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Common practices and several studies have demonstrated the positive effect of biochar amendment on climate change mitigation, soil properties, and plant growth. We performed a greenhouse experiment to assess the potential of wood biochar to improve the properties of the growing media and the growth of seedlings in boreal tree species. We added willow biochar (0%, 5%, 10%, and 20%) to raw peat and measured the growth of Norway spruce (Picea abies (L.) H. Karst.), Scots pine (Pinus sylvestris L.), and silver birch (Betula pendula Roth) seedlings. In addition, the co-effect of biochar amendment with 0%, 50%, and 100% fertilization was estimated. We found that using up to 10% of biochar did not reduce the water retention capacity of the growing media significantly. Moreover, biochar amendment significantly increased carbon, nitrogen, potassium, and phosphorus concentrations and had a significant liming effect on the growing media. The biochar amendment increased the aboveground growth of spruce seedlings and root biomass, as well as the root collar diameter, of birch seedlings. Biochar amendment did not affect the quality of seedlings, estimated by the Dickson’s quality index, for spruce and pine, while the quality of birch increased. Based on our results, biochar has potential in forest seedling production.

The temperature changes for the future climate are predicted to be the most pronounced in boreal and arctic regions, affecting the stability of permafrost and fire dynamics of these areas. Fires can affect soil microbiome (archaea, bacteria, fungi, and protists) directly via generated heat, whereas fire-altered soil properties have an indirect effect on soil microbiome. Fires usually decrease microbial biomass and alter microbial community composition. These changes can take decades to recover to prefire states. As the fire occurrence times are expected to change in the future, and the fire return intervals, intensity, and severity are expected to increase in boreal environments, the fire-related changes in the soil microbiome, including its recovery and resilience, are inevitable.

Wildfires burn approximately 1% of boreal forest yearly, being one of the most significant factors affecting soil organic matter (SOM) pools. Boreal forests are largely situated in the permafrost zone, which contains half of global soil carbon (C). Wildfires advance thawing of permafrost by burning the insulating organic layer and decreasing surface albedo, thus increasing soil temperatures. Fires also affect SOM quality through chemical and physical changes, such as the formation of resistant C compounds. The long-term post-fire effects on SOM quality, degradability and isotopic composition are not well known in permafrost forests. We studied the effect of forest fires on the proportional sizes of SOM pools with chemical fractionation (extracting with water, ethanol and acid) of soil samples (5, 30 and 50cm depths) collected from a fire chronosequence in the upland mineral soils of the Canadian permafrost zone. We also determined the C-13 and N-15 isotopic composition of soil after fire. In the topsoil horizon (5cm) recent fire areas contained a smaller fraction of labile SOM and were slightly more enriched with N-15 and C-13 than older fire areas. The SOM fraction ratios reverted towards pre-fire status with succession. Changes in SOM were less apparent deeper in the soil. Best predictors for the size of recalcitrant SOM fraction were active layer depth, vegetation biomass and soil C/N ratio, whereas microbial biomass was best predicted by the size of the recalcitrant SOM fraction. Results indicated that SOM in upland mineral soils at the permafrost surface could be mainly recalcitrant and its decomposition not particularly sensitive to changes resulting from fire.

Key message During the first summer, wood biochar amendments increased soil temperature, pH, and soil CO2effluxes in a xeric boreal Scots pine forest. The increase of soil CO2efflux could be largely explained by increases in by soil temperature. Higher biochar application rates (1.0 vs 0.5 kg m−2) led to higher soil CO2efflux while the pyrolysis temperature of biochar (500 or 650 °C) had no effect on soil CO2efflux. Context Using biochar as a soil amendment has been proposed to increase the carbon sequestration in soils. However, a more rapid soil organic matter turnover after biochar application might reduce the effectiveness of biochar applications for carbon sequestration. By raising the pyrolysis temperature, biochar with lower contents of labile carbohydrates can be produced. Aims To better understand the effects of biochar on boreal forest soil, we applied two spruce biochar with different pyrolysis temperatures (500 °C and 650 °C) at amounts of 1.0 and 0.5 kg m−2 in a young xeric Scots pine forest in southern Finland. Methods Soil CO2, microbial biomass, and physiochemical properties were measured to track changes after biochar application during the first summer. Results Soil CO2 increased 14.3% in 1.0 kg m−2 treatments and 4.6% in 0.5 kg m−2. Soil temperature and pH were obviously higher in the 1.0 kg m−2 treatments. Differences in soil CO2 among treatments disappear after correcting by soil temperature and soil moisture. Conclusion Biochar increased soil CO2 mainly by raising soil temperature in the short term. Higher biochar application rates led to higher soil CO2 effluxes. The increase in soil CO2 efflux may be transient. More studies are needed to get the optimum biochar amount for carbon sequestration in boreal forest.

Rising air temperatures and changes in precipitation patterns in boreal ecosystems are changing the fire occurrence regimes (intervals, severity, intensity, etc.). The main impacts of fires are reported to be changes in soil physical and chemical characteristics, vegetation stress, degradation of permafrost, and increased depth of the active layer. Changes in these characteristics influence the dynamics of carbon dioxide (CO2) and methane (CH4) fluxes. We have studied the changes in CO2 and CH4 fluxes from the soil in boreal forest areas in central Siberia underlain by continuous permafrost and the possible impacts of the aforementioned environmental factors on the emissions of these greenhouse gases. We have used a fire chronosequence of areas, with the last fire occurring 1, 23, 56, and more than 100 years ago. The soils in our study acted as a source of CO2. Emissions of CO2 were lowest at the most recently burned area and increased with forest age throughout the fire chronosequence. The CO2 flux was influenced by the pH of the top 5cm of the soil, the biomass of the birch (Betula) and alder (Duschekia) trees, and by the biomass of vascular plants in the ground vegetation. Soils were found to be a CH4 sink in all our study areas. The uptake of CH4 was highest in the most recently burned area (forest fire one year ago) and the lowest in the area burned 56 years ago, but the difference between fire chronosequence areas was not significant. According to the linear mixed effect model, none of the tested factors explained the CH4 flux. The results confirm that the impact of a forest fire on CO2 flux is long-lasting in Siberian boreal forests, continuing for more than 50 years, but the impact of forest fire on CH4 flux is minimal.

The addition of biochar to soil may offer a chance to mitigate climate change by increasing soil carbon stocks, improving soil fertility and enhancing plant growth. The impacts of biochar in cold environments with limited microbial activity are still poorly known. In order to understand to what extent different types and application rates of biochar affect carbon (C) and nitrogen (N) fluxes in boreal forests, we conducted a field experiment where two different spruce biochars (pyrolysis temperatures 500 °C and 650 °C) were applied at the rate of 0, 5 and 10 t ha−1 to Pinus sylvestris forests in Finland. During the second summer after treatment, soil CO2 effluxes showed no clear response to biochar addition. Only in June, the 10 t ha−1 biochar (650 °C) plots had significantly higher CO2 effluxes compared to the control plots. The pyrolysis temperature of biochar did not affect soil CO2 effluxes. Soil pH increased in the plots receiving 10 t ha−1 biochar additions. Biochar treatments had no significant effect on soil microbial biomass and biological N fixation. Nitrogen mineralization rates in the organic layer tended to increase with the amount of biochar, but no statistically significant effect was detected. The results suggest that wood biochar amendment rates of 5–10 t ha−1 to boreal forest soil do not cause large or long-term changes in soil CO2 effluxes or reduction in native soil C stocks. Furthermore, the results imply that biochar does not adversely affect soil microbial biomass or key N cycling processes in boreal xeric forests, at least within this time frame. Thus, it seems that biochar is a promising tool to mitigate climate change and sequester additional C in boreal forest soils.

SPECIAL ISSUE ARTICLE Editor Dr. Chris J. Barrow Reindeer (Rangifer tarandus L.) is considered to be an important mammalian herbivore, strongly influencing Arctic lichen-dominated ecosystems. There is no wide knowledge about the effect of reindeer on greenhouse gas (GHG) fluxes in northern boreal forests. Ground vegetation plays an important role in absorbing nitrogen (N) and carbon dioxide (CO2) from the atmosphere. Lately, it has also been found to be a significant source of nitrous oxide (N2O) and a small source of methane (CH4). We investigated the influence of reindeer grazing on field layer GHG (CO2, CH4, and N2O) fluxes, ground vegetation coverage and biomass, and soil physical properties (temperature and moisture) in a northern boreal forest. At our study site, the reindeer-induced replacement of lichen by mosses had contrasting effects on the GHG fluxes originating from the field layer. Field layer CO2 efflux was significantly higher in grazed areas. The field layer was a CH4 sink in all areas, but grazed areas absorbed more CH4 compared to non-grazed areas. Although total N2O fluxes remained around 0 in grazed areas, a small N2O sink occurred in non-grazed areas with lower moss biomass. Our results indicated that grazing by reindeer in northern boreal forests affects GHG fluxes from the forest field layer both positively and negatively, and these emissions largely depend on grazing-induced changes in vegetation composition.

In this work, we investigated how the reindeer grazing in subarctic boreal Scots pine forests and climate (air temperature and the amount of precipitation) affects soil temperature, soil water content, and ultimately the CO2 efflux from forest soils. The study was carried out in years 2013 and 2014, where 2013 was an extremely dry year (especially the summer), while 2014 was a “normal” year. Our work showed that in subarctic mature pine forests, soil temperatures were higher, and soil water content was fluctuating more on grazed areas compared to non-grazed areas in both years. On both years, the soil water content on the grazed area was higher in June and the situation changed in the second half of July when the moisture content in the non-grazed area remained higher. There was a negative correlation between soil water content and soil temperature. The soil CO2 effluxes were mostly affected by the year of measurement, time of measurement (different months through growing season), soil temperature and also by the management (grazed or non-grazed) resulting in higher CO2 emissions on the grazed areas. Soil moisture content was not affecting the soil CO2 efflux. The average soil CO2 efflux values were significantly higher in the year 2014 compared to the year 2013 mainly due to differences in soil temperature at the beginning of season.

Wildfires strongly regulate carbon (C) cycling and storage in boreal forests and account for almost 10% of global fire C emissions. However, the anticipated effects of climate change on fire regimes may destabilize current C-climate feedbacks and switch the systems to new stability domains. Since most of these forests are located in upland soils where permafrost is widespread, the expected climate warming and drying combined with more active fires may alter the greenhouse gas (GHG) budgets of boreal forests and trigger unprecedented changes in the global C balance. Therefore, a better understanding of the effects of fires on the various spatial and temporal patterns of GHG fluxes of different physical environments (permafrost and nonpermafrost soils) is fundamental to an understanding of the role played by fire in future climate feedbacks. While large amounts of C are released during fires, postfire GHG fluxes play an important role in boreal C budgets over the short and long term. The timescale over which the vegetation cover regenerates seems to drive the recovery of C emissions after both low- and high-severity fires, regardless of fire-induced changes in soil decomposition. In soils underlain by permafrost, fires increase the active layer depth for several years, which may alter the soil dynamics regulating soil GHG exchange. In a scenario of global warming, prolonged exposition of previously immobilized C could result in higher carbon dioxide emission during the early fire succession. However, without knowledge of the contribution of each respiration component combined with assessment of the warming and drying effects on both labile and recalcitrant soil organic matter throughout the soil profile, we cannot advance on the most relevant feedbacks involving fire and permafrost. Fires seem to have either negligible effects on methane (CH4) fluxes or a slight increase in CH4 uptake. However, permafrost thawing driven by climate or fire could turn upland boreal soils into temporary CH4 sources, depending on how fast the transition from moist to drier soils occurs. Most studies indicate a slight decrease or no significant change in postfire nitrous oxide (N2O) fluxes. However, simulations have shown that the temperature sensitivity of denitrification exceeds that of soil respiration; thus, the effects of warming on soil N2O emissions may be greater than on C emissions.

Climate warming in arctic/subarctic ecosystems will result in increased frequency of forest fires, elevated soil temperatures and thawing of permafrost, which have implications for soil organic matter (SOM) decomposition rates, the CO2 emissions and globally significant soil C stocks in this region. It is still unclear how decomposability and temperature sensitivity of SOM varies in different depths and different stages of succession following forest fire in permafrost regions and studies on long term effects of forest fires in these areas are lacking. To study this question, we took soil samples from 5, 10 and 30 cm depths from forest stands in Northwest Canada, underlain by permafrost, that were burnt by wildfire 3, 25 and over 100 years ago. We measured heterotrophic soil respiration at 1, 7, 13 and 19 °C. Fire had a significant effect on the active layer depth, and it increased the temperature sensitivity (Q10) of respiration in the surface (5 cm) and in the deepest soil layer (30 cm) in the 3-year-old area compared to the 25- and more than 100-year-old areas. Also the metabolic quotient (qCO2) of soil microbes was increased after fire. Though fires may facilitate the SOM decomposition by increasing active layer depth, they also decreased SOM quality, which may limit the rate of decomposition. After fire all of these changes reverted back to original levels with forest succession.

One of the effects of climate change on boreal forest will be more frequent forest wildfires and permafrost thawing. These will increase the availability of soil organic matter (SOM) for microorganisms, change the ground vegetation composition and ultimately affect the emissions of biogenic volatile organic compounds (BVOCs), which impact atmospheric chemistry and climate. BVOC emissions from boreal forest floor have been little characterized in southern boreal region, and even less so in permafrost soil, which underlies most of the northern boreal region. Here, we report the long-term effects of wildfire on forest floor BVOC emission rates along a wildfire chronosequence in a Larix gmelinii forest in central Siberia. We determined forest floor BVOC emissions from forests exposed to wildfire 1, 23 and > 100 years ago. We studied how forest wildfires and the subsequent succession of ground vegetation, as well as changes in the availability of SOM along with the deepened and recovered active layer, influence BVOC emission rates. The forest floor acted as source of a large number of BVOCs in all forest age classes. Monoterpenes were the most abundant BVOC group in all age classes. The total BVOC emission rates measured from the 23- and >100-year-old areas were ca. 2.6 times higher than the emissions from the 1-year-old area. Lower emissions were related to a decrease in plant coverage and microbial decomposition of SOM after wildfire. Our results showed that forest wildfires play an important indirect role in regulating the amount and composition of BVOC emissions from post-fire originated boreal forest floor. This could have a substantial effect on BVOC emissions if the frequency of forest wildfires increases in the future as a result of climate warming. (C) 2019 Elsevier B.V. All rights reserved.

Forest ecosystems are an important carbon (C) pool, and the decomposition of dead wood plays a key role in its C cycle. Based on the United Nations Framework Convention on Climate Change (UNFCCC) and the Kyoto Protocol, IPCC Guidelines for National Greenhouse Gas Inventories were established. Nations that have signed the agreements are encouraged to quantify C pools and fluxes in their forests, including its proportion occurring as dead wood. There are significant differences in density and C concentration of dead wood among tree species. In managed hemiboreal forests of Estonia the dead wood density, and C and N concentration changes in different decay classes for Scots pine ( Pinus sylvestris L.), Norway spruce ( Picea abies (L.) Karst.), silver and downy birch ( Betula pendula Roth. and Betula pubescens Ehrh.), black alder ( Alnus glutinosa (L.) Gaertn.), grey alder ( Alnus incana (L.) Moench.) and European aspen ( Populus tremula L.) have been assessed. All together 548 sample discs taken from logs (measurements were restricted to fallen dead trees only) were collected. The results revealed a decrease in mean dead wood density with progressing decay state for all studied tree species. Pine, spruce and grey alder had the smallest wood density reduction with progressing decay state, retaining 37%, 30%, and 36% of initial density, respectively. Other broadleaved tree species (birch, black alder and aspen) had the greatest density reduction during decomposition, retaining 24%, 23%, and 16% of initial density, respectively. For all studied tree species there were no significant differences of wood density between sites with different moisture conditions (dry, medium or wet areas). In case of pine, spruce and birch the C concentrations were significantly affected by the decay class, while in case of both alders and aspen the C concentrations were not significantly affected by changes in decay classes. For all the assessed tree species the N concentration in dead wood was increasing with increasing decay class.

Reindeer (Rangifer tarandus L.) are the most important large mammalian herbivores in Lapland, strongly affecting the dynamics of vegetation by grazing and trampling, and this is likely in turn to have consequences for the soil processes. We have investigated the changes occurring in above- and belowground biomasses, and soil C dynamics (CO2 efflux, soil C content, soil microbial biomass) among areas grazed and not grazed by reindeer. Our study areas are located in the northern boreal subarctic coniferous forest (undisturbed Scots pine [Pinus sylvestris L.] forests that are naturally lichen-dominated). Our study showed that grazing by reindeer decreased the biomass and cover of lichens in the area significantly. Also the tree regeneration was affected by grazing, as we had much less tree regeneration in the grazed areas. In subarctic mature pine forest, grazing did not affect the soil temperature or the soil moisture. We found no statistically significant effect of grazing on the soil CO2 efflux, ...

In boreal forests, wild fire is an important factor influencing the greenhouse gas (GHG) fluxes and emission rates. As the frequency and severity of forest fires are predicted to increase with a changing climate, the knowledge on the influence of GHG fluxes from fires and burned areas on the atmospheric warming will be of high importance. We studied the effect of forest fire and the time since the last fire on fluxes of three important long-lived GHGs from soil: carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) in northern boreal subarctic Scots pine forest stands with 5 to 155 years since the last forest fire. There was a seasonal pattern in all measured GHG fluxes: emissions of CO 2 and N 2 O, and uptake of CH 4 were increasing towards the end of the growing season throughout all fire chronosequence classes. Emission of CO 2 was lowest in June, in area where the fire was 5 years ago (0.061 mg CO 2 s − 1 m − 2 ) and highest in August in the area which burnt 155 years ago (0.120 mg CO 2 s − 1 m − 2 ). We also found that the soil was a CH 4 sink throughout the post-fire chronosequence in all stand ages. The uptake of methane was compensating the increase in radiative forcing resulting from the carbon dioxide emissions at the recently burned sites, but the compensating effect was very small. The influence and duration of the impact of forest fire were strongest on the carbon dioxide fluxes, but on the methane and nitrous oxide fluxes the effects were less evident.

Fire is one of the most important natural disturbances in the boreal forest. It strongly influences boreal forest structure and function, and alters microbial biomass and species composition e.g. by significantly reducing the abundance of decomposing fungi. We measured carbon stocks and estimated fungal biomass and soil carbon turnover in Pinus sylvestris stands on a fire chronosequence from 2 to 152 years post-fire. Results show that the turnover time of soil carbon was longest in the area where fire occurred 2 years ago (117 years) and approximately two times shorter (ca. 60 years) in the areas where fire occurred 42, 60 and 152 years ago. The soil carbon turnover time in our study areas was associated with very slow fungal biomass recovery. The fungal biomass (ergosterol content), soil carbon content and soil CO 2 efflux reached the same levels as in the oldest site approximately 60 years after the fire. Our results indicate that the slow multi-decadal post-fire recovery of fungal biomass drives the soil carbon balance of boreal forests, and disturbances such as fires have a profound influence on soil carbon turnover for decades. On the other hand, the longer post-fire residence time of carbon in the soil compared to older forests suggests that fire effects on the soil carbon pool in boreal forests might be less dramatic than previously thought.

Forest fires are one of the most important natural disturbances in boreal forests, and their occurrence and severity are expected to increase as a result of climate warming. A combination of factors induced by fire leads to a thawing of the near-surface permafrost layer in subarctic boreal forest. Earlier studies reported that an increase in the active layer thickness results in higher carbon dioxide (CO2) and methane (CH4) emissions. We studied changes in CO2, CH4 and nitrous oxide (N2O) fluxes in this study, and the significance of several environmental factors that influence the greenhouse gas (GHG) fluxes at three forest sites that last had fires in 2012, 1990 and 1969, and we compared these to a control area that had no fire for at least 100 years. The soils in our study acted as sources of CO2 and N2O and sinks for CH4. The elapsed time since the last forest fire was the only factor that significantly influenced all studied GHG fluxes. Soil temperature affected the uptake of CH4, and the N2O fluxes were significantly influenced by nitrogen and carbon content of the soil, and by the active layer depth. Results of our study confirm that the impacts of a forest fire on GHGs last for a rather long period of time in boreal forests, and are influenced by the fire induced changes in the ecosystem. (C) 2017 Elsevier B.V. All rights reserved.

Fire is a major natural disturbance factor in boreal forests, and the frequency of forest fires is predicted to increase due to climate change. Nitrogen (N) is a key determinant of carbon sequestration in boreal forests because the shortage of N limits tree growth. We studied changes in N pools and fluxes, and the overall N balance across a 155-year non stand-replacing fire chronosequence in sub-arctic Pinus sylvestris forests in Finland. Two years after the fire, total ecosystem N pool was 622 kg ha-1 of which 16% was in the vegetation, 8% in the dead biomass and 76% in the soil. 155 years after the fire, total N pool was 960 kg ha-1, with 27% in the vegetation, 3% in the dead biomass and 69% in the soil. This implies an annual accumulation rate of 2.28 kg ha-1 which was distributed equally between soil and biomass. The observed changes in N pools were consistent with the computed N balance +2.11 kg ha-1 yr-1 over the 155-year post-fire period. Nitrogen deposition was an important component of the N balance. The biological N fixation increased with succession and constituted 9% of the total N input during the 155 post-fire years. N2O fluxes were negligible (≤ 0.01 kg ha-1 yr-1) and did not differ among post-fire age classes. The number and intensity of microbial genes involved in N cycling were lower at the site 60 years after fire compared to the youngest and the oldest sites indicating potential differences in soil N cycling processes. The results suggest that in sub-arctic pine forests, the non-stand-replacing, intermediate-severity fires decrease considerably N pools in biomass but changes in soil and total ecosystem N pools are slight. Current fire-return interval does not seem to pose a great threat to ecosystem productivity and N status in these sub-arctic forests., published version, peerReviewed

We compared the changes in aboveground biomass and initial recovery of C pools and CO2 efflux following fire disturbances in Scots pine (Pinus sylvesteris L.) stands with different time since stand-replacing fire. The study areas are located in hemiboreal vegetation zone, in north-western Estonia, in Vihterpalu. Six areas where the last fire occurred in the year 1837, 1940, 1951, 1982, 1997, and 2008 were chosen for the study. Our results show that forest fire has a substantial effect on the C content in the top soil layer, but not in the mineral soil layers. Soil respiration showed a chronological response to the time since the forest fire and the values were lowest in the area where the fire was in the year 2008. The respiration values also followed seasonal pattern being highest in August and lowest in May and November. The CO2 effluxes were lowest on the newly burned area through the entire growing season. There was also a positive correlation between soil temperature and soil respiration values in our study areas.

In boreal forest ecosystems fire, fungi and bacteria, and their interactions, have a pronounced effect on soil carbon dynamics. In this study we measured enzymatic activities, litter decomposition rates, carbon stocks and fungal and microbial biomasses in a boreal subarctic coniferous forest on a four age classes of non-stand replacing fire chronosequence (2, 42, 60 and 152 years after the fire). The results show that microbial activity recovered slowly after fire and the decomposition of new litter was affected by the disturbance. The percent mass loss of Scots pine litter increased with time from the last fire. Slow litter decomposition during the first post-fire years accelerates soil organic matter accumulation that is essential for the recovery of soil biological activities. Fire reduced the enzymatic activity across all the enzyme types measured. Carbon-degrading, chitin-degrading and phosphorus-dissolving enzymes showed different responses with the time elapsed since the fire disturbance. Microbial and enzymatic activity took decades before recovering to the levels observed in old forest stands. Our study demonstrates that slower post-fire litter decomposition has a pronounced impact on the recovery of soil organic matter following forest fires in northern boreal coniferous forests.

In boreal forest ecosystems fire, fungi and bacteria, and their interactions, have a pronounced effect on soil carbon dynamics. In this study we measured enzymatic activities, litter decomposition rates, carbon stocks and fungal and microbial biomasses in a boreal subarctic coniferous forest on a four age classes of non-stand replacing fire chronosequence (2, 42, 60 and 152 years after the fire). The results show that microbial activity recovered slowly after fire and the decomposition of new litter was affected by the disturbance. The percent mass loss of Scots pine litter increased with time from the last fire. Slow litter decomposition during the first post-fire years accelerates soil organic matter accumulation that is essential for the recovery of soil biological activities. Fire reduced the enzymatic activity across all the enzyme types measured. Carbon-degrading, chitin-degrading and phosphorus-dissolving enzymes showed different responses with the time elapsed since the fire disturbance. Microbial and enzymatic activity took decades before recovering to the levels observed in old forest stands. Our study demonstrates that slower post-fire litter decomposition has a pronounced impact on the recovery of soil organic matter following forest fires in northern boreal coniferous forests.