Enhancing boreal forest resilience: A four-year impact of biochar on soil quality and fungal communities.

Boreal forests commonly suffer from nutrient deficiency due to restricted biological activity and decomposition. Biochar has been used as a promising strategy to improve soil quality, yet its impacts on forest soil microbes, particularly in cold environment, remains poorly understood. In this study, we investigated the effects of biochar, produced at different pyrolysis temperatures (500 °C and 650 °C) and applied at different amounts (0.5 kg·m-2 and 1.0 kg·m-2), on soil property, soil enzyme activity, and fungal community dynamics in a boreal forest over a span of two to four years. Our results showed that, four-year post-application of biochar produced at 650 °C and applied at 1.0 kg·m-2, significantly increased the relative abundance of Mortierellomycota and enhanced fungal species richness, α-diversity and evenness compared to the control (CK) (P < 0.05). Notably, the abundance of Phialocephala fortinii increased with the application of biochar produced at 500 °C and applied at 0.5 kg·m-2, exhibiting a positively correlation with the carbon cycling-related enzyme ß-cellobiosidase. Functionally, distinct fungal gene structures were formed between different biochar pyrolysis temperatures, and between application amounts in four-year post-biochar application (P < 0.05). Additionally, correlation analyses revealed the significance of the duration post-biochar application on the soil properties, soil extracellular enzymes, soil fungal dominant phyla, fungal community and gene structures (P < 0.01). The interaction between biochar pyrolysis temperature and application amount significantly influenced fungal α-diversity (P < 0.01). Overall, these findings provide theoretical insights and practical application for biochar as soil amendment in boreal forest ecosystems.

Temporal dynamics of soil dissolved organic carbon in temperate forest managed by prescribed burning in Northeast China.

Dissolved organic carbon (DOC) is an important function of soil organic carbon and sensitive to environmental disturbance. Few studies have explored the variations in soil DOC dynamics and effects on soil physicochemical properties following prescribed burnings. In this study, Pinus koraiensis plantation forests in Northeast China were selected and subjected to prescribed burning in early November 2018. Soil DOC and different soil physicochemical and biological properties in the 0-10 cm and 10-20 cm soil layers were sampled six times within two years after a prescribed burning. In this study, some soil physicochemical (SOC, TN, and ST) and microbial biomass properties (MBC) recovered within two years after a prescribed burning. Compared to the unburned control stands, the post-fire soil DOC concentrations in the upper and lower soil layers increased by 16% and 12%, respectively. Soil DOC concentrations varied with sampling time, and peaked one year after the prescribed burning. Our results showed that soil chemical properties (NH4+-N and pH) rather than biological properties (microbial biomass) were the main driving factors for changes in post-fire soil DOC concentrations. Current study provides an important reference for post-fire and seasonal soil C cycling in plantation forests of Northeast China.

The impact of biochar on wood-inhabiting bacterial community and its function in a boreal pine forest.

Biochar is considered to be a possible means of carbon sequestration to alleviate climate change. However, the dynamics of the microbial community during wood decomposition after biochar application remain poorly understood. In this study, the wood-inhabiting bacterial community composition and its potential functions during a two-year decomposition period after the addition of different amounts of biochar (0.5 kg m-2 and 1.0 kg m-2), and at different biochar pyrolysis temperatures (500 °C and 650 °C), in a boreal Scots pine forest, were analyzed using Illumina NovaSeq sequencing combined with Functional Annotation of Prokaryotic Taxa (FAPROTAX). The results showed that the wood decomposition rates increased after biochar addition to the soil surface in the second year. Treatment with biochar produced at high temperatures increased the diversity of wood-inhabiting bacteria more than that produced at low temperatures (P < 0.05). The wood-inhabiting bacterial diversity and species richness decreased with decomposition time. The biochar treatments changed the wood-inhabiting bacterial community structure during the decomposition period. The pyrolysis temperature and the amount of applied biochar had no effect on the bacterial community structure but shifted the abundance of certain bacterial taxa. Similarly, biochar application shifted the wood-inhabiting bacterial community function in the first year, but not in the second year. The wood-inhabiting bacterial community and function were affected by soil pH, soil water content, and soil total nitrogen. The results provide useful information on biochar application for future forest management practices. Long-term monitoring is needed to better understand the effects of biochar application on nutrient cycling in boreal forests.

Long-term effects of forest fires on fungal community and soil properties along a hemiboreal Scots pine forest fire chronosequence.

We studied long-term effects of forest fires on the dynamics of soil fungal community along a post-fire chronosequence in hemiboreal Scots pine stands in north-western Estonia. Effects of fire on soil and fungi were studied on six sites that differed in time since fire (10, 21, 36, 67, 78 and 181 years ago), without further management interventions. Soil fungal communities along the chronosequence were dominated by soil saprotrophs and ectomycorrhizal (EcM) fungi. Across the chronosequence, the most dominant phylum was Ascomycota. The most abundant OTUs were identified as Umbelopsis sp., Hyaloscyphaceae sp. and Pezoloma ericae with relative abundances of 9.5, 8.9 and 6.8 %, respectively. Fungal species richness was similar among sample areas except in the area where fire occurred 36 years ago, where it was significantly lower. There were considerable differences in EcM fungal species composition along the chronosequence. The most recently burned site had Piloderma sphaerosporum, Pseudotomentella sp. and Clavulinaceae sp. as most abundant EcM OTUs while in three oldest burned areas Clavulinaceae sp. and Cortinarius sp. were abundant. Soil C and N stocks were lower in the most recently burned area but differences with other areas were not statistically significant. Soil pH had a significant effect on fungal species composition. Older areas had substantially lower pH compared to more recently burned areas.

The effect of natural disturbances on forest biodiversity: an ecological synthesis.

Disturbances alter biodiversity via their specific characteristics, including severity and extent in the landscape, which act at different temporal and spatial scales. Biodiversity response to disturbance also depends on the community characteristics and habitat requirements of species. Untangling the mechanistic interplay of these factors has guided disturbance ecology for decades, generating mixed scientific evidence of biodiversity responses to disturbance. Understanding the impact of natural disturbances on biodiversity is increasingly important due to human-induced changes in natural disturbance regimes. In many areas, major natural forest disturbances, such as wildfires, windstorms, and insect outbreaks, are becoming more frequent, intense, severe, and widespread due to climate change and land-use change. Conversely, the suppression of natural disturbances threatens disturbance-dependent biota. Using a meta-analytic approach, we analysed a global data set (with most sampling concentrated in temperate and boreal secondary forests) of species assemblages of 26 taxonomic groups, including plants, animals, and fungi collected from forests affected by wildfires, windstorms, and insect outbreaks. The overall effect of natural disturbances on α-diversity did not differ significantly from zero, but some taxonomic groups responded positively to disturbance, while others tended to respond negatively. Disturbance was beneficial for taxonomic groups preferring conditions associated with open canopies (e.g. hymenopterans and hoverflies), whereas ground-dwelling groups and/or groups typically associated with shady conditions (e.g. epigeic lichens and mycorrhizal fungi) were more likely to be negatively impacted by disturbance. Across all taxonomic groups, the highest α-diversity in disturbed forest patches occurred under moderate disturbance severity, i.e. with approximately 55% of trees killed by disturbance. We further extended our meta-analysis by applying a unified diversity concept based on Hill numbers to estimate α-diversity changes in different taxonomic groups across a gradient of disturbance severity measured at the stand scale and incorporating other disturbance features. We found that disturbance severity negatively affected diversity for Hill number q = 0 but not for q = 1 and q = 2, indicating that diversity-disturbance relationships are shaped by species relative abundances. Our synthesis of α-diversity was extended by a synthesis of disturbance-induced change in species assemblages, and revealed that disturbance changes the ß-diversity of multiple taxonomic groups, including some groups that were not affected at the α-diversity level (birds and woody plants). Finally, we used mixed rarefaction/extrapolation to estimate biodiversity change as a function of the proportion of forests that were disturbed, i.e. the disturbance extent measured at the landscape scale. The comparison of intact and naturally disturbed forests revealed that both types of forests provide habitat for unique species assemblages, whereas species diversity in the mixture of disturbed and undisturbed forests peaked at intermediate values of disturbance extent in the simulated landscape. Hence, the relationship between α-diversity and disturbance severity in disturbed forest stands was strikingly similar to the relationship between species richness and disturbance extent in a landscape consisting of both disturbed and undisturbed forest habitats. This result suggests that both moderate disturbance severity and moderate disturbance extent support the highest levels of biodiversity in contemporary forest landscapes.

Data sharing practices and data availability upon request differ across scientific disciplines.

Data sharing is one of the cornerstones of modern science that enables large-scale analyses and reproducibility. We evaluated data availability in research articles across nine disciplines in Nature and Science magazines and recorded corresponding authors' concerns, requests and reasons for declining data sharing. Although data sharing has improved in the last decade and particularly in recent years, data availability and willingness to share data still differ greatly among disciplines. We observed that statements of data availability upon (reasonable) request are inefficient and should not be allowed by journals. To improve data sharing at the time of manuscript acceptance, researchers should be better motivated to release their data with real benefits such as recognition, or bonus points in grant and job applications. We recommend that data management costs should be covered by funding agencies; publicly available research data ought to be included in the evaluation of applications; and surveillance of data sharing should be enforced by both academic publishers and funders. These cross-discipline survey data are available from the plutoF repository.

Soil Fungal Community Structure in Boreal Pine Forests: From Southern to Subarctic Areas of Finland.

The boreal forest environment plays an important role in the global C cycle due to its high carbon storage capacity. However, relatively little is known about the forest fungal community at a regional scale in boreal forests. In the present study, we have re-analyzed the data from our previous studies and highlighted the core fungal community composition and potential functional groups in three forests dominated by Scots pine (Pinus sylvestris L.) in Finland, and identified the fungal generalists that appear across geographic locations despite differences in local conditions. The three forests represent subarctic, northern and southern boreal forest, and are all in an un-managed state without human interference or management. The subarctic and northern areas are subject to reindeer grazing. The results showed that the three locations formed distinct fungal community structures (P < 0.05). Compared to the two northern locations, the southern boreal forest harbored a greater abundance of Zygomycota, Lactarius, Mortierella Umbelopsis, and Tylospora, in which aspect there were no differences between the two northern forests. Cortinarius, Piloderma, and Suillus were the core fungal genera in the boreal Scots pine forest. Functionally, the southern boreal forest harbored a greater abundance of saprotroph, endophytes and fungal parasite-lichen, whereas a greater abundance of ectomycorrhizal fungi was observed in the northern boreal forests. Moreover, the pathotroph and wood saprotrophs were commonly present in these three regions. The three locations formed two distinct fungal community functional structures, by which the southern forest was clearly separated from the two northern forests, suggesting a distance-decay relationship via geographic location. This study provides useful information for better understanding the common fungal communities and functions in boreal forests in different geographical locations.

North American boreal forests are a large carbon source due to wildfires from 1986 to 2016.

Wildfires are a major disturbance to forest carbon (C) balance through both immediate combustion emissions and post-fire ecosystem dynamics. Here we used a process-based biogeochemistry model, the Terrestrial Ecosystem Model (TEM), to simulate C budget in Alaska and Canada during 1986-2016, as impacted by fire disturbances. We extracted the data of difference Normalized Burn Ratio (dNBR) for fires from Landsat TM/ETM imagery and estimated the proportion of vegetation and soil C combustion. We observed that the region was a C source of 2.74 Pg C during the 31-year period. The observed C loss, 57.1 Tg C year-1, was attributed to fire emissions, overwhelming the net ecosystem production (1.9 Tg C year-1) in the region. Our simulated direct emissions for Alaska and Canada are within the range of field measurements and other model estimates. As burn severity increased, combustion emission tended to switch from vegetation origin towards soil origin. When dNBR is below 300, fires increase soil temperature and decrease soil moisture and thus, enhance soil respiration. However, the post-fire soil respiration decreases for moderate or high burn severity. The proportion of post-fire soil emission in total emissions increased with burn severity. Net nitrogen mineralization gradually recovered after fire, enhancing net primary production. Net ecosystem production recovered fast under higher burn severities. The impact of fire disturbance on the C balance of northern ecosystems and the associated uncertainties can be better characterized with long-term, prior-, during- and post-disturbance data across the geospatial spectrum. Our findings suggest that the regional source of carbon to the atmosphere will persist if the observed forest wildfire occurrence and severity continues into the future.

How do forest fires affect soil greenhouse gas emissions in upland boreal forests? A review.

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.

Wildfire effects on BVOC emissions from boreal forest floor on permafrost soil in Siberia.

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.

Long-term effects of forest fires on soil greenhouse gas emissions and extracellular enzyme activities in a hemiboreal forest.

Fire is the most important natural disturbance in boreal forests, and it has a major role regulating the carbon (C) budget of these systems. With the expected increase in fire frequency, the greenhouse gas (GHG) budget of boreal forest soils may change. In order to understand the long-term nature of the soil-atmosphere GHG exchange after fire, we established a fire chronosequence representing successional stages at 8, 19, 34, 65, 76 and 179 years following stand-replacing fires in hemiboreal Scots pine forests in Estonia. Changes in extracellular activity, litter decomposition, vegetation biomass, and soil physicochemical properties were assessed in relation to carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) emissions. Soil temperature was highest 8 years after fire, whereas soil moisture varied through the fire chronosequences without a consistent pattern. Litter decomposition and CO2 efflux were still lower 8 years after fire compared with pre-fire levels (179 years after fire). Both returned to pre-fire levels before vegetation re-established, and CO2 efflux was only strongly responsive to temperature from 19 years after fire onward. Recovery of CO2 efflux in the long term was associated with a moderate effect of fire on enzyme activity, the input of above- and below-ground litter carbon, and the re-establishment of vegetation. Soil acted as a CH4 sink and N2O source similarly in all successional stages. Compared with soil moisture and time after fire, soil temperature was the most important predictor for both GHGs. The re-establishment of overstorey and vegetation cover (mosses and lichens) might have caused an increase in CH4 and N2O effluxes in the studied areas, respectively.

Temperature sensitivity of soil organic matter decomposition after forest fire in Canadian permafrost region.

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.

Impact of post-fire management on soil respiration, carbon and nitrogen content in a managed hemiboreal forest.

Boreal forests are an important carbon (C) sink and fire is the main natural disturbance, directly affecting the C-cycle via emissions from combustion of biomass and organic matter and indirectly through long-term changes in C-dynamics including soil respiration. Carbon dioxide (CO2) emission from soil (soil respiration) is one of the largest fluxes in the global C-cycle. Recovery of vegetation, organic matter and soil respiration may be influenced by the intensity of post-fire management such as salvage logging. To study the impact of forest fire, fire and salvage, and recovery time on soil respiration and soil C and N content, we sampled two permanent research areas in north-western Estonia that were damaged by fire: Vihterpalu (59°13' N 23°49' E) in 1992 and Nõva (59°10' N 23°45' E) in 2008. Three types of sample plots were established: 1) unburned control with no harvesting (CO); 2) burned and uncleared (BU); and 3) burned and cleared (BC). Measurements were made in 2013, 21 years after wildfire in Vihterpalu and 5 years after wildfire in Nõva. Soil respiration ranged from 0.00 to 1.38 g CO2 m-2 h-1. Soil respiration in the burned and cleared areas (BC) was not reduced compared to burned and uncleared (BU) areas but the average soil respiration in unburned control areas was more than twice the value in burned areas (average soil respiration in CO areas was 0.34 CO2 m-2 h-1, versus 0.16 CO2 m-2 h-1, the average soil respiration of BC and BU combined). Recovery over 20 years was mixed; respiration was insignificantly lower on younger than older burned sites (when BC and BU values were combined, the average values were 0.15 vs. 0.17 g CO2 m-2 h-1, respectively); soil-C was greater in the older burned plots than the younger (when BC and BU values were combined, the average values were 9.71 vs. 5.99 kg m-2, respectively); but root biomass in older and recently burned areas was essentially the same (average 2.23 and 2.11 kg m-2, respectively); soil-N was highest on burned areas 20 years after fire. Twenty years post-fire may be insufficient time for carbon dynamics to fully recover on these low productivity sandy sites.

Changes in fluxes of carbon dioxide and methane caused by fire in Siberian boreal forest with continuous permafrost.

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 5 cm 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.

Carbon dioxide, methane and nitrous oxide fluxes from a fire chronosequence in subarctic boreal forests of Canada.

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 100years. 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.

Nitrogen balance along a northern boreal forest fire chronosequence.

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.

Bacterial community structure and function shift across a northern boreal forest fire chronosequence.

Soil microbial responses to fire are likely to change over the course of forest recovery. Investigations on long-term changes in bacterial dynamics following fire are rare. We characterized the soil bacterial communities across three different times post fire in a 2 to 152-year fire chronosequence by Illumina MiSeq sequencing, coupled with a functional gene array (GeoChip). The results showed that the bacterial diversity did not differ between the recently and older burned areas, suggesting a concomitant recovery in the bacterial diversity after fire. The differences in bacterial communities over time were mainly driven by the rare operational taxonomic units (OTUs < 0.1%). Proteobacteria (39%), Acidobacteria (34%) and Actinobacteria (17%) were the most abundant phyla across all sites. Genes involved in C and N cycling pathways were present in all sites showing high redundancy in the gene profiles. However, hierarchical cluster analysis using gene signal intensity revealed that the sites with different fire histories formed separate clusters, suggesting potential differences in maintaining essential biogeochemical soil processes. Soil temperature, pH and water contents were the most important factors in shaping the bacterial community structures and function. This study provides functional insight on the impact of fire disturbance on soil bacterial community.

Fungal Community Shifts in Structure and Function across a Boreal Forest Fire Chronosequence.

Forest fires are a common natural disturbance in forested ecosystems and have a large impact on the microbial communities in forest soils. The response of soil fungal communities to forest fire is poorly documented. Here, we investigated fungal community structure and function across a 152-year boreal forest fire chronosequence using high-throughput sequencing of the internal transcribed spacer 2 (ITS2) region and a functional gene array (GeoChip). Our results demonstrate that the boreal forest soil fungal community was most diverse soon after a fire disturbance and declined over time. The differences in the fungal communities were explained by changes in the abundance of basidiomycetes and ascomycetes. Ectomycorrhizal (ECM) fungi contributed to the increase in basidiomycete abundance over time, with the operational taxonomic units (OTUs) representing the genera Cortinarius and Piloderma dominating in abundance. Hierarchical cluster analysis by using gene signal intensity revealed that the sites with different fire histories formed separate clusters, suggesting differences in the potential to maintain essential biogeochemical soil processes. The site with the greatest biological diversity had also the most diverse genes. The genes involved in organic matter degradation in the mature forest, in which ECM fungi were the most abundant, were as common in the youngest site, in which saprotrophic fungi had a relatively higher abundance. This study provides insight into the impact of fire disturbance on soil fungal community dynamics.