Abstract
Abstract. Fire is a common ecosystem perturbation that affects many soil properties. As global fire regimes continue to change with climate change, we investigated thermal alteration of soils' physical and chemical properties after they are exposed to a range of temperatures that are expected during prescribed and wildland fires. For this study, we used topsoils collected from 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 had significant differences in soil organic matter (SOM) concentration and mineralogy owing to the effects of climate on soil development. 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 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 increasing 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 temperature. Most of the statistically significant changes (p < 0.05) occurred between 350 and 450 °C. We observed relatively smaller changes at temperature ranges below 250 °C. This study identifies critical temperature thresholds for significant physico-chemical changes in soils that developed under different climate regimes. Our findings will be of interest to studies of inferences for how soils are likely to respond to different fire intensities under anticipated climate change scenarios.
Highlights
Fire is a common, widespread phenomenon in many ecosystems around the world (Bowman et al, 2009)
Among the variables we investigated in this study, we observed changes along two general lines: (1) mass loss, specific surface area (SSA) and pH which showed a progressive increase with heating temperature, and (2) % C, % N, C : N ratio, cation exchange capacity (CEC), and wet aggregate stability that showed a progressive decrease with increase in temperature (Fig. 10), with the most significant changes in all cases being recorded in all soils between 250 and 450 ◦C
As we found in this study, more than 80 % of the variability in mass loss, aggregate strength, SSA, pH, CEC and N concentrations is associated with changes in C concentration at the different heating temperatures (Table 3)
Summary
Widespread phenomenon in many ecosystems around the world (Bowman et al, 2009). Vegetation fires burn an estimated 300 to 400 million hectares of land globally every year (FAO, 2005). In the US alone, over 80 000 fires were reported in 2014, including about 63 000 wildland fires, and 17 000 prescribed burns that burned over 1.5 million and 970 000 ha of land, respectively (National Interagency Fire Center, 2015). Significant changes in global fire regimes are anticipated because of climate change including an increase in frequency of fires in the coming decades (Pechony and Shindell, 2010; Westerling et al, 2006). Our understanding of how climate change and changes in fire regimes will interact to influence topsoils in fire affected ecosystems is limited
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