It is not the first year that forests have been burning in Russia, we can regularly hear the news that people and wild animals are losing their homes, being forced to look for safe places. In addition, we can see with our own eyes smog spreading for tens and hundreds of kilometers from the centers of spontaneous burning. These are the manifestations of fires, but there are also long-term consequences of these natural disasters. A researcher at the Laboratory of Biogeochemical Cycles in Forest Ecosystems of the V.N.Sukachev Institute of Forest  KSC SB RAS, Candidate of Biological Sciences, Oksana Viktorovna Masyaguina.

- Why is the post-fire forest attracting the attention of scientists?

- Siberian forests, which suffer from fires every year, have a significant amount of the so-called old soil organic carbon buried in permafrost. Due to changes in microclimatic conditions caused by earlier exposure to fire, it can undergo microbial oxidation to carbon dioxide (CO2), to be released into the atmosphere. This process will aggravate climatic changes in high latitudes (close to the poles, in our case - to the north pole), which are recognized as the most vulnerable.  All this will negatively influence the global climate. In addition, forest fires greatly affect the local climate. On the one hand, aerosols released during these processes have cooling properties and also reduce the greenhouse effect. On the other hand, a decrease in the albedo (reflectivity) of the surface in snow-covered areas due to the destruction of the tree and shrub cover can lead to the permafrost melting. These changes can directly affect the flow of CO2 and other greenhouse gases between soil and atmosphere in the long term. Of particular concern is the fact that over the past decades, forest fires in Siberia have become more frequent, and in all its parts: Western, Middle and Eastern. The 2019-2020 fire seasons became unprecedented in scale in the Arctic due to the earlier start of the fire season caused by climate change, such as warm winters.

- What is old organic carbon and how is it released from the soil?

- Old soil organic carbon, the so-called old soil organic carbon, is organic matter buried in permafrost, often under anaerobic conditions at low temperature and humidity, which do not contribute to its decomposition by soil microorganisms. It is, so to say, conserved until the moment when conditions change and it becomes available to soil microbiota, which will oxidize it to CO2. Carbon dioxide is considered to be the main greenhouse gas affecting climate change, including the ecosystems of high latitudes in Siberia and in the Arctic. In northern ecosystems, radiocarbon studies showed that thermokarst lakes, well-drained soils with a thick active layer formed due to fires, as well as water bodies which appeared in areas with thermal erosion, are the sources of old soil organic carbon released from permafrost. For example, the study of the Yedoma deposits (the uplands surrounded by river valleys or lake depressions in the Arctic and subarctic plains of Eastern Siberia) showed that the older soil material preserved in permafrost have a high rate of soil microbial respiration per unit of incubated carbon as compared to younger and less decomposed samples.

Soil respiration, or CO2 flux from the soil into the atmosphere, measured using the chamber method, has several sources: autotrophic respiration (root respiration), soil microbial respiration, and CO2 fluxes resulting from soil physicochemical processes. Various environmental factors such as temperature and rainfall, as well as destabilizing factors such as fires, can influence the contribution of each process to soil respiration. In postfire boreal ecosystems, including Siberian forests, fires cause significant changes in soil respiration itself, and also affect the included processes. The chamber method refers to a group of methods for directly determining the rate of CO2 emissions from soil, or soil respiration. It is non-destructive, as it excludes disturbances or destruction of plant communities of the North which are highly vulnerable and difficult to restore. The method consists in registering, using a portable infrared gas analyzer, the CO2 flux rising from the surface of the living ground cover. At each site selected for the analysis, plastic rings are installed, whose number depends on many factors (16–20, on average), for example, on the presence of microrelief, type of vegetation, and homogeneity of the area. The greater the variation in these factors, the more rings are required. The ring penetration depth depends on the surface type, but they must be immersed in the mineral layer, otherwise the diffusion of CO2 outside the chamber is possible. Further, the soil chamber is installed on the ring, connected to the gas analyzer, for example Li-Cor 6200. The basic principle of the installation is to measure the increase in the CO2 concentration in the measuring chamber in order to assess CO2 gas exchange in natural conditions. Before the measurement, the necessary parameters are entered in the gas analyzer menu for the correct estimation of soil respiration, for example, the current atmospheric pressure, chamber area, flow rate. The flow rate is manually set to ensure stable conditions inside the chamber and it is dependent on the soil respiration rate. For example, when studying permafrost soils in Evenkia, the flow rate was set from 0.1 to 2 liters per minute. Soil respiration is calculated from the increase in the CO2 concentration with time, volume of the entire system (1,120 cm3) and area occupied by the chamber on the soil surface (78.5 cm2). In addition to soil respiration, a number of micrometeorological parameters are usually also measured, for example, soil temperature, humidity.

- How do CO2 emissions depend on the time passed after the fire?

- According to the latest published data, soil respiration after a fire can vary greatly, but, in general, it takes about 10-30 years after this natural disaster for soil CO2 emissions to stabilize at the pre-fire level. However, the rate of the soil respiration restoration and vegetation regeneration is significantly influenced by the type of fire. In Siberian regions, there are different types of them: most often these are ground fires due to the low density of larch forests, the main tree species in Siberia. The abundance of dense moss and lichen cover, which have increased flammability during dry summer, enhances the intensity of ground fires, and they can spread over several million hectares. In permafrost regions, due to the shallow distribution depth of larch roots, whose growth is limited by permafrost, ground fires can be transformed into crown fires, as, for example, in Central and Eastern Siberia. The impact of forest fires on soil cover can range from direct destruction of the topsoil, mostly organic, to thermochemical transformation of organic matter. It should be noted that the presence of permafrost introduces additional uncertainty in the estimates of soil respiration after fires. For example, an increase in the active soil layer due to a decrease in the albedo of the soil surface resulting from a fire and the associated heating of the soil can stimulate soil respiration. In any case, in the available publications, the minimum period when the values of soil respiration in postfire ecosystems cease to significantly differ from those in undamaged forests is 23 years.