Quality or decomposer efficiency – which is most important in the temperature response of litter decomposition? A modelling study using the GLUE methodology

Abstract. We still lack full mechanistic understanding of how the temperature history affects the future decomposition rate of litter and soil organic matter. To explore that, we used the GLUE modelling framework together with the Q-model and data from a needle litter incubation experiment to compare a differential temperature response of litter qualities to a temperature-dependent decomposer efficiency. The needle litter incubation was a full factorial design with the initial and final temperatures 5, 15 and 25 °C. Samples were moved from the initial to the final temperature when approximately 12% of the initial carbon had been respired and the experiment terminated when an additional 12% had been lost. We used four variations of the Q-model; the litter was described as having one or two initial quality values and the decomposer efficiency was either fixed or allowed to vary with temperature. All variations were calibrated with good fits to the data subsets with equal initial and final temperatures. Evaluation against temperature shift subsets also showed good results, except just after the change in temperature where all variations predicted a smaller response than observed. The effects of having one or two initial litter quality values (fixed decomposer efficiency) on end-of-experiment litter quality and respiration were marginal. Letting decomposer efficiency vary with temperature resulted in a decrease in efficiency between 5 and 15 °C but no change between 15 and 25 °C and in substantial differences in litter quality at the end of the initial incubation in response to incubation temperature. The temperature response of decomposition through temperature dependent decomposer efficiency proved, therefore, to be more important than the differential response to different substrate qualities. These results suggests that it may be important to consider other factors (e.g. microbial efficiency, changing substrate composition) than the temperature sensitivity coupled to substrate quality when evaluating effects of temperature changes on soil organic matter stability.