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 (CO) and methane (CH) emissions. We studied changes in CO, CH and nitrous oxide (NO) 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 CO and NO and sinks for CH. 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 CH, and the NO 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.

Background and aims 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. Methods 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. Results 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. Conclusions 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.

As the number of drought occurrences has been predicted to increase with increasing temperatures, it is believed that boreal forests will become particularly vulnerable to decreased growth and increased tree mortality caused by the hydraulic failure, carbon starvation and vulnerability to pests following these. Although drought-affected trees are known to have stunted growth, as well as increased allocation of carbon to roots, still not enough is known about the ways in which trees can acclimate to drought. We studied how drought stress affects belowground and aboveground carbon dynamics, as well as nitrogen uptake, in Scots pine (Pinus sylvestris L.) seedlings exposed to prolonged drought. Overall 40 Scots pine seedlings were divided into control and drought treatments over two growing seasons. Seedlings were pulse-labelled with 13CO2 and litter bags containing 15N-labelled root biomass, and these were used to follow nutrient uptake of trees. We determined photosynthesis, biomass distribution, root and rhizosphere respiration, water potential, leaf osmolalities and carbon and nitrogen assimilation patterns in both treatments. The photosynthetic rate of the drought-induced seedlings did not decrease compared to the control group, the maximum leaf specific photosynthetic rate being 0.058 and 0.045 µmol g-1 s-1 for the drought and control treatments, respectively. The effects of drought were, however, observed as lower water potentials, increased osmolalities as well as decreased growth and greater fine root-to-shoot ratio in the drought-treated seedlings. We also observed improved uptake of labelled nitrogen from soil to needles in the drought-treated seedlings. The results indicate acclimation of seedlings to long-term drought by aiming to retain sufficient water uptake with adequate allocation to roots and root-associated mycorrhizal fungi. The plants seem to control water potential with osmolysis, for which sufficient photosynthetic capability is needed.

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