EFFECTS OF DRAINAGE AND TEMPERATURE ON CARBON BALANCE OF TUSSOCK TUNDRA MICROCOSMS

We examined the importance of temperature (7 degrees C or 15 degrees C ) and soil moisture regime (saturated or field capacity) on the carbon (C) balance of arctic tussock tundra microcosms (intact blocks of soi l and vege tation) in growth chambers over an 81-day simulated growing season. We measured gaseous CO2 exchanges, methane (CH4) emissions, a nd dissolved C losses on intact blocks of tussock (Eriophorum vaginatu m) and in tertussock (moss-dominated). We hypothesized that under incr eased temperature and/or enhanced drainage, C losses from ecosystem re spiration (CO2 respired by plants and heterotrophs) would exceed gains from gross photosynthesis causing tussock tundra to become a net sour ce of C to the atmosphere. The field capacity moisture regime caused a decrease in net CO2 storage (NEP) in tussock tundra microcosms. This resulted from a stimulation of ecosystem respiration (probably mostly microbial) with enhanced drainage, rather than a decrease in gross pho tosynthesis. Elevated temperature alone had no effect on NEP because C O2 losses from increased ecosystem respiration at elevated temperature were compensated by increased CO2 uptake (gross photosynthesis). Alth ough CO2 losses from ecosystem respiration were primarily limited by d rainage, CH4 emissions, in contrast, were dependent on temperature. Fu rthermore, substantial dissolved C losses, especially organic C, and i mportant microhabitat differences must be considered in estimating C b alance for the tussock tundra system. As much as similar to 20% of tot al C fixed in photosynthesis was lost as dissolved organic C. Tussocks stored similar to 2x more C and emitted 5x more methane than intertus socks. In spite of the limitations of this microcosm experiment, this study has further elucidated the critical role of soil moisture regime and dissolved C losses in regulating net C balance of arctic tussock tundra.