Responses of soil physico-chemical properties to combustion: a space for time substitution study to infer how changes in climate are likely to affect response of topsoil to fires

Abstract

Fire is a common ecosystem perturbation that affects many soil properties. As global fire regimes continue to change with climate change, we investigated the effect of fire heating temperatures on the physical and chemical properties of soils across a climosequence transect along the Western slope of the Sierra Nevada that spans from 210 to 2865 m.a.s.l. All the soils we studied were formed on a granitic parent material and have sign ificant differences in soil organic matter (SOM) concentration and mineralogy owing to the effects of climate on soil development. The dominant vegetation from lowest to highest elevation across the transect range from oak woodland, oak/mixed-conifer forest, mixed-conifer forest and subalpine mixed-conifer forest. Topsoils (0–5 cm depth) from the Sierra Nevada climosequence were heated in a muffle furnace at six set temperatures that cover the range of major fire intensity classes (150, 250, 350, 450, 550 and 650 °C). We determined the effects of fire heating temperature on soil aggregate strength, aggregate size distribution, specific surface area (SSA), mineralogy, pH, cation exchange capacity (CEC), and carbon (C) and nitrogen (N) concentrations. With increase of temperature, we found significant reduction of total C, N and CEC. Aggregate strength also decreased with further implications for loss of C protected inside aggregates. Soil pH and SSA increased with increase in temperature. Most of the statistically significant changes ( p < 0.05) occurred at temperature ranges of 350 to 450 °C. We observed relatively smaller changes at typical temperature ranges of prescribed fires (i.e. less than 250 °C). This study identifies critical combustion temperature thresholds for significant physico-chemical changes in soils that developed under different climate regimes, allowing inferences for how soils are likely to respond to different fire intensities under anticipated climate change scenarios.