Thermal alteration of soil organic matter properties: a systematic study to infer response of Sierra Nevada climosequence soils to forest fires

Abstract

Abstract. Fire is a major driver of soil organic matter (SOM) dynamics, and contemporary global climate change is changing global fire regimes. We investigated thermal alteration of SOM properties by exposing five different topsoils (0 to 5 cm depth) from the western Sierra Nevada Climosequence to a range of temperatures that are expected during prescribed and wild fires (150, 250, 350, 450, 550 and 650 °C), and determined temperature thresholds for major shifts in SOM properties. With increase in temperature, we found that the concentrations of C and N decreased in a similar pattern among all five soils that varied considerably in their original SOM concentrations and mineralogies. Soils were separated into discrete size classes by dry sieving. The C and N concentrations in the larger aggregate size fractions (2–0.25 mm) decreased with increase in temperature that at 450 °C temperature, the remaining C and N were almost entirely associated with the smaller aggregate size fractions (