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.
N
2
O, a greenhouse gas, is increasingly emitted from degrading permafrost mounds of palsa mires because of the global warming effects on microbial activity. In the present study, we hypothesized ...that N
2
O emission could be affected by a change in pH conditions because the collapse of acidic palsa mounds (pH 3.4–4.6) may result in contact with minerogenic ground water (pH 4.8–6.3), thereby increasing the pH. We compared the effects of pH change on N
2
O emission from cultures inoculated with peat suspensions. Peat samples were collected on a transect from a still intact high part to the collapsing edge of a degrading palsa mound in northwestern Finland, assuming the microbial communities could be different. We adjusted the pH of peat suspensions prepared from a collapsing palsa mound and compared the N
2
O emission in a pH gradient from 4.5 to 8.5. The collapsing edge had the highest N
2
O emission from the peat suspensions among all points on the transect under natural acidic conditions (pH 4.5). The N
2
O emission was reduced with a moderate rise in pH (pH 5.0–6.0) by approximately 85% compared with natural acidic level (pH 4.5). The bacterial communities in acidic cultures differed considerably from those in alkaline cultures. When pH was adjusted to alkaline conditions, N
2
O–emitting bacteria different from those present in acidic conditions appeared to emit N
2
O. The bacterial communities could be characterized by changing pH conditions after thawing and collapse of permafrost have contrasting impacts on N
2
O production that calls for further attention in future studies.
Northern mires (fens and bogs) have significant climate feedbacks and contribute to biodiversity, providing habitats to specialized biota. Many studies have found drying and degradation of bogs in ...response to climate change, while northern fens have received less attention. Rich fens are particularly important to biodiversity, but subject to global climate change, fen ecosystems may change via direct response of vegetation or indirectly by hydrological changes. With repeated sampling over the past 20 years, we aim to reveal trends in hydrology and vegetation in a pristine boreal fen with gradient from rich to poor fen and bog vegetation. We resampled 203 semi‐permanent plots and compared water‐table depth (WTD), pH, concentrations of mineral elements, and dissolved organic carbon (DOC), plant species occurrences, community structure, and vegetation types between 1998 and 2018. In the study area, the annual mean temperature rose by 1.0°C and precipitation by 46 mm, in 20‐year periods prior to sampling occasions. We found that wet fen vegetation decreased, while bog and poor fen vegetation increased significantly. This reflected a trend of increasing abundance of common, generalist hummock species at the expense of fen specialist species. Changes were the most pronounced in high pH plots, where Sphagnum mosses had significantly increased in plot frequency, cover, and species richness. Changes of water chemistry were mainly insignificant in concentration levels and spatial patterns. Although indications toward drier conditions were found in vegetation, WTD had not consistently increased, instead, our results revealed complex dynamics of WTD as depending on vegetation changes. Overall, we found significant trend in vegetation, conforming to common succession pattern from rich to poor fen and bog vegetation. Our results suggest that responses intrinsic to vegetation, such as increased productivity or altered species interactions, may be more significant than indirect effects via local hydrology to the ecosystem response to climate warming.
We studied global change effects on vegetation and hydrology in a pristine boreal mire in a decadal time‐span. We repeated over 200 measurements of plant species cover, water‐table depth, and several water chemistry variables, after a marked shift to warmer conditions. We did not find significant changes in water chemistry, while remarkable vegetation changes were observed, suggesting that responses intrinsic to vegetation were more significant than indirect effects via local hydrology to the ecosystem response to recent warming.
•Multispectral and RGB UAV-data provide vegetation maps for different purposes.•K-means spectral classes reflect plant entities with shared wetness preference.•RF classification reaches moderate to ...good results for plant community mapping.•Plant communities with distinct microtopography are most accurately mapped.•Hierarchical clustering of vegetation data can be used for image classification.
Plant communities of mires can be linked to important ecological processes, such as carbon storage and gas fluxes. As indicators of ecosystem dynamics, knowledge about their distribution and condition can support ecosystem assessment. Mapping mire vegetation enables monitoring at ecosystem-scale, which can be done with UAVs (Unmanned Aerial Vehicles). Depending on the mounted sensor and the spectral signals recorded, various attributes of plant communities can be retrieved. However, it is uncertain to what extent plant communities can be derived, as mapping vegetation on detailed level remains challenging due to overlapping spectral signatures of plant species. Advancing technology offers the choice between low cost RGB and multispectral sensors as well as a variety of classification methods to overcome these challenges. Therefore, we used K-means unsupervised classification and Random Forest supervised classification with different input variables to map microtopographical patterns and plant communities of two aapa mires as resolved by hierarchical clustering. This extensive approach allowed the assessment of both classifier’s strength and weaknesses, as well as the criteria of selecting suitable input data. UAV- RGB and multispectral imagery with associated spectral and topographical indices of both 0.05 m and 0.30 m spatial resolution were used for the K-means method. We assessed the relationship between these generated spectral classes and plant community clusters. The clusters further served as training and validation labels for to classify the high resolution, multispectral UAV-data (0.05 m) using Random Forest. Our study demonstrates that maps reflecting microtopograpical patterns and a wetness gradient can be produced with low-cost RGB imagery and unsupervised classification. Despite this linkage to plant communities, enhanced and detailed maps of plant community distribution can only be achieved with multispectral data and robust machine learning techniques. Random forest classifications showed good overall accuracies (0.59 – 0.82) in mapping microtopographical patterns and plant communities based on hierarchical clustering of vegetation data. While the strength of both classifiers lies in the distinction of bog hummock communities, classification performance was weaker between different transition and lawn community types. Casual misclassifications occurred also for communities along the transition of microtopographical patterns. The main obstacle for accurate mapping remains the overlap of spectral signatures from species and spectral noise originating from wetness in mires that lead to misclassification. Future studies addressing plant community and diversity mapping should therefore consider the origin of spectral variation with further sensors.
•We detected spatial patterns of plant community clusters, plant functional types, and plant species in two northern peatlands.•We utilized uncrewed aerial vehicle (UAV) hyperspectral and ...multispectral imagery and topography data.•The combination of multispectral and topography data yielded the best performance in most cases.•The UAV hyperspectral data aided in the identification of some vegetation characteristics.
Northern peatland vegetation exhibits fine-scale spatial and spectral heterogeneity that can potentially be captured with uncrewed aerial vehicle (UAV) data due to their ultra-high spatial resolution (<10 cm). From this perspective, the contribution of different spectral sensors in mapping various vegetation characteristics has not been thoroughly investigated. We delineated spatial patterns of plant community clusters, plant functional types (PFTs), and selected plant species with UAV hyperspectral (HS), UAV multispectral (MS), and airborne LiDAR (light detection and ranging) topography (TP) data in two northern peatlands. We conducted random forest (RF) regressions in a geographic object-based image analysis (GEOBIA) framework and compared the relative contributions of the different datasets. In the best regression models, the percentage of explained variance was 24–74 % (RMSE:0.24–0.31), 40–90 % (RMSE:0.12–0.41), and 18–90 % (RMSE:0.03–0.40) for plant community clusters, PFTs, and plant species, respectively. The MS-TP combination had, in many cases, the highest performance, while HS-based models had better performance for some plant community clusters, PFTs, and plant species. TP features were useful only in certain situations. Overall, our results suggest that UAV MS imagery combined with TP data outperformed or performed at least almost as well as the models using UAV HS data and while all data combinations are capable for fine-scale detection of vegetation in northern peatlands. A more comprehensive investigations of data processing and methodology selection is needed to conclude if there is an added value of UAV HS data for peatland vegetation monitoring.
1. Hydrological changes due to drainage and climate warming can have great impact on the ecosystem balance of boreal mires. The possibility of ombrotrophication, i.e. the development from fen to bog, ...in response to altered hydrology has not been previously tested. Here, recent changes in vegetation and surface peat are studied in an aapa mire, a typical boreal mire system dominated by fen vegetation. Drainage in the catchment from 1968 onwards led to the change from richly minerogenous to ombrogenous hydrology, thus providing a long-term ombrotrophication experiment. 2. A sequence of aerial photographs (1941, 1953, 1965, 1974, 1984, 1995, 2005) revealed a dramatic shift from fen vegetation to the nearly complete dominance of peat mosses (Sphagnum) within two decades after the catchment disturbance. 3. A distinct change from Carex peat to Sphagnum peat at the average depth of 23.3 cm (SE 0.8 cm) was found in 18 peat cores. All of the new Sphagnum peat had accumulated within the last four decades. This was verified by the relationship of age and rooting depth of 37 small pines (Pinus sylvestris) and by two pollen density profiles. The ratio Ca/Mg diminished towards the surface of peat profiles indicating change from minerogenous to ombrogenous hydrology. In accordance, extremely low pH (range 3.8-4.2) and conductivity (average 14.5 μs cm⁻¹) were measured in the surface pore water. 4. The average total dry mass of new Sphagnum peat was 7042 g m⁻² (SE 442) and the recent apparent rate of carbon accumulation was 100.6 g m⁻² year⁻¹ (SE 6.3), as calculated for a 35-year period and 50% carbon content. 5. Synthesis. Remarkable potential for vegetation change and increase of peat growth is demonstrated in boreal aapa mires. Ombrotrophication can be initiated within a few decades in response to reduced input of minerogenous water. Future changes in the hydrological cycle, as indicated by climate change models, are similar to the impact of catchment disturbance in aapa mires. Diminished total water budgets during the summer cause a decrease of minerogenous input and a draw-down of water level, both of which may promote the growth of Sphagnum over fen vegetation.
Background and aims
In forestry-drained peatlands, drying leads to changes in C cycling which could affect peat δ
13
C. Furthermore, the δ
13
C profile of the entire peat column may reveal effects of ...earlier climatic periods.
Methods
We measured peat δ
13
C and C inventories in adjacent peat profiles, two collected from undrained and two from the drained side of a bog that was partially ditch-drained 37 years earlier. The cores were sliced into 10-cm subsamples for analyses; matching of the profiles based on surface levelling, peat stratigraphic correlation and a horizontal ash layer found in both profiles.
Results
Surface subsidence of 30 cm was observed in the dried site and the uppermost 160 cm in the undrained site contained an excess of 5.9 kg m
−2
of C compared with the corresponding strata of the ditch-drained site. The δ
13
C values increased but markedly only in the thin surface layer of the drained site, indicating low δ
13
C of the missing C (ca. –30‰). In the deeper strata, dating to Mid-Holocene, high dry bulk density, C%, N%, humification index and low C/N ratio were connected to low δ
13
C of peat.
Conclusions
Drainage of 37 years increased δ
13
C values in the upper peat profile of the drained bog and led to the selective loss of
13
C depleted C. Results indicate that C balance studies can be aided by C isotope analyses. Low δ
13
C values in the peat profile indicate the existence of a wet fen stage during the moist and warm period during Mid-Holocene.
Peat bogs have historically represented exceptional carbon (C) sinks because of their extremely low decomposition rates and consequent accumulation of plant remnants as peat. Among the factors ...favoring that peat accumulation, a major role is played by the chemical quality of plant litter itself, which is poor in nutrients and characterized by polyphenols with a strong inhibitory effect on microbial breakdown. Because bogs receive their nutrient supply solely from atmospheric deposition, the global increase of atmospheric nitrogen (N) inputs as a consequence of human activities could potentially alter the litter chemistry with important, but still unknown, effects on their C balance. Here we present data showing the decomposition rates of recently formed litter peat samples collected in nine European countries under a natural gradient of atmospheric N deposition from approximately equal to 0.2 to 2 g·m⁻²·yr⁻¹. We found that enhanced decomposition rates for material accumulated under higher atmospheric N supplies resulted in higher carbon dioxide (CO₂) emissions and dissolved organic carbon release. The increased N availability favored microbial decomposition (i) by removing N constraints on microbial metabolism and (ii) through a chemical amelioration of litter peat quality with a positive feedback on microbial enzymatic activity. Although some uncertainty remains about whether decay-resistant Sphagnum will continue to dominate litter peat, our data indicate that, even without such changes, increased N deposition poses a serious risk to our valuable peatland C sinks.
Melting permafrost mounds in subarctic palsa mires are thawing under climate warming and have become a substantial source of N
2
O emissions. However, mechanistic insights into the permafrost ...thaw-induced N
2
O emissions in these unique habitats remain elusive. We demonstrated that N
2
O emission potential in palsa bogs was driven by the bacterial residents of two dominant
Sphagnum
mosses especially of
Sphagnum capillifolium
(SC) in the subarctic palsa bog, which responded to endogenous and exogenous
Sphagnum
factors such as secondary metabolites, nitrogen and carbon sources, temperature, and pH. SC's high N
2
O emission activity was linked with two classes of distinctive hyperactive N
2
O emitters, including
Pseudomonas
sp. and
Enterobacteriaceae
bacteria, whose hyperactive N
2
O emitting capability was characterized to be dominantly pH-responsive. As the
nosZ
gene-harboring emitter,
Pseudomonas
sp. SC-H2 reached a high level of N
2
O emissions that increased significantly with increasing pH. For emitters lacking the
nosZ
gene,
an Enterobacteriaceae
bacterium SC-L1 was more adaptive to natural acidic conditions, and N
2
O emissions also increased with pH. Our study revealed previously unknown hyperactive N
2
O emitters in
Sphagnum capillifolium
found in melting palsa mound environments, and provided novel insights into SC-associated N
2
O emissions.
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