Soil respiration under prolonged soil warming: are rate reductions caused by acclimation or substrate loss?

The world's soils contain a large amount of carbon so that even a fractionally small loss or gain could have a quantitatively important feedback effect on net CO2 emissions to the atmosphere. It is therefore important to fully understand the temperature dependence of soil-carbon decomposition. Evidence from various observations can be used to quantify the temperature dependence of carbon efflux, but it is important to ensure that confounding factors, such as changing water relations or availability of readily decomposable substrate, are fully considered in inferring an underlying temperature response from observed response patterns. A number of recent findings from soil-warming experiments have led to the suggestion that stimulation of soil-carbon efflux by increasing temperature is only transitory before acclimation takes place and carbon efflux rates return to similar rates as before the increase in temperature. It is shown here that this response pattern can be explained through a simple two-pool soil-carbon model with no acclimation response needing to be invoked. The temporal pattern is, instead, due to depletion of readily decomposable substrate. It shows that findings of reduced respiration rate in soil-warming experiments are consistent with unchanged high temperature sensitivity of organic carbon decomposition and affirms that there is, indeed, a danger of positive feedback between global warming and the release of soil organic carbon that can lead to further warming.