Temperature sensitivity and substrate quality in soil organic matter decomposition: results of an incubation study with three substrates

Kinetic theory suggests that the temperature sensitivity of decomposition of soil organic matter should increase with increasing recalcitrance. This 'temperature-quality hypothesis' was tested in a laboratory experiment. Microcosms with wheat straw, spruce needle litter and mor humus were initially placed at 5, 15 and 25 degrees C until the same cumulative amount of CO2 had been respired. Thereafter, microcosms from each single temperature were moved to a final set of incubation temperatures of 5, 15 and 25 degrees C. Straw decomposed faster than needle litter at 25 and 15 degrees C, but slower than needle litter at 5 degrees C, and showed a higher temperature sensitivity (expressed as Q(10)) than needle litter at low temperatures. When moved to the same temperature, needle litter initially incubated at 5 and 15 degrees C had significantly higher respiration rates in the final incubation than litters initially placed at 25 degrees C. Mor humus placed at equal temperatures during the initial and final incubations had higher cumulative respiration during the final incubation than humus experiencing a shift in temperature, both up- and downwards. These results indicate that other factors than substrate quality are needed to fully explain the temperature dependence. In agreement with the hypothesis, Q(10) was always higher for the temperature step between 5 and 15 degrees C than between 15 and 25 degrees C. Also in agreement with the temperature-quality hypothesis, Q(10) significantly increased with increasing degree of decomposition in five out of the six constant temperature treatments with needle litter and mor humus. Q(10)s for substrates moved between temperatures tended to be higher than for substrates remaining at the initial temperature and an upward shift in temperature increased Q(10) more than a downward shift. This study largely supports the temperature-quality hypothesis. However, other factors like acclimation and synthesis of recalcitrant compounds can modify the temperature response.