While peatland C cycling is generally well covered, understanding of the role of soil fertility in driving the spatial variation of C fluxes within peatlands remains scattered. Our aim was to examine the relative effects of fertility and microtopography on CO2 and CH4 exchange within a boreal fen and to link these effects to the spatial variation in plant and soil attributes. Fertility zones (eutrophic, mesotrophic, oligotrophic) were judged by moss species appearances, and the growing season CO2 and CH4 exchange was measured by static chambers for microforms (string, Sphagnum lawn, flark) and fertility zones and by eddy covariance technique for the entire ecosystem in three years. Plant leaf area index, plant functional type biomasses, soil C and N concentrations and litter decomposition were measured at study plots placed on the microforms and fertility zones. We found that higher fertility led to greater fluxes in both gases: the eutrophic zone had 111% higher net ecosystem CO2 exchange, 102% higher gross primary production, 83% higher ecosystem respiration and 93% higher CH4 emissions than the oligotrophic zone. Peat N concentration was lowest in the eutrophic zone, indicating fast N cycling. The relative importance of microtopography and fertility differed between the two gases: while microform explained 31-39% and fertility 10-15% of total variation in CO2 exchange, microform explained 14% and fertility 36% of variation in CH4 exchange. These results show that growing season CO2 and CH4 fluxes can be significantly affected by within-fen variation of fertility and that CH4 emissions can be more closely associated with fertility than microtopography. It seems that understanding of within-site variation in soil nutrient availability is highly relevant for predicting current and future C exchange in peatlands.

Laboratory and field studies with other grazer species suggest that reindeer (Rangifer tarandus L.) grazing on northern peatlands could shape the peat soil microbial communities and lead to higher ecosystem methane (CH4) emissions. We investigated this at two sedge fens in northern Finland, Lompoloj & auml;nkk & auml; and Halssiaapa, in experiments where reindeer grazing presence or absence was achieved with exclosure fences, and the effects of reindeer droppings were evaluated comparing dropping additions either on peat surface or trampled into the peat to controls with no droppings. Active soil methanogen and methanotroph communities were analyzed by metatranscriptomics. Soil CH4 fluxes were quantified with manual chambers and portable gas analyzer. Reindeer presence and dropping additions were both connected to differences in the soil communities as compared to controls (no presence or no droppings). The responses differed between the two fens. Activity of rumen microbes in peat could not be detected. Structural equation models indicated that the ecosystem CH4 flux in both fens depended on measurement year and sedge leaf area. At Halssiaapa trampled droppings, and at Lompoloj & auml;nkk & auml; both surface and trampled droppings reduced the sedge leaf area. While at Halssiaapa the dropping effect was not altogether statistically significant, in Lompoloj & auml;nkk & auml; surface droppings reduced the CH4 flux both directly and through the reduced leaf area. In conclusion, while both reindeer presence and dropping addition were diversely reflected in the active soil communities, reindeer effects on the CH4 flux were indirect and mediated via vegetation. The results contrast our earlier laboratory findings, and i) caution against liberal generalizations from lab studies to field conditions in peatlands, as well as ii) point to a need for rigorous multivariate analyses for deciphering the complex interactions governing the functions of these ecosystems.

Continuous cover forestry, where forests are managed only by partial harvestings, has been proposed to mitigate forestry -induced carbon and nutrient exports to receiving water courses. We studied the effects of two partial harvest treatments, strip -cutting at five sites and single -tree harvesting at the other site, on nutrient and dissolved organic carbon (DOC) exports from drained peatland forests. We found that, as well as clear -cutting, partial harvesting may also increase exports from peatland forests. The comparison of our results with earlier studies suggested that other factors than the harvest method, such as harvested stem volume per catchment area, are more important in controlling nutrient and DOC exports. Future research is still needed to produce exact export estimates for partially harvested drained peatland forests.

Climate change may affect the carbon sink function of peatlands through warming and drying. Fine-root biomass production (FRBP) of sedge fens, a widespread peatland habitat, is important in this context, since most of the biomass is below ground in these ecosystems. We examined the response of fine-root biomass production, depth distribution (10 cm intervals down to 60 cm), chemical characteristics, and decomposition along with other main litter types (sedge leaves, Sphagnum moss shoots) to an average May-to-October warming of 1.7 °C above ambient daily mean temperature and drying of 2-8 cm below ambient soil water-table level (WL) in two sedge fens situated in Northern and Southern Boreal zones. Warming was induced with open top chambers and drying with shallow ditching. Finally, we simulated short-term organic matter (OM) accumulation using net primary production and mass loss data. Total FRBP, and FRBP in deeper layers, was clearly higher in southern than northern fen. Drying significantly increased, and warming marginally increased, total FRBP, while warming significantly increased, and drying marginally increased, the proportional share of FRBP in deeper layers. Drying, especially, modified root chemistry as the relative proportions of fats, wax, lipids, lignin and other aromatics increased while the proportion of polysaccharides decreased. Warming did not affect the decomposition of any litter types, while drying reduced the decomposition of sedge leaf litter. Although drying increased OM accumulation from root litter at both fens, total OM accumulation decreased at the southern fen, while the northern fen with overall lower values showed no such pattern. Our results suggest that in warmer and/or modestly drier conditions, sedge fen FRBP will increase and/or be allocated to deeper soil layers. These changes along with the altered litter inputs may sustain the soil carbon sink function through OM accumulation, unless the WL falls below a tipping point.

This package contains the data used in the research article: "Partial cutting of a boreal nutrient-rich peatland forest causes radically less on-site CO2 emissions than clear-cutting" published in Agricultural and Forest Meteorology. LAI_data.xlsx - Contains Leaf Area Index data and their standard deviations for all the measured areas WTL_data.csv - Contains the mean water table level data for pre-harvest, partial harvest and clearcut areas. Lettosuo_2010-2015_Section_A_fluxes.csv - Contains the pre-harvest (2010-2015) carbon flux data for Section A. Lettosuo_2010-2015_Section_BCD_fluxes.csv - Contains the pre-harvest carbon flux data for Section BCD. Lettosuo_2016-2021_Section_AB_(partialcut).csv - Contains the carbon flux data for the partial cut area (2016-2021, Section AB). Lettosuo_2016-2021_Section_D_(Clearcut).csv - Contains the carbon flux data for the clear-cut area (2016-2021, Section D) The carbon flux data files contain the following columns: Gapfilled PAR - Gapfilled photsynthetically active radiation Gapfilled air temperature - Gapfilled air temperature Measured NEE - Filtered NEE data Modelled TER - Modelled total ecosystem respiration Modelled GPP - Modelled gross primary production Modelled NEE - Modelled NEE calculated from the modelled TER and GPP Gapfilled NEE - A combination of measured and modelled NEE. Gaps in the measured data are filled with modelled NEE Modelling uncertainty - Uncertainty of the modelled NEE Measurement uncertainty - An estimation of the uncertainty of the measured NEE

Introduction Peatlands are terrestrial-carbon hotspots, where changes in carbon pools and fluxes potentially caused by drying or warming may have significant feedbacks to climate change. In forested peatlands, fine-root biomass (FRB), and production (FRP) are important carbon pools and fluxes, but they and their depth distribution and plant functional type (PFT) composition are poorly known. Methods We studied the effects of persistent water-table level (WTL) drawdown on these characteristics in four forested boreal peatland site types that varied in soil nutrient and WTL regimes, ground vegetation and tree stand characteristics. Each site type was represented by a pair of one undrained and one drained site. Two pairs were nutrient-poor, Scots pine dominated sites, one very wet and one relatively dry in their undrained condition. The other two pairs were nutrient-rich, Norway spruce dominated sites, again one wetter and one drier in the undrained condition. FRB was estimated by separating and visually identifying roots from soil cores extending down to 50 cm depth. FRP was estimated using ingrowth cores covering the same depth, and the separated roots were identified using infrared spectroscopy. Results and discussion Both FRB and FRP varied widely both within and among the different types of boreal forested peatland. In FRB, the clearest differences were seen in the two originally wettest sites, nutrient-poor tall-sedge pine fen and nutrient-rich herb-rich spruce swamp: FRB was smaller in the drained site compared to the undrained site in the pine fen, but the opposite was true in the spruce swamp. FRP was generally higher in the nutrient-poor, pine-dominated sites than the nutrient-rich, spruce-dominates sites. The depth distribution of FRB was more superficial than that of FRP, except for the most nutrient-rich spruce swamp. Tree and shrub roots dominated both FRB and FRP, except for the undrained pine fen, where graminoids and forbs dominated. Even there, these PFTs were replaced by trees and shrubs at the drained site. Site wetness and nutrient regime both thus clearly regulated FRB and FRP of the forested peatland site types studied, and both need to be considered when making any generalizations.

Fine-root production (FRP) data along with climatic variables (annual precipitation, temperature sum, and latitude) and stand variables (tree stand stem volume; tree stand basal area; stand basal area of tree species including Scots pine, Norway spruce and deciduous trees; site type; peat type; peat depth; C:N ratio of topmost 20 cm peat layer; grouping of sites to nutrient rich and nutrient poo; average soil water-table level) from 28 forestry-drained peatland forest sites in Finland, FRP and its depth distribution were estimated using ingrowth cores. The ingrowth cores were installed between October 15th and November 27th, 2013, and recovered after two years in late November 2015.

CH4 flux (mg m-2 day-1), soil water-table level, soil temperature, and LAI data from two subarctic fens, Halssiaapa and Lompolojänkkä, located in northern Finland. These data were used for evaluating the impacts of reindeer grazing presence (measurement locations inside and outside exclosure fences) and reindeer droppings (measurement locations with no added reindeer droppings, droppings added on fen surface, droppings added and trampled below the fen surface) on the CH4 flux.

About 15 million ha of peatlands has been drained for forestry to increase the volume of aerobic peat available for tree roots. Simultaneously, the peat that was earlier in anoxic conditions has become available for aerobic decomposers, resulting in increased CO2 emissions through respiration. To mitigate these CO2 emissions a proposed solution is to switch from clearcuts to continuous cover forestry targeting to regulate water table (WT) with tree stand evapotranspiration and maintain an optimum WT low enough for tree growth and high enough to protect peat from decomposition. However, it is yet unknown how tree stand volume and site properties interact in controlling WT and peat decomposition. We aimed at quantifying the effect of tree harvest intensity on WT and heterotrophic peat respiration (RHP) at seven drained peatland forests of varying fertility. We conducted chamber measurements of RHP at plots where autotrophic root respiration was excluded by trenching and the respiration of mosses and fresh litter was excluded by temporarily removing the upmost 5 cm soil layer before the measurements. Seasonal RHP measurements were conducted up to nine years after tree harvesting, with intensities varying from clearcut to different selection harvest regimes and unharvested controls. Unharvested controls had an average WT depth of about 50 cm, which rose after tree harvesting with the largest increase of 22 cm observed in clearcut sites and a smaller effect found in the selection harvest treatments. The highest RHP rates of 307 mg m2 h-1 were measured at the most nutrient rich site types. Although deeper WT in general increased RHP, tree harvest intensity had only a limited impact on RHP. This suggests that although WT depth of drained peatland forests can be regulated with the tree stand volume, the resulting shallower WT may still be too deep to markedly reduce RHP.

Purpose In peatlands, fine-root biomass (FRB) and production (FRP) and their depth distribution and plant functional type (PFT) composition are poorly known. We studied the effects of persistent water-table level drawdown on these characteristics in four forested boreal peatland site types that varied in soil nutrient and WTL regimes, ground vegetation and tree stand characteristics. Methods FRB was estimated by separating and visually identifying roots from soil cores extending down to 50-cm depth. FRP was estimated using ingrowth cores covering the same depth and the separated roots were identified using infrared spectroscopy. Results In both FRB and FRP, the differences between undrained and drained sites were small. In FRB, the clearest differences were seen in the originally wettest site, oligotrophic tall-sedge pine fen, and in the most-nutrient-rich site, herb-rich spruce swamp: in the pine fen FRB was smaller, and in the spruce swamp greater in the drained than in the undrained site. FRP was generally higher in nutrient-poor, pine-dominated sites. The depth distribution of FRB was more superficial than that of FRP, except for the most nutrient-rich spruce swamp. Tree and shrub roots dominated both FRB and FRP, except for the undrained pine fen, where graminoids and forbs dominated. Even there, these PFTs were replaced by trees and shrubs at the drained site. Conclusion Fine-root biomass and production vary widely both within and among different types of boreal forested peatlands. Lowering of WTL large enough to support greater-volume tree stands affects FRB and FRP relatively little.

Boreal upland forests are generally considered methane (CH4) sinks due to the predominance of CH4 oxidizing bacteria over the methanogenic archaea. However, boreal upland forests can temporarily act as CH4 sources during wet seasons or years. From a landscape perspective and in annual terms, this source can be significant as weather conditions may cause flooding, which can last a considerable proportion of the active season and because often, the forest coverage within a typical boreal catchment is much higher than that of wetlands. Processes and conditions which change mineral soils from acting as a weak sink to a strong source are not well understood. We measured soil CH4 fluxes from 20 different points from regularly irrigated and control plots during two growing seasons. We also estimated potential CH4 production and oxidation rates in different soil layers and performed a laboratory experiment, where soil microcosms were subjected to different moisture levels and glucose addition simulating the fresh labile carbon (C) source from root exudates. The aim was to find the key controlling factors and conditions for boreal upland soil CH4 production. Probably due to long dry periods in both summers, we did not find occasions of CH4 production following the excess irrigation, with one exception in July 2019 with emission of 18 200 µg CH4 m−2 h−1. Otherwise, the soil was always a CH4 sink (median CH4 uptake rate of 260–290 and 150–170 µg CH4 m−2 h−1, in control and irrigated plots, respectively). The median soil CH4 uptake rates at the irrigated plot were 88 % and 50 % lower than at the control plot in 2018 and 2019, respectively. Potential CH4 production rates were highest in the organic layer (0.2–0.6 nmol CH4 g−1 d−1), but some production was also observed in the leaching layer, whereas in other soil layers, the rates were negligible. Potential CH4 oxidation rates varied mainly within 10–40 nmol CH4 g−1 d−1, except in deep soil and the organic layer in 2019, where potential oxidation rates were almost zero. The laboratory experiment revealed that high soil moisture alone does not turn upland forest soil into a CH4 source. However, a simple C source, e.g., substrates coming from root exudates with high moisture, switched the soil into a CH4 source. Our unique study provides new insights into the processes and controlling factors on CH4 production and oxidation, and the resulting net efflux that should be incorporated in process models describing global CH4 cycling.

Boreal upland forests are generally considered methane (CH4) sinks due to high methane consumption rates. However, some studies have shown a boreal upland forest soil turning from a CH4 sink to a source after long-term abundant precipitation but the factors affecting this change remain largely unknown. In a simulated rainfall experiment, we investigated soil moisture effects on CH4 flux. We also evaluated the influence of simultaneous soil temperature increase, organic litter addition and root exclusion on CH4 flux. The study was conducted in a northern boreal upland forest soil (Kenttärova forest in Kittilä, Finland) in summer 2018. Split-plot design was used in the experiment with soil moisture being the main treatment variable and soil warming (T), organic litter addition (A) and organic litter and root exclusion (E) subtreatment variables. The design included two main plots: irrigation (I) and control (C), within which each subtreatment was replicated three times. In addition to the T, A and E manipulations, plots without additional manipulations (O) were included for the assessment of the effect of only soil moisture increase (n=4). CH4 flux was measured at least once a week using chambers. Soil moisture and temperature were also continuously measured. Contrary to our expectation, the soil remained a CH4 sink after experimental rainfall increase but irrigation and control sites differed significantly in their CH4 fluxes, indicating a strong decreasing effect of soil moisture on soil CH4 sink. All treatments had lowest CH4 uptake rates in August, possibly as a result of high soil moisture. The IA treatment had the lowest uptake rates possibly due to a reduction in gas diffusion. The IE treatment had contrasting results to all other treatments and showed a positive relationship between uptake rate and soil moisture, but the causes remained unsolved and the results were highly uncertain. T treatment had no effect on uptake likely due to a failure to create soil temperature differences and thus the interactions were not reliably analyzed. Our results suggest that the CH4 flux response to soil moisture may have been more related to changes in CH4 consumption than production. Based on these results, future irrigation experiments should especially assess the combined effect of organic litter addition and soil moisture on soil CH4 sink over multiple years. Improvements in the manipulation methods, experimental design and sensors will also be needed.

Peatlands, especially fens, are known to emit methane. Reindeer (Rangifer tarandus) use mires mainly as spring and summer pastures. In this work we observed that adding reindeer droppings to fen peat increased the potential methane production by 40%. This became apparent when droppings originating from reindeer kept in pen or pasture in winter were added to methanogenic fen peat samples. The droppings introduced Methanobacteriaceae (Methanobrevibacter; > 90% of the mcrA MiSeq reads) to the peat, which was originally populated by Methanosarcinaceae, Methanosaetaceae, Methanoregulaceae, Methanobacteriaceae, Methanomassiliicoccaceae, Methanocellaceae and Methanomicrobiaceae. The original community structure did not explain the induced methane production and neither did the origin of the droppings. Instead, the increment in methane production was explained by the increased methanogenic abundance, measured by mcrA qPCR, due to the addition of droppings. The result confirms that methanogens from the reindeer rumen participate in peat methane production. This finding suggests that reindeer grazing may increase methane emissions in northern fens.

Subarctic ecohydrological processes are changing rapidly, but detailed and integrated ecohydrological investigations are not as widespread as necessary. We introduce an integrated research catchment site (Pallas) for atmosphere, ecosystems, and ecohydrology studies in subarctic conditions in Finland that can be used for a new set of comparative catchment investigations. The Pallas site provides unique observational data and high‐intensity field measurement datasets over long periods. The infrastructure for atmosphere‐ to landscape‐scale research in ecosystem processes in a subarctic landscape has recently been complemented with detailed ecohydrological measurements. We identify three dominant processes in subarctic ecohydrology: (a) strong seasonality drives ecohydrological regimes, (b) limited dynamic storage causes rapid stream response to water inputs (snowmelt and intensive storms), and (c) hydrological state of the system regulates catchment‐scale dissolved carbon dynamics and greenhouse (GHG) fluxes. Surface water and groundwater interactions play an important role in regulating catchment‐scale carbon balances and ecosystem respiration within subarctic peatlands, particularly their spatial variability in the landscape. Based on our observations from Pallas, we highlight key research gaps in subarctic ecohydrology and propose several ways forward. We also demonstrate that the Pallas catchment meets the need for sustaining and pushing the boundaries of critical long‐term integrated ecohydrological research in high‐latitude environments.

Water flows in peatland margins is an under-researched topic. This study examines recharge from a peatland to an esker aquifer in an aapa mire complex of northern Finland. Our objective was to study how the aapa mire margin is hydrogeologically connected to the riverside aquifer and spatial and temporal variations in the recharge of peatland water to groundwater (GW). Following geophysical studies and monitoring of the saturated zone, a GW model (MODFLOW) was used in combination with stable isotopes to quantify GW flow volumes and directions. Peatland water recharge to the sandy aquifer indicated a strong connection at the peatland–aquifer boundary. Recharge volumes from peatland to esker were high and rather constant (873 m3 d−1) and dominated esker recharge at the study site. The peat water recharging the esker boundary was rich in dissolved organic carbon (DOC). Stable isotope studies on water (δ18O, δ2H, and d-excess) from GW wells verified the recharge of DOC-rich water from peatlands to mineral soil esker. Biogeochemical analysis revealed changes from DOC to dissolved inorganic carbon in the flow pathway from peatland margin to the river Kitinen. This study highlights the importance of careful investigation of aapa mire margin areas and their potential role in regional GW recharge patterns.

A substantial amount of below-ground carbon (C) is suggested to be associated with fungi, which may significantly affect the soil C balance in forested ecosystems. Ergosterol from in-growth mesh bags and litterbags was used to estimate fungal biomass production and community composition in drained peatland forests with differing fertility. Extramatrical mycelia (EMM) biomass production was generally higher in the nutrient-poor site, increased with deeper water table level and decreased along the length of the recovery time. EMM biomass production was of the same magnitude as in mineral-soil forests. Saprotrophic fungal biomass production was higher in the nutrient-rich site. Both ectomycorrhizal (ECM) and saprotrophic fungal community composition changed according to site fertility and water table level. ECM fungal community composition with different exploration types may explain the differences in fungal biomass production between peatland forests. Melanin-rich Hyaloscypha may indicate decreased turnover of biomass in nutrient-rich young peatland forest. Genera Lactarius and Laccaria may be important in nutrient rich and Piloderma in the nutrient-poor conditions, respectively. Furthermore, Paxillus involutus and Cortinarius sp. may be important generalists in all sites and responsible for EMM biomass production during the first summer months. Saprotrophs showed a functionally more diverse fungal community in the nutrient-rich site.

Continuous-cover forestry (CCF) is expected to reduce the negative environmental impacts of peatland forestry in comparison with rotation forestry (RF), but the unknown profitability of CCF on peatlands limits its application in practice. The profitability of CCF was analyzed by simulating management scenarios with a process-based ecosystem model, EFIMOD, which was complemented to describe the interplay between tree growth and water table depth, which is typical of peatland forests. A variety of harvest intervals and post-harvest basal areas for a mature Norway spruce (Picea abies (L.) Karst.) dominated stand was simulated on a nutrient-rich peatland site. Conventional RF was simulated for comparison. CCF provided a higher profit than RF. The best financial performance was obtained with a 15-year harvest interval regardless of interest rate, although the overall profitability of CCF depended on the interest rate used. Ditch network maintenance was needed to maintain the stand growth only when the post-harvest basal area was smaller than 10 m2·ha−1. There were many CCF scenarios in which the difference in the net present value of harvest revenues was within 10% compared with the best CCF scenario. Hence, there are many relatively profitable CCF harvesting alternatives for forest management in boreal spruce-dominated peatland forests.

Abstract Northern mires have sequestered substantial amounts of atmospheric carbon since the last glacial period forming one of the largest carbon pools in the biosphere (Hugelius et al., 2020). Current global warming is causing the subarctic and arctic regions warm rapidly, two to three times as fast as the rest of the world (Masson-Delmotte et al., 2018), which will affect the carbon balance of these mires. In Kaamanen, northern Finland, we studied carbon dioxide (CO2) and methane (CH4) exchange between patterned mesotrophic fen and the atmosphere, both on ecosystem and plant community level. The ecosystem level measurements were conducted by utilizing eddy covariance method, while the fluxes on plant community scale were measured with flux chambers. The studied fen can be described as a mosaic of strings and flarks (or hummocks and hollows, respectively). The microtopography of the string-flark continuum form four main plant community types with varying water table conditions and vegetation composition. The measurements took place in 2017–2018. The two years in question were contrasting in their meteorological and environmental conditions. The 2017 growing season had average temperature, but high precipitation sum, while 2018 growing season was warm and dry. In July 2018 a north-western Europe-wide heatwave caused a month-long drought period at the site. Compared to 2017, the annual carbon balance of the Kaamanen fen was affected by earlier onset of photosynthesis in spring and the drought event during summer 2018. We found that the annual carbon balance of the fen did not differ markedly between the studied years, even though the meteorological and environmental conditions did. The earlier onset of growing season in 2018 strengthened the CO2 sink of the ecosystem, but this gain was counterbalanced by the later drought period. Additionally, we found strong spatial variation in CO2 and CH4 dynamics between the main plant communities. Most of the variation in ecosystem level carbon exchange could be explained by the variation in water table level, soil temperature and vegetation characteristics, which were also the environmental factors that varied between the plant community types. References Hugelius, G., Loisel, J., Chadburn, S., Jackson, R. B., Jones, M., MacDonald, G., Marushchak, M., Olefeldt, D., Packalen, M., Siewert, M. B., Treat, C., Turetsky, M., Voigt, C. and Yu, Z.: Large stocks of peatland carbon and nitrogen are vulnerable to permafrost thaw, Proceedings of the National Academy of Sciences - PNAS, 117, 20438–20446, doi:10.1073/pnas.1916387117, 2020. Masson-Delmotte, V., Zhai, P., Pörtner, H.-O., Roberts, D., Skea, J., Shukla, P. R., Pirani, A., Moufouma-Okia, W., Péan, C., Pidcock, R., Connors, S., Matthews, J. B. R., Chen, Y., Zhou, X., Gomis, M. I., Lonnoy, E., Maycock, T., Tignor, M. and Waterfield T. (Eds.): Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty, World Meteorological Organization, Geneva, Switzerland, 2018.