DEPENDENCE OF GAS EXCHANGE ON THE LEVEL OF GOUND WATER IN SWAMP COTTON GRASS-SPHAGNUM PINE FORESTS

Abstract

Introduction. Peat bogs play a very important role in climate regulation, while peat moss bogs can significantly contribute to stable concentration of CO₂ in the atmosphere. Goals and objectives. The work is aimed at the evaluation of CO₂ gas exchange in pine bog cotton grass peat moss biogeocoenosis, as a result of ground water drawdown. Materials and methods. The research was carried out in the bog pine biogeocoenosis in the southern taiga (the Yaroslavl Region, Uglich district) during the vegetation period in 2009 and 2011 in different conditions of soil moisture and temperature. The measurement of CO2 gas exchange was carried out using infrared gas analyzer “LI-820” (Li-Cor, USA) with the open scheme. Research results. It has been defined that the dependence of the CO₂ exchange between the surface of sphagnum cover including soil and air is largely determined by the natural water table. Solar radiation incurs ambiguous effect on CO₂ exchange at the value of natural water table variations from 4 to 21 cm. This dependence is represented in the logarithmic form. However, at the value of natural water table below 21 cm the carbon dioxideis emitted, rather than absorbed.  Further reduction of natural water table below 30 cm results in independence of CO₂ exchange on solar radiation.  Further reduction leads to suppression of emission. With the increasing temperature and natural water table varying from 7 to 21 cm there is a decrease of CO₂ emissions, and at the value of natural water table below 33 cm some increase is observed. When the natural water table is below 43 cm, the effect of temperature is significantly weaker, and CO₂ emissions are reduced by half or more. Using the developed dependence of CO₂ gas exchange emitted from the surface of sphagnum on the environmental factors (natural water table, solar radiation, air temperature and humidity) our research revealed that when the level of natural water table varies from 0 to 8 cm the absorption of CO₂ sphagnum is comparable with  the absorption of CO₂ by Ia quality class pine forest. However in the second half of the vegetation period when the natural water table falls below 20-30 cm there are emissions of CO₂ that cannot be compensated by the pine forest of class Ia. Conclusion. When the natural water table amounts to 1-7 cm during the vegetation period, the net ecosystem exchange (NEE) of the bog cotton grass-sphagnum pine forest under study amounts to 7.6 t CO₂ / ha during the vegetation season, including 6.1 t CO₂ / ha from Sphagnum mosses and 3.2 t / ha from pine trees (needles photosynthesis minus trunk respiration and night needle respiration). By reducing the natural water table from mid-summer to 20-30 cm, the net ecosystem exchange becomes negative and amounts to -17.5 t CO₂ / ha during the vegetation period. As a result, at present, due to greenhouse effect observed in the subzone of the southern taiga in the second half of summer there is often a decrease in natural water table below 20 cm. In this case, the net ecosystem exchange of  cotton grass-sphagnum biogeocoenosis becomes negative and even pine forest of Ia quality class cannot compensate for these emissions produced on the same area. However, when the natural water table stands at 2-7 cm the net ecosystem exchange in bog pine cotton grass-sphagnum biogeocoenose is close to net ecosystem exchange in  bilberry-pine of Ia quality class.