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 conducted laboratory heating experiments on soils from five locations across the western Sierra Nevada climosequence to investigate thermal alteration of SOM properties and determine temperature thresholds for major shifts in SOM properties. Topsoils (0 to 5 cm depth) were exposed to a range of temperatures that are expected during prescribed and wild fires (150, 250, 350, 450, 550, and 650 • C). With increase in temperature, we found that the concentrations of carbon (C) and nitrogen (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 an increase in temperature, so that at 450 • C the remaining C and N were almost entirely associated with the smaller aggregate size fractions (< 0.25 mm). We observed a general trend of 13 C enrichment with temperature increase. There was also 15 N enrichment with temperature increase, followed by 15 N depletion when temperature increased beyond 350 • C. For all the measured variables, the largest physical, chemical, elemental, and isotopic changes occurred at the mid-intensity fire temperatures, i.e., 350 and 450 • C. The magnitude of the observed changes in SOM composition and distribution in three aggregate size classes, as well as the temperature thresholds for critical changes in physical and chemical properties of soils (such as specific surface area, pH, cation exchange capacity), suggest that transformation and loss of SOM are the principal responses in heated soils. Findings from this systematic investigation of soil and SOM response to heating are critical for predicting how soils are likely to be affected by future climate and fire regimes.

Fire severity, time since fire, and site-level characteristics influence streamwater chemistry at baseflow conditions in catchments of the Sierra Nevada, California, USA

“…In studies conducted two to three years following a fire in the Sierra Nevada, high-severity fires were reported to decrease total C stocks in organic horizons relative to unburned treatments (Maestrini et al 2017, Adkins et al 2019. During high-severity fires, increases in soil heating can also decrease mineral soil C concentrations (Almendros et al 2003, Araya et al 2017) and lead to low molecular weight DOM (Norwood et al 2013, Santos et al 2016 transported from soils to streams. Our results suggest that an increase in fire severity decreased C and N concentrations and molecular size of DOM over time under baseflow conditions, likely due to the thermal degradation of topsoil OM.…”

“…Fires can alter the cycling, concentration, and chemical composition of OM in soil (Santín et al 2015, Araya et al 2016, Santos et al 2016, Araya et al 2017 and aquatic ecosystems (Jaffé et al 2013, Masiello and. The thermal degradation of soil organic C and biomass produces a heterogeneous mixture of organic residues that are collectively referred to as pyrogenic (fire-derived) C (PyC).…”

Post-wildfire Erosion in Mountainous Terrain Leads to Rapid and Major Redistribution of Soil Organic Carbon

“…The MIR data indicate that both the control and 10-year post-fire plots showed an increase in peak heights in the -OH function group (3,700 cm −1 ) with depth and a decrease in the −CH (2,940 cm −1 ). The ester and phenol regions (1,159 and 995 cm −1 ) also showed a decrease in peak heights with depth into the soil profile in both the pre-and post-fire spectra based on DRIFT spectra designations of Araya et al (2017). The most important differences in the MIR spectra in the soils from the burned and control plots, for regions of interest to SOM composition, (Figure 9) were observed in the spectral regions representing asymmetric and symmetric C-H stretching vibrations in aliphatic compounds (2,924 and 2,850 cm −1 , respectively); C = C stretching vibrations of aromatic compounds (1,650 cm −1 ); N-H bending vibrations and C = N stretching vibrations in amides (1,575 cm −1 ); C-H bending (1,390, 1,405, and 1,470 cm −1 ) in aliphatic groups; C-O stretching and asymmetric stretching vibrations in carboxylic and phenolic groups (1,270 cm −1 ); and C-O symmetric vibrations in polysaccharides (1,080 and 1110 cm −1 ).…”

“…Spectra were background corrected against silicon carbide and then baseline corrected. Peak region assignments were adapted from Araya et al (2017).…”

Pyrogenic Carbon Erosion: Implications for Stock and Persistence of Pyrogenic Carbon in Soil

“…Fires can lead to destabilization of topsoil, reducing aggregation and aggregate protected C, increased soil hydrophobicity, which can lead to increased erodibility of soil post-fires through both water and wind-driven erosion processes (Shakesby and Doerr, 2006;Johnson et al, 2007;Ravi et al, 2007;Al-Hamdan et al, 2012;Miller et al, 2012;Araya et al, 2017). Albalasmeh et al (2013), for example, showed that low severity fires can have a lasting impact on soil aggregate stability.…”