Climate models project that global mean annual temperature will increase by 1.1–6.4°C by the end of the 21st century. Whether terrestrial ecosystem feedbacks will mitigate or enhance warming represents a significant uncertainty in such projections of future climate. One of the largest potential positive feedbacks to climate warming is that arising from enhanced release of CO2 from warmer soils. Soils contain approximately twice as much carbon (C) as the atmosphere and microbial decomposition of soil C is expected to increase with climate warming, as temperature is often the most important factor in determining rates of soil organic matter decomposition. Thus, the aim of this investigation is to determine the in situ soil respiration response to ecosystem warming in an open-air warming experiment at two southern boreal forest sites in northern Minnesota. The experiment uses infrared heat lamps and soil heating cables (at 10-cm depth) to heat 7-m2 plots continuously by during the growing season by 2 or 4°C above ambient. Additionally, there are two ambient temperature treatments, one undisturbed, and the other with inactive soil warming cables (n=6). We measured soil CO2 flux every two weeks during the snow free season in 2009 using a portable infrared gas analyzer with soil chamber attached.
Results/Conclusions
To date, we found slight increases in soil respiration in the two heated treatments relative to the disturbed soil control, corresponding to a 14% (Q10 = 1.4) increase in soil CO2 flux in the +4°C treatment. These treatment effects are smaller than initial warming effects on soil respiration in many other studies. Several untested mechanisms could explain the smaller than expected warming effects on soil respiration. One likely explanation is that warming has significantly reduced soil moisture in the well drained upland soils of these southern boreal forest sites during the drier-than-average 2009 growing season, offsetting positive effects of warming on respiration. These results suggest that warming-induced feedbacks from increased soil respiration may not be as large as would be predicted from simple temperature responses of respiration.
