Permafrost soils contain more than 1300 Pg of carbon (C), twice the amount of C in the atmosphere. Temperatures in higher latitudes are increasing, inducing permafrost thaw and subsequent microbial ...decomposition of previously frozen C, which will most likely feed back to climate warming through release of the greenhouse gases CO2 and CH4. Understanding the temperature sensitivity (Q10) and dynamics of soil organic matter (SOM) decomposition under warming is essential to predict the future state of the climate system. Alaskan tundra soils from the discontinuous permafrost zone were exposed to in situ experimental warming for two consecutive winters, increasing soil temperature by 2.3 °C down to 40 cm in the soil profile. Soils obtained at three depths (0–15, 15–25 and 45–55 cm) from the experimental warming site were incubated under aerobic conditions at 15 °C and 25 °C over 365 days in the laboratory. Carbon fluxes were measured periodically and dynamics of SOM decomposition, C pool sizes, and decay rates were estimated. Q10 was estimated using both a short-term temperature manipulation (Q10-ST) performed at 14, 100 and 280 days of incubation and via the equal C method (Q10-EC, ratio of time taken for a soil to respire a given amount of C), calculated continuously. At the same time points, functional diversities of the soil microbial communities were monitored for all incubation samples using a microbial functional gene array, GeoChip 5.0. Each array contains over 80,000 probes targeting microbial functional genes involved in biogeochemical cycling of major nutrients, remediation strategies, pathogenicity and other important environmental functions. Of these, over 20,000 probes target genes involved in the degradation of varying C substrates and can be used to quantify the relative gene abundances and functional gene diversities related to soil organic matter turnover. The slow decomposing C pool (CS), which represented close to 95% of total C in the top 25 cm soils, had a higher Q10 than the fast decomposing C pool (CF) and also dominated the total amount of C released by the end of the incubation. Overall, CS had temperature sensitivities of Q10-ST = 2.55 ± 0.03 and Q10-EC = 2.19 ± 0.13, while the CF had a temperature sensitivity of Q10-EC = 1.16 ± 0.30. In contrast to the 15 °C incubations, the 25 °C microbial communities showed reduced diversities of C-degradation functional genes in the early stage of the incubations. However, as the incubations continued the 25 °C communities more closely paralleled the 15 °C communities with respect to the detection of microbial genes utilized in the degradation of labile to recalcitrant C substrates. Two winter seasons of experimental warming did not affect the dynamics and temperature sensitivity of SOM decomposition or the microbial C-degradation genes during incubation. However, under the projected sustained warming attributable to climate change, we might expect increased contribution of CS to organic matter decomposition. Because of the higher Q10 and the large pool size of CS, increased soil organic matter release under warmer temperatures will contribute towards accelerating climate change.
•Q10 and kinetics of SOM decomposition estimated in incubated permafrost soils.•Higher Q10 exhibited by slow than fast decomposing C pools.•Decomposition driven by C quantity, quality, temperature and time of incubation.•The microbial functional community structure varied with environmental parameters.•Increases in Q10 as C was respired was convoyed by shifts in microbial communities.
•The phenology schemes in ELM of the E3SM were improved using PhenoCam observations.•Earlier spring onset and stronger temperature responses of phenology were produced in new ELM.•Higher carbon and ...water fluxes and stronger flux responses to warming were produced in new ELM.•Selected phenology parameters are critical to uncertainty in key carbon and water cycle variables.
Phenological transitions determine the timing of changes in land surface properties and the seasonality of exchanges of biosphere-atmosphere energy, water, and carbon. Accurate mechanistic modeling of phenological processes is therefore critical to understand and correctly predict terrestrial ecosystem feedbacks with changing atmospheric and climate conditions. However, the phenological components in the land model of the US Department of Energy's (DOE) Energy Exascale Earth System Model (ELM of E3SM) were previously unable to accurately capture the observed phenological responses to environmental conditions in a well-studied boreal peatland forest. In this research, we introduced new seasonal-deciduous phenology schemes into version 1.0 of ELM and evaluated their performance against the PhenoCam observations at the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment in northern Minnesota from 2015 to 2018. We found that phenology simulated by the revised ELM (i.e., earlier spring onsets and stronger warming responses of spring onsets and autumn senescence) was closer to observations than simulations from the original algorithms for both the deciduous conifer (Larix laricina) and mixed shrub layers. Moreover, the revised ELM generally produced higher carbon and water fluxes (e.g., photosynthesis and evapotranspiration) during the growing season and stronger flux responses to warming than the default ELM. A parameter sensitivity analysis further indicated the significant contribution of phenology parameters to uncertainty in key carbon and water cycle variables, underscoring the importance of precise phenology parameterization. This phenological modeling effort demonstrates the potential to enhance the E3SM representation of land-climate interactions at broader spatiotemporal scales, especially under anticipated elevated CO2 and warming conditions.
Higher temperatures in northern latitudes will increase permafrost thaw and stimulate above- and belowground plant biomass growth in tundra ecosystems. Higher plant productivity increases the input ...of easily decomposable carbon (C) to soil, which can stimulate microbial activity and increase soil organic matter decomposition rates. This phenomenon, known as the priming effect, is particularly interesting in permafrost because an increase in C supply to deep, previously frozen soil may accelerate decomposition of C stored for hundreds to thousands of years. The sensitivity of old permafrost C to priming is not well known; most incubation studies last less than one year, and so focus on fast-cycling C pools. Furthermore, the age of respired soil C is rarely measured, even though old C may be vulnerable to labile C inputs. We incubated soil from a moist acidic tundra site in Eight Mile Lake, Alaska for 409 days at 15 °C. Soil from surface (0–25 cm), transition (45–55 cm), and permafrost (65–85 cm) layers were amended with three pulses of uniformly 13C-labeled glucose or cellulose every 152 days. Glucose addition resulted in positive priming in the permafrost layer 7 days after each substrate addition, eliciting a two-fold increase in cumulative soil C loss relative to unamended soils with consistent effects across all three pulses. In the transition and permafrost layers, glucose addition significantly decreased the age of soil-respired CO2-C with Δ14C values that were 115‰ higher. Previous field studies that measured the age of respired C in permafrost regions have attributed younger Δ14C ecosystem respiration values to higher plant contributions. However, the results from this study suggest that positive priming, due to an increase in fresh C supply to deeply thawed soil layers, can also explain the respiration of younger C observed at the ecosystem scale. We must consider priming effects to fully understand permafrost C dynamics, or we risk underestimating the contribution of soil C to ecosystem respiration.
•Glucose addition increased permafrost soil C loss two-fold.•Glucose addition to surface soils did not elicit a priming response.•Cellulose addition to surface soils elicited a negative priming response.•Glucose addition decreased the age of soil-respired carbon in deep layers.
In the last few decades, temperatures in the Arctic have increased twice as much as the rest of the globe. As permafrost thaws in response to this warming, large amounts of soil organic matter may ...become vulnerable to decomposition. Microbial decomposition will release carbon (C) from permafrost soils, however, warmer conditions could also lead to enhanced plant growth and C uptake. Field and modeling studies show high uncertainty in soil and plant responses to climate change but there have been few studies that reconcile field and model data to understand differences and reduce uncertainty. Here, we evaluate gross primary productivity (GPP), ecosystem respiration (Reco), and net ecosystem C exchange (NEE) from eight years of experimental soil warming in moist acidic tundra against equivalent fluxes from the Community Land Model during simulations parameterized to reflect the field conditions associated with this manipulative field experiment. Over the eight-year experimental period, soil temperatures and thaw depths increased with warming in field observations and model simulations. However, the field and model results do not agree on warming effects on water table depth; warming created wetter soils in the field and drier soils in the models. In the field, initial increases in growing season GPP, Reco, and NEE to experimentally-induced permafrost thaw created a higher C sink capacity in the first years followed by a stronger C source in years six through eight. In contrast, both models predicted linear increases in GPP, Reco, and NEE with warming. The divergence of model results from field experiments reveals the role subsidence, hydrology, and nutrient cycling play in influencing the C flux responses to permafrost thaw, a complexity that the models are not structurally able to predict, and highlight challenges associated with projecting C cycle dynamics across the Arctic.
The magnitude of carbon (C) loss to the atmosphere via microbial
decomposition is a function of the amount of C stored in soils, the quality
of the organic matter, and physical, chemical, and ...biological factors that
comprise the environment for decomposition. The decomposability of C is
commonly assessed by laboratory soil incubation studies that measure
greenhouse gases mineralized from soils under controlled conditions. Here,
we introduce the Soil Incubation Database (SIDb) version 1.0, a compilation
of time series data from incubations, structured into a new, publicly
available, open-access database of C flux (carbon dioxide, CO2, or
methane, CH4). In addition, the SIDb project also provides a platform
for the development of tools for reading and analysis of incubation data as
well as documentation for future use and development. In addition to
introducing SIDb, we provide reporting guidance for database entry and the
required variables that incubation studies need at minimum to be included in
SIDb. A key application of this synthesis effort is to better characterize
soil C processes in Earth system models, which will in turn reduce our
uncertainty in predicting the response of soil C decomposition to a changing
climate. We demonstrate a framework to fit curves to a number of incubation
studies from diverse ecosystems, depths, and organic matter content using a
built-in model development module that integrates SIDb with the existing
SoilR package to estimate soil C pools from time series data. The database
will help bridge the gap between point location measurements, which are
commonly used in incubation studies, and global remote-sensed data or data
products derived from models aimed at assessing global-scale rates of
decomposition and C turnover. The SIDb version 1.0 is archived and publicly
available at https://doi.org/10.5281/zenodo.3871263 (Sierra et al., 2020), and the database is managed
under a version-controlled system and centrally stored in GitHub (https://github.com/SoilBGC-Datashare/sidb, last access: 26 June 2020).
Abstract
The continental shelves of the Arctic Ocean and surrounding seas contain large stocks of organic matter (OM) and methane (CH
4
), representing a potential ecosystem feedback to climate ...change not included in international climate agreements. We performed a structured expert assessment with 25 permafrost researchers to combine quantitative estimates of the stocks and sensitivity of organic carbon in the subsea permafrost domain (i.e. unglaciated portions of the continental shelves exposed during the last glacial period). Experts estimated that the subsea permafrost domain contains ∼560 gigatons carbon (GtC; 170–740, 90% confidence interval) in OM and 45 GtC (10–110) in CH
4
. Current fluxes of CH
4
and carbon dioxide (CO
2
) to the water column were estimated at 18 (2–34) and 38 (13–110) megatons C yr
−1
, respectively. Under Representative Concentration Pathway (RCP) RCP8.5, the subsea permafrost domain could release 43 Gt CO
2
-equivalent (CO
2
e) by 2100 (14–110) and 190 Gt CO
2
e by 2300 (45–590), with ∼30% fewer emissions under RCP2.6. The range of uncertainty demonstrates a serious knowledge gap but provides initial estimates of the magnitude and timing of the subsea permafrost climate feedback.
Losses of C from decomposing permafrost may be offset by increased productivity of tundra plants, but nitrogen availability partially limits plant growth in tundra ecosystems. In this soil incubation ...experiment carbon (C) and nitrogen (N) cycling dynamics were examined from the soil surface down through upper permafrost. We found that losses of CO2 were negatively correlated to net N mineralization because C‐rich surface soils mineralized little N, while deep soils had low rates of C respiration but high rates of net N mineralization. Permafrost soils released a large flush of inorganic N when initially thawed. Depth‐specific rates of N mineralization from the incubation were combined with thaw depths and soil temperatures from a nearby manipulative warming experiment to simulate the potential magnitude, timing, and depth of inorganic N release during the process of permafrost thaw. Our calculations show that inorganic N released from newly thawed permafrost may be similar in magnitude to the increase in N mineralized by warmed soils in the middle of the profile. The total release of inorganic N from the soil profile during the simulated thaw process was twice the size of the observed increase in the foliar N pool observed at the manipulative experiment. Our findings suggest that increases in N availability are likely to outpace the N demand of tundra plants during the first 5 years of permafrost thaw and may increase C losses from surface soils as well as induce denitrification and leaching of N from these ecosystems.
Plain Language Summary
Arctic plants are rooted in an active layer of soil that thaws during the summer months and is often nutrient‐poor because of slow decomposition in these cold ecosystems. Beneath the active layer, there is a layer of soil that remains frozen year‐round (permafrost). In this experiment, we collected soil cores that spanned the entire active layer and upper permafrost and incubated these soils in the lab so we could monitor their decomposition. We focus on nitrogen cycling because this is a key nutrient for the growth of arctic plants and soil microbes. We found nitrogen availability was low in shallow surface soils but high deep in the active layer and permafrost. Our results show that arctic warming will impact nitrogen release from two locations in the soil profile: at the bottom of the soil profile when nitrogen‐rich permafrost soil thaws for the first time and with the active layer when decomposition is accelerated by warmer temperatures. Our calculations suggest that these two sources of nitrogen are similar in size during the first five years of permafrost thaw, exceed plant demand for nitrogen, and are likely to contribute to losses of nitrogen from warming arctic ecosystems.
Key Points
Soil microbes are nitrogen (N) limited in surface soils, but deeper layers are N rich; permafrost thaw will alter this stratification
Warming will release inorganic N (1) by thawing N‐rich permafrost and (2) by accelerating N mineralization within active layer soils
These two sources of inorganic N are similar in magnitude during the first 5 years of thaw and together exceed plant N demand
Abstract
Permafrost thaw causes the seasonally thawed active layer to deepen, causing the Arctic to shift toward carbon release as soil organic matter becomes susceptible to decomposition. Ground ...subsidence initiated by ice loss can cause these soils to collapse abruptly, rapidly shifting soil moisture as microtopography changes and also accelerating carbon and nutrient mobilization. The uncertainty of soil moisture trajectories during thaw makes it difficult to predict the role of abrupt thaw in suppressing or exacerbating carbon losses. In this study, we investigated the role of shifting soil moisture conditions on carbon dioxide fluxes during a 13‐year permafrost warming experiment that exhibited abrupt thaw. Warming deepened the active layer differentially across treatments, leading to variable rates of subsidence and formation of thermokarst depressions. In turn, differential subsidence caused a gradient of moisture conditions, with some plots becoming consistently inundated with water within thermokarst depressions and others exhibiting generally dry, but more variable soil moisture conditions outside of thermokarst depressions. Experimentally induced permafrost thaw initially drove increasing rates of growing season gross primary productivity (GPP), ecosystem respiration (
R
eco
), and net ecosystem exchange (NEE) (higher carbon uptake), but the formation of thermokarst depressions began to reverse this trend with a high level of spatial heterogeneity. Plots that subsided at the slowest rate stayed relatively dry and supported higher CO
2
fluxes throughout the 13‐year experiment, while plots that subsided very rapidly into the center of a thermokarst feature became consistently wet and experienced a rapid decline in growing season GPP,
R
eco
, and NEE (lower carbon uptake or carbon release). These findings indicate that Earth system models, which do not simulate subsidence and often predict drier active layer conditions, likely overestimate net growing season carbon uptake in abruptly thawing landscapes.
Permafrost thaw is typically measured with active layer thickness, or the maximum seasonal thaw measured from the ground surface. However, previous work has shown that this measurement alone fails to ...account for ground subsidence and therefore underestimates permafrost thaw. To determine the impact of subsidence on observed permafrost thaw and thawed soil carbon stocks, we quantified subsidence using high‐accuracy GPS and identified its environmental drivers in a permafrost warming experiment near the southern limit of permafrost in Alaska. With permafrost temperatures near 0°C, 10.8 cm of subsidence was observed in control plots over 9 years. Experimental air and soil warming increased subsidence by five times and created inundated microsites. Across treatments, ice and soil loss drove 85–91% and 9–15% of subsidence, respectively. Accounting for subsidence, permafrost thawed between 19% (control) and 49% (warming) deeper than active layer thickness indicated, and the amount of newly thawed carbon within the active layer was between 37% (control) and 113% (warming) greater. As additional carbon thaws as the active layer deepens, carbon fluxes to the atmosphere and lateral transport of carbon in groundwater could increase. The magnitude of this impact is uncertain at the landscape scale, though, due to limited subsidence measurements. Therefore, to determine the full extent of permafrost thaw across the circumpolar region and its feedback on the carbon cycle, it is necessary to quantify subsidence more broadly across the circumpolar region.
Plain Language Summary
Permafrost soils, which are perennially frozen soils found throughout cold regions, contain vast quantities of carbon and ice. When permafrost thaws, carbon can be lost to the atmosphere, contributing to climate change. This means it is important to track permafrost thaw, which is often done using active layer thickness, or the depth of the seasonally thawed surface layer of soil. However, ice volume can be lost from thawing permafrost, causing the soil surface to drop. Conventional measurements do not account for this surface drop, and the rate of thaw could therefore be underestimated. We found that experimentally warmed soils dropped at a rate of 6 cm year−1, mostly due to loss of ice volume and also due to the loss of soil mass. When accounting for the change in soil surface height over time, the full depth of permafrost thaw was 49% greater. The increased depth of thaw resulted in more than twice as much carbon being thawed as was estimated with standard methods that did not account for subsidence. These findings suggest that permafrost is thawing more quickly than long‐term records indicate and that this could result in additional carbon release contributing to climate change.
Key Points
Subsidence causes a shifting reference frame for measurements of permafrost thaw
The rate of permafrost carbon thaw doubles when subsidence is accounted for
Subsidence of up to 6 cm year−1 was observed in a permafrost warming experiment, due to both ice and soil loss
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