Natural reforestation effect on soil organic carbon and СО2 flux dynamics in southern taiga ecosystems with Albeluvisols

Introduction. Natural reforestation is the widespread trend in the modern land-use changes in the southern taiga zone at the European territory of Russia after 1990s. Its total area is more than 10 million ha. At the same time forest natural regeneration is mutual usual process in the long-term land-use changes in this part of Russia with complex history of its development during millennium. It plays crucial role in soil successions, their fertility and environmental function dynamics including bio productivity support and carbon sequestration, which is given special attention against the background of global climate change challenges in the twenty-first century. This paper presents the results of a round-the-year monitoring of soil CO₂ emission in comparable sites of the fallow lands chrono sequences in conditions of a representative area of the Central Forest Reserve with background wood-sorrel spruce forests which are typical for the southern taiga zone of Central Russia.

Objects and methods. The dominant soil type is sandy-loam Albeluvisols (by WRB, or Orthopodzolic soil by Russia Taxonomy, or Alfisols by Soil Taxonomy, or Podzoluvisols by FAO). The studies have been done in the representative 5 sites at different age of natural reforestation: (1) Fallow meadow grassland – “0-moment”; (2) Forest-fallow birch domination stage of 10-15 year; (3) Birch domination stage of 20-30 year with young spruce participation; (4) Birch-forest stage of 50-60 year with spruce participation; (5) Spruce-forest after fallow stage of more than 100 year with birch participation. CO₂ fluxes seasonal and diurnal dynamics measuring were carried out in situ using a mobile gas analyzer Li-820 and soil exposure chambers with parallel measurements of air temperature, soil temperature and moisture. Also, biomass, soil organic carbon and bulk density were analyzed in their topsoil and subsoil horizons with C stock calculation.

Results and discussion. Analysis of the successional dynamics of the topsoil organic carbon stock showed the maximum rate of their increasing in the first stages of natural reforestation by a thick undergrowth of birch (more than 30 g m^-2∙year^-1) that agrees well with the maximum intensity of the woody biomass growth in case of dominant birch forest up to 50-60 years (more than 100 g m^-2∙year^-1). Research revealed the maximum intensity of soil CO₂ emission (up to 11-12 g C-CO₂ m^-2∙day^-1) in the meadow fallow land and its gradual decreasing in process of reforestation down to values close to background ecosystems in 4-5 g C-CO₂ m^-2∙day^-1 in the last investigated succession study with wood-sorrel spruce older than 100 years, which is in good correlation with the gradual humus accumulation in topsoil due to reduced mineralization of organic compounds from dying vegetation. The seasonal and daily dynamics of soil CO₂ emissions are determined by soil temperature (K_TS 0.77 - 0.99), air temperature (K_TA 0.42 - 0.99), and soil moisture in spring and fall (K_WS -0.55 - -0.98).

Conclusions. Investigation of forest natural regeneration impacts on the level of soil organic carbon accumulation and CO₂ fluxes in the representative southern taiga ecosystems is important element of their soil environmental monitoring and management.