interestingly, warming together with WLD decreased archaeal 16S rRNA copy numbers in general, and fungal ITS copy numbers in the northern fen. Expectedly, many results indicated that microbial response on warming may be linked to the moisture regime. Results indicated that microbial community in the northern fen representing Arctic soils would be more sensitive to environmental changes. The response to future climate change clearly may vary even within a habitat type, exemplified here by boreal sedge fen.
Climate change affects peatlands directly through increased air temperatures and indirectly through changes in water-table level (WL). The interactions of these two still remain poorly known. We determined experimentally the separate and interactive effects of temperature and WL regime on factors of relevance for the inputs to the carbon cycle: plant community composition, phenology, biomass production, and shoot:root allocation in two wet boreal sedge-dominated fens, "southern" at 62°N and "northern" at 68°?. Warming (1.5°C higher average daily air temperature) was induced with open-top chambers and WL drawdown (WLD; 3-7 cm on average) by shallow ditches. Total biomass production varied from 250 to 520 g/m2 , with belowground production comprising 25%-63%. Warming was associated with minor effects on phenology and negligible effects on community composition, biomass production, and allocation. WLD clearly affected the contribution of different plant functional types (PFTs) in the community and the biomass they produced: shrubs benefited while forbs and mosses suffered. These responses did not depend on the warming treatment. Following WLD, aboveground biomass production decreased mainly due to reduced growth of mosses in the southern fen. Aboveground vascular plant biomass production remained unchanged but the contribution of different PFTs changed. The observed changes were also reflected in plant phenology, with different PFTs showing different responses. Belowground production increased following WLD in the northern fen only, but an increase in the contributions of shrubs and forbs was observed in both sites, while sedge contribution decreased. Moderate warming alone seems not able to drive significant changes in plant productivity or community composition in these wet ecosystems. However, if warming is accompanied by even modest WL drawdown, changes should be expected in the relative contribution of PFTs, which could lead to profound changes in the function of fens. Consequently, hydrological scenarios are of utmost importance when estimating their future function.
Northern peatlands form a major soil carbon (C) stock. With climate change, peatland C mineralization is expected to increase, which in turn would accelerate climate change. A particularity of peatlands is the importance of soil aeration, which regulates peatland functioning and likely modulates the responses to warming climate. Our aim is to assess the impacts of warming on a southern boreal and a sub-arctic sedge fen carbon dioxide (CO2 ) exchange under two plausible water table regimes: wet and moderately dry. We focused this study on minerotrophic treeless sedge fens, as they are common peatland types at boreal and (sub)arctic areas, which are expected to face the highest rates of climate warming. In addition, fens are expected to respond to environmental changes faster than the nutrient poor bogs. Our study confirmed that CO2 exchange is more strongly affected by drying than warming. Experimental water level draw-down (WLD) significantly increased gross photosynthesis and ecosystem respiration. Warming alone had insignificant impacts on the CO2 exchange components, but when combined with WLD it further increased ecosystem respiration. In the southern fen, CO2 uptake decreased due to WLD, which was amplified by warming, while at northern fen it remained stable. As a conclusion, our results suggest that a very small difference in the WLD may be decisive, whether the C sink of a fen decreases, or whether the system is able to adapt within its regime and maintain its functions. Moreover, the water table has a role in determining how much the increased temperature impacts the CO2 exchange.