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. Boreal forest soils play an important role in the global carbon cycle (1) and are a net sink for atmospheric CO 2 (2, 3). Approximately 16% of the terrestrial carbon (C) stock is estimated to be stored in the boreal forest ecosystem (1). Fire is a natural disturbance in most forest ecosystems (4), and about 1% of the boreal forest burns annually (5). The frequency of forest fires in the northern boreal zone is expected to increase because of rising temperatures and more frequent dry periods resulting from ongoing climate change (6).Soil microbes perform essential ecological functions in forested ecosystems via nutrient cycling and decomposition of organic matter. Microbial activities control the turnover of organic C in soil and thus contribute to global C cycling (7). Fungi are the predominant decomposers in boreal soils and play a central role in the turnover of carbon and nitrogen (8). Ectomycorrhizal fungi form symbioses with both trees and ground vegetation in boreal forests. Many species of ground vegetation can additionally form symbioses with ericoid mycorrhizal fungi. Boreal forest ecosystem productivity is linked to plant nutrient acquisition from the microbial community via soil organic matter (SOM) decomposition. Forest fires result in the loss of mycorrhizal host plants and may modify the SOM chemistry, leading to dramatic changes in soil microbial activity (9, 10). Fire disturbance often increases soil pH and causes changes in soil temperature due to loss of canopy cover, bo...

Abstract. Boreal forest floor emits biogenic volatile organic compounds (BVOCs) from the understorey vegetation and the heterogeneous soil matrix, where the interactions of soil organisms and soil chemistry are complex. Earlier studies have focused on determining the net exchange of VOCs from the forest floor. This study goes one step further, with the aim of separately determining whether the photosynthesized carbon allocation to soil affects the isoprenoid production by different soil organisms, i.e., decomposers, mycorrhizal fungi, and roots. In each treatment, photosynthesized carbon allocation through roots for decomposers and mycorrhizal fungi was controlled by either preventing root ingrowth (50 µm mesh size) or the ingrowth of roots and fungi (1 µm mesh) into the soil volume, which is called the trenching approach. Isoprenoid fluxes were measured using dynamic (steady-state flow-through) chambers from the different treatments. This study aimed to analyze how important the understorey vegetation is as a VOC sink. Finally, a statistical model was constructed based on prevailing temperature, seasonality, trenching treatments, understory vegetation cover, above canopy photosynthetically active radiation (PAR), soil water content, and soil temperature to estimate isoprenoid fluxes. The final model included parameters with a statistically significant effect on the isoprenoid fluxes. The results show that the boreal forest floor emits monoterpenes, sesquiterpenes, and isoprene. Monoterpenes were the most common group of emitted isoprenoids, and the average flux from the non-trenched forest floor was 23 µg m −2 h −1 . The results also show that different biological factors, including litterfall, carbon availability, biological activity in the soil, and physico-chemical processes, such as volatilization and absorption to the surfaces, are important at various times of the year. This study also discovered that understorey vegetation is a strong sink of monoterpenes. The statistical model, based on prevailing temperature, seasonality, vegetation effect, and the interaction of these parameters, explained 43 % of the monoterpene fluxes, and 34-46 % of individual α-pinene, camphene, β-pinene, and 3 -carene fluxes.

Boreal forests are ecosystems with low nitrogen (N) availability that store globally significant amounts of carbon (C), mainly in plant biomass and soil organic matter (SOM). Although crucial for future climate change predictions, the mechanisms controlling boreal C and N pools are not well understood. Here, using a three-year field experiment, we compare SOM decomposition and stabilization in the presence of roots, with exclusion of roots but presence of fungal hyphae and with exclusion of both roots and fungal hyphae. Roots accelerate SOM decomposition compared to the root exclusion treatments, but also promote a different soil N economy with higher concentrations of organic soil N compared to inorganic soil N accompanied with the build-up of stable SOM-N. In contrast, root exclusion leads to an inorganic soil N economy (i.e., high level of inorganic N) with reduced stable SOM-N build-up. Based on our findings, we provide a framework on how plant roots affect SOM decomposition and stabilization.

See also the Commentary on this article by Hättenschwiler et al., 223: 5–7.

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Fungal communities are important for carbon (C) transformations in boreal forests that are one of the largest C pools in terrestrial ecosystems, warranting thus further investigation of fungal community dynamics in time and space. We investigated fungal diversity and community composition seasonally and across defined soil horizons in boreal Scots pine forest in Finland using 454 pyrosequencing. We collected a total of 120 samples from five vertical soil horizons monthly from March to October; in March, under snow. Boreal forest soil generally harbored diverse fungal communities across soil horizons. The communities shifted drastically and rapidly over time. In late winter, saprotrophs dominated the community and were replaced by ectomycorrhizal fungi during the growing season. Our studies are among the first to dissect the spatial and temporal dynamics in boreal forest ecosystems and highlights the ecological importance of vertically distinct communities and their rapid seasonal dynamics. As climate change is predicted to result in warmer and longer snow-free winter seasons, as well as increase the rooting depth of trees in boreal forest, the seasonal and vertical distribution of fungal communities may change. These changes are likely to affect the organic matter decomposition by the soil-inhabiting fungi and thus alter organic C pools.

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Carbon dioxide is released from the soil to the atmosphere in heterotrophic respiration when the dead organic matter is used for substrates for soil micro-organisms and soil animals. Respiration of roots and mycorrhiza is another major source of carbon dioxide in soil CO 2 efflux. The partitioning of these two fluxes is essential for understanding the carbon balance of forest ecosystems and for modelling the carbon cycle within these ecosystems. In this study, we determined the carbon balance of three common tree species in boreal forest zone, Scots pine, Norway spruce, and Silver birch with gas exchange measurements conducted in laboratory in controlled temperature and light conditions. We also studied the allocation pattern of assimilated carbon with 14 C pulse labelling experiment. The photosynthetic light responses of the tree species were substantially different. The maximum photosynthetic capacity (P max ) was 2.21 lg CO 2 s -1 g -1 in Scots pine, 1.22 lg CO 2 s -1 g -1 in Norway spruce and 3.01 lg CO 2 s -1 g -1 in Silver birch seedlings. According to the pulse labelling experiments, 43-75% of the assimilated carbon remained in the aboveground parts of the seedlings. The amount of carbon allocated to root and rhizosphere respiration was about 9-26%, and the amount of carbon allocated to root and ectomycorrhizal biomass about 13-21% of the total assimilated CO 2 . The 14 CO 2 pulse reached the root system within few hours after the labelling and most of the pulse had passed the root system after 48 h. The transport rate of carbon from shoot to roots was fastest in Silver birch seedlings.