Soil Structural Degradation During Low-Severity BurnsM. Jian1 , M. Berli2 , and T. A. Ghezzehei1

1Life and Environmental Sciences Department, University of California, Merced, CA, USA, 2Hydrologic Sciences Division,Desert Research Institute, Las Vegas, Nevada, USA

Abstract Low-severity wildfires and prescribed burns have been steadily increasing for over threedecades, currently accounting for more than half of total burned area in the southwestern United States.Most observations immediately after low-severity burns report little adverse impacts on soil properties andprocesses. In a few studies, however, significant deterioration of soil structure has been observed severalmonths after such fires. Here we show that rapid vaporization of pore water during low-severity burns raisespneumatic gas pressure inside large aggregates (20–30 mm) to damaging levels, on the order of aggregatetensile strength and high enough to cause viscoplastic deformation. However, the impact on soil structurewas not immediately perceptible. This suggests that other natural forces, such as wetting-drying and thermalcycles, are required to disrupt the weakened aggregates. Thus, adverse consequences of the suggestedmechanism on soil processes and services (e.g., infiltration, erodibility, and organic matter protection) arelikely overlooked.

Plain Language Summary Fire researchers and the public are often concerned about theincreasing threat of fires that are considered moderate to high in intensity—a measure fuel energy—andin severity—a measure of impact on ecosystems. What does not get much attention is, however, that the areaimpacted by low-severity and low-intensity fires is also on the rise. Currently, more than half of the burnedareas in the southwestern United States are low-severity wildfires and controlled burns. These fires lastonly for a fewminutes, and the soil surface temperature rarely exceeds 200 °C and does not heat more than afew centimeters of soil. Because of this low energy input, the effect of low-severity burns on soil qualityhas been generally assumed negligible. This study was motivated by long-term monitoring studies inSpain and United States that showed significant disaggregation of surface soil occurred several months afterlow-severity burns. Here we show that soil water is vaporized very rapidly when moist soil aggregates aresubjected to rapid heating. The escaping vapor momentarily creates disruptive stress that may exceed thebonds that hold soil aggregates together, causing disaggregation and loss of soil functions such infiltrability,resistance to erosion, and protection of organic matter.

1. Introduction

Half of the combined wildfire and prescribed burn area reported in the United States between 1984 and 2016(Figure S1 in the supporting information) was characterized as low in intensity (time-averaged energy flux)and severity (degree of ecological effects; Eidenshink et al., 2007; Keeley, 2009; MTBS, 2017). The impact offire severity on soil processes is expressed in terms of peak temperature and duration of elevated tempera-ture in the soil profile (Mataix-Solera et al., 2011). Fires with soil surface temperature exceeding 250 °C aregenerally categorized as medium to high in severity. The impact of such fires on intensification of runoffand erosion is widely recognized (Carroll et al., 2007; Certini, 2005; DeBano et al., 1977; Knicker, 2007). Theincreased soil erosion after such fires has been attributed to soil structure deterioration, loss of permeableorganic materials, and increased hydrophobicity caused by combustion and volatilization of soil organic mat-ter (Mataix-Solera et al., 2011). In contrast, low-severity burns are presumed to have no or minimal adverseeffects on soil processes (Debano, 2000; DeBano et al., 1977; Mataix-Solera et al., 2011). Thus, only limitedstudies have been directed at investigating the impacts of low-severity burns on soil processes. Whilemost of these investigations reported no significant change of soil structure immediately after low-intensityburns (Arcenegui et al., 2008; Jordán et al., 2011; Mataix-Solera et al., 2002; O’Dea, 2007), a few found slight-to-moderate increase in aggregate stability that was attributed to addition of organic matter and/or desiccation-induced hardening of organic and inorganic cements (García-Corona et al., 2004; Mataix-Solera et al., 2002;Úbeda & Bernia, 2005). However, long-term monitoring of soil structure following low intensity fires inNortheastern Spain (Úbeda & Bernia, 2005) and Great Basin region of Nevada (Chief et al., 2012; Kavouras

JIAN ET AL. 1

Geophysical Research Letters

RESEARCH LETTER10.1029/2018GL078053

Key Points:• We show that rapid vaporization ofpore water increases pneumatic porepressure inside pores to a level thatcan weaken soil aggregates

• Destabilization occurs over narrowmoisture range, when aggregates aresufficiently pliable to yield to pressuregenerated by vapor

• Postburn disaggregation occursslowly making its impact on soilfunction ecosystem services easyto overlook

Supporting Information:• Supporting Information S1

Correspondence to:T. A. Ghezzehei,taghezzehei@ucmerced.edu

Citation:Jian, M., Berli, M., & Ghezzehei, T. A.(2018). Soil structural degradationduring low-severity burns. GeophysicalResearch Letters, 45. https://doi.org/10.1029/2018GL078053

Received 23 MAR 2018Accepted 22 MAY 2018Accepted article online 31 MAY 2018

©2018. American Geophysical Union.All Rights Reserved.

et al., 2012) revealed significant deterioration of soil aggregate stabilityfor 8 to 13 months after low-severity burns.

Previously, we hypothesized that low-severity burns can lead to signifi-cant weakening of soil aggregates if rapid vaporization of pore waterraises the pore level pneumatic pressure momentarily to disruptivelevels (Albalasmeh et al., 2013). We expected the weakened aggregatesto be prone to disruption by natural factors that take place after theburn. We tested this hypothesis indirectly by showing that the water sta-bility of moist aggregates rapidly heated to 175 °C is lower than aggre-gates slowly heated to the same temperature. The latter case wasassumed to be less damaging because the vapor has sufficient time toescape without elevating the internal pressure to damaging levels.Albalasmeh et al. (2013) was the first and only study to suggest andindirectly test this mechanism of soil aggregate degradation duringlow intensity burns (Urbanek, 2013).

The goal of this study was to provide a direct proof of the hypothesis bymeasuring the pressure inside individually burned soil aggregates.Mechanistic explanation of the impact of low-severity burns on soilstructure is crucial in advancing our understanding and managementdecisions of large swaths of delicate ecosystems worldwide.

2. Materials and Methods2.1. Soil Collection

Soils were collected from two distinct ecosystems that experience low-severity fires in the western United States: (a) a sandy loam from anundisturbed pine forest in Mariposa County, California and (b) loam soilfrom an unburned shrubland (adjacent to the burn boundary of theCarpenter 1 Fire) in Clark County, Nevada. There were no known firesat the sampling sites in the last 10 years. In the remainder of this paper,these soils will be referred to as forest and shrubland soils, respectively.Soil samples were collected from 0 to 10 cm depth then air-dried beforelarger aggregates (1.3–5.3 g) were separated out by hand. Basic soilcharacteristics are provided in Table S1 in the supporting information.To test the main hypothesis we conducted two complimentary experi-ments: (a) direct measurement of pneumatic pressure inside aggregates

during simulated low-severity burn and (b) measurement of tensile strength of individual aggregates. Forboth experiments, soil aggregates were equilibrated to different levels of matric potentials prior to start ofmeasurements (see S1 for detailed protocols).

2.2. Pneumatic Pressure Measurement

Pneumatic pressure inside aggregates was measured using a custom-made aggregate pressure sensor(Figure 1a). The sensor involves a pressure transducer (Honeywell sensor number 26PCCFA6G) connectedto a BD 0.45 mm × 10 mm hypodermic needle. Pneumatic pressure was recorded at frequency of l Hz usinga data logger (Keithley 2700 Multimeter/Data Acquisition System). The surface temperature of the soil aggre-gates was continuously recorded with an infrared thermometer (Omega infrared thermometer OS1327D) atthe same frequency. The aggregates were heated with a 360-W precision noncontact heat gun (Milwaukeeheat gun model 1400) for 15 min. The power output was controlled by connecting the heat gun to a variableautotransformer (POWERSTAT Type 2PF136). A power output setting that raises aggregate surface tempera-ture to maximum of 175 °C (that simulates low-severity burn) was selected during preliminary experiments.

2.3. Tensile Strength Measurement

To measure the tensile strength of the soil aggregates, thin strings were attached to opposite sides ofthe aggregates using epoxy, leaving a narrow band of uncoated soil (Figures 1b and 1c). The epoxy was

Figure 1. Pictures of experiments conducted. (a) Direct measurement ofpneumatic pressure inside aggregate during heating experiment. (b) Directmeasurement of tensile strength by pulling apart aggregates withepoxied ends (c) until they break apart at the failure plane.

10.1029/2018GL078053Geophysical Research Letters

JIAN ET AL. 2

allowed to cure for 72 hr. Afterward, a micrometer caliper was used to measure the length and width of thepredefined failure plane, and the cross-sectional area of the failure plane was approximated as an ellipse.

After the soil aggregates were equilibrated to the desired matric potentials, the soil aggregates were sus-pended (Figure 1b) and tensile stress was applied by adding weights to the string at the bottom of theaggregate until the aggregates ruptured along the predefined failure plane (Figures 1b and 1c). The ten-sile stress at failure was then calculated with the mass required to rupture the aggregates over the areaof the predefined tensile shear plane. A summary of the soil aggregates used in these experiments is pro-vided in Table S2.

2.4. Transient Deformation During Heating Experiment

The experiments described in the preceding sections showed how maximum pneumatic pore pressure riseand aggregate tensile strength are related to the initial moisture level of the soil aggregates. However, theaggregate tensile strength measurements indicate only the aggregate strength at failure, that is, the maxi-mum stress required to fully break the soil aggregates. During the course of a low intensity burn, soil aggre-gates may not fully break, but their strength may be reduced because of the pore pressure increasedisrupting the microscopic bonds between soil particles. Moreover, the soil aggregates dry during the courseof the heating experiment; thus, the tensile strength of the aggregates is expected to increase in the course ofthe heating experiment. A significant portion of the aggregate weakening is likely to occur during the earlystage of burn, while the pore pressure is high and the aggregates are moist and weak.

To semiquantitatively analyze the transient effect of pore pressure on aggregate stability, we apply a rheolo-gical soil stress-deformation model. Pneumatic stress, strain, and soil properties are assumed to be uniformlydistributed within an aggregate volume. The soil matrix within aggregates is assumed to behave as aBingham viscoplastic material, where the viscous shear strain rate is given as (Ghezzehei & Or, 2001)

dγdt

¼0 P tð Þ < S tð Þ

P tð Þ � S tð Þη tð Þ P tð Þ ≥ S tð Þ

8<: (1)

where γ is the shear strain and P is the stress applied, here assumed to be the pneumatic pressured mea-sured during burn treatment. The yield stress, S, and plastic viscosity, η, are soil-specific rheological char-acteristics that vary with water content. The above model dictates that viscoplastic deformation occursonce the applied stress exceeds the yield stress of the material, while reversible elastic deformation isto be expected for applied stresses lower than the yield stress. The applied pressure does not necessarilycause volume expansion of the whole aggregate. Rather, it is more likely that the pressure is dissipated bymany localized shear deformations that ultimately weaken the overall integrity of the bonds that holdaggregates together.

Ghezzehei and Or (2001) observed that the dependence of plastic viscosity and yield stress on water contentcan be explained using empirical power law relationships:

η tð Þ ¼ η0w tð Þα (2a)

S tð Þ ¼ S0w tð Þb (2b)

where w is the gravimetric water content (g/g), S0 and η0 are yield stress and plastic-viscosity of 1:1 soilwater slurries, and a and b are shape factors. For illustrative purposes, we assumed that the rheologicalcharacteristics of Millville silt loam soil reported by Ghezzehei and Or (2001) are applicable to the two studysoils (Table S3).

The transient nature of soil deformation during burning stems from the rapid loss of soil water, which causesrapid increases in soil yield stress and viscosity as well. Tomodel the water content decline during the heatingexperiment, mass of soil aggregates equilibrated at the selected matric potentials (�6, �10, �30, and�100 kPa) was monitored during heating experiments. The wetting of aggregates and heating was per-formed in identical manner as heating experiment above. The gravimetric water contents of unburned soilsat the different matric potential levels are reported in Table S2.

10.1029/2018GL078053Geophysical Research Letters

JIAN ET AL. 3

The decline in water content during heating for all aggregates and all initial matric potential levels were fittedwith exponential curves

w tð Þ ¼ w0e�kt (3)

where w0 is the initial gravimetric water content of the soil aggregate and k is the decay constant.The best fit values of k ranged from 2.64 × 10�3 s�1 to 4.89 × 10�3 s�1 for the forest soil and4.24 × 10�3 s�1 to 6.11 × 10�3 s�1 for the shrubland soil (see S4 for determining k). The viscous strainis evaluated by integrating the strain rate (equation (1)) during the times when the applied pneumatic stressexceeded the yield stress.

The above model is neither expected nor intended to provide accurate prediction of strain because ofthe uncertainties underlying the model assumptions and parameters. Nevertheless, because these uncer-tainties are equally applicable to all moisture levels of any given soil, the model provides valuable insighton the susceptibility of aggregates to disruption while both pneumatic pressure rise and soil moistureevolve rapidly.

3. Results

Representative measurements of aggregate surface temperatures and internal pneumatic pore pressureduring the heating experiments at five initial matric potential levels are shown in Figure 2. These aggregateswere selected from a total of 80 forest soils aggregates and 64 shrubland soil aggregates (complete data setwill be published with the paper). The energy output from the heat source and the distance between theaggregates and the heat source were kept constant for all aggregates. However, the total energy inter-cepted by soil aggregates could vary due to the size and shape of individual aggregates.

0

50

100

150

200

°CS

urfa

ce T

empe

ratu

re,

(a) Forest

0 200 400 600 800 1000

0

1

2

3(c) Forest

Por

e P

ress

ure,

kPa

(b) Shrubland

0 200 400 600 800 1000

(d) Shrubland

Heating Time, sec

ψ (−kPa)

61030100air−dry

Figure 2. Representative plots of soil surface temperature of the (a) forest and (b) shrubland soil and pneumatic gaspressure rise inside of the (c) forest and (d) shrubland soils during heating experiment. Matric potential of the air-driedaggregates were �94,230 and �123,603 kPa for the forest and shrubland soils, respectively.

10.1029/2018GL078053Geophysical Research Letters

JIAN ET AL. 4

The tip of the pressure transducer was positioned in the center of the aggregate, where the pressure rise wasexpected to be the highest due to limited escape path for the vapor and hot gas. The peak-pressure reachedduring the burning experiment could serve as an indicator of the maximum disruptive impact of the rapidlyvaporizing pore water. In Figure 3 the peak pneumatic pressure observed inside the soil aggregates and thetensile strength of individual aggregates are shown. For all the data points, the standard error of multiplereplicates is indicated with error bars. For the forest soils, a total of 24 tensile-strength measurements weretaken over the five matric potentials. For the shrubland soils, a total of 27 measurements were taken over

the five matric potentials. The measured pneumatic pressure valueswere generally higher in aggregates at higher matric potentials. This isto be expected as vaporization of pore water is the primary sourceof the observed increase in pressure. The forest soils, which have higherwater holding capacity at all the measured matric potentials (Table S2),sustained higher pressure for much longer time than the shrublandsoils. For both soils, the air-dry aggregates warmed up much morerapidly than the rest of the aggregates, due to the lower heat capacity.For the wet aggregates (�6 kPa) of the forest and shrubland soils,approximately 60% and 40%, respectively, of the total energy input fromthe heat source were dissipated by vaporization. While for the air-dryaggregates only ~8% of the energy was spent in vaporization of theminiscule residual water content. Energy partitioning calculations arereported in S2.

The duration of sustained high pneumatic pressure is an important fac-tor that influences the extent to which the bonds that hold aggregatestogether will be strained. Viscoplastic behavior is a good surrogate forprocesses with time-dependent ductile or transient deformation. Theestimated range of strain due to viscoplastic deformation while thepneumatic pressure inside the aggregates exceeds the yield stress isshown for the forest soil aggregates in Figure 4. Recall that while soilsdry, both the pneumatic pressure and the yield strength increase.Because the yield stress increase is very rapid and eventually exceedsthe pneumatic pressure, the window during which viscoplastic

Figure 3. Measured peak internal pressure, tensile strength, and estimated yield stress of (a) forest and (b) shrubland soilover multiple matric potentials. Averages and standard error bars were calculated based on 7 to 20 replicates for thepeak pneumatic pressure measurements and 3 to 11 replicates for the tensile strength measurements. Yield stress wasestimated based on rheological properties of Millville silt loam soil (Ghezzehei & Or, 2001).

Figure 4. The range of predicted viscous strain experienced by forest soilaggregates due to pneumatic gas pressure increase during the heatingexperiment. The pneumatic pressure in shrubland soils (not shown) didexceed the yield stress at any time, so no viscous strain was expected.

10.1029/2018GL078053Geophysical Research Letters

JIAN ET AL. 5

deformation is possible is rather narrow. In addition, increase in viscosity during drying reduces the tendencyof soil aggregates to deform in ductile fashion. As shown in Figure 3, the pneumatic pressure due to rapidlyvaporizing pore water is substantially lower than the tensile-strength for both soils. Therefore, brittle failureunder low-severity burns in unlikely.

Similar calculations for the shrubland soil aggregates revealed that the internal pressure never exceededthe yield stress; hence, the soils were not prone to viscous deformation. This is to be expected as thewater holding capacity of the shrubland soils is very low.

4. Discussions and Implications4.1. Micromechanical Experiment and Modeling of Aggregate Degradation

Our tensile strength data are comparable to previously reported measurements of aggregate strength by dif-ferent methods (Causarano, 1993; Dexter & Kroesbergen, 1985). For example, Munkholm et al. (2002)reported tensile strength of 7 kPa for large sandy loam soil aggregates (8–16 mm) at matric potential of�30 kPa, which is comparable to the 2.85 kPa tensile strength of the forest soil aggregates at the samematricpotential in our study. The results of our tensile strength test confirm that moist soil aggregates are inherentlyweaker than dry soil aggregates and can possibly break down when subjected to pneumatic gas pressureincrease from rapidly vaporized soil moisture.

Although the two soils exhibited markedly different water retention characteristics (see Figure S4), the mag-nitudes and trends of peak pressure for both soils were remarkably similar (Figure 3). It seems, therefore, thatthe size of water filled pores, not the volumetric water content, plays more important role in restricting thatmaximum escape route for water vapor. However, the role of initial volumetric water content was clearly evi-dent in the temporal evolution of pneumatic pressure and surface temperature (Figure 3). The duration ofelevated pressure and the time to peak pressure and peak temperature were longer for the forest soil, whichhas higher water retention capacity at all matric potentials, than the shrubland soil. Note that the period ofpeak pressure could be briefer if the duration of an actual fire is shorter.

Initial water content influences structural stability of soil aggregates during low-severity burns in three ways:(a) Higher initial water content results in higher pneumatic pressure that is sustained for a longer period oftime; (b) aggregate wetness reduces the yield stress—threshold stress for onset of viscoplastic, ductile defor-mation (Ghezzehei & Or, 2001); and (c) wetter aggregates exhibit lower tensile-strength—minimum stressrequired for tensile rupture. It is important to note that the yield stress is much lower than the tensilestrength. Therefore, initially wet aggregates are more likely to experience viscoplastic strain than tensile rup-ture. Overall initially moist soil aggregates are more susceptible to degradation by rapidly vaporized porewater as shown by Albalasmeh et al. (2013).

Our results shown in Figure 3 confirm that pneumatic gas pressure is significantly higher for initially moist soilaggregates than initially dry soil aggregates. Conversely, the tensile strength of soil aggregates increases assoil aggregates dry down. Albalasmeh et al. (2013) predicted that that rapid vaporization of soil moistureproduced sufficient pneumatic pore pressure at matric potential as low as �300 kPa to induce measurablydecreased soil aggregate stability. In this study we showed that pneumatic pressure produced in the rangeof �6 to �100 kPa was sufficient to weaken the soil aggregates.

We assume that wet aggregates behave as Bingham viscoplastic material (Barre & Hallett, 2009; Ghezzehei &Or, 2000; Markgraf et al., 2006). As soils dry, loss of plasticity gives way to brittle failure (Snyder & Miller, 1989).The transition from viscoplastic rheology to brittle behavior and its dependence on soil texture, organic mat-ter content, and initial structure remains poorly understood. Here we make a conservative assumption thatviscoplastic deformation is possible only if the applied stress (pneumatic pressure) exceeds the measuredtensile strength.

At stresses lower than the tensile strength of the aggregates, moist aggregates may undergo viscoplasticdeformation that can trigger microscopic degradation with long-term ramifications to aggregate stability(see Figure 4). However, the predicted viscous strain for the shrubland soil aggregates was zero. These find-ings suggest that susceptibility of aggregate disruption is determined by the balance between the pneumaticpressure and the yield stress during the early stages of burn, both of which are predicated upon the initialwater content. It is important to note that the modeling exercise is a semiquantitative indication of the

10.1029/2018GL078053Geophysical Research Letters

JIAN ET AL. 6

deformation of the soil aggregates, as the model parameters used for predicting viscoplastic strain are basedon fitting the model parameters to previous literature values in Ghezzehei and Or (2001). Nonetheless, thestrain caused by the stress will most likely still be evident, but with varying degree depending on the rheo-logical properties of the soil.

We acknowledge that there are other physical factors that may contribute to micromechanical breakdownof bonds between soil particles that were not addressed in this article. Some of these factors include desic-cation of soil organic matter and other binding agents, thermal expansion of soil pore water, and differentialthermal expansion of mineral constituents. All of these can create localized stresses at the particle-to-particle contact points, which can lead to failure of aggregate bonds. However, it may be a combinationof all of these factors and the pore pressure rise within the aggregate that contribute to micromechanicalbreakdown of aggregates. In a prior study (Albalasmeh et al., 2013), we compared the stability of soil aggre-gates subjected to rapid and slow heating (up to identical maximum temperatures) and concluded thatrapid heating caused significantly higher deterioration. Because the secondary factors listed above werepresent in both the rapid and slow heating experiments, we conclude that rapid pressure rise must play adominant role in disaggregation. Nevertheless, we recommend future studies to test for the combinationof some or all of the listed factors.

4.2. Implications of Low Intensity Burns on Ecosystem Processes and Human Society

Controlled burns are generally low in intensity and severity to minimize the negative effects on soils and eco-system processes. This is typically done by conducting the burns in conditions when soil moisture levels aremoderate to high (Carter & Foster, 2004; Certini, 2005; USDA, 2012). However, the experimental results sug-gest that it is best to conduct controlled burns in drier soil moisture conditions to minimize soil structuraldegradation from pneumatic pore pressure from rapidly vaporized soil moisture.

This underappreciated mechanism of soil aggregate degradation can have significant repercussions onecosystem processes that may be overlooked. Aggregates that are weakened by pneumatic pressure duringlow-severity burns will likely be more prone to disaggregating factors such as slaking, raindrop impact, andsurface runoff. For example, the loss of soil structure through disaggregation and collapse of large pores canresult in decrease in total porosity and increase in bulk density (Chief et al., 2012). Additionally, the sealing ofmacropores following burns can result in decrease in hydraulic conductivity and infiltration (Scott & Burgy,1956). This postburn soil sealing can further be aggravated by breakdown of already weakened soil aggre-gates due to raindrop impacts that result in soil surface compaction (Hoogmoed & Stroosnijder, 1984;Poesen & Savat, 1981). The weakening and breakdown of aggregates can also play a significant role in soilorganic matter decomposition, as soil organic matter decomposition is primarily controlled by physical acces-sibility of organic matter to decomposers (Gaillard et al., 1999; Schmidt et al., 2011). Besides ecosystem pro-cesses, such mechanisms of soil disaggregation can also have significant effects on human society. Forexample, the decreased infiltrability and postfire sealing can result in higher soil erosion rate in the burnedlandscape. This increase in erosion can be of concern because of potential decrease in site productivityand adverse effects on downstream resources such as water quality and transported sediment (Benavides-Solorio & MacDonald, 2005). Soil aggregate degradation by this mechanism can also potentially occur inslash-and-burn agriculture, which is practiced by many rural and indigenous communities in developingcountries (Kleinman et al., 1995), thereby adversely impacting moisture and nutrient retention in agriculturalsoils (Horn & Smucker, 2005).

Improved understanding of how low intensity fires alter soil aggregation and the associated landscape andecosystem processes is crucial in designing protocols of prescribed burns that minimize negative effects tosoil aggregation. Furthermore, low-severity burns have been shown to account approximately half of thecombined wildfire and prescribed burn areas reported in the United States between 1984 and 2016(Eidenshink et al., 2007; MTBS, 2017). Therefore, soil alterations caused by low intensity burns should notbe treated as a negligible effect, and we recommend that more studies look into the effects of low intensityfires on soil alteration.

The MTBS data tend to favor canopy burn severity and may not accurately reflect soil burn severity. Thus,improved assessment of soil burn severity is needed to understand the full range of impact of the proposedmechanism in soil disaggregation. A recent catchment-scale fire experiment by Stoof et al. (2013) showed,

10.1029/2018GL078053Geophysical Research Letters

JIAN ET AL. 7

contrary to what is generally expected, soils to be cooler in areas with high fuel load and fire intensity andvice versa. The lower-than-expected soil temperature in high fuel load areas may be attributed to protec-tive effect of high soil moisture (Hartford & Frandsen, 1992; Hille & den Ouden, 2005; Stoof et al., 2013).Therefore, it is also possible that the proposed mechanism can play a role in high-intensity and high-moisture burn areas.

5. Conclusions

Both the forest and shrubland soil aggregates, pneumatic pressure increases due to trapped vaporized soilmoisture, were observed during the heating experiment. Moreover, a minimal amount of pressure increasewas observed for the air-dried soil aggregates, whereas maximum pressure observed increased with initialsoil moisture content. This suggests that vaporized moisture trapped within soil aggregates is the main causeof the pressure increase. This pressure increase can induce stresses within the soil aggregates that exceed thetensile strength of the aggregate and cause the aggregate to fall apart. Our study also shows that pressure-induced stresses smaller than the tensile strength of the aggregates may lead to aggregate deterioration overtime due to microscale changes within the aggregates. These aggregate breakdowns have implications forecosystem processes, such as decreases in hydraulic conductivity, infiltrability and erodibility, as well aschange in soil organic matter decomposition. These types of low-intensity fire effects will most likely be pro-minent in areas where controlled burns are conducted, since these burns are generally conducted during thewettest soil conditions. The results in this study suggest that it may be advantageous to conduct controlledburns during drier soil conditions. But it is important to weigh in the possibility of unintended fire spread ifcontrolled burn is performed under dry soil conditions.

ReferencesAlbalasmeh, A. A., Berli, M., Shafer, D. S., & Ghezzehei, T. A. (2013). Degradation of moist soil aggregates by rapid temperature rise under low

intensity fire. Plant and Soil, 362(1–2), 335–344. https://doi.org/10.1007/s11104-012-1408-zArcenegui, V., Mataix-Solera, J., Guerrero, C., Zornoza, R., Mataix-Beneyto, J., & García-Orenes, F. (2008). Immediate effects of wildfires on water

repellency and aggregate stability in Mediterranean calcareous soils. Catena, 74(3), 219–226. https://doi.org/10.1016/j.catena.2007.12.008Barre, P., & Hallett, P. D. (2009). Rheological stabilization of wet soils by model root and fungal exudates depends on clay mineralogy.

European Journal of Soil Science, 60(4), 525–538. https://doi.org/10.1111/j.1365-2389.2009.01151.xBenavides-Solorio, J. D. D., & MacDonald, L. H. (2005). Measurement and prediction of post-fire erosion at the hillslope scale, Colorado Front

Range. International Journal of Wildland Fire, 14(4), 457–474. https://doi.org/10.1071/WF05042Carroll, E. M., Miller, W. W., Johnson, D. W., Saito, L., Qualls, R. G., & Walker, R. F. (2007). Spatial analysis of a large magnitude erosion event

following a Sierran wildfire. Journal of Environmental Quality, 36(4), 1105–1111. https://doi.org/10.2134/jeq2006.0466Carter, M. C., & Foster, C. D. (2004). Prescribed burning and productivity in southern pine forests: A review. Forest Ecology and Management,

191(1-3), 93–109. https://doi.org/10.1016/j.foreco.2003.11.006Causarano, H. (1993). Factors affecting the tensile strength of soil aggregates. Soil & Tillage Research, 28(1), 15–25. https://doi.org/10.1016/

0167-1987(93)90052-QCertini, G. (2005). Effects of fire on properties of forest soils: A review. Oecologia, 143(1), 1–10. https://doi.org/10.1007/s00442-004-1788-8Chief, K., Young, M. H., & Shafer, D. S. (2012). Changes in soil structure and hydraulic properties in a wooded-shrubland ecosystem following a

prescribed fire. Soil Science Society of America Journal, 76(6), 1965–1977. https://doi.org/10.2136/sssaj2011.0072Debano, L. F. (2000). The role of fire and soil heating on water repellency in wildland environments: A review. Journal of Hydrology, 231–232,

195–206. https://doi.org/10.1016/S0022-1694(00)00194-3DeBano, L. F., Dunn, P. H., & Conrad, C. E. (1977). Fire’s effects on physical and chemical properties of chaparral soils. In Proceedings of the

Symposium on the Environmental Consequences of Fire and Fuel Management in Mediterranean Ecosystems (Vol. Environmen, pp. 65–74).Palo Alto, CA: Forest Service, USDA.

Dexter, A. R., & Kroesbergen, B. (1985). Methodology for determination of tensile strength of soil aggregates. Journal of AgriculturalEngineering Research, 31(2), 139–147. https://doi.org/10.1016/0021-8634(85)90066-6

Eidenshink, J., Schwind, B., Brewer, K., Zhu, Z., Quayle, B., & Howard, S. (2007). A project for monitoring trends in burn severity. Fire Ecology,3(1), 3–21. https://doi.org/10.4996/fireecology.0301003

Gaillard, V., Chenu, C., Recous, S., & Richard, G. (1999). Carbon, nitrogen and microbial gradients induced by plant residues decomposing insoil. European Journal of Soil Science, 50(4), 567–578. https://doi.org/10.1046/j.1365-2389.1999.00266.x

García-Corona, R., Benito, E., de Blas, E., & Varela, M. E. (2004). Effects of heating on some soil physical properties related to its hydrologicalbehaviour in two north-western Spanish soils. International Journal of Wildland Fire, 13(2), 195–199. https://doi.org/10.1071/WF03068

Ghezzehei, T. A., & Or, D. (2000). Dynamics of soil aggregate coalescence governed by capillary and rheological processes. Water ResourcesResearch, 36(2), 367–379. https://doi.org/10.1029/1999WR900316

Ghezzehei, T. A., & Or, D. (2001). Rheological properties of wet soils and clays under steady and oscillatory stresses. Soil Science Society ofAmerica Journal, 65, 624–637.

Hartford, R. A., & Frandsen, W. H. (1992). When it’s hot, it’s hot... or maybe it’s not! (surface flaming may not portend extensive soil heating).International Journal of Wildland Fire, 2(3), 139–144. https://doi.org/10.1071/WF9920139

Hille, M. G., & den Ouden, J. (2005). Fuel load, humus consumption and humus moisture dynamics in Central European Scots pine stands.International Journal of Wildland Fire, 14(2), 153–159. https://doi.org/10.1071/WF04026

Hoogmoed, W. B., & Stroosnijder, L. (1984). Crust formation on sandy soils in the Sahel I. Rainfall and infiltration. Soil and Tillage Research, 4(1),5–23. https://doi.org/10.1016/0167-1987 (84)90013-8

10.1029/2018GL078053Geophysical Research Letters

JIAN ET AL. 8

AcknowledgmentsThe authors acknowledge funding bythe National Science Foundation underawards EAR-1324919 and EAR-1324894.The authors thank Ammar Albalasmeh,Chelsea Arnold, and Rose Sillito for theirsupport with sample collection. Theauthors do not have any financial con-flict of interest that impacts this worknor do they have affiliations that may beperceived as having a conflict of interestwith respect to the results of this paper.Processed data tables and backgrounddata are provided in the supportinginformation. All data and accompanyingcode that were used to generate theresults may be accessed at https://doi.org/10.6084/m9.figshare.6349469.v1.

Horn, R., & Smucker, A. (2005). Structure formation and its consequences for gas and water transport in unsaturated arable and forest soils.Soil and Tillage Research, 82(1), 5–14. https://doi.org/10.1016/j.still.2005.01.002

Jordán, A., Zavala, L. M., Mataix-Solera, J., Nava, A. L., & Alanís, N. (2011). Effect of fire severity on water repellency and aggregate stability onMexican volcanic soils. Catena, 84(3), 136–147. https://doi.org/10.1016/j.catena.2010.10.007

Kavouras, I. G., Nikolich, G., Etyemezian, V., DuBois, D. W., King, J., & Shafer, D. (2012). In situ observations of soil minerals and organic matterin the early phases of prescribed fires. Journal of Geophysical Research, 117, D12313. https://doi.org/10.1029/2011JD017420

Keeley, J. E. (2009). Fire intensity, fire severity and burn severity: A brief review and suggested usage. International Journal of Wildland Fire,18(1), 116. https://doi.org/10.1071/WF07049

Kleinman, P. J. A., Pimentel, D., & Bryant, R. B. (1995). The ecological sustainability of slash-and-burn agriculture. Agriculture, Ecosystems andEnvironment, 52(2-3), 235–249. https://doi.org/10.1016/0167-8809(94)00531-I

Knicker, H. (2007). How does fire affect the nature and stability of soil organic nitrogen and carbon? A review. Biogeochemistry, 85(1), 91–118.https://doi.org/10.1007/s10533-007-9104-4

Markgraf, W., Horn, R., & Peth, S. (2006). An approach to rheometry in soil mechanics—Structural changes in bentonite, clayey and silty soils.Soil and Tillage Research, 91(1–2), 1–14. https://doi.org/10.1016/j.still.2006.01.007

Mataix-Solera, J., Cerdà, A., Arcenegui, V., Jordán, A., & Zavala, L. M. (2011). Fire effects on soil aggregation: A review. Earth-Science Reviews,109(1-2), 44–60. https://doi.org/10.1016/j.earscirev.2011.08.002.

Mataix-Solera, J., Gómez, I., Navarro-Pedreño, J., Guerrero, C., & Moral, R. (2002). Soil organic matter and aggregates affected by wildfire in aPinus halepensis forest in a Mediterranean environment. International Journal of Wildland Fire, 11(2), 107–114. https://doi.org/10.1071/WF02020

MTBS (2017). Monitoring trends in burn severity. Retrieved from http://www.mtbs.gov/Munkholm, L. J., Schjønning, P., Debosz, K., Jensen, H. E., & Christensen, B. T. (2002). Aggregate strength andmechanical behaviour of a sandy

loam soil under long-term fertilization treatments. European Journal of Soil Science, 53(1), 129–137. https://doi.org/10.1046/j.1365-2389.2002.00424.x

O’Dea, M. E. (2007). Fungal mitigation of soil erosion following burning in a semi-arid Arizona savanna. Geoderma, 138(1–2), 79–85. https://doi.org/10.1016/j.geoderma.2006.10.017

Poesen, J., & Savat, J. (1981). Detachment and transportation of loess sediments by rainfall splash. Catena, 8(1), 19–41. https://doi.org/10.1016/S0341-8162(81)80002-1

Schmidt, M. W. I., Torn, M. S., Abiven, S., Dittmar, T., Guggenberger, G., Janssens, I. A., et al. (2011). Persistence of soil organic matter as anecosystem property. Nature, 478(7367), 49–56. https://doi.org/10.1038/nature10386

Scott, V. H., & Burgy, R. H. (1956). Effects of heat and brush burning on the physical properties of certain upland soils that influenceinfiltration. Soil Science Society of America Journal, 82(1), 63–70. https://doi.org/10.1097/00010694-195607000-00008

Snyder, V. A., & Miller, R. D. (1989). Soil deformation and fracture under tensile forces. In W. E. Larson, G. R. Blake, R. R. Allmaras, W. B. Voorhees,& S. C. Gupta (Eds.), Mechanics and related processes in structured agricultural soils, NATO ASI Series (Series E: Applied Sciences) (Vol. 172,pp. 23–35). Dordrecht: Springer. https://doi.org/10.1007/978-94-009-2421-5_3

Stoof, C. R., Moore, D., Fernandes, P. M., Stoorvogel, J. J., Fernandes, R. E., Ferreira, A. J., & Ritsema, C. J. (2013). Hot fire, cool soil.Geophysical Research Letters, 40, 1534–1539. https://doi.org/10.1002/grl.50299

Úbeda, X., & Bernia, S. (2005). The effect of wildfire intensity on soil aggregate stability in the Cadiretes Massif, NE Spain, (May 2004), 37–45.Urbanek, E. (2013). Why are aggregates destroyed in low intensity fire? Plant and Soil, 362(1–2), 33–36. https://doi.org/10.1007/

s11104-012-1470-6USDA (2012). Introduction to prescribed fire in southern ecosystems. Southern Resesarch Station, (August).

10.1029/2018GL078053Geophysical Research Letters

JIAN ET AL. 9