Decomposition of soil organic matter

Sammanfattning: The climate is changing and with it the capacity of soils to store carbon in all likelihood, since there is strong evidence that decomposition increases with increasing temperature. The soil contains about twice as much carbon as the atmosphere, so decreasing soil carbon will increase atmospheric CO₂ concentrations proportionally more, further contributing to temperature increase. Different types of soil carbon may respond differently to climate change. A central theory in explaining temperature sensitivity is the activation energy concept sensu Arrhenius. Applied to soil organic matter (SOM) it suggests that low quality SOM will respond more strongly to increased temperature than high quality SOM. Much of the carbon in soil is old and believed to be of low quality. This thesis examines the effects of temperature and SOM quality, separately and in combination. A model of decomposition as three processes in series resulted in unexpected temperature sensitivity, which may explain why the temperature sensitivity is so variable in the literature. A subsequent incubation experiment with different types of organic matter subjected to combinations of different initial and final temperatures showed that the temperature sensitivity increased with time, in agreement with existing theory. In contrast, the effects of past temperatures on present respiration were different than expected. When the decomposition experiment was modelled using the Q-model, decomposer efficiency decreased with temperature, greatly affecting the AOM quality decrease and indirectly affecting temperature sensitivity. Other results presented in the thesis showed that high quality SOM can be stored for hundreds of years and still be made available to decomposers simply by subjecting the soil to drying/wetting cycles. The results presented in this thesis have theoretical and experimental implications and indicate that quality might be less important than previously believed in terms of SOM susceptibility to increasing temperatures. The other processes involved need further attention.