The priming potential of environmentally weathered pyrogenic carbon during land‐use transition to biomass crop production

Since land‐use change (LUC) to lignocellulosic biomass crops often causes a loss of soil organic carbon (SOC), at least in the short term, this study investigated the potential for pyrogenic carbon (PyC) to ameliorate this effect. Although negative priming has been observed in many studies, most of these are long‐term incubation experiments which do not account for the interactions between environmentally weathered PyC and native SOC. Here, the aim was to assess the impact of environmentally weathered PyC on native SOC mineralization at different time points in LUC from arable crops to short rotation coppice (SRC) willow. At eight SRC willow plantations in England, with ages of 3–22 years, soil amended 18–22 months previously with PyC was compared with unamended control soil. Cumulative CO₂ flux was measured weekly from incubated soil at 0–5 cm depth, and soil‐surface CO₂ flux was also measured in the field. For the incubated soil, cumulative CO₂ flux was significantly higher from soil containing weathered PyC than the control soil for seven of the eight sites. Across all sites, the mean cumulative CO₂ flux was 21% higher from soil incubated with weathered PyC than the control soil. These results indicate the potential for positive priming in the surface 5 cm of soil independent of changes in soil properties following LUC to SRC willow production. However, no net effect on CO₂ flux was observed in the field, suggesting this increase in CO₂ is offset by a contrasting PyC‐induced effect at a different soil depth or that different effects were observed under laboratory and field conditions. Although the mechanisms for these contrasting effects remain unclear, results presented here suggest that PyC does not reduce LUC‐induced SOC losses through negative priming, at least for this PyC type and application rate.     

Towards a global assessment of pyrogenic carbon from vegetation fires

The production of pyrogenic carbon (PyC; a continuum of organic carbon (C) ranging from partially charred biomass and charcoal to soot) is a widely acknowledged C sink, with the latest estimates indicating that ~50% of the PyC produced by vegetation fires potentially sequesters C over centuries. Nevertheless, the quantitative importance of PyC in the global C balance remains contentious, and therefore, PyC is rarely considered in global C cycle and climate studies. Here we examine the robustness of existing evidence and identify the main research gaps in the production, fluxes and fate of PyC from vegetation fires. Much of the previous work on PyC production has focused on selected components of total PyC generated in vegetation fires, likely leading to underestimates. We suggest that global PyC production could be in the range of 116–385 Tg C yr^?1, that is ~0.2–0.6% of the annual terrestrial net primary production. According to our estimations, atmospheric emissions of soot/black C might be a smaller fraction of total PyC (<2%) than previously reported. Research on the fate of PyC in the environment has mainly focused on its degradation pathways, and its accumulation and resilience either in situ (surface soils) or in ultimate sinks (marine sediments). Off‐site transport, transformation and PyC storage in intermediate pools are often overlooked, which could explain the fate of a substantial fraction of the PyC mobilized annually. We propose new research directions addressing gaps in the global PyC cycle to fully understand the importance of the products of burning in global C cycle dynamics.     

Ecosystem Carbon Remains Low for Three Decades Following Fire and Constrains Soil CO2 Responses to Precipitation in Southwestern Ponderosa Pine Forests

The fire regime of ponderosa pine forests in the southwestern United States has shifted over the past century from historically frequent, low-intensity surface fires to infrequent, stand-replacing crown fires. We quantified plant and soil carbon (C) responses to this new fire regime and assessed interactions between changes in fire regime and changes in precipitation regime predicted by some climate models (specifically, an earlier monsoon rain season). We hypothesized that soil C pools and carbon dioxide (CO₂) efflux rates would decrease initially following stand-replacing fires (due to low plant C inputs and the loss of the soil surficial organic (O) horizon), but then increase with time-after-fire (as plant C inputs increase). Water availability often limits soil biological activity in these forests, but we predicted that low soil C availability following fire would constrain soil CO₂ efflux responses to precipitation. In a series of sites with histories of stand-replacing fires that burned between 2 and 34?years prior to sampling, burned patches had lower soil C pools and fluxes than adjacent unburned patches, but there was no evidence of a trend with time-after-fire. Burned forests had 7,500?g C m^?2 less live plant biomass C (P?<?0.001), 1,600?g C m^?2 less soil total C (P?<?0.001) and 90?g C m^?2 less soil labile C (P?<?0.001) than unburned forests. Lower soil labile C in burned patches was due to both a loss of O horizon mass with fire and lower labile C concentrations (g labile C kg^?1 soil total C) in the mineral soil. During the annual drought that precedes summer monsoon rains, both burned and unburned patches had soil CO₂ efflux rates ranging from 0.9 to 1.1?g CO₂-C m^?2 day^?1. During the monsoon season, soil CO₂ efflux in unburned patches increased to approximately 4.8?g CO₂-C m^?2 day^?1 and rates in paired burned patches (3.4?g CO₂-C m^?2 day^?1) were lower (P?<?0.001). We also used field irrigation to experimentally create an earlier and longer monsoon season, and soil CO₂ efflux rates at both burned and unburned plots increased initially in response to watering, but decreased to below control (plots without irrigation) rates within weeks. Watering did not significantly change cumulative growing season soil CO₂ efflux, supporting our prediction that C availability constrains soil CO₂ efflux responses to precipitation. This research advances our understanding of interactions among climate, fire, and C in southwestern forests, suggesting that climate-induced shifts toward more stand-replacing fires will decrease soil C for decades, such that a single fire can constrain future soil biological responses to precipitation regime changes.     

Dramatic loss of inorganic carbon by nitrogen‐induced soil acidification in Chinese croplands

Intensive crop production systems worldwide, particularly in China, rely heavily on nitrogen (N) fertilization, but left more than 50% of fertilizer N in the environment. Nitrogen (over) fertilization and atmospheric N deposition induce soil acidification, which is neutralized by soil inorganic carbon (SIC; carbonates), and carbon dioxide (CO₂) is released to the atmosphere. For the first time, the loss of SIC stocks in response to N‐induced soil acidification was estimated for Chinese croplands from 1980 to 2020 and forecasts were made up to 2100. The SIC stocks in croplands in 1980 were 2.16 Pg C (16.3 Mg C/ha) in the upper 40 cm, 7% (0.15 Pg C; 1.1 Mg C/ha) of which were lost from 1980 to 2020. During these 40 years, 7 million ha of cropland has become carbonate free. Another 37% of the SIC stocks may be lost up to 2100 in China, leaving 30 million ha of cropland (37.8%) without carbonates if N fertilization follows the business‐as‐usual (BAU) scenario. Compared to the BAU scenario, the reduction in N input by 15%–30% after 2020 (scenarios S1 and S2) will decrease carbonate dissolution by 18%–41%. If N input remains constant as noted in 2020 (S3) or decreases by 1% annually (S4), a reduction of up to 52%–67% in carbonate dissolution is expected compared to the BAU scenario. The presence of CaCO₃ in the soil is important for various processes including acidity buffering, aggregate formation and stabilization, organic matter stabilization, microbial and enzyme activities, nutrient cycling and availability, and water permeability and plant productivity. Therefore, optimizing N fertilization and improving N‐use efficiency are important for decreasing SIC losses from acidification. N application should be strictly calculated based on crop demand, and any overfertilization should be avoided to prevent environmental problems and soil fertility decline associated with CaCO₃ losses.     

Increased soil release of greenhouse gases shrinks terrestrial carbon uptake enhancement under warming

Warming can accelerate the decomposition of soil organic matter and stimulate the release of soil greenhouse gases (GHGs), but to what extent soil release of methane (CH₄) and nitrous oxide (N₂O) may contribute to soil C loss for driving climate change under warming remains unresolved. By synthesizing 1,845 measurements from 164 peer‐reviewed publications, we show that around 1.5°C (1.16–2.01°C) of experimental warming significantly stimulates soil respiration by 12.9%, N₂O emissions by 35.2%, CH₄ emissions by 23.4% from rice paddies, and by 37.5% from natural wetlands. Rising temperature increases CH₄ uptake of upland soils by 13.8%. Warming‐enhanced emission of soil CH₄ and N₂O corresponds to an overall source strength of 1.19, 1.84, and 3.12 Pg CO₂‐equivalent/year under 1°C, 1.5°C, and 2°C warming scenarios, respectively, interacting with soil C loss of 1.60 Pg CO₂/year in terms of contribution to climate change. The warming‐induced rise in soil CH₄ and N₂O emissions (1.84 Pg CO₂‐equivalent/year) could reduce mitigation potential of terrestrial net ecosystem production by 8.3% (NEP, 22.25 Pg CO₂/year) under warming. Soil respiration and CH₄ release are intensified following the mean warming threshold of 1.5°C scenario, as compared to soil CH₄ uptake and N₂O release with a reduced and less positive response, respectively. Soil C loss increases to a larger extent under soil warming than under canopy air warming. Warming‐raised emission of soil GHG increases with the intensity of temperature rise but decreases with the extension of experimental duration. This synthesis takes the lead to quantify the ecosystem C and N cycling in response to warming and advances our capacity to predict terrestrial feedback to climate change under projected warming scenarios.     

Net biome production of the Amazon Basin in the 21st century

Global change includes multiple stressors to natural ecosystems ranging from direct climate and land‐use impacts to indirect degradation processes resulting from fire. Humid tropical forests are vulnerable to projected climate change and possible synergistic interactions with deforestation and fire, which may initiate a positive feedback to rising atmospheric CO₂. Here, we present results from a multifactorial impact analysis that combined an ensemble of climate change models with feedbacks from deforestation and accidental fires to quantify changes in Amazon Basin carbon cycling. Using the LPJmL Dynamic Global Vegetation Model, we modelled spatio‐temporal changes in net biome production (NBP); the difference between carbon fluxes from fire, deforestation, soil respiration and net primary production. By 2050, deforestation and fire (with no CO₂ increase or climate change) resulted in carbon losses of 7.4–20.3 Pg C with the range of uncertainty depending on socio‐economic storyline. During the same time period, interactions between climate and land use either compensated for carbon losses due to wetter climate and CO₂ fertilization or exacerbated carbon losses from drought‐induced forest mortality (?20.1 to +4.3 Pg C). By the end of the 21st century, depending on climate projection and the rate of deforestation (including its interaction with fire), carbon stocks either increased (+12.6 Pg C) or decreased (?40.6 Pg C). The synergistic effect of deforestation and fire with climate change contributed up to 26–36 Pg C of the overall decrease in carbon stocks. Agreement between climate projections (n=9), not accounting for deforestation and fire, in 2050 and 2098 was relatively low for the directional change in basin‐wide NBP (19–37%) and aboveground live biomass (13–24%). The largest uncertainty resulted from climate projections, followed by implementation of ecosystem dynamics and deforestation. Our analysis partitions the drivers of tropical ecosystem change and is relevant for guiding mitigation and adaptation policy related to global change.     

Vulnerability of carbon storage in North American boreal forests to wildfires during the 21st century

The boreal forest contains large reserves of carbon. Across this region, wildfires influence the temporal and spatial dynamics of carbon storage. In this study, we estimate fire emissions and changes in carbon storage for boreal North America over the 21st century. We use a gridded data set developed with a multivariate adaptive regression spline approach to determine how area burned varies each year with changing climatic and fuel moisture conditions. We apply the process‐based Terrestrial Ecosystem Model to evaluate the role of future fire on the carbon dynamics of boreal North America in the context of changing atmospheric carbon dioxide (CO₂) concentration and climate in the A2 and B2 emissions scenarios of the CGCM2 global climate model. Relative to the last decade of the 20th century, decadal total carbon emissions from fire increase by 2.5–4.4 times by 2091–2100, depending on the climate scenario and assumptions about CO₂ fertilization. Larger fire emissions occur with warmer climates or if CO₂ fertilization is assumed to occur. Despite the increases in fire emissions, our simulations indicate that boreal North America will be a carbon sink over the 21st century if CO₂ fertilization is assumed to occur in the future. In contrast, simulations excluding CO₂ fertilization over the same period indicate that the region will change to a carbon source to the atmosphere, with the source being 2.1 times greater under the warmer A2 scenario than the B2 scenario. To improve estimates of wildfire on terrestrial carbon dynamics in boreal North America, future studies should incorporate the role of dynamic vegetation to represent more accurately post‐fire successional processes, incorporate fire severity parameters that change in time and space, account for human influences through increased fire suppression, and integrate the role of other disturbances and their interactions with future fire regime.     

Optimal Fire Regimes for Soil Carbon Storage in Tropical Savannas of Northern Australia

Modification of fire regimes in tropical savannas can have significant impacts on the global carbon (C) cycle, and therefore, on the climate system. In Australian tropical savannas, there has been recent, large-scale implementation of fire management that aims to decrease Kyoto-compliant non-CO₂ greenhouse gas emissions by reducing late dry season intense fires through strategic early dry season burning. However, there is no accounting for changes to soil C stocks resulting from changes to savanna fire management, although impacts on these pools may be considerable. We present a hypothesis that soil C storage is greatest under low intensity fires with an intermediate fire return interval. Simulations using the CENTURY Soil Organic Matter Model confirmed this hypothesis with greatest soil C storage under a fire regime of one low intensity fire every 5 years. Key areas of uncertainty for CENTURY model simulations include fine root dynamics, charcoal production and nitrogen (N) cycling, and better understanding of these processes could improve model predictions. Soil C stocks measured in the field after 5 years of annual, 3 year and unburned fire treatments were not significantly different (range 41–58 t ha⁻¹), but further CENTURY modelling suggests that changes in fire management will take up to 100 years to have a detectable impact (+4 t ha⁻¹) on soil C stocks. However, implementation of fire management that reduces fire frequency and intensity within the large area of intact savanna landscapes in northern Australia could result in emissions savings of 0.17 t CO₂-e ha⁻¹ y⁻¹, four times greater than reductions of non-CO₂ emissions.     

Soil GHG fluxes are altered by N deposition: New data indicate lower N stimulation of the N2O flux and greater stimulation of the calculated C pools

The effects of nitrogen (N) deposition on soil organic carbon (C) and greenhouse gas (GHG) emissions in terrestrial ecosystems are the main drivers affecting GHG budgets under global climate change. Although many studies have been conducted on this topic, we still have little understanding of how N deposition affects soil C pools and GHG budgets at the global scale. We synthesized a comprehensive dataset of 275 sites from multiple terrestrial ecosystems around the world and quantified the responses of the global soil C pool and GHG fluxes induced by N enrichment. The results showed that the soil organic C concentration and the soil CO₂, CH₄ and N₂O emissions increased by an average of 3.7%, 0.3%, 24.3% and 91.3% under N enrichment, respectively, and that the soil CH₄ uptake decreased by 6.0%. Furthermore, the percentage increase in N₂O emissions (91.3%) was two times lower than that (215%) reported by Liu and Greaver (Ecology Letters, 2009, 12:1103–1117). There was also greater stimulation of soil C pools (15.70 kg C ha^?1 year^?1 per kg N ha^?1 year^?1) than previously reported under N deposition globally. The global N deposition results showed that croplands were the largest GHG sources (calculated as CO₂ equivalents), followed by wetlands. However, forests and grasslands were two important GHG sinks. Globally, N deposition increased the terrestrial soil C sink by 6.34 Pg CO₂/year. It also increased net soil GHG emissions by 10.20 Pg CO₂‐Geq (CO₂ equivalents)/year. Therefore, N deposition not only increased the size of the soil C pool but also increased global GHG emissions, as calculated by the global warming potential approach.     

Linking forest fires to lake metabolism and carbon dioxide emissions in the boreal region of Northern Québec

Natural fires annually decimate up to 1% of the forested area in the boreal region of Québec, and represent a major structuring force in the region, creating a mosaic of watersheds characterized by large variations in vegetation structure and composition. Here, we investigate the possible connections between this fire‐induced watershed heterogeneity and lake metabolism and CO₂ dynamics. Plankton respiration, and water–air CO₂ fluxes were measured in the epilimnia of 50 lakes, selected to lie within distinct watershed types in terms of postfire terrestrial succession in the boreal region of Northern Québec. Plankton respiration varied widely among lakes (from 21 to 211 μg C L^?1 day^?1), was negatively related to lake area, and positively related to dissolved organic carbon (DOC). All lakes were supersaturated in CO₂ and the resulting carbon (C) flux to the atmosphere (150 to over 3000 mg C m² day^?1) was negatively related to lake area and positively to DOC concentration. CO₂ fluxes were positively related to integrated water column respiration, suggesting a biological component in this flux. Both respiration and CO₂ fluxes were strongly negatively related to years after the last fire in the basin, such that lakes in recently burnt basins had significantly higher C emissions, even after the influence of lake size was removed. No significant differences were found in nutrients, chlorophyll, and DOC between lakes in different basin types, suggesting that the fire‐induced watershed features influence other, more subtle aspects, such as the quality of the organic C reaching lakes. The fire‐induced enhancement of lake organic C mineralization and C emissions represents a long‐term impact that increases the overall C loss from the landscape as the result of fire, but which has never been included in current regional C budgets and future projections. The need to account for this additional fire‐induced C loss becomes critical in the face of predictions of increasing incidence of fire in the circumboreal landscape.     

Emission of CO2 from biochar‐amended soils and implications for soil organic carbon

Soil amendment with pyrogenic organic matter (PyOM), also named biochar, is claimed to sequester carbon (C). However, possible interactions between PyOM and native soil organic carbon (SOC) may accelerate the loss of SOC, thus reducing PyOM's C sequestration potential. We combined the results of 46 studies in a meta‐analysis to investigate changes in CO₂ emission of PyOM‐amended soils and to identify the causes of these changes and the possible factors involved. Our results showed a statistically significant increase of 28% in CO₂ emission from PyOM‐amended soils. When grouped by PyOM C (PyC):SOC ratios, the group of studies with a ratio >2 showed a significant increase in CO₂ emissions, but those with a ratio <2 showed no significant effect of PyOM application on CO₂ emission. Our data are consistent with the hypothesis that increased CO₂ emission after PyOM addition is additive and mainly derived from PyOM's labile C fractions. The PyC:SOC ratio provided the best predictor of increases in CO₂ production after PyOM addition to soil. This meta‐analysis highlights the importance of taking into account the amount of applied PyC in relation to SOC for designing future decomposition experiments.     

Carbon for nutrient exchange between arbuscular mycorrhizal fungi and wheat varies according to cultivar and changes in atmospheric carbon dioxide concentration

Arbuscular mycorrhizal fungi (AMF) form symbioses with most crops, potentially improving their nutrient assimilation and growth. The effects of cultivar and atmospheric CO₂ concentration ([CO₂]) on wheat–AMF carbon‐for‐nutrient exchange remain critical knowledge gaps in the exploitation of AMF for future sustainable agricultural practices within the context of global climate change. We used stable and radioisotope tracers (¹⁵N, ³³P, ¹⁴C) to quantify AMF‐mediated nutrient uptake and fungal acquisition of plant carbon in three wheat (Triticum aestivum L.) cultivars. We grew plants under current ambient (440 ppm) and projected future atmospheric CO₂ concentrations (800 ppm). We found significant ¹⁵N transfer from fungus to plant in all cultivars, and cultivar‐specific differences in total N content. There was a trend for reduced N uptake under elevated atmospheric [CO₂]. Similarly, ³³P uptake via AMF was affected by cultivar and atmospheric [CO₂]. Total P uptake varied significantly among wheat cultivars and was greater at the future than current atmospheric [CO₂]. We found limited evidence of cultivar or atmospheric [CO₂] effects on plant‐fixed carbon transfer to the mycorrhizal fungi. Our results suggest that AMF will continue to provide a route for nutrient uptake by wheat in the future, despite predicted rises in atmospheric [CO₂]. Consideration should therefore be paid to cultivar‐specific AMF receptivity and function in the development of climate smart germplasm for the future.     

Maximum impacts of future reforestation or deforestation on atmospheric CO2

There is scope for land‐use changes to increase or decrease CO₂ concentrations in the atmosphere over the next century. Here we make simple but robust calculations of the maximum impact of such changes. Historical land‐use changes (mostly deforestation) and fossil fuel emissions have caused an increase in atmospheric concentration of CO₂ of 90 ppm between the pre‐industrial era and year 2000. The projected range of CO₂ concentrations in 2100, under a range of emissions scenarios developed for the IPCC, is 170–600 ppm above 2000 levels. This range is mostly due to different assumptions regarding fossil fuel emissions. If all of the carbon so far released by land‐use changes could be restored to the terrestrial biosphere, atmospheric CO₂ concentration at the end of the century would be about 40–70 ppm less than it would be if no such intervention had occurred. Conversely, complete global deforestation over the same time frame would increase atmospheric concentrations by about 130–290 ppm. These are extreme assumptions; the maximum feasible reforestation and afforestation activities over the next 50 years would result in a reduction in CO₂ concentration of about 15–30 ppm by the end of the century. Thus the time course of fossil fuel emissions will be the major factor in determining atmospheric CO₂ concentrations for the foreseeable future.     

Experimental fire increases soil carbon dioxide efflux in a grassland long‐term multifactor global change experiment

Numerous studies have demonstrated that soil respiration rates increase under experimental warming, although the long‐term, multiyear dynamics of this feedback are not well constrained. Less is known about the effects of single, punctuated events in combination with other longer‐duration anthropogenic influences on the dynamics of soil carbon (C) loss. In 2012 and 2013, we assessed the effects of decadal‐scale anthropogenic global change – warming, increased nitrogen (N) deposition, elevated carbon dioxide (CO₂), and increased precipitation – on soil respiration rates in an annual‐dominated Mediterranean grassland. We also investigated how controlled fire and an artificial wet‐up event, in combination with exposure to the longer‐duration anthropogenic global change factors, influenced the dynamics of C cycling in this system. Decade‐duration surface soil warming (1–2 °C) had no effect on soil respiration rates, while +N addition and elevated CO₂ concentrations increased growing‐season soil CO₂ efflux rates by increasing annual aboveground net primary production (NPP) and belowground fine root production, respectively. Low‐intensity experimental fire significantly elevated soil CO₂ efflux rates in the next growing season. Based on mixed‐effects modeling and structural equation modeling, low‐intensity fire increased growing‐season soil respiration rates through a combination of three mechanisms: large increases in soil temperature (3–5 °C), significant increases in fine root production, and elevated aboveground NPP. Our study shows that in ecosystems where soil respiration has acclimated to moderate warming, further increases in soil temperature can stimulate greater soil CO₂ efflux. We also demonstrate that punctuated short‐duration events such as fire can influence soil C dynamics with implications for both the parameterization of earth system models (ESMs) and the implementation of climate change mitigation policies that involve land‐sector C accounting.     

Evidence for large carbon sink and long residence time in semiarid forests based on 15 year flux and inventory records

The rate of change in atmospheric CO₂ is significantly affected by the terrestrial carbon sink, but the size and spatial distribution of this sink, and the extent to which it can be enhanced to mitigate climate change are highly uncertain. We combined carbon stock (CS) and eddy covariance (EC) flux measurements that were collected over a period of 15 years (2001–2016) in a 55 year old 30 km² pine forest growing at the semiarid timberline (with no irrigating or fertilization). The objective was to constrain estimates of the carbon (C) storage potential in forest plantations in such semiarid lands, which cover ~18% of the global land area. The forest accumulated 145–160 g C m^?2 year^?1 over the study period based on the EC and CS approaches, with a mean value of 152.5 ± 30.1 g C m^?2 year^?1 indicating 20% uncertainty in carbon uptake estimates. Current total stocks are estimated at 7,943 ± 323 g C/m² and 372 g N/m². Carbon accumulated mostly in the soil (~71% and 29% for soil and standing biomass carbon, respectively) with long soil carbon turnover time (59 years). Regardless of unexpected disturbances beyond those already observed at the study site, the results support a considerable carbon sink potential in semiarid soils and forest plantations, and imply that afforestation of even 10% of semiarid land area under conditions similar to that of the study site, could sequester ~0.4 Pg C/year over several decades.     

Post‐fire carbon dynamics in the tropical peat swamp forests of Brunei reveal long‐term elevated CH4 flux

Tropical peatlands hold about 15%–19% of the global peat carbon (C) pool of which 77% is stored in the peat swamp forests (PSFs) of Southeast Asia. Nonetheless, these PSFs have been drained, exploited for timber and land for agriculture, leading to frequent fires in the region. The physico‐chemical characteristics of peat, as well as the hydrology of PSFs are affected after a fire, during which the ecosystem can act as a C source for decades, as C emissions to the atmosphere exceed photosynthesis. In this work, we studied the longer‐term impact of fires on C cycling in tropical PSFs, hence we quantified the magnitude and patterns of C loss (CO₂, CH₄ and dissolved organic carbon) and soil‐water quality characteristics in an intact and a degraded burnt PSF in Brunei Darussalam affected by seven fires over the last 40 years. We used natural tracers such as ¹⁴C to investigate the age and sources of C contributing to ecosystem respiration (R_eco) and CH₄, while we continuously monitored soil temperature and water table (WT) level from June 2017 to January 2019. Our results showed a major difference in the physico‐chemical parameters, which in turn affected C dynamics, especially CH₄. Methane effluxes were higher in fire‐affected areas (7.8 ± 2.2 mg CH₄ m^?2 hr^?1) compared to the intact PSF (4.0 ± 2.0 mg CH₄ m^?2 hr^?1) due to prolonged higher WT and more optimal methanogenesis conditions. On the other hand, we did not find significant differences in R_eco between burnt (432 ± 83 mg CO₂ m^?2 hr^?1) and intact PSF (359 ± 76 mg CO₂ m^?2 hr^?1). Radiocarbon analysis showed overall no significant difference between intact and burnt PSF with a modern signature for both CO₂ and CH₄ fluxes implying a microbial preference for the more labile C fraction in the peat matrix.     

Accumulation of soil carbon under elevated CO2 unaffected by warming and drought

Elevated atmospheric CO₂ concentration and climate change may substantially alter soil carbon (C) dynamics, which in turn may impact future climate through feedback cycles. However, only very few field experiments worldwide have combined elevated CO₂ (eCO₂) with both warming and changes in precipitation in order to study the potential combined effects of changes in these fundamental drivers of C cycling in ecosystems. We exposed a temperate heath/grassland to eCO₂, warming, and drought, in all combinations for 8 years. At the end of the study, soil C stocks were on average 0.927 kg C/m² higher across all treatment combinations with eCO₂ compared to ambient CO₂ treatments (equal to an increase of 0.120 ± 0.043 kg C m^?2 year^?1), and showed no sign of slowed accumulation over time. However, if observed pretreatment differences in soil C are taken into account, the annual rate of increase caused by eCO₂ may be as high as 0.177 ± 0.070 kg C m^?2 year^?1. Furthermore, the response to eCO₂ was not affected by simultaneous exposure to warming and drought. The robust increase in soil C under eCO₂ observed here, even when combined with other climate change factors, suggests that there is continued and strong potential for enhanced soil carbon sequestration in some ecosystems to mitigate increasing atmospheric CO₂ concentrations under future climate conditions. The feedback between land C and climate remains one of the largest sources of uncertainty in future climate projections, yet experimental data under simulated future climate, and especially including combined changes, are still scarce. Globally coordinated and distributed experiments with long‐term measurements of changes in soil C in response to the three major climate change‐related global changes, eCO₂, warming, and changes in precipitation patterns, are, therefore, urgently needed.     

Carbon emissions from fires in tropical and subtropical ecosystems

Global carbon emissions from fires are difficult to quantify and have the potential to influence interannual variability and long‐term trends in atmospheric CO₂ concentrations. We used 4 years of Tropical Rainfall Measuring Mission (TRMM) Visible and Infrared Scanner (VIRS) satellite data and a biogeochemical model to assess spatial and temporal variability of carbon emissions from tropical fires. The TRMM satellite data extended between 38°N and 38°S and covered the period from 1998 to 2001. A relationship between TRMM fire counts and burned area was derived using estimates of burned area from other satellite fire products in Africa and Australia and reported burned areas from the United States. We modified the Carnegie‐Ames‐Stanford‐Approach (CASA) biogeochemical model to account for both direct combustion losses and the decomposition from fire‐induced mortality, using both TRMM and Sea‐viewing Wide Field of view Sensor (SeaWiFS) satellite data as model drivers. Over the 1998–2001 period, we estimated that the sum of carbon emissions from tropical fires and fuel wood use was 2.6 Pg C yr^?1. An additional flux of 1.2 Pg C yr^?1 was released indirectly, as a result of decomposition of vegetation killed by fire but not combusted. The sum of direct and indirect carbon losses from fires represented 9% of tropical and subtropical net primary production (NPP). We found that including fire processes in the tropics substantially alters the seasonal cycle of net biome production by shifting carbon losses to months with low soil moisture and low rates of soil microbial respiration. Consequently, accounting for fires increases growing season net flux by ～12% between 38°N and 38°S, with the greatest effect occurring in highly productive savanna regions.     

Periglacial fires and trees in a continental setting of Central Canada,Upper Pleistocene

Fire is a key factor controlling global vegetation patterns and carbon cycling. It mostly occurs under warm periods during which fuel builds up with sufficient moisture, whereas such conditions stimulate fire ignition and spread. Biomass burning increased globally with warming periods since the last glacial era. Data confirming periglacial fires during glacial periods are very sparse because such climates are likely too cold to favour fires. Here, tree occurrence and fires during the Upper Pleistocene glacial periods in Central Canada are inferred from botanical identification and calibrated radiocarbon dates of charcoal fragments. Charcoal fragments were archived in sandy dunes of central Saskatchewan and were dated >50 000–26 600 cal BP. Fragments were mostly gymnosperms. Parallels between radiocarbon dates and GISP2‐δ¹⁸O records deciphered relationships between fire and climate. Fires occurred either hundreds to thousands of years after Dansgaard–Oeschger (DO) interstadial warming events (i.e., the time needed to build enough fuel for fire ignition and spread) or at the onset of the DO event. The chronological uncertainties result from the dated material not precisely matching the fires and from the low residual ¹⁴C associated with old sample material. Dominance of high‐pressure systems and low effective moisture during post‐DO coolings likely triggered flammable periglacial ecosystems, while lower moisture and the relative abundance of fuel overshadowed lower temperatures for fire spread. Laurentide ice sheet (LIS) limits during DO events are difficult to assess in Central Canada due to sparse radiocarbon dates. Our radiocarbon data set constrains the extent of LIS. Central Saskatchewan was not covered by LIS throughout the Upper Pleistocene and was not a continental desert. Instead, our results suggest long‐lasting periods where fluctuations of the northern tree limits and fires after interstadials occurred persistently.     

Soil carbon and nitrogen cycling and storage throughout the soil profile in a sweetgum plantation after 11 years of CO2‐enrichment

Increased partitioning of carbon (C) to fine roots under elevated [CO₂], especially deep in the soil profile, could alter soil C and nitrogen (N) cycling in forests. After more than 11 years of free‐air CO₂ enrichment in a Liquidambar styraciflua L. (sweetgum) plantation in Oak Ridge, TN, USA, greater inputs of fine roots resulted in the incorporation of new C (i.e., C with a depleted δ¹³C) into root‐derived particulate organic matter (POM) pools to 90‐cm depth. Even though production in the sweetgum stand was limited by soil N availability, soil C and N contents were greater throughout the soil profile under elevated [CO₂] at the conclusion of the experiment. Greater C inputs from fine‐root detritus under elevated [CO₂] did not result in increased net N immobilization or C mineralization rates in long‐term laboratory incubations, possibly because microbial biomass was lower in the CO₂‐enriched plots. Furthermore, the δ¹³CO₂ of the C mineralized from the incubated soil closely tracked the δ¹³C of the labile POM pool in the elevated [CO₂] treatment, especially in shallower soil, and did not indicate significant priming of the decomposition of pre‐experiment soil organic matter (SOM). Although potential C mineralization rates were positively and linearly related to total SOM C content in the top 30 cm of soil, this relationship did not hold in deeper soil. Taken together with an increased mean residence time of C in deeper soil pools, these findings indicate that C inputs from relatively deep roots under elevated [CO₂] may increase the potential for long‐term soil C storage. However, C in deeper soil is likely to take many years to accrue to a significant fraction of total soil C given relatively smaller root inputs at depth. Expanded representation of biogeochemical cycling throughout the soil profile may improve model projections of future forest responses to rising atmospheric [CO₂].