Soils can be sources or sinks of carbon depending on the balance between carbon inputs from plants and losses from the decomposition of soil organic matter (SOM). A good understanding of the temperature sensitivity of SOM decomposition is critical for forecasting whether soils in a warming world will lose or gain carbon, and therefore accelerate or mitigate the rate of increasing atmospheric carbon dioxide (CO₂) concentration.

We provide new evidence to show that the response of SOM decomposition to temperature may be constrained by substrate availability to microbial decomposers. We used laboratory incubations of a grassland soil to compare the temperature sensitivity of SOM decomposition with unmodified substrate availability with that of the same soil in which substrate availability was reduced by adding allophane, a clay-size mineral with a high capacity for binding SOM. In the soil with no added allophane, the decomposition rate increased about 7-fold over the temperature range from 1 to 40 °C. With added allophane, decomposition rate increased only about 3- fold over the same temperature range.

We then used a non-disruptive, natural abundance isotopic technique at our field site to partition total soil respiration into CO₂ efflux from newly released, ¹³C-depleted SOM (root respiration and rhizosphere decomposition) from CO₂ efflux from older ¹³C-enriched SOM from the decomposition of more stable SOM. We found no increase in the decomposition rate of the ¹³C-enriched pool of SOM between 11 and 28°C. That finding contrasts with most previous studies that have generally reported strong increases in SOM decomposition with temperature. We hypothesised that the large temperature sensitivity observed in laboratory incubations was due to substrate becoming readily available as a result of the disturbance involved in collecting soil samples. In undisturbed field conditions, the limiting step for the decomposition of the more stable SOM pool may be the rate at which decomposable substrate becomes available for decomposition.

Our findings will have important implications for the feedbacks between soil carbon storage and the rate of increase in atmospheric CO₂ concentration mediated by global warming.

Keywords: Decomposition; temperature sensitivity; heterotrophic respiration; substrate availability; carbon stable isotopes