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
Fire is a common, widespread global phenomenon (Bowman et al., 2009) that conditions the dynamics of soil and soil organic matter (SOM). Vegetation fires burn an estimated 300 to 400 million ha 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). In the Sierra Nevada, vegetation fires have a major influence on the landscape. Ecological functions such as plant regeneration, habitat revitalization, biomass accumulation, and nutrient cycling are influenced by fires (McKelvey et al., 1996). Historically most fires were caused by lightning fires, and vegetation fires play an important role in maintaining the health of many ecosystems around the world (Harrison et al., 2010). In recent decades, anthropogenic activities have become major causes of vegetation fires (Caldararo, 2002). Moreover, climate and climatic variations exert a strong influence on the distribution, frequency, and severity of fires (Harrison et al., 2010). Significant changes in global fire regimes are anticipated because of climate change, including increased frequency of fires in the coming decades (Pechony and Shindell, 2010; Westerling et al., 2006). However, our understanding of how climate change and changes in fire regimes will interact to influence topsoils in fire-affected ecosystems is limited.
In addition to combustion of aboveground biomass and alteration of vegetation dynamics, fires also affect the physical, chemical, and biological properties of soils (Certini, 2005; González-Pérez et al., 2004; Mataix-Solera et al., 2011). The degree of alteration caused by fires depends on the fire intensity and duration, which in turn depend on factors such as the amount and type of fuels, properties of aboveground biomass, air temperature and humidity, wind, topography, and soil properties such as moisture content, texture, and SOM content (DeBano et al., 1998). The first-order effects of fire on soil are caused by the input of heat, causing extreme soil temperatures in topsoil (Badía and Martí, 2003b; Neary et al., 1999), which results in loss and transformation of SOM, changes in soil hydrophobicity, changes in soil aggregation, loss of soil mass, and addition of charred material and other combustion products (Albalasmeh et al., 2013; Araya et al., 2016; Mataix-Solera et al., 2011; Rein et al., 2008; Santos et al., 2016).
The duration of burning regulates the amount of energy transferred through
the soil. Fires with longer residence time and lower temperature typically
impact the soil and SOM more than fires with shorter residence time that burn
at a higher temperature (Frandsen and Ryan, 1986; González-Pérez et
al., 2004). Penetration of heat down a soil profile depends on intensity and
duration of fire as well as the thermal conductivity of the soil (Steward et
al., 1990). Soils have low thermal conductivity and only experience extreme temperatures in the top few centimeters of soil during fires. For example, in
short-duration or low-severity fires temperatures typically reach only
100–150
Fire has multiple complex effects on carbon (C) dynamics in soil. Wildfires
alone lead to the release of up to 4.1 Pg C year
The aim of this study is to determine the effects of heating temperatures on important SOM properties. We used a laboratory heating experiment on five soils from a well-characterized climosequence in the western Sierra Nevada mountain range (Dahlgren et al., 1997). We analyzed changes in SOM quantity and quality following heating treatment with the aim to (1) determine magnitudes of change in SOM properties associated with different fire heating temperatures, (2) identify critical thresholds for these changes, and (3) infer the implications of changing climate on topsoil SOM properties that might experience changing fire regime. This study aims to contribute to the systematic evaluation and development of the ability to predict the effect of fires of different intensities on soil properties under changing climate and fire regimes.
Following the laboratory heating of five soils from the western Sierra Nevada to
temperatures ranging from 150 to 650
Soil classification and site description for the five sites along an elevational transect of the western slopes of the Sierra Nevada (adapted from Dahlgren et al., 1997).
For this study, we collected soils from five sites across an elevation transect along the western slope of the central Sierra Nevada, California (Fig. 1); the sites were previously characterized by Dahlgren et al. (1997). We selected four forested sites that are likely to experience forest fires and a fifth lower-elevation grassland site. The thermal alterations in bulk soil physical and chemical properties from the same study soils were previously reported in Araya et al. (2016).
All the sites have a Mediterranean climate characterized by warm-to-hot, dry
summers and cool-to-cold, wet winters. Mean annual air temperature ranges
from 16.7
The lower elevation woodlands of the Sierra Nevada experience less frequent fires than further upslope and the fires are often fast moving and lower severity (Skinner and Chang, 1996). At the middle-elevation zone of the Sierran forest, the mixed conifer zones, frequent fires are low to moderate severity at lower altitudes, but fire frequency generally increases with altitude towards the upper elevation of the mixed conifer forest (Caprio and Swetnam, 1993). Fires are infrequent and low severity within the high altitude, subalpine zone of the Sierra Nevada (Skinner and Chang, 1996).
Bulk density, water content, pH, C concentration, cation exchange
capacity (CEC), specific surface area (SSA), and particle size distribution
for the five soils (mean
Soils from the lowest elevation site, Vista series soils (210 m a.s.l.),
fall within the oak woodland zone (elevations
The western slope of the central Sierra Nevada presents a remarkable climosequence of soils that developed under similar granitic parent material and are located in landscapes of similar age, relief, slope, and aspect (Trumbore et al., 1996), with significant developmental differences attributed to climate. The soils at mid-elevation range (1000 to 2000 m a.s.l.) tend to be highly weathered, while soils at high and low elevations are relatively less developed (Dahlgren et al., 1997; Harradine and Jenny, 1958; Huntington, 1954; Jenny et al., 1949). Among the most important changes in soil properties along the climosequence are changes in soil organic carbon (SOC) concentration, base saturation, mineral desilication, and hydroxyl-Al interlayering of 2 : 1 layer silicates. Soil pH generally decreases with elevation and the concentrations of clay and secondary iron oxides show a step change at the elevation of the present-day average effective winter snow line, i.e., 1600 m elevation (Tables 1 and 2) (California Department of Water Resources, 1952–1962; Dahlgren et al., 1997).
Triplicate samples (0 to 5 cm depth) were collected at the five sites,
approximately 10 m apart from each other. Any overlaying organic layer was
removed prior to sampling so that only mineral soil was collected. The soils
were air dried at room temperature and passed through a 2 mm sieve. Prior to
furnace heating, the soils were oven dried at 60
Subsamples from each soil were heated in a muffle furnace to one of six
selected maximum temperatures (150, 250, 350, 450, 550, and 650
The six heating temperatures were selected to correspond with fire intensity
categories that are based on maximum surface temperature (DeBano et al.,
1977; Janzen and Tobin-Janzen, 2008; Neary et al., 1999), that is, low
intensity (150 and 250
Dry-aggregate size distribution was measured by sieving. Samples were dry
sieved into three aggregate size classes: 2–0.25 mm (macroaggregates),
0.25–0.053 mm (microaggregates), and
C and N concentrations and stable isotope ratios were measured using an
elemental combustion system (Costech ECS 4010 CHNSO Analyzer, Costech
Analytical Technologies, Valencia, CA, USA) that was interfaced with a mass
spectrometer (DELTA V Plus Isotope Ratio Mass Spectrometer, Thermo Fisher
Scientific, Inc., Waltham, MA, USA). For the analyses, air-dried soil samples
were ground to powder consistency on a ball mill (8000M Mixer/Mill, with a
55 mL tungsten carbide vial, SPEX SamplePrep, LLC, Metuchen, NJ, USA) and
oven dried at 60
Bulk soil carbon and nitrogen concentrations, C : N atomic ratio,
and
Bulk soil organic matter composition was analyzed using FTIR spectroscopy on
a Bruker IFS 66v/S vacuum FT-IR spectrometer (Bruker Biosciences Corporation,
Billerica, MA, USA). We used the diffuse reflectance infrared Fourier
transform (DRIFT) technique (Ellerbrock and Gerke, 2013; Parikh et al.,
2014). Powder samples were dried overnight at 60
All quantitative results are expressed as means of three replicates
The initial concentration of C ranged from 1.5 % (Vista soil, 210 m) to
7.7 % (Musick soils, 1384 m). Soil C concentration continuously
decreased with increasing temperature. The largest decrease occurred between
temperatures of 250 and 450
The loss of C and N from soils due to heating showed a similar response among
all five soils (Fig. 2). After 250
The
C and N concentrations as well as
The distribution of C in the three aggregate size fractions followed the
same general pattern with increase in the heating temperatures. The macroaggregate size fraction (2–0.25 mm) had the least C concentration and
silt–clay-sized particles (
C and N concentrations, C : N atomic ratio, and
C and N distributions in macroaggregates (2–0.25 mm), microaggregates
(0.25–0.053 mm), and silt–clay-sized (
The distribution of C and N in different size aggregates did not change
noticeably except at 450
FTIR spectra of the five soils at the different heating temperatures. Heating temperatures, in Celsius, are shown to the right of each spectrum.
The stable isotope composition of
Changes in chemical composition of SOM due to heating were analyzed by
infrared spectroscopy using the DRIFT technique. The spectra and peaks after contrasting levels of thermal
treatments exhibited qualitative similarities among the different soils. FTIR
spectra for the soils are shown in Fig. 5. One notable change that occurred
in the functional group composition of SOM with heating is the lowered
absorbance intensity of aliphatic methylene groups (as represented by the
aliphatic C–H stretching peak that appears at bands between
2950 and 2850 cm
Our results show significant effects of combustion temperature on
concentration, distribution, and composition of SOM in topsoils that
experience the most intense heating during vegetation fires. Topsoils have
relatively high OM and low clay content that render them more sensitive to
heating since the SOM experiences significant changes during heating. In our
study system, the effect of fire heating on SOM ranged from slight
distillation (volatilization of minor constituents), typically at temperatures
below 150
We observed significant changes in concentration, distribution, and
composition of SOM with increasing heating temperature. The steep decline in
concentration of C in soil that we observed in this study is consistent
with a decrease of about 25 % C at 250
SOM has a C isotopic composition that reflects the
Linear correlation coefficients of changes in soil properties with
changes in C concentration. All correlation coefficients have
Fires tend to lead to enrichment of
The alterations in and loss of SOM are likely more important causes of soil property changes rather than alterations in soil minerals. SOM is vulnerable to temperatures, while soil minerals are only affected at much higher temperatures (Araya et al., 2016). In addition, all of the soils in our study are characterized by low clay content and low concentration of reactive minerals, but they have a high concentration of SOM, especially in the topsoil, leading to strong relationships between SOM concentrations and soil physical properties.
Degradation of lignin and hemicellulose begins between 130 and
190
Loss of OM from soil due to combustion has multiple implications for soil
physical and chemical properties. Simple linear correlation between C
concentration changes and other soil physical and chemical changes that we
observed with heating (reported here and in Araya et al., 2016) show that
more than 80 % of the variability in mass loss, aggregate strength, specific surface area (SSA),
pH, cation exchange capacity (CEC), and N concentrations is associated with changes in C concentration at
the different heating temperatures. Table 3 summarizes the correlation
coefficients of soil property changes with change in C concentration.
Analyses of associations between C concentration and several soil properties
showed a linear association between C and N (
In this study, the greatest changes in SOM occurred between the temperatures 250
and 450
Investigation of the response of climosequence soils to different heating
temperatures in this study enables us to infer how changes in climate are
likely to alter the effect of fires on topsoil physical and chemical
properties in the long term. Along our study climosequence, we observed
critical differences in response of topsoils based mostly on concentration of
OM in soil and soil development stages of each soil. Soil OM concentration
and composition in particular have been shown to respond to changes in
precipitation amount and distribution, as is expected in the Sierra Nevada
(Berhe et al., 2012b). Consequently, changes in soil C storage associated
with climate change are expected to lead to different amounts of C loss due
to fires. This is evidenced by the observed highest total mass of C loss from
the mid-elevation Musick soil that had the highest carbon stock, compared to
soils on either side of that elevation range. Anticipated changes in climate
in the Sierra Nevada mountain range are expected to include upward movement
of the rain–snow transition line, exposing areas that now receive most of
their precipitation as snow to rainfall and associated runoff (Arnold et al.,
2015, 2014; Stacy et al., 2015). Upward movement of the rain–snow transition
zone under anticipated climate change scenarios and associated more intense
weathering at higher elevation zones can render more C to
be lost during fires. More than
80 % of the variability in mass loss, aggregate strength, SSA, pH, CEC,
and N concentrations is associated with changes in C concentration (Table 3).
Hence, as the vulnerability of these ecosystems to increased fire frequency
increases due to climate change (Westerling et al., 2006), we can expect more
soil C loss with fires, along with associated changes in soil chemical and
physical properties. In particular, our findings of important changes in soil
physical and chemical properties occurring between 250 and 450
The different responses of soil aggregation in our climosequence to the treatment temperatures also suggest potential loss and transformation of the physically protected C pool in topsoil. Degradation of aggregates during fire (Albalasmeh et al., 2013) is likely to render aggregate-protected C susceptible to potential losses through oxidative decomposition, leaching, and erosion. Moreover, in systems such as the Sierra Nevada, which are dominated by steep slopes, movement of the rain–snow transition zone upward is likely to increase proportion of precipitation that occurs as rain. The kinetic energy of raindrops and the observed increase in hydrophobicity of soils after fires (Johnson et al., 2007, 2004) can lead to higher rates of erosional redistribution, especially for the free light fraction or particulate C that is not associated with soil minerals (Berhe et al., 2012a; Berhe and Kleber, 2013; McCorkle et al., 2016; Stacy et al., 2015).
A considerable amount of work has been published to demonstrate how
fires affect OM concentration and composition in biomass. This study fills
critical gaps by determining how and to what extent OM in soil experiences
changes due to heating. The findings of this study also showed that changes
in soil properties during heating are closely related to changes in C
concentrations in soil. The temperatures most critical to C loss and
alteration were found to be 250
This study presented the effects of heat input on topsoil properties. The study is necessary for understanding thermally induced changes in soil properties in isolation from other variables that accompany vegetation fires, such as the addition of pyrolysis products from plants and ash and the fire-induced soil moisture dynamics. Findings from this study will contribute towards estimating the amount and rate of change in carbon and nitrogen loss and other essential soil properties that can be expected from topsoil exposure to fires of different intensities under anticipated climate change scenarios.
The data for this article are available online at
The authors declare that they have no conflict of interest.
The authors would like to thank Randy A. Dahlgren for providing georeferences for the study sites, background data, and for his comments on an earlier version of this paper. We thank Christina Bradley for her help and expertise in analysis of C and N and Samuel Traina for his comments on an earlier version of this paper. Funding for this work was provided by a UC Merced Graduate Research Council grant and the National Science Foundation (CAREER EAR – 1352627) award to A. A. Berhe. Edited by: A. Jordán Reviewed by: A. Bento-Gonçalves and three anonymous referees