The carbon quality-temperature hypothesis does not consistently predict temperature sensitivity of soil organic matter mineralization in soils from two manipulative ecosystem experiments

The temperature sensitivity of soil organic matter (SOM) decomposition is a source of uncertainty in models of soil-climate feedbacks. However, empirical studies have given contradictory results concerning the temperature response of SOM fractions, even as the understanding of the chemical nature of SOM is evolving. The carbon-quality temperature (CQT) hypothesis states that more ‘recalcitrant’ SOM should have higher temperature sensitivity. Incubation studies have often shown a negative correlation between soil respiration rates and temperature sensitivity. However, there have been important exceptions to these results which challenge the assumption that older SOM is necessarily more chemically complex. We asked whether we would expect a universal relationship between temperature sensitivity and soil respiration given that SOM decomposition is influenced by factors other than chemical complexity. We examined temperature sensitivity in long-term incubations of four soils representing two biomes and two ecosystem-level manipulations. Soils from a manipulative climate experiment in Pacific Northwest grasslands demonstrated an increase in temperature sensitivity with incubation duration, but soil from a 20-year input manipulation study in a Northeastern forest showed no relationship of temperature sensitivity with either carbon depletion or incubation time. Furthermore, across all four soils, the temperature sensitivity of soil respiration was frequently inconsistent with indices of carbon quality and did not show a negative correlation with soil respiration rate. We conclude that the CQT hypothesis fails to universally capture the temperature sensitivity of SOM decomposition across environmental contexts, consistent with an emerging understanding of the multiplicity of factors that control soil carbon cycling.