Permafrost in the Arctic is thawing, exposing large carbon and nitrogen stocks for decomposition. Gaseous carbon release from Arctic soils due to permafrost thawing is known to be substantial, but ...growing evidence suggests that Arctic soils may also be relevant sources of nitrous oxide (N₂O). Here we show that N2O emissions from subarctic peatlands increase as the permafrost thaws. In our study, the highest postthaw emissions occurred from bare peat surfaces, a typical landform in permafrost peatlands, where permafrost thaw caused a fivefold increase in emissions (0.56 ± 0.11 vs. 2.81 ± 0.6 mg N₂O m−2 d−1). These emission rates match those from tropical forest soils, the world’s largest natural terrestrial N₂O source. The presence of vegetation, known to limit N₂O emissions in tundra, did decrease (by ∼90%) but did not prevent thaw-induced N₂O release, whereas waterlogged conditions suppressed the emissions. We show that regions with high probability for N₂O emissions cover one-fourth of the Arctic. Our results imply that the Arctic N₂O budget will depend strongly on moisture changes, and that a gradual deepening of the active layer will create a strong noncarbon climate change feedback.
Lakes account for about 10% of the boreal landscape and are responsible for approximately 30% of biogenic methane emissions that have been found to increase under changing climate. However, the ...quantification of this climate-sensitive methane source is fraught with large uncertainty under warming climate conditions. Only a few studies have addressed the mechanism of climate impact on the increase of northern lake methane emissions. This study uses a large observational dataset of lake methane concentrations in Finland to constrain methane emissions with an extant process-based lake biogeochemical model. We found that the total current diffusive emission from Finnish lakes is 0.12 ± 0.03 Tg CH4 yr−1 and will increase by 26%-59% by the end of this century depending on different warming scenarios. We discover that while warming lake water and sediment temperature plays an important role, the climate impact on ice-on periods is a key indicator of future emissions. We conclude that these boreal lakes remain a significant methane source under the warming climate within this century.
Estimates of regional and global freshwater N2O emissions have remained inaccurate due to scarce data and complexity of the multiple processes driving N2O fluxes the focus predominantly being on ...summer time measurements from emission hot spots, agricultural streams. Here, we present four‐season data of N2O concentrations in the water columns of randomly selected boreal lakes covering a large variation in latitude, lake type, area, depth, water chemistry, and land use cover. Nitrate was the key driver for N2O dynamics, explaining as much as 78% of the variation of the seasonal mean N2O concentrations across all lakes. Nitrate concentrations varied among seasons being highest in winter and lowest in summer. Of the surface water samples, 71% were oversaturated with N2O relative to the atmosphere. Largest oversaturation was measured in winter and lowest in summer stressing the importance to include full year N2O measurements in annual emission estimates. Including winter data resulted in fourfold annual N2O emission estimates compared to summer only measurements. Nutrient‐rich calcareous and large humic lakes had the highest annual N2O emissions. Our emission estimates for Finnish and boreal lakes are 0.6 and 29 Gg N2O‐N/year, respectively. The global warming potential of N2O from lakes cannot be neglected in the boreal landscape, being 35% of that of diffusive CH4 emission in Finnish lakes.
Up‐scaling of freshwater N2O emissions at regional to global scales has remained challenging due to sparse data based on summer measurements. We collected seasonal data on N2O concentrations from 112 randomly selected boreal lakes in Finland and determined a representative set of possible drivers. Our data underline the key role of nitrate in regulating seasonal and spatial N2O concentrations. Nitrate explained 78% of the variation in N2O across all lakes. The Global Warming Potential of N2O in our data was 35% of that of diffusive CH4 emission underlining the importance to include N2O in landscape GHG evasion estimates.
Despite the fact that heterotrophic nitrification was identified more than 100 years ago, the biochemistry of heterotrophic nitrifiers is poorly known and their contribution to nitrification in soil ...is still speculative. Heterotrophic nitrifiers need organic compounds as their energy source in contrast to the chemolithotrophic nitrifiers. Most of the potential pathways for nitrite/nitrate production by heterotrophs can be considered as secondary metabolism. Only nitrification and simultaneous denitrification by some heterotrophic bacteria is known to have connection to the energy metabolism. Evidently, the nitrification pathways of bacteria and fungi differ. Some heterotrophic bacteria oxidizing ammonia have ammonia monooxygenase (AMO), but there are also heterotrophic bacteria oxidizing ammonia without AMO. The structure of AMO of chemolithotrophic ammonia oxidizers and heterotrophs differs. AMO has not been found in nitrifying fungi. The conditions for heterotrophic nitrification in soil highly differ from those in waste waters where heterotrophic nitrification activity can be high. Heterotrophic nitrification in soil is limited by the low availability of easily decomposable organic substrates. Possible nitrification by heterotrophs in the rhizosphere and endophytic root microbes gaining a good supply of substrates from plants is not known. Fungi are of special interest in heterotrophic nitrification in soil because fungi can evidently nitrify not only easily decomposable substrates but also when decomposing recalcitrant organic compounds (like lignin) which are abundant in soil. Nitrite/nitrate production from easily decomposable nitrogenous organic compounds, such as amino acids, is common among heterotrophic bacteria and fungi. The general conclusion that heterotrophs use solely an “organic pathway” in their nitrification is, however, not valid because also ammonia can be oxidized. Owing to the diverse, poorly known biochemistry of heterotrophic nitrification and methodological difficulties to differentiate heterotrophic and chemolithotrophic nitrification in soil, the role of heterotrophic nitrification in soil nitrogen cycle remains uncertain.
•Heterotrophic nitrification involves several poorly known mechanisms.•Heterotrophic bacteria, unlike fungi can possess ammonia monooxygenase.•Fungi are able to nitrify easily decomposable and recalcitrant organic compounds.•The methods available do not allow accurate determination of nitrification in soil.
Rapidly rising temperatures in the Arctic might cause a greater release of greenhouse gases (GHGs) to the atmosphere. To study the effect of warming on GHG dynamics, we deployed open‐top chambers in ...a subarctic tundra site in Northeast European Russia. We determined carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes as well as the concentration of those gases, inorganic nitrogen (N) and dissolved organic carbon (DOC) along the soil profile. Studied tundra surfaces ranged from mineral to organic soils and from vegetated to unvegetated areas. As a result of air warming, the seasonal GHG budget of the vegetated tundra surfaces shifted from a GHG sink of −300 to −198 g CO2–eq m−2 to a source of 105 to 144 g CO2–eq m−2. At bare peat surfaces, we observed increased release of all three GHGs. While the positive warming response was dominated by CO2, we provide here the first in situ evidence of increasing N2O emissions from tundra soils with warming. Warming promoted N2O release not only from bare peat, previously identified as a strong N2O source, but also from the abundant, vegetated peat surfaces that do not emit N2O under present climate. At these surfaces, elevated temperatures had an adverse effect on plant growth, resulting in lower plant N uptake and, consequently, better N availability for soil microbes. Although the warming was limited to the soil surface and did not alter thaw depth, it increased concentrations of DOC, CO2, and CH4 in the soil down to the permafrost table. This can be attributed to downward DOC leaching, fueling microbial activity at depth. Taken together, our results emphasize the tight linkages between plant and soil processes, and different soil layers, which need to be taken into account when predicting the climate change feedback of the Arctic.
Experimental air warming increased emissions of all three greenhouse gases (GHGs), including the highly understudied N2O, clearly demonstrating the need to include N2O in future Arctic GHG budgets. Increased GHG fluxes were regulated by changes in plant functioning and biogeochemical processes, leading to an enhanced soil input of labile carbon compounds via leaching. Plants were also identified as the main regulator of arctic N2O emissions. Importantly, we highlight the tight linkages between plant and soil processes, and the interactions between the top‐soil and deeper soil layers, in regulating arctic GHG exchange.
Permafrost peatlands are biogeochemical hot spots in the Arctic as they store vast amounts of carbon. Permafrost thaw could release part of these long‐term immobile carbon stocks as the greenhouse ...gases (GHGs) carbon dioxide (CO2) and methane (CH4) to the atmosphere, but how much, at which time‐span and as which gaseous carbon species is still highly uncertain. Here we assess the effect of permafrost thaw on GHG dynamics under different moisture and vegetation scenarios in a permafrost peatland. A novel experimental approach using intact plant–soil systems (mesocosms) allowed us to simulate permafrost thaw under near‐natural conditions. We monitored GHG flux dynamics via high‐resolution flow‐through gas measurements, combined with detailed monitoring of soil GHG concentration dynamics, yielding insights into GHG production and consumption potential of individual soil layers. Thawing the upper 10–15 cm of permafrost under dry conditions increased CO2 emissions to the atmosphere (without vegetation: 0.74 ± 0.49 vs. 0.84 ± 0.60 g CO2–C m−2 day−1; with vegetation: 1.20 ± 0.50 vs. 1.32 ± 0.60 g CO2–C m−2 day−1, mean ± SD, pre‐ and post‐thaw, respectively). Radiocarbon dating (14C) of respired CO2, supported by an independent curve‐fitting approach, showed a clear contribution (9%–27%) of old carbon to this enhanced post‐thaw CO2 flux. Elevated concentrations of CO2, CH4, and dissolved organic carbon at depth indicated not just pulse emissions during the thawing process, but sustained decomposition and GHG production from thawed permafrost. Oxidation of CH4 in the peat column, however, prevented CH4 release to the atmosphere. Importantly, we show here that, under dry conditions, peatlands strengthen the permafrost–carbon feedback by adding to the atmospheric CO2 burden post‐thaw. However, as long as the water table remains low, our results reveal a strong CH4 sink capacity in these types of Arctic ecosystems pre‐ and post‐thaw, with the potential to compensate part of the permafrost CO2 losses over longer timescales.
Permafrost peatlands are biogeochemical hot spots in the Arctic as they store vast amounts of carbon. Permafrost thaw could release part of these long‐term immobile carbon stocks as the greenhouse gases (GHGs) carbon dioxide (CO2) and methane (CH4) to the atmosphere, but how much, at which time‐span and as which gaseous carbon species is still highly uncertain. A novel experimental approach using intact plant–soil systems (mesocosms) allowed us to simulate permafrost thaw under near‐natural conditions. We show here that peatlands may strengthen the permafrost–carbon feedback by adding to the atmospheric CO2 burden post‐thaw.
The photolysis of nitrous acid (HONO) is an important source of OH radical, the key oxidizing agent in the atmosphere. Recently it has been reported that nitrite (NO2−) in soil can also be a source ...of atmospheric HONO, in addition to the other pathways, e.g. the hydrolysis of nitrogen dioxide (NO2) on wet surfaces. However, the production rates of HONO in various soils are unknown. We selected a range of dominant northern acidic soils and showed in microcosm experiments that soils which have the highest nitrous oxide (N2O) and nitric oxide (NO) emissions (drained peatlands) also have the highest HONO production rates. It is known that in natural peatlands with high water table and in boreal coniferous forest soils, low nitrification activity (microbial production of nitrite and nitrate) limits their N2O production. Low availability of nitrite in these soils is the likely reason also for their low HONO production rates. Lowering of water table of peatlands as a result of drainage or climate change enhances their nitrification activity and N2O/NO production and as shown here also their HONO production. Also nitrogen deposition, or nitrogen fertilization and other land-use practices increasing availability of mineral nitrogen can be expected to enhance HONO emissions from soils.
•Soil nitrite (NO2−) can be a source of atmospheric nitrous acid (HONO).•In acidic soils NO2− it is partly released as HONO.•Northern acidic soils are emitting HONO to the atmosphere.•HONO emissions from acidic soils are connected to N-cycle, N2O and NO emissions and soil C/N ratio.
Abstract
The paradigm that permafrost-affected soils show restricted mineral nitrogen (N) cycling in favor of organic N compounds is based on the observation that net N mineralization rates in these ...cold climates are negligible. However, we find here that this perception is wrong. By synthesizing published data on N cycling in the plant-soil-microbe system of permafrost ecosystems we show that gross ammonification and nitrification rates in active layers were of similar magnitude and showed a similar dependence on soil organic carbon (C) and total N concentrations as observed in temperate and tropical systems. Moreover, high protein depolymerization rates and only marginal effects of C:N stoichiometry on gross N turnover provided little evidence for N limitation. Instead, the rather short period when soils are not frozen is the single main factor limiting N turnover. High gross rates of mineral N cycling are thus facilitated by released protection of organic matter in active layers with nitrification gaining particular importance in N-rich soils, such as organic soils without vegetation. Our finding that permafrost-affected soils show vigorous N cycling activity is confirmed by the rich functional microbial community which can be found both in active and permafrost layers. The high rates of N cycling and soil N availability are supported by biological N fixation, while atmospheric N deposition in the Arctic still is marginal except for fire-affected areas. In line with high soil mineral N production, recent plant physiological research indicates a higher importance of mineral plant N nutrition than previously thought. Our synthesis shows that mineral N production and turnover rates in active layers of permafrost-affected soils do not generally differ from those observed in temperate or tropical soils. We therefore suggest to adjust the permafrost N cycle paradigm, assigning a generally important role to mineral N cycling. This new paradigm suggests larger permafrost N climate feedbacks than assumed previously.
Peatlands are a major natural source of atmospheric methane (CH
4
). Emissions from
Sphagnum-
dominated mires are lower than those measured from other mire types. This observation may partly be due ...to methanotrophic (i.e., methane-consuming) bacteria associated with
Sphagnum
. Twenty-three of the 41
Sphagnum
species in Finland can be found in the peatland at Lakkasuo. To better understand the
Sphagnum
-methanotroph system, we tested the following hypotheses: (1) all these
Sphagnum
species support methanotrophic bacteria; (2) water level is the key environmental determinant for differences in methanotrophy across habitats; (3) under dry conditions,
Sphagnum
species will not host methanotrophic bacteria; and (4) methanotrophs can move from one
Sphagnum
shoot to another in an aquatic environment. To address hypotheses 1 and 2, we measured the water table and CH
4
oxidation for all
Sphagnum
species at Lakkasuo in 1-5 replicates for each species. Using this systematic approach, we included
Sphagnum
spp. with narrow and broad ecological tolerances. To estimate the potential contribution of CH
4
to moss carbon, we measured the uptake of δ
13
C supplied as CH
4
or as carbon dioxide dissolved in water
.
To test hypotheses 2-4, we transplanted inactive moss patches to active sites and measured their methanotroph communities before and after transplantation. All 23
Sphagnum
species showed methanotrophic activity, confirming hypothesis 1. We found that water level was the key environmental factor regulating methanotrophy in
Sphagnum
(hypothesis 2). Mosses that previously exhibited no CH
4
oxidation became active when transplanted to an environment in which the microbes in the control mosses were actively oxidizing CH
4
(hypothesis 4). Newly active transplants possessed a
Methylocystis
signature also found in the control
Sphagnum
spp. Inactive transplants also supported a
Methylocystis
signature in common with active transplants and control mosses, which rejects hypothesis 3. Our results imply a loose symbiosis between
Sphagnum
spp. and methanotrophic bacteria that accounts for potentially 10-30% of
Sphagnum
carbon.
Increased use of bioenergy has increased the production of wood ash in power plants. Wood ash is used as a fertilizer to improve tree stand growth in nitrogen-rich peatland forests. Nutrients removed ...during tree harvest are recycled back to the forest by wood ash fertilization. However, there is a risk that ash enhances microbial processes and associated nitrous oxide (N2O) production in peat. Because there are difficulties in spreading loose ash, ash is nowadays granulated or hardened in other ways before it is applied as fertilizer. Granulation process changes not only the physical but also the chemical properties of ash. In previous studies wood ash has showed variable effects on N2O emissions. Nevertheless, there are indications that in laboratory conditions granulated wood ash decreases N2O production. We conducted laboratory experiments to investigate the capability of granulated wood ash to decrease N2O production in peat soils and the role of nitrification and denitrification in N2O production. Acetylene inhibition experiments showed that N2O production was dominated by nitrification over denitrification. The rate of nitrification as well as N2O production from nitrification and denitrification decreased by the addition of granulated wood ash. Wood ash addition increased peat electrical conductivity while peat pH remained unchanged. The inhibitory effect of ash was associated to ion content because isolated ion addition experiments produced a similar decline in N2O production. The inhibition of ions was more evident in acidic conditions because when peat pH was increased by addition of lime, the ions or granulated ash inhibited N2O production significantly less than they did at natural peat pH. Our results show that manipulation of ion strength could offer a new tool to slow nitrification and denitrification processes and decrease the associated N2O emissions in acidic soils.
•Granulated wood ash decreased nitrous oxide (N2O) production in acidic peat soil.•Nitrification dominated over denitrification in N2O production in peat soil.•Ions leached from granulated wood ash inhibited N2O production in acidic peat soil.•Inhibitory effect of granulated ash and ions was eliminated by increasing peat pH.
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