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SOIL PHYSICS

Application of Thermal Analysis to Elucidate Water-Repellency Changes in Heated Soils

^a Dep. of Soil Science, Faculty of Natural Sciences, Comenius Univ., Mlynska dolina B-2, 842 15 Bratislava, Slovak Republic
^b Dep. of Geography, Univ. of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK
^c CSIRO Land & Water, GPO Box 1666, Canberra ATC 2601, Australia
^d Institute of Chemistry, Slovak Academy of Sciences, Dubravska cesta 9, 845 38 Bratislava, Slovak Republic
^e Institute of Landscape Ecology, Slovak Academy of Sciences, Stefanikova 3, 81499 Bratislava, Slovak Republic
^f GEA– Grupo de Edafología Ambiental, Dep. of Agrochemistry and Environment, Univ. Miguel Hernández, Avda de la Universidad s/n, E-03202 Elche, Alicante, Spain

^* Corresponding author (dlapa{at}fns.uniba.sk).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
It is well established that during wildfires, the volatilization and condensation of hydrophobic compounds, as well as the thermal energy input itself, can markedly change the wettability of soils. This study evaluated the effects of thermal energy input on soil water repellency of sandy forest soils from Slovakia and explored the processes underlying the changes observed using thermal analysis. Initial sample water drop penetration time values ranged from <1 to >43,200 s. Heating induced distinct increases in water repellency to >3600 s in most samples, with its elimination occurring at 175°C or higher. The thermal analysis allowed evaluation of the relationship between the destruction of soil water repellency and thermal changes affecting soil organic matter (SOM). Differences in the thermal resistance of soil water repellency correspond to the thermal stability of SOM. Kinetic analysis showed that water repellency elimination due to soil heating is linked with thermal decomposition of a more thermally labile pool of SOM. The results suggest that under nonisothermal conditions, the degree of SOM decomposition depends on both the soil temperature reached and the soil heating rate. The temperature at which a certain level of SOM decomposition is reached increases with increasing heating rate. Heating experiments and the kinetic evaluation of thermogravimetric data for isothermal conditions also demonstrated an exponential relationship between heating durations and temperature thresholds.

Abbreviations: DTA, differential thermal analysis • DTG, derivative thermogravimetric analysis • SOM, soil organic matter • TGA, thermogravimetric analysis • WDPT, water drop penetration time

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The effects of heating on the water repellency of soils have seen increased attention in recent years. DeBano and Krammes (1966) were among the first to report that exposure of soils to temperatures typically reached during wildfires can have a remarkable effect on their wettability, inducing, enhancing, or destroying water repellency depending on the temperatures reached. They noted that, depending on heating duration, heating to temperatures below 175°C caused little alteration in repellency, between 175 and 200°C repellency increased considerably, but it was destroyed between 280 and 400°C. Building on this landmark study, the effects of heating duration, post-heating equilibration times, O₂ availability, and the applicability of previous findings to soils from other environments have been examined in a range of studies. These studies broadly confirmed the above effects on repellency and the relevant temperature thresholds (e.g., Savage, 1974; Scholl, 1975; Robichaud and Hungerford, 2000; Doerr et al., 2004; Garcia-Corona et al., 2004; Bryant et al., 2005).

In these previous studies on thermal elimination of soil water repellency, however, a detailed investigation of thermal changes affecting soil organic matter (SOM) was lacking. Therefore an information gap exists on the change in SOM properties leading to water-repellency destruction. In the current study, we incorporated thermal analysis to provide such information on the SOM changes during soil heating. Thermal analysis is the study of the relationship between temperature and a change of sample's mass or heat flux. Previous studies not related to soil water repellency have shown that thermogravimetry allows SOM pools differing in thermal stability to be identified and quantified. It has been used to characterize whole soil (Siewert, 2004), chemical and physical soil organic matter fractions (Dell'Abate et al., 2002; Lopez-Capel et al., 2005), and composting processes (Dell'Abate et al., 2000).

Grisi et al. (1998) used differential thermal analysis and thermogravimetric analysis to quantify the differences in humification between soils. Their data indicated that the SOM can be considered to consist of two fractions having different thermal stabilities. The first more thermally labile fraction tends to decrease and the second to increase with greater humification. Thermal investigations by Schulten and Leinweber (1999) demonstrated that stabilization of SOM components is determined by their interaction with mineral particles. Dell'Abate et al. (2002) used thermal analysis to provide evidence of different levels of stabilization of humic substances. Dell'Abate et al. (2003) used thermal analysis to identify different thermal behaviors of two groups of samples in which the humic substances from tropical soils were characterized by organic fractions more thermally stable than the humic substances from temperate regions. Lopez-Capel et al. (2005) applied thermal analysis to different pools of SOM isolated using density fractionation. They attributed an exothermic weight loss between 300 and 350°C to a relatively labile portion comprising carboxylic and aliphatic compounds and weight loss between 400 and 450°C to the decomposition of material rich in aromatic components. Plante et al. (2005) used thermogravimetry to characterize changes in clay-associated organic matter quality in terms of thermal properties. They distinguished mass losses in the thermally labile (180–310°C) exothermic region and in the more thermally resistant (310–450°C) exothermic region. Based on the combination of thermal analysis with nuclear magnetic resonance analysis and pyrolysis, the initial weight loss, around 300°C, can be assigned to the exothermic decomposition of labile aliphatic and carboxylic groups while the exothermic decomposition of aromatic structures occurs at higher temperatures, around 450°C (Leinweber and Schulten, 1992; Lopez-Capel et al., 2005). Thermal analysis also provides a useful tool for the investigation of the thermal decomposition kinetics of organic substances (Galwey and Browne, 1998; Carrasco and Pages, 2004). The knowledge of SOM decomposition kinetics also allows elucidation of the specific temperature thresholds at which water repellency is eliminated from soils during heating.

In this study, we used thermal analysis (thermogravimetric analysis, TGA, differential thermal analysis, DTA, and derivative thermogravimetric analysis, DTG) as a tool for the investigation of thermal stability of soil water repellency in conjunction with heating experiments on sandy forest soils from Slovakia under a standard atmosphere. To our knowledge, this is the first application of thermal analysis to examine the processes involved in the changes to soil water repellency and SOM associated with various thermal energy inputs that could be expected during wildfires.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Site and Sample Characteristics
Samples were taken from soils under pine forests (Pinus sylvestris L.) in the northern part of the Borska Nizina lowland, the largest area of aeolian sand in Slovakia. The sampled sites have not experienced fire for at least 60 yr; however, forest fires in Slovakia are not infrequent. For example, between 1998 and 2004, 155 to 1056 forest fires per year have occurred (Osvald et al., 2005). Three different soil types, known to exhibit different levels of wettability when dry, were sampled. They were a Typic Ustipsamment, a Humic Dystrustept, and a Typic Psammaquent (Soil Survey Staff, 2006) developed in Pleistocene dune sands. Samples (2–3 kg each) were taken from different genetic horizons of each profile, air dried for 90 d, and passed through a 2-mm sieve before analysis to remove large plant residues.

Soil chemical properties and particle size distribution are listed in Table 1 . The aeolian sands consist mostly of fine sand with <15% silt and clay. They typically contain 87 to 89% quartz, up to 10% feldspars, and 1 to 3% heavy minerals (Pelisek, 1963). Organic C contents of 2.2, 2.4, and 0.9% are present in the A horizons (topsoils) of the Typic Ustipsamment, Humic Dystrustept, and Typic Psammaquent, respectively, decreasing to 0.23, 0.15, and 0.05%, respectively, in subsurface horizons. The occurrence of hydro- and hygrophilous plant species gives evidence of distinct differences in soil moisture regimes of the three selected soils. Relatively dry conditions occur in the whole profile of the Typic Ustipsamment, whereas the Typic Psammaquent is saturated due to a shallow groundwater table. For the Humic Dystrustept, the relatively deep humus horizon is typical for a profile affected by wet conditions in the past.

Soil water-repellency persistence was determined using the water drop penetration time (WDPT) test (Letey, 1969). Laboratory measurements were performed at 21 to 23°C and 50 to 60% relative humidity. Air-dried soil samples (10 g) were placed in petri dishes, three drops of distilled water (volume: 58 ± 5 µL) were applied from a medicinal dropper onto the sample surface, the petri dishes were covered to reduce evaporation, and the actual time required for complete droplet infiltration was recorded. Tests were terminated after 43,200 s (12 h). In subsequent processing, the average of three WDPT values was used. The following classes of the persistence of water repellency were distinguished: <5 s, wettable or not water repellent; 5 to 60 s, slightly water repellent; 60 to 600 s, strongly water repellent; 600 to 3600 s, severely water repellent, and >3600 s, extremely water repellent (Dekker and Ritsema, 1995).

For heat treatments, each 10-g sample (a 5-mm-thick soil layer in an open petri dish) was placed into an oven that had been preheated to the selected temperature. Samples were heated at temperatures of 125, 175, 200, 225, 250, and 275°C for durations from 8 min to 157 h, depending on the time required for water-repellency elimination at the selected temperature. Heated samples were equilibrated for 48 h (at 21–23°C and 50–60% relative humidity) before water-repellency measurement. Instead of establishing replicability of results by replication of individual heat treatments on the same subsamples, we chose instead to increase the reliability of any trend observed by maximizing the number of temperature and heating duration steps for each sample type accordingly. Thermal analysis was performed on samples from the Typic Ustipsamment, Humic Dystrustept, and Typic Psammaquent soil profiles, as these provided interesting contrasts with respect to moisture regime and soil genesis. Thermal analysis of soil samples (20 mg) was performed on a TGA–DTA system (SDT 2960, TA Instruments, New Castle, DE) with a 90 cm³ min^–1 standard air flow rate from 20 to 1000°C. Measurements were performed at five different heating rates: 2.5, 5, 7.5, 10, and 15°C min^–1. The results are presented as TGA (thermogravimetric analysis), DTA (differential thermal analysis), and DTG (derivative thermogravimetric analysis) curves. Similarly to Dell'Abate et al. (2000, 2002), Lopez-Capel et al. (2005), and Plante et al. (2005), we expressed total weight loss associated with thermal decomposition of organic matter as Exotot and the proportions of Exotot associated with the first and the second exotherms as Exo1 and Exo2, respectively.

Kinetic parameters were determined using the isoconversional method of Friedman, described in more detail in Friedman (1965) or Galwey and Browne (1998). The method is based on the following equations:

[1]

[2]

where t is the reaction time (min), is the SOM conversion degree, d/dt is the decomposition rate of SOM (min^–1), β is the heating rate, A is the pre-exponential factor (min^–1), E is the activation energy (J mol^–1), R is the gas constant (8.314 J mol^–1 K^–1), and T is the temperature (K). The pre-exponential factor (A) provides a measure of the frequency of the occurrence of a condition that may lead to reaction. The activation energy (E) is a measure of the energy barrier to reaction. The plot of ln(d/dt) against 1/T for different heating rates is linear with a slope of –E/R and an intercept of lnA + n ln(1 – ) for a given value of . Thermal decomposition of SOM is a complex process, for which it is difficult to define a simple reaction mechanism and to evaluate reaction order. We therefore assumed first-order kinetics because it is most commonly used in the kinetic data assessment. The pre-exponential factor, A, was calculated for the particular case of assuming first-order kinetics (n = 1). Considering SOM decomposition, the SOM conversion degree () is the fraction of Exotot that has reacted up to a defined moment and is related to the degree of reaction advancement. It was calculated as

[3]

where Exotot and Exo are the final and actual mass losses, respectively, associated with the exothermic reaction.

From the known kinetic parameters, a quantitative estimation of the thermal decomposition of SOM can be calculated. A first-order reaction is described by the equation

[4]

where k is the reaction rate constant at a given temperature. The reaction rate constant, k, is a function of the absolute temperature, T, according to equation

[5]

[6]

The same function was already included in Eq. [1] and [2] for SOM conversion rate. At fixed temperature, Eq. [4] can be integrated, giving

[7]

According to Eq. [7], the SOM conversion degree can be calculated as a function of heating duration at a given temperature.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Effects of Heating on Soil Water Repellency
Although there were large differences in the water repellency of the air-dried samples before heat treatments (WDPT <1 to >43,200 s; Table 1), the heating led to a marked increase in WDPT at 125 and 175°C (Fig. 1 ). A gradual increase in WDPT during heating at 125°C occurred in samples of the A horizons of the Typic Psammaquent and Humic Dystrustept, which were originally wettable and slightly water repellent, respectively. The third, the Typic Ustipsamment, was already extremely water repellent so that no further increase could be observed. For the other two samples, when heated to 175°C, a very rapid increase in soil water repellency was observed and WDPT values >3600 s (extremely water repellent) were reached after 10 min of heating in all samples, except in the Cg horizon of the Typic Psammaquent, which is probably due to the very low SOM content.

View larger version (11K): [in this window] [in a new window]   Fig. 1. The effects of heating duration at selected temperatures (125 and 175°C) on water drop penentration time (WDPT) in A horizons of a Humic Dystrustept and a Typic Psammaquent. Values for the WDPT were obtained as the mean of three determinations. The WDPTs in the A horizon of a Typic Ustipsamment are not included as they were >43,200 s before heating.

View larger version (14K): [in this window] [in a new window]   Fig. 2. Effect of heating temperature on heating duration necessary for water repellency elimination for samples from individual soil horizons of a Typic Ustipsamment, a Humic Dystrustept, and a Typic Psammaquent.

Relationship of Thermal Analysis Results to the Increase of Water Repellency
Thermal analysis results in standard air are given in Fig. 3 . The TGA mass loss curves of soil samples consist of several overlapping steps that correspond to physical and chemical processes. There are four characteristic peaks in the DTA curves (Table 3 ). For the first endotherm and the first and second exotherms, DTA maxima were very close to DTG peaks (Fig. 3).

View larger version (12K): [in this window] [in a new window]   Fig. 3. Results of thermal analysis in standard air for A horizons of the Typic Ustipsamment, Humic Dystrustept, and Typic Psammaquent. Plotted curves correspond to thermogravimetric analysis (TGA), derivative thermogravimetric analysis (DTG), and differential thermal analysis (DTA) at a heating rate of 10°C min–1.

The observed effect of an increase in soil water repellency for heating at lower temperatures is commonly explained by the idea that the amphiphilic organic substances responsible for soil water repellency may change orientation and align more strongly (e.g., Ma'shum and Farmer, 1985). Heating of soils may also allow more complex processes to take place, such as enhanced migration, redistribution, or structural change of organic substances (Doerr et al., 2000; Franco et al., 2000), glass transition behavior (Schaumann and Antelmann, 2000; DeLapp and LeBoeuf, 2004; Schaumann and LeBoeuf, 2005), or even esterification (Todoruk et al., 2003). Water repellency increased by these processes, however, can be preserved during heating only when the temperature of SOM decomposition is not reached. Based on the TGA results, such a condition is fulfilled when the heating temperature is below 200°C for short to medium heating durations. The processes occurring during heating below 100°C, including loss of water (Fig. 3), are therefore worthy of further study as they may play a crucial role in water repellency development or increase.

Results of Thermal Analysis Related to the Elimination of Water Repellency
In the temperature region where water repellency elimination becomes dominant, processes of SOM decomposition take place: above 200°C, a steep loss of mass is evident in the TGA diagrams and two exotherms in the DTA diagrams were observed (Fig. 3). The two exothermic reactions are thought to result from the thermal oxidative decomposition of two groups of SOM components with distinct thermal stability. For the five heating rates used in the thermal analysis, the temperature ranges for two DTA peaks were 282 to 336 and 328 to 365°C for the Typic Ustipsamment, 295 to 342 and 341 to 389°C for the Humic Dystrustept, and 296 to 337 and 384 to 419°C for the Typic Psammaquent, respectively (Table 3). The peak temperatures generally increased with increasing heating rate (2.5–15°C min^–1) during thermal analysis. For the first exotherm temperatures, a significant difference was observed (P = 0.019) only between the Typic Ustipsamment and the Humic Dystrustept. But all differences were significant for the second exotherm temperatures (P < 0.001). The obtained results are typical for thermal analysis of SOM. Temperature ranges for exothermic reactions correspond well to those reported in the literature (Leinweber and Schulten, 1992; Lopez-Capel et al., 2005). The weight loss associated with the first exotherm is commonly associated with the volatilization of labile aliphatic and carboxylic groups as derived from pyrolysis and nuclear magnetic resonance studies (Schulten and Leinweber, 1999; Almendros et al., 2003; Lopez-Capel et al., 2005). The second exotherm is attributed to aromatic structure decomposition at higher temperatures. Additionally, there was a small sharp endotherm at 569°C (Table 3). This endothermic peak with no weight loss can be attributed to the inversion from low to high quartz (Smykatz-Kloss and Klinke, 1997).

In the approach used in this work, we separately evaluated weight losses associated with the first and second exotherms as Exo1 and Exo2, respectively. Based on Lopez-Capel et al. (2005), we attribute Exo1 to aliphatic and carboxyl C and Exo2 to aromatic C content. The TGA data (Table 3) show that SOM from the Typic Psammaquent was characterized by the dominance of the less thermally stable fraction (Exo1), giving the first exothermic peak, while in the Typic Ustipsamment and Humic Dystrustept, the contents of the Exo1 and Exo2 fractions were approximately equal (average Exo1/Exo2 ratios 1.23 and 0.95, respectively). Differences in Exo1/Exo2 ratios between three samples were significant (P < 0.001). Differences in Exo1/Exo2 ratios also indicate significant differences in SOM humification (Grisi et al., 1998) among the three soils, and it is the lowest in the Typic Psammaquent.

We can investigate the thermal stability of SOM directly according to the temperature from the TGA results at which 50% of SOM conversion was reached. In Table 4 , temperatures at which 50% of the Exotot mass was decomposed are listed for each heating rate. For an individual soil, the isoconversional temperature increased with increasing heating rate applied in thermal analysis (Table 4). For A horizons, thermal stabilities of the soils examined increased in the order Typic Ustipsamment < Typic Psammaquent < Humic Dystrustept, but only the differences related to the Typic Ustipsamment were significant (P < 0.05; Table 5 ). Such results indicate that SOM components in the Typic Ustipsamment are thermally more labile than those in the Humic Dystrustept and Typic Psammaquent. Among other factors (such as mineralogy, exchangeable cations present, etc.), at least the relation of the specific mineral surface to total C content is also a reason for site-specific adsorption, which may have a significant impact on the thermal stability. The increase in thermal stability in the A horizons of the Typic Psammaquent and Humic Dystrustept can be attributed to adsorption of organic matter onto the surfaces of mineral particles, which appears to be mediated through temporary anaerobic conditions (Dell'Abate et al., 2002).

Kinetics of Soil Organic Matter Decomposition and their Relationship with Heating Duration and Temperature Thresholds
To examine the thermal decomposition kinetics for the A horizons of the soils, the logarithms of conversion rate vs. the reciprocal value of temperature at different heating rates (2.5, 7.5, and 15°C min^–1) were plotted for the A horizons of the three soils (Fig. 4 ). Organic matter content for zero conversion degree was taken to be equal to the Exotot. Dashed lines interpolate five points (determined for five heating rates) with the same conversion degree of SOM (0.1, 0.3, 0.5, 0.7, and 0.9). From the results (Fig. 4), implications arise for nonisothermal conditions, where a change in soil temperature proceeds at a variable heating rate. Such nonisothermal conditions are common under field conditions where the soil is heated due to biomass burning. The slope of the dashed lines implies that the temperature at which a certain SOM conversion degree is reached increases with heating rate. Thus, under nonisothermal conditions, the SOM decomposition level depends both on the soil temperature reached and on the soil heating rate. This conclusion agrees with experimentally determined values in Table 4. From Fig. 4 it also follows that the rate of organic matter decomposition increases with increasing heating rate and vice versa under nonisothermal conditions.

View larger version (11K): [in this window] [in a new window]   Fig. 4. Logarithm of the conversion rate of soil organic matter (d/dt) vs. the reciprocal value of temperature (T) at different heating rates (2.5, 7.5, and 15°C min–1) for A horizons of the three soils. Dashed lines interpolate points with the same conversion degree of soil organic matter (0.1, 0.3, 0.5, 0.7 and 0.9).

View larger version (13K): [in this window] [in a new window]   Fig. 5. Activation energies (E) and logarithms of the pre-exponential factor (lnA) at selected degrees of soil organic matter (SOM) conversion () as determined by the Friedman method for A horizons of the three soils. The pre-exponential factor was determined assuming first-order kinetics.

View larger version (12K): [in this window] [in a new window]   Fig. 6. Calculated heating durations necessary to destroy 50% of soil organic matter (conversion = 0.5) as a function of temperature.

View larger version (10K): [in this window] [in a new window]   Fig. 7. Calculated remainder of soil organic matter (SOM) at the time of water-repellency destruction as a function of heating temperature.

View larger version (9K): [in this window] [in a new window]   Fig. 8. Calculated conversions () of soil organic matter at the time of water-repellency destruction as a function of heating temperature.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

Thermal analysis indicates that water-repellency elimination is associated with thermal decomposition of a more labile pool of SOM. The thermal resistance of soil water repellency corresponds to an experimentally determined thermal stability of SOM. The kinetic analysis suggests that the temperature required to achieve a given level of SOM decomposition under nonisothermal conditions increases with heating rate. Thus the SOM decomposition level depends on both the soil temperature reached and on the soil heating rate. This theoretical assumption should be considered when evaluating the impact of fire on SOM decomposition under field conditions where the soil temperature gradually increases at variable heating rates.

The observed relationship between heating temperature causing water-repellency elimination and SOM decomposition kinetics derived from thermal analysis indicates that, under isothermal conditions, the heating duration required for repellency elimination depends exponentially on heating temperature. Thus, a thorough evaluation of heating effects on the water repellency of soils (or other materials) requires temperature thresholds to be related to the selected heating duration and vice versa. Although, the soil samples were derived from sites under the same vegetation (Pinus sylvestris), the thermal analysis results demonstrate a different resistance of the SOM to thermal degradation present between samples from three genetically different soil profiles.

	ACKNOWLEDGMENTS

 
The financial support from the Science and Technology Assistance Agency Project no. APVT-51-017804, the Slovak Scientific Grant Agency Project no. 1/3058/06, the Centre of Excellence for Degradation of Biopolymers of the Slovak Academy of Sciences, the CICYT co-financed FEDER project (REN2003-08424-C02-01/GLO), NERC (Adv. Fellowship NER/J/S/2002/00662 and Grant NE/C003985/1), and the Generalitat Valenciana project (GV05/018) is gratefully acknowledged.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
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Received for publication August 13, 2006.

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