Environmental assessment of soil greenhouse gases fluxes in forest – fallow succession at the Central Forest Reserve in European Russia
The problem of global climate change is one of the key modern environmental problems. It is generally accepted that greenhouse gases (CO2, CH4, N2O) accumulation is the main factor of Global climate change. One of the principal factors influencing the current level of the greenhouse fluxes are land-use changes, including the forest restoration in fallow lands – widespread at the Central Region of Russia.
We conducted the comprehensive environmental studies of soil greenhouse fluxes in comparable sites with different stages of the forest-fallow successions in the southern part of the Central Forest Reserve with spruce domination in the mature forest - representative southern-taiga ecosystems.
The principal studies have been done in 5 representative sites at different ages of succession:
1. Fallow meadow grassland («0» - moment);
2. Forest-fallow stage of 10-15 years with young birch domination;
3. Forest-fallow stage of 20-30 years with birch domination and young spruce participation;
4. Birch-forest after fallow stage of 80-100 years with spruce participation;
5. Spruce-forest after fallow stage of 120-150 years with birch participation.
Seasonal and diurnal dinamics CO2 fluxes measurements were carried out in situ using a mobile gas analyzer Li-820 with soil exposure chambers and parallel observation of air temperature, soil temperature and moisture. Also, every ten days the soil air has been sampled in the vials for further CO2, CH4 and N2O flux measurements by the stationary gas chromatograph.
The dominant soil type is Orthopodzolic soil (by Russia Taxonomy, or Alfisols by Soil Taxonomy, or Albeluvisols by WRB, or Sandy-loam podzoluvisols by FAO). Within forest-fallow successions there are shown the litter gradual development, humus-accumulative horizon differentiation, soil acidity and bulk density increasing. At the same time there is enough obvious in the down part of past-arable horizon gradual restoration of the podzolic horizon.
The monitoring results shown the essential decreasing of soil CO2 fluxes «in 2 times» in frame of successions. The maximum CO2 fluxes have been fixed in July with optimal temperature / moisture ratio. In the medial of July the maximum CO2 emission is observed in fallow grassland (34,1 g CO2 / m2day), that is almost «in 2 – times» more than in spruce-forest after fallow stage of 120-150 years. It is important that soil CO2 fluxes essentially increase with soil temperature rise (with up to R = 0,75) and drop soil moisture (with up to R = - 0,66).
During the day, the most intense soil CO2 fluxes have been observed from case of 12:00 to 18:00. The maximum CO2 flux has been recorded at 15:00 in the fallow grassland (23 g CO2 / m2 day). In the forest-fallow stage of 10-15 years the maximum soil CO2 flux observed at 12 hours was (16 - 17 g CO2 / m2 day). There were not strong differences in soil CO2 fluxes of these two investigated sites in the « night» time from 21:00 to 9:00. The essential daily dynamics must be taken into attention for assessment the seasonal fluxes of greenhouse gases and carbon balance.
The maximum CH4 flux has been fixed in the fallow grassland and forest-fallow stage of 10 - 15 years - in stark contrast to stable soil sink CH4 in the spruce-forest after forest-fallow older than 120 years (final stage overgrowth fallow). In the fallow meadow grassland there are observed CH4 emission in July and sink in June and August, with a maximum flux in late June. The level of N2O fluxes usually does not exceed 0,2 mg N2O /m2*day with the maximum flux in mid-August and light sink in early June.