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• Immediately after the fire, soil CO 2 effluxes increased compared to pre-fire conditions. • Through the post-fire growing season, fire had negative impact on soil CO 2 fluxes. • Unburned areas were sinks of CH 4 through the entire measurement period. • Shortly after the fire, the burned areas turned from CH 4 sink to source. • Shortly after the fire, there was a marked increase in post-fire soil C and N. Although forest fires are one of the main natural disturbance types in boreal forests, there is limited information regarding surface fires (dominant in Northern Europe), and how surface fires of different severities could affect post-fire soil greenhouse gas emissions. The results of our study show that fire severity, time since fire and post-fire changes in soil temperature were the main factors driving soil carbon dioxide (CO 2) flux (forest floor ecosystem respiration) from burned boreal forest soils. Approximately two hours after the fire, soil CO 2 emissions from burned areas were significantly higher compared to pre-fire conditions, and areas with high-severity fires had significantly higher soil CO 2 emissions compared to those with low-severity fires. Later (days, months) after the fire, the unburned control areas always had higher soil CO 2 emission values compared to burned areas. In the case of methane (CH 4), time since fire and post-fire changes in soil temperatures were the main factors driving soil CH 4 fluxes. Unburned study areas were sinks of CH 4 through the entire measurement period, while immediately after the fire, the burned areas turned from CH 4 sink to CH 4 source. For nitrous oxide (N 2 O) measurements, time since fire was the only factor that significantly affected soil N 2 O fluxes. Shortly after the fire, N 2 O emissions increased significantly from both low- and high-intensity study plots. Two days after the fire, post-fire soil C and N content decreased in the O-horizon and increased within the first 5 cm of the soil mineral layer, and the trend was visible both in low- and high-severity fire plots. Samples collected four months after the fire, showed similar total soil C and N content as there was before the fire. [ABSTRACT FROM AUTHOR]

• Large fire years in boreal Europe did not increase their occurrence over 20th and 21st centuries. • Fire-prone years are consistently associated with high pressure atmospheric cells. • Climatological fire hazard in spring increased in the western and northern parts of the study area. • There is an association of the fire activity with June SNAO (Summer North Atlantic Oscillation). Fire remains one of the main natural disturbance factors in the European boreal zone and understanding climatic forcing on fire activity is important for projecting effects of climate change on ecosystem services in this region. We analyzed records of annually burned areas in 16 administrative regions of the European boreal zone (countries or administrative units within countries) and fire weather variability to test for their spatio-temporal patterns over the 1901-2017 period. Over the 1992-2017 period, the region exhibited large variability in forest fire activity with the fire cycles varying from ~1600 (St. Petersburg region) to ~37000 years (Finland). The clustering of administrative units in respect to their burned area, suggested the presence of sub-regions with synchronous annual variability in burned areas. Large fire years (LFYs) in each of the clusters were associated with the development of the high pressure cell over or in immediate proximity of the regions in question in July, indicating climatic forcing of LFYs. Contingency analysis indicated that there was no long-term trend in the synchrony of LFYs observed simultaneously in several administrative units. We documented a trend towards higher values of Monthly Drought Code (MDC) for the months of April and May in the western (April) and northern (April and May) sections. The significant positive correlation between biome-wide fire activity index and June SNAO (Summer North Atlantic Oscillation) (r = 0.53) pointed to the importance of large-scale atmospheric circulation, in particular the summer European blocking pattern, in controlling forest fires across EBZ. The forest fire activity of the European boreal zone remains strongly connected to the annual climate variability. Higher frequency of strongly positive SNAO states in the future will likely synchronize years with a large area burned across the European boreal zone. [ABSTRACT FROM AUTHOR]