Peatlands contain one-third of soil carbon (C), mostly buried in deep, saturated anoxic zones (catotelm). The response of catotelm C to climate forcing is uncertain, because prior experiments have ...focused on surface warming. We show that deep peat heating of a 2 m-thick peat column results in an exponential increase in CH
emissions. However, this response is due solely to surface processes and not degradation of catotelm peat. Incubations show that only the top 20-30 cm of peat from experimental plots have higher CH
production rates at elevated temperatures. Radiocarbon analyses demonstrate that CH
and CO
are produced primarily from decomposition of surface-derived modern photosynthate, not catotelm C. There are no differences in microbial abundances, dissolved organic matter concentrations or degradative enzyme activities among treatments. These results suggest that although surface peat will respond to increasing temperature, the large reservoir of catotelm C is stable under current anoxic conditions.
Peatlands contain one-third of the world's soil carbon (C). If destabilized, decomposition of this vast C bank could accelerate climate warming; however, the likelihood of this outcome remains ...unknown. Here, we examine peatland C stability through five years of whole-ecosystem warming and two years of elevated atmospheric carbon dioxide concentrations (eCO
). Warming exponentially increased methane (CH
) emissions and enhanced CH
production rates throughout the entire soil profile; although surface CH
production rates remain much greater than those at depth. Additionally, older deeper C sources played a larger role in decomposition following prolonged warming. Most troubling, decreases in CO
:CH
ratios in gas production, porewater concentrations, and emissions, indicate that the peatland is becoming more methanogenic with warming. We observed limited evidence of eCO
effects. Our results suggest that ecosystem responses are largely driven by surface peat, but that the vast C bank at depth in peatlands is responsive to prolonged warming.
We examined rates of C, N, and P mineralization in soils from 16 northern Minnesota wetlands that occur across an ombrotrophic-minerotrophic gradient. Soils were incubated at 30 degrees C under ...aerobic and anaerobic conditions for 59 wk, and the results were fit with a two-pool kinetic model. Additionally, 39 different soil quality variables were used in a principal components analysis (PCA) to predict mineralization rates. Mineralization of C, N, and P differed significantly among wetland types, aeration status (aerobic vs. anaerobic), and their interaction term. Despite low total soil N and P, there was a rapid turnover of the nutrient pools in ombrotrophic sites, particularly under aerobic conditions. On a volumetric basis, C and N mineralization increased in a predictable manner across the ombrotrophic-minerotrophic gradient, largely due to increasing soil bulk density. However, P mineralization per cubic centimeter remained relatively high in the bogs. The higher total P content of more minerotrophic soils appears to be offset by greater P immobilization due to geochemical sorption, yielding overall lower availability. Total C turnover rates were relatively similar among sites, despite large differences in soil quality. We suggest that, over time, the decay rates of organic matter in different wetland communities converge to a common rate. In contrast, CH4 production was extremely low in ombrotrophic peats. The apparent labile pools of N (N0), P (P0), and C (C0) were generally <10% of their respective total pool sizes, except for P0 in the bogs, which constituted up to 33% of total soil P. From 10% to 87% of the N, P, and C mineralized after 59 wk was derived from their respective labile pools. A simple group of variables describing the physical degree of decomposition of organic matter was often as good as, or superior to, more complicated chemical analyses in predicting C, N, and P mineralization. Because peats are classified and mapped according to these variables, it should make scaling efforts in landscape analyses much more tractable. Large differences in mineralization rates in northern wetland communities demonstrate that climate change models should not consider these areas as homogeneous entities. Our C mineralization results suggest that soil respiratory response to climate change (as CO2 and CH4) will vary considerably in different wetland communities. Our results also suggest that the common perception that more ombrotrophic sites are inherently more nutrient deficient needs to be reassessed.
The majority of the Earth's terrestrial carbon is stored in the soil. If anthropogenic warming stimulates the loss of this carbon to the atmosphere, it could drive further planetary warming. Despite ...evidence that warming enhances carbon fluxes to and from the soil, the net global balance between these responses remains uncertain. Here we present a comprehensive analysis of warming-induced changes in soil carbon stocks by assembling data from 49 field experiments located across North America, Europe and Asia. We find that the effects of warming are contingent on the size of the initial soil carbon stock, with considerable losses occurring in high-latitude areas. By extrapolating this empirical relationship to the global scale, we provide estimates of soil carbon sensitivity to warming that may help to constrain Earth system model projections. Our empirical relationship suggests that global soil carbon stocks in the upper soil horizons will fall by 30 ± 30 petagrams of carbon to 203 ± 161 petagrams of carbon under one degree of warming, depending on the rate at which the effects of warming are realized. Under the conservative assumption that the response of soil carbon to warming occurs within a year, a business-as-usual climate scenario would drive the loss of 55 ± 50 petagrams of carbon from the upper soil horizons by 2050. This value is around 12-17 per cent of the expected anthropogenic emissions over this period. Despite the considerable uncertainty in our estimates, the direction of the global soil carbon response is consistent across all scenarios. This provides strong empirical support for the idea that rising temperatures will stimulate the net loss of soil carbon to the atmosphere, driving a positive land carbon-climate feedback that could accelerate climate change.
Peatlands occupy approximately 15% of boreal and sub-arctic regions, contain approximately one third of the world's soil carbon pool, and supply most of the dissolved organic carbon (DOC) entering ...boreal lakes and rivers and the Arctic Ocean. The high latitudes occupied by these peatlands are expected to see the greatest amount of climatic warming in the next several decades. In addition to increasing temperatures, climatic change could also affect the position of the water-table level and discharge from these peatlands. Changes in temperature, water tables, and discharge could affect delivery of DOC to downstream ecosystems where it exerts significant control over productivity, biogeochemical cycles, and attenuation of visible and UV radiation. We experimentally warmed and controlled water tables while measuring discharge in a factorial experiment in large mesocosms containing peat monoliths and intact plant communities from a bog and fen to determine the effects of climate change on DOC budgets. We show that the DOC budget is controlled largely by changes in discharge rather than by any effect of warming or position of the water-table level on DOC concentrations. Furthermore, we identify a critical discharge rate in bogs and fens for which the DOC budget switches from net export to net retention. We also demonstrate an exponential increase in trace gas CO2- C and CH4- C emissions coincident with increased retention of dissolved organic carbon from boreal peatlands.
In this Brief Communications Arising Reply, the affiliation for author P. H. Templer was incorrectly listed as 'Department of Ecology & Evolutionary Biology, University of California Irvine, Irvine, ...California 92697, USA' instead of 'Department of Biology, Boston University, Boston, Massachusetts 02215, USA'. This has been corrected online.
Boreal peatlands may be particularly vulnerable to climate change, because temperature regimes that currently constrain biological activity in these regions are predicted to increase substantially ...within the next century. Changes in peatland plant community composition in response to climate change may alter nutrient availability, energy budgets, trace gas fluxes, and carbon storage. We investigated plant community response to warming and drying in a field mesocosm experiment in northern Minnesota, USA. Large intact soil monoliths removed from a bog and a fen received three infrared warming treatments crossed with three water‐table treatments (n = 3) for five years. Foliar cover of each species was estimated annually.
In the bog, increases in soil temperature and decreases in water‐table elevation increased cover of shrubs by 50% and decreased cover of graminoids by 50%. The response of shrubs to warming was distinctly species‐specific, and ranged from increases (for Andromeda glaucophylla) to decreases (for Kalmia polifolia). In the fens, changes in plant cover were driven primarily by changes in water‐table elevation, and responses were species‐ and lifeform‐specific: increases in water‐table elevation increased cover of graminoids – in particular Carex lasiocarpa and Carex livida– as well as mosses. In contrast, decreases in water‐table elevation increased cover of shrubs, in particular A. glaucophylla and Chamaedaphne calyculata. The differential and sometimes opposite response of species and lifeforms to the treatments suggest that the structure and function of both bog and fen plant communities will change – in different directions or at different magnitudes – in response to warming and/or changes in water‐table elevation that may accompany regional or global climate change.
Although methane (CH₄) dynamics are known to differ at broad scales among peatland types and with climate, there is limited understanding of the variability associated with anaerobic carbon (C) ...cycling, and, the mechanisms that control that variability, among low pH, Sphagnum moss-dominated peatlands within a geographical region with similar climate. This is important because upscaling of CH₄ emissions to regional and global scales often considers peatlands as a single, or at most two, ecosystem type (s). Here, we report the results from two studies exploring the controls of CH₄ cycling in peatlands from the Upper Midwest (USA). Potential CH₄ production and resultant CO₂:CH₄ ratios varied by several orders-of-magnitude among these soils. These differences were only partially explained by pH and fiber content (a measure of degree of decomposition in peat), suggesting other, more complicated controls may drive CH₄ cycling in ombrotrophic peat soils. Based in part on the results from this survey, we more intensively examined CH₄ dynamics in three bog-like, acidic, Sphagnum-dominated peatlands in northern Minnesota that differed in their degree of ombrotrophy. Net CH₄ flux was lowest in the peatland with well-developed hummocks, and the isotopic composition of the CH₄ along with methanotroph gene expression indicated a strong role for CH₄ oxidation in controlling net CH₄ flux. There were limited differences in porewater chemistry (CH₄ and dissolved inorganic C concentrations) or microbial community composition among sites, and potential CH₄ production was also similar among the sites. Taken together, these experiments demonstrate that high variation in CH₄ cycling in seemingly similar peatlands within a single geographical region is common. We suggest a one peatland represents all approach is inappropriate—even among Sphagnum-dominated peatlands—and caution must be used when extrapolating data from a single site to the landscape scale, even for outwardly very similar peatlands. Instead, the macroscale development of peatlands, and concomitantly their microtopography as expressed in the proportion of hummocks, hollows, lawns and pools, need to be considered as central controls over CH₄ emissions.
Northern wetlands may be a potential carbon source to the atmosphere upon global warming, particularly with regard to methane. However, recent conclusions have largely been based on short-term field ...measurements. We incubated three wetland soils representing a range of substrate quality for 80 wk in the laboratory under both aerobic and anaerobic conditions at 15@? and 30@?C. The soils were obtained from a Scirpus-Carex-dominated meadow in an abandoned beaver pond and from the surface and at 1 m depth of a spruce (Picea)-Sphagnum bog in Voyageurs National Park, Minnesota. Substrate quality was assessed by fractionation of carbon compounds and summarized using principal components analysis. Nitrogen and carbon mineralization, the partitioning of carbon between carbon dioxide and methane, pH, and Eh were measured periodically over the course of the incubation. The responses of nitrogen mineralization, carbon mineralization, and trace gas partitioning to both temperature and aeration depended strongly on the substrate quality of the soils. Sedge meadow soil had the highest nitrogen and carbon mineralization rates and methane production under anaerobic conditions, and carbon mineralization under aerobic conditions, but the surface peats had the highest nitrogen mineralization rates under aerobic conditions. Methanogenesis was highest in the sedge soil but less sensitive to temperature than in the peats. A double exponential model showed that most of the variation in nitrogen and carbon mineralization among the soils and treatments was accounted for by differences in the size and kinetics of a relatively small labile pool. The kinetics of this pool were more sensitive to changes in temperature and aeration than that of the larger recalcitrant pool. Principal components analysis separated the soils on the basis of labile and recalcitrant carbon fractions. Total C and N mineralization correlated positively with the factor representing labile elements, while methanogenesis also showed a negative correlation with the factor representing recalcitrant elements. Estimates of atmospheric feedbacks from northern wetlands upon climatic change must account for extreme local variation in substrate quality and wetland type; global projections based on extrapolations from a few field measurements do not account for this local variation and may be in error.
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