Boreal ecosystems contain a substantial fraction of the earth's soil carbon stores and are prone to frequent and severe wildfires. In this study, we examine changes in element and organic matter ...stocks due to a 1999 wildfire in Alaska. One year after the wildfire, burned soils contained between 1071 and 1420 g/m
2
less carbon than unburned soils. Burned soils had lower nitrogen than unburned soils, higher calcium, and nearly unchanged potassium, magnesium, and phosphorus stocks. Burned surface soils tended to have higher concentrations of noncombustible elements such as calcium, potassium, magnesium, and phosphorus compared with unburned soils. Combustion losses of carbon were mostly limited to surface dead moss and fibric horizons, with no change in the underlying mineral horizons. Burning caused significant changes in soil organic matter structure, with a 12% higher ratio of carbon to combustible organic matter in surface burned horizons compared with unburned horizons. Pyrolysis gas chromatography mass spectroscopy also shows preferential volatilization of polysaccharide-derived organic matter and enrichment of lignin- and lipid-derived compounds in surface soils. The chemistry of deeper soil layers in burned and unburned sites was similar, suggesting that immediate fire impacts were restricted to the surface soil horizon.
On a sequence of soils developed under similar vegetation, temperature, and precipitation conditions, but with variations in mineralogical properties, we use organic carbon and 14C inventories to ...examine mineral protection of soil organic carbon. In these soils, 14C data indicate that the creation of slow‐cycling carbon can be modeled as occurring through reaction of organic ligands with Al3+ and Fe3+ cations in the upper horizons, followed by sorption to amorphous inorganic Al compounds at depth. Only one of these processes, the chelation of Al3+ and Fe3+ by organic ligands, is linked to large carbon stocks. Organic ligands stabilized by this process traverse the soil column as dissolved organic carbon (both from surface horizons and root exudates). At our moist grassland site, this chelation and transport process is very strongly correlated with the storage and long‐term stabilization of soil organic carbon. Our 14C results show that the mechanisms of organic carbon transport and storage at this site follow a classic model previously believed to only be significant in a single soil order (Spodosols), and closely related to the presence of forests. The presence of this process in the grassland Alfisol, Inceptisol, and Mollisol soils of this chronosequence suggests that this process is a more significant control on organic carbon storage than previously thought.
Growing season CH4 fluxes were monitored over a two year period following the start of ecosystem‐scale manipulations of water table position and surface soil temperatures in a moderate rich fen in ...interior Alaska. The largest CH4 fluxes occurred in plots that received both flooding (raised water table position) and soil warming, while the lowest fluxes occurred in unwarmed plots in the lowered water table treatment. A combination of treatment and soil hydroclimate variables explained more than 70% of the variation in ln‐transformed CH4 fluxes, with mean daily water table position representing the strongest predictor. We used quantitative PCR of the α‐subunit of mcr operon to explore the influence of soil climate manipulations on methanogen abundances. Methanogen abundances were greatest in warmed plots, and showed a positive relationship with mean daily CH4 fluxes. Our results show that water table manipulations that led to soil inundation (flooding) had a stronger effect on CH4 fluxes than water table drawdown. Seasonal CH4 fluxes increased by 80–300% under the combined wetter and warmer soil climate treatments. Thus, while warming is expected to increase CH4 emissions from Alaskan wetlands, higher water table positions caused by increases in precipitation or disturbances such as permafrost thaw that lead to thermokarst and flooding in wetlands will stimulate CH4 emissions beyond the effects of soil warming alone. Consequently, we argue that modeling the effects of climate change on Alaskan wetland CH4 emissions needs to consider the interactive effects of soil warming and water table position on CH4 production and transport.
Carbon (C) dynamics of boreal forest ecosystems have substantial implications for efforts to mitigate the rise of atmospheric CO
2
and may be substantially influenced by warming and changing wildfire ...regimes. In this study we applied a large-scale ecosystem model that included dynamics of organic soil horizons and soil organic matter characteristics of multiple pools to assess forest C stock changes of the Yukon River Basin (YRB) in Alaska, USA, and Canada from 1960 through 2006, a period characterized by substantial climate warming and increases in wildfire. The model was calibrated for major forests with data from long-term research sites and evaluated using a forest inventory database. The regional assessment indicates that forest vegetation C storage increased by 46 Tg C, but that total soil C storage did not change appreciably during this period. However, further analysis suggests that C has been continuously lost from the mineral soil horizon since warming began in the 1970s, but has increased in the amorphous organic soil horizon. Based on a factorial experiment, soil C stocks would have increased by 158 Tg C if the YRB had not undergone warming and changes in fire regime. The analysis also identified that warming and changes in fire regime were approximately equivalent in their effects on soil C storage, and interactions between these two suggests that the loss of organic horizon thickness associated with increases in wildfire made deeper soil C stocks more vulnerable to loss via decomposition. Subbasin analyses indicate that C stock changes were primarily sensitive to the fraction of burned forest area within each subbasin and that boreal forest ecosystems in the YRB are currently transitioning from being sinks to sources at ∼0.7% annual area burned. We conclude that it is important for international mitigation efforts focused on controlling atmospheric CO
2
to consider how climate warming and changes in fire regime may concurrently affect the CO
2
sink strength of boreal forests. It is also important for large-scale biogeochemical and earth system models to include organic soil dynamics in applications to assess regional C dynamics of boreal forests responding to warming and changes in fire regime.
Radiocarbon is a critical constraint on our estimates of
the timescales of soil carbon cycling that can aid in identifying mechanisms
of carbon stabilization and destabilization and improve the ...forecast of soil
carbon response to management or environmental change. Despite the wealth of
soil radiocarbon data that have been reported over the past 75 years, the
ability to apply these data to global-scale questions is limited by our
capacity to synthesize and compare measurements generated using a variety of
methods. Here, we present the International Soil Radiocarbon Database
(ISRaD; http://soilradiocarbon.org, last access: 16 December 2019), an open-source archive of soil data that
include reported measurements from bulk soils, distinct soil carbon pools
isolated in the laboratory by a variety of soil fractionation methods,
samples of soil gas or water collected interstitially from within an intact
soil profile, CO2 gas isolated from laboratory soil incubations, and
fluxes collected in situ from a soil profile. The core of ISRaD is a relational
database structured around individual datasets (entries) and organized
hierarchically to report soil radiocarbon data, measured at different
physical and temporal scales as well as other soil or environmental
properties that may also be measured and may assist with interpretation and
context. Anyone may contribute their own data to the database by entering it
into the ISRaD template and subjecting it to quality assurance protocols.
ISRaD can be accessed through (1) a web-based interface, (2) an R package
(ISRaD), or (3) direct access to code and data through the GitHub
repository, which hosts both code and data. The design of ISRaD allows for
participants to become directly involved in the management, design, and
application of ISRaD data. The synthesized dataset is available in two
forms: the original data as reported by the authors of the datasets and an
enhanced dataset that includes ancillary geospatial data calculated within
the ISRaD framework. ISRaD also provides data management tools in the
ISRaD-R package that provide a starting point for data analysis; as an
open-source project, the broader soil community is invited and encouraged
to add data, tools, and ideas for improvement. As a whole, ISRaD provides
resources to aid our evaluation of soil dynamics across a range of spatial
and temporal scales. The ISRaD v1.0 dataset is
archived and freely available at https://doi.org/10.5281/zenodo.2613911 (Lawrence et al., 2019).
The fate of carbon (C) contained within permafrost in boreal forest environments is an important consideration for the current and future carbon cycle as soils warm in northern latitudes. Currently, ...little is known about the microbiology or chemistry of permafrost soils that may affect its decomposition once soils thaw. We tested the hypothesis that low microbial abundances and activities in permafrost soils limit decomposition rates compared with active layer soils. We examined active layer and permafrost soils near Fairbanks, AK, the Yukon River, and the Arctic Circle. Soils were incubated in the lab under aerobic and anaerobic conditions. Gas fluxes at −5 and 5 °C were measured to calculate temperature response quotients (Q10). The Q10 was lower in permafrost soils (average 2.7) compared with active layer soils (average 7.5). Soil nutrients, leachable dissolved organic C (DOC) quality and quantity, and nuclear magnetic resonance spectroscopy of the soils revealed that the organic matter within permafrost soils is as labile, or even more so, than surface soils. Microbial abundances (fungi, bacteria, and subgroups: methanogens and Basidiomycetes) and exoenzyme activities involved in decomposition were lower in permafrost soils compared with active layer soils, which, together with the chemical data, supports the reduced Q10 values. CH4 fluxes were correlated with methanogen abundance and the highest CH4 production came from active layer soils. These results suggest that permafrost soils have high inherent decomposability, but low microbial abundances and activities reduce the temperature sensitivity of C fluxes. Despite these inherent limitations, however, respiration per unit soil C was higher in permafrost soils compared with active layer soils, suggesting that decomposition and heterotrophic respiration may contribute to a positive feedback to warming of this eco region.
Peatlands store 30% of the world's terrestrial soil carbon (C) and those located at northern latitudes are expected to experience rapid climate warming. We monitored growing season carbon dioxide ...(CO₂) fluxes across a factorial design of in situ water table (control, drought, and flooded plots) and soil warming (control vs. warming via open top chambers) treatments for 2 years in a rich fen located just outside the Bonanza Creek Experimental Forest in interior Alaska. The drought (lowered water table position) treatment was a weak sink or small source of atmospheric CO₂ compared to the moderate atmospheric CO₂ sink at our control. This change in net ecosystem exchange was due to lower gross primary production and light-saturated photosynthesis rather than increased ecosystem respiration. The flooded (raised water table position) treatment was a greater CO₂ sink in 2006 due largely to increased early season gross primary production and higher light-saturated photosynthesis. Although flooding did not have substantial effects on rates of ecosystem respiration, this water table treatment had lower maximum respiration rates and a higher temperature sensitivity of ecosystem respiration than the control plot. Surface soil warming increased both ecosystem respiration and gross primary production by approximately 16% compared to control (ambient temperature) plots, with no net effect on net ecosystem exchange. Results from this rich fen manipulation suggest that fast responses to drought will include reduced ecosystem C storage driven by plant stress, whereas inundation will increase ecosystem C storage by stimulating plant growth.
Approximately 1700 Pg of soil carbon (C) are stored in the northern circumpolar permafrost zone, more than twice as much C than in the atmosphere. The overall amount, rate, and form of C released to ...the atmosphere in a warmer world will influence the strength of the permafrost C feedback to climate change. We used a survey to quantify variability in the perception of the vulnerability of permafrost C to climate change. Experts were asked to provide quantitative estimates of permafrost change in response to four scenarios of warming. For the highest warming scenario (RCP 8.5), experts hypothesized that C release from permafrost zone soils could be 19–45 Pg C by 2040, 162–288 Pg C by 2100, and 381–616 Pg C by 2300 in CO
2
equivalent using 100-year CH
4
global warming potential (GWP). These values become 50 % larger using 20-year CH
4
GWP, with a third to a half of expected climate forcing coming from CH
4
even though CH
4
was only 2.3 % of the expected C release. Experts projected that two-thirds of this release could be avoided under the lowest warming scenario (RCP 2.6). These results highlight the potential risk from permafrost thaw and serve to frame a hypothesis about the magnitude of this feedback to climate change. However, the level of emissions proposed here are unlikely to overshadow the impact of fossil fuel burning, which will continue to be the main source of C emissions and climate forcing.
The vast amount of organic carbon (OC) stored in soils of the northern circumpolar permafrost region is a potentially vulnerable component of the global carbon cycle. However, estimates of the ...quantity, decomposability, and combustibility of OC contained in permafrost-region soils remain highly uncertain, thereby limiting our ability to predict the release of greenhouse gases due to permafrost thawing. Substantial differences exist between empirical and modeling estimates of the quantity and distribution of permafrost-region soil OC, which contribute to large uncertainties in predictions of carbon-climate feedbacks under future warming. Here, we identify research challenges that constrain current assessments of the distribution and potential decomposability of soil OC stocks in the northern permafrost region and suggest priorities for future empirical and modeling studies to address these challenges.
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