Temperature sensitivity of decomposition in a peat profile

Soil microbes favour easily decomposable organic compounds, and, consequently, soil organic matter tends to enrich with recalcitrant carbon compounds. In theory, this leads to an increasing temperature sensitivity of decomposition towards older fractions of soil organic matter. We tested this theory in a Sphagnum peat profile. This profile was particularly suitable for this purpose because the age of organic matter increased with depth. The age correlated closely with the degree of decomposition in the oxic layer and soil minerals did not interfere with the relationship between the age and the temperature sensitivity of decomposition. We sampled and analysed the peat profile by layer. We took laboratory measurements of chemical characteristics, decomposability and temperature sensitivity of decomposition. We used the bomb-14C isotopic tracer to differentiate the sources of respired CO2 into age-classes and to estimate the temperature sensitivity of the oldest fraction in each layer. We also used the natural abundance ratio of stable carbon isotopes as an indicator of the recalcitrance of the decomposing carbon fraction. The measurements showed that the decomposition of the older and more recalcitrant carbon compounds was enhanced more in response to increasing temperature than was the decomposition of the younger and more labile compounds. Our results support the theory tested and indicate that the temperature sensitivity of decomposition increases with depth, age and recalcitrance in the oxic layer of the studied peat profile.