Effects of wildfire and laboratory heating on soil aggregate stability of pine forests in Galicia: The role of lithology, soil organic matter content and water repellency

Catena 83 (2010) 127–134

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Catena j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t e n a

Effects of wildfire and laboratory heating on soil aggregate stability of pine forests in Galicia: The role of lithology, soil organic matter content and water repellency M.E. Varela a,b,⁎, E. Benito a, J.J. Keizer b a b

Departamento de Biología Vegetal y Ciencias del Suelo, Facultad de Biología, Universidad de Vigo, Campus Lagoas-Marcosende, 36310 Vigo, Spain Centre for Environmental and Marine Studies (CESAM), Department of Environmental and Planning, University of Aveiro, 3810-193 Aveiro, Portugal

a r t i c l e

i n f o

Article history: Received 3 June 2009 Received in revised form 22 July 2010 Accepted 12 August 2010 Keywords: Wildfire Aggregate stability Organic carbon Water repellency Laboratory heating experiments

a b s t r a c t The present work aims to assess the effects of wildfires on soil aggregate stability and the role therein of contrasting lithologies as well as of fire-induced changes in organic matter content and soil water repellency. To this end, a pair-wise comparison of neighbouring burned and unburned soils was carried out and complemented by laboratory heating experiments to clarify the role of fire intensity. In total, 18 pairs of adjacent burned and unburned pine forest soils were sampled within one month after wildfire. At each site, five samples were collected of the top 5 cm of the A horizon at randomly selected sample points and were mixed in the field to obtain one composite sample per site. Three additional samples were collected at each site but stored separately, and those of three sites were selected for the laboratory heating experiments. Laboratory heating involved five different temperatures ranging from 25 to 460 °C. Aggregate stability of the field and laboratory samples was determined using the water drop impact test, organic carbon content using a modified Sauerlandt method and soil water repellency using the ‘Molarity of an Ethanol Droplet’ test. The wildfire effects on field aggregate stability were highly variable and results indicated that these changes depend primarily on organic matter combustion and, thus, fire intensity. Controlled heating up to 220 °C either did not alter aggregate stability or increased it with increasing temperature, possibly due to the development of a protective coating of organic compounds inducing water repellency. Heating at 380 and 460 °C, by contrast, produced considerable to massive combustion of organic matter and, thereby, very pronounced reduction of aggregate stability as well as water repellency. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The Spanish region of Galicia, located in the NW corner of the Iberian Peninsula, is one of the most densely forested regions in Europe, reflecting the region's favourable, humid temperate climate conditions. Afforestation is widely regarded in Galicia as an important measure of soil protection, since the risk of soil erosion is high in many rural areas due to the pronounced terrain relief in combination with the elevated rainfall erosivity (Díaz-Fierros et al., 1987). During the past few decades, fast-growing tree species (predominantly pines and eucalypt) have been planted as a massive scale. This has, however, contributed to a strong increase in the occurrence of wildfires, especially in the last few years. According to the wildfire statistics for Spain for the period 1991–2006, approximately half of the wildfires occurred in Galicia, whilst Galicia only corresponds to in an area 16 % of the national forest area. These wildfires consumed a total area of ⁎ Corresponding author. Departamento de Biología Vegetal y Ciencias del Suelo, Facultad de Biología, Universidad de Vigo, Campus Lagoas-Marcosende, 36310 Vigo, Spain. Tel.: + 34 986 812561; fax: +34 986 812556. E-mail address: [email protected] (M.E. Varela). 0341-8162/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2010.08.001

501,184 ha in Galicia. August 2006 was especially dramatic, with many wildfires closely approaching urban centres and with roughly 75,000 ha burnt by almost 2000 wildfires in no more than 12 days. One of the most serious consequences of this ecological disaster was the subsequent soil erosion in these burned areas and the resulting off-site damages caused by runoff and transported sediments in downstream terrestrial and aquatic ecosystems (Carballas, 2007). This was caused by the torrential rainfall events that occurred at the end of the summer of 2006, in combination with the lack of immediate soil conservation measures. In unburned soils, aggregate stability is one of the factors influencing infiltration capacity and susceptibility to erosion. The occurrence of a wildfire, however, through the removal of the protective vegetation and litter cover in particular turns aggregate stability into a key consideration in post-fire measures of water, soil and nutrient conservation (Mataix-Solera and Guerrero, 2007). The effect of fire on soil aggregate stability is nonetheless far from clear. Authors like Ibáñez et al. (1983), Boix Fayos (1997) and Arcenegui et al. (2008) claim that burning enhances structural stability, whereas for example Giovannini and Lucchesi (1997), García-Oliva et al. (1999) and Mataix-Solera et al. (2002) reported the opposite.

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Fire-induced combustion of soil organic matter can be expected to induce the partial or complete destruction of soil aggregates. Namely, whilst soil organic matter plays a marked role in the formation of aggregates in many soils (Oades, 1993; Roldán et al., 1994), it is the prevalent aggregation factor in the forest soils of Galicia (Díaz-Fierros et al., 1994; Varela, 2007). However, fire does not necessarily reduce topsoil organic matter content in a significant manner and lowintensity fires have even been reported to increase organic matter content (Mataix-Solera et al., 1996; Giovannini and Lucchesi, 1997; Guerrero et al., 2001). Such a fire-induced increase in organic matter content could explain why some fires are exceptional in enhancing aggregate stability rather than decreasing it (Mataix-Solera and Guerrero, 2007). An alternative explanation for fire increasing aggregate stability is through its impact on soil water repellency (Shakesby and Doerr, 2006; Varela et al., 2005; Keizer et al., 2008). Hydrophobic substances can form a thin film that partially or entirely covers aggregates and, thereby, increases their resistance to disaggregating (slaking) when wetted (Chenu et al., 2000; Mataix-Solera and Doerr, 2004; García-Corona et al., 2004; Goebel et al., 2005; Arcenegui et al., 2008). Differences in fire intensity are likely to be of crucial importance in the contrasting findings by earlier studies on the impacts of wildfire on aggregate stability, notwithstanding the well-established fact that aggregation also depends on soil type and soil physico-chemical properties like, for example, clay and calcium carbonate content (Mataix-Solera and Cerdá, 2009) . The intensity of a wildfire, however, is not easily assessed. There exist a variety of fire intensity indicators, for example related to the colour of the ashes or the consumption of woody plan parts (e.g. Shakesby and Doerr, 2006), but their relationship with the heating regime of the soil itself is tentative at best. Therefore, various prior studies have resorted to controlled heating experiments in the laboratory to study the influence of temperature on soil properties (Giovannini et al., 1988; Soto et al., 1991; García-Corona et al., 2004). The main aim of the present work is to determine the direct impacts of wildfires on the aggregate stability of forest soils and to clarify the role therein of fire-induced changes in soil organic matter content and water repellency. To this end, a two-fold approach is employed. Wildfire effects are studied by means of a paired sampling strategy comparing recently burned and long unburned soils at similar neighbouring sites. Heating effects are addressed by means of controlled laboratory experiments, and compared with the wildfire effects to shed further light on the importance therein of differences in fire severity. This study concerns pine forest plantations for being one of the most prevalent and, at the same time, most fire-prone forest types in Galicia. Special attention is given to comparing the region's two predominant lithologies. 2. Materials and methods 2.1. Study area and site characteristics The study area is located in the southern part of Galicia, NW Spain (Fig. 1). The climate of this temperate–humid Atlantic zone has a pronounced oceanic character. Mean annual rainfall is high (rainfall is about 1400 mm), and has a marked seasonal pattern, of rainy autumns and winters. Summer droughts can be pronounced with water balance deficits up to 400 mm. The temperature regime is that of temperate areas, with mean annual temperatures ranging from 8 to 15 °C. At present, forests constitute the predominant land cover in Galicia, covering roughly 70%. About 35% of this area is dominated by needle-leaved species, mainly Pinus pinaster Ait. Towards the end of the summers of 1999 and 2000, a total of 18 sites were selected where pine forest had burned by wildfire that same summer. A crucial first criterion in the selection of these sites was the existence of nearby pine forest sites that were long unburned

but comparable in terms of parent material, slope aspect and angle, and vegetation understory. Furthermore, site selection was restricted to the two lithologies that clearly prevail in Galicia, i.e. granites and low-grade metamorphic rocks (especially slates and schists). These two types of parent material are associated with soils of clearly distinct textures, i.e. sandy-loam soils overlying granites and loamy or silty-loam soils overlying slates and schists. According to Díaz-Fierros and Benito (1991), this contrast in texture is accompanied by differences in aggregate stability, at least in long unburned soils. Therefore, this study assesses if also the fire-induced changes in aggregate stability differ for both lithologies. Unfortunately, it proved impossible to achieve an entirely balanced sampling of granitesversus slates/schists-overlying soils. Table 1 gives a general description of the 18 study locations, each comprising a pair of comparable burned and unburned pine forest plantations. The soils in the region of Galicia studied here are typically Leptosols, Regosols and Umbrisols (WRB, 2006). The soils at locations G1 to G12 overlie granite and affine parent material and, as expected, have a coarser texture than the soils at locations S1 to S6 which are developed on metamorphic rocks. At the majority of locations, the soil is extremely acidic and has a low effective cation exchange capacity. The understory vegetation of the unburned sites appeared to be similar at the various study locations, with Pteridium aquilinum, Ulex europeus and Erica spp. being predominant floristic elements. Fire severity indicators were not described in detail but the prevalence of black ashes and the partial consumption of the ground vegetation suggested that fire severity had been low to moderate at all study sites (e.g. Shakesby and Doerr, 2006; Keizer et al., 2008). 2.2. Field sampling Field sample collection was carried out as soon as possible after the wildfires but always within one month's time. This was done to avoid as much as possible the occurrence of significant rainfall between the wildfire and the sampling. Unfortunately, however, we are unable to corroborate this for the individual sites with quantitative rainfall information, since the available data from neighbouring meteorological stations concern monthly rainfall and monthly maximum daily rainfall. Although no noteworthy phenomena of splash erosion or surface wash/deposition were observed during field sampling, differences in post-fire rainfall can play a somewhat confounding role in between-site differences reported here. At each of the 36 study sites, a representative sample of the top 5 cm of the A horizon was obtained as a composite of five or more subsamples collected at randomly selected spots. Any litter or ashes were removed prior to the actual sampling. Three study sites were selected for the controlled laboratory heating experiments. Two of these sites (G2 and G3) concern granitetype parent material and the remaining one (S3) metamorphic parent material. This selection aimed first and foremost at contrasting differences in the observed responses to wildfire, as is further detailed in the initial part of section on laboratory heating results. The heating experiment was limited to three sites as a trade-off against replicate tests of the various heating regimes for each soil. Three replicate tests were carried out as a minimum number for statistical testing. The three samples of the three selected sites were collected during the two above-mentioned sampling campaigns of late summer 1999 and 2000. This was done at three additional, randomly selected sampling spots at each site, and involved the collection of the top 5 cm of the A horizon after removal of any litter and/or ashes. 2.3. Laboratory analyses Aggregate stability was measured using the ‘Water Drop Impact’ (WDI) test of Low (1954), which simulates the impact of water drops on aggregates. From each composite or individual soil sample,

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Fig. 1. Map of the 18 study locations in the southern part of Galicia, NW Spain.

twenty-five aggregates of 4–5 mm in diameter were selected. Each individual aggregate was then subjected to the impact of up to 200 water drops of 0.1 g that were released from a height of 1 m. The results are expressed as the percentage of aggregates surviving the impact of 200 water drops (Imeson and Vis, 1984). Water repellency severity was determined using the ‘Molarity of an Ethanol Droplet’ (MED) test (e.g. King, 1981; Doerr, 1998). The test was applied to the fraction smaller than 2 mm obtained by sieving. Following air-drying of the samples, the test was carried out by applying droplets of increasing ethanol concentrations until infiltration of the droplets occurred within 10 s. Test results are given in ethanol concentrations (vol.%). The concentrations used in this study are given in Table 2, together with the corresponding ethanol classes and repellency severity ratings.

The particle size distribution of the samples was determined using the pipette method, whereas their organic carbon content was measured using the Sauerlandt method as modified by Guitián and Carballas (1976). 2.4. Laboratory heating experiments The laboratory heating experiments were carried out in a muffle furnace equipped with a timer and heating rate control, with a thermocouple being placed in the samples to control their temperatures. The experiments were performed on the fraction smaller than 10 mm. The sample material was put in ceramic crucibles with a diameter of 155 mm and a height of 30 mm as a layer with a constant thickness of 1 cm.

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Table 1 Sites, parent material, slope angle and general characteristics of 18 soils examined. Site Parent code material

Slope (% sand, silt, clay) angle (%) Texture

pH

CECe (cmol(+) kg− 1)

G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G12 S1 S2 S3 S4 S5 S6

30 31 20 45 40 27 32 30 40 45 35 n.d. 14 12 24 35 25 22

4.19 4.16 3.91 4.09 4.56 4.18 4.93 4.06 3.99 4.13 4.24 4.48 3.92 4.25 4.36 5.40 6.38 6.45

5.61 6.21 6.07 3.51 4.33 6.88 2.42 5.39 6.48 7.78 4.50 5.31 4.65 5.00 4.83 5.48 9.15 4.84

Granite Granodiorite Granite Granite Gneis Granite Granodiorite Granite Granite Granite Granite Granodiorite Schist Schist Schist Slate Slate Slate

(66, (74, (60, (76, (56, (61, (65, (72, (68, (69, (71, (59, (40, (50, (36, (45, (45, (40,

22, 12) Sandy loam 17, 9) Sandy loam 23, 17) Sandy loam 15, 9) Sandy loam 29, 15) Sandy loam 25, 14) Sandy loam 20, 15) Sandy loam 20, 8) Sandy loam 19, 12) Sandy loam 19, 12) Sandy loam 17, 12) Sandy loam 28, 13) Sandy loam 49, 11) Loam 40, 10) Loam 51, 13) Silt loam 39, 16) Loam 37, 18) Loam 38, 22) Loam

Table 3 Aggregate stability, organic carbon content and water repellency of the unburned (UB) and burned (B) soils at the 18 study locations. Site code

G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G12 S1 S2 S3 S4 S5 S6

n.d.: not determined.

Five heating temperatures were selected for this study, entailing maximum temperatures of 25, 170, 220, 380 and 460 °C. They correspond to the temperatures that produce the most typical thermal reactions of soils, as evidenced by the differential thermal analyses of Giovannini et al. (1988) and Soto et al. (1991). The heating rate was set at 3 °C min− 1 to prevent sudden combustion when the soil's ignition temperature is being reached (Fernández et al., 1997). Once the pre-established maximum temperature was reached, samples were kept at this temperature for a period of 30 min. This period was chosen based on the review paper by Giovannini (1994), in which this period emerges as standard procedure in laboratory heating experiments. Following an equilibration period of two days under constant laboratory conditions of 20 °C and 50% relative air humidity (as suggested by Doerr et al., 2002), the samples were analysed with respect to aggregate stability, organic carbon content and water repellency as described in the preceding section. 2.5. Data analysis The role of the two lithologies in the aggregate stability of the unburned soils was assessed using the Mann–Whitney U-Test. The relationship with the other soil parameters included in this study was determined using Spearman's rank correlation coefficient. The effect of wildfire on soil aggregate stability as well as on organic carbon content and water repellency was assessed using the Wilcoxon Signed Ranks Test. This test is based on direct comparison of the neighbouring pairs of unburned and recently burned sites. The pair-wise differences in aggregate stability, organic carbon content and water repellency between neighbouring sites were further compared using Spearman's rank correlation coefficient. The laboratory heating results were analysed using parametric statistics, i.e. one-way ANOVA and Pearson's product-moment correlation coefficient. Namely, the Kolmogorov–Smirnov and Levene Tests indicated that the conditions of normality and homoscedasticity, could be assumed.

MED (% ethanol)

Stable aggregates (%)

Organic Carbon (g kg− 1)

UB

B

UB

B

UB

B

28 52 69 60 72 20 88 45 52 64 44 60 8 24 31 92 36 76

38 52 16 46 90 56 48 36 76 52 38 64 32 18 48 96 48 78

34.6 53.1 101.9 102.7 62.9 76.0 48.4 103.7 106.0 115.0 82.3 48.5 50.3 80.6 87.7 55.2 53.0 65.4

71.3 59.5 57.2 98.2 49.8 73.2 56.1 66.0 81.2 63.5 79.0 75.5 112.5 73.2 115.0 58.7 65.4 63.2

13.4 19.1 21.8 18.7 14.0 16.3 10.5 21.0 23.3 21.6 18.7 15.8 1.7 21.6 10.9 5.8 2.3 8.2

15.8 19.3 5.8 20.4 12.3 15.8 17.5 22.8 19.3 16.3 21.0 18.1 15.2 5.3 17.5 19.3 14.0 9.3

The software package SPSS 17.0 was used for the various statistical analyses, and testing was done against a significance level α of 0.01 and/or 0.05.

3. Results and discussion 3.1. Unburned soils The main topsoil characteristics at the unburned sites are given in Table 3. As expected, the soils overlying granite at sites G1 to G12 are coarser than those overlying metamorphic rocks at sites S1 to S6. The former have a sandy-loam texture, the latter a loam or silt loam texture. This contrast is accompanied by marked and statistically significant differences in the sand and silt fractions but not the clay fractions. The sand fraction is significantly higher at sites G1 to G12 than sites S1 to S6 (Mann–Whitney U-Test: U = 0.0; p b 0.01), median values being 67 and 24 %. The opposite is true for the silt fraction (Mann–Whitney U-Test: U = 0.0; p b 0.01), median values being 20 and 40%. The above-mentioned textural differences between the two lithologies are not associated with a significant difference in aggregate stability of the unburned soils (Mann–Whitney U-Test: U = 28.0; p = 0.49), i.e. contrary to what was expected. Nonetheless, median aggregate stability is clearly lower for the coarser soils at sites G1 to G12 (56%) than for the finer-textured soil at sites S1 to S6 (33%). This agrees with the well-known fact that soils with high contents of lime and fine sand are most susceptible to disaggregation by raindrop impact (Morgan, 2005). That the marked difference in median aggregate stability lacks statistical significance can be attributed to the elevated between-site variability. This especially applies to the soils overlying metamorphic rocks, as their inter-quartile range of aggregate stability is twice that of the soils overlying granite (S1–S6: 20%; G1–G12: 40%).

Table 2 Resume of the volumetric ethanol percentage concentrations used in the ‘Molarity of an Ethanol Droplet’ (MED) tests, and corresponding ethanol classes and water repellency severity rating (based on King, 1981; Doerr, 1998). Ethanol concentrations (%)

0

3

5

8.5

13

24

36

Ethanol classes Repellency Severity rating

1 Hydrophilic Very

2

3 Hydrophobic Slight

4

5

6

7

Moderately

Strongly

Very strongly

Extremely

Normal

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Table 3 also presents the results obtained for the recently burned topsoils. The overall effect of the different wildfires on aggregate stability is minor as the median value for the 18 recently burned soils (48%) is practically the same as that for the 18 unburned soils (52%). At the level of the individual study locations, however, aggregate stability can be strongly modified by the passage of fire and, furthermore, in opposite manners. The observed differences between the neighbouring unburned and burned soils range from a supposed increase in aggregate stability of 36% in the case of site G6 to an apparent decrease of 53% in the case of site G3. As mentioned before, prior studies have equally reported fire-induced increases (Ibáñez et al., 1983; Boix Fayos, 1997; Arcenegui et al., 2008) as well as decreases (Giovannini and Lucchesi, 1997; García-Oliva et al., 1999; Mataix-Solera et al., 2002). The 18 pairs of neighbouring unburned and recently burned soils do not differ significantly in aggregate stability (Wilcoxon Signed Ranks Test: Z = 0.47; p = 0.64). This is also easily perceived from Fig. 2, where seven pairs of adjacent soils are depicted below the 1:1 line and ten above it. The graph provides some suggestion that the effect of wildfire on aggregate stability depends on parent material. Namely, from the six study locations on metamorphic rocks (S1–S6)

five involve a higher aggregate stability for the burned than unburned soil. Also, median aggregate stability at these sites is notably higher following wildfire than before (48 vs. 33%). This tendency is not statistically significant (Wilcoxon Signed Ranks Test: Z = 1.57; p = 0.12) but that is hardly surprising since the number of samples is limited. By contrast, the locations overlying granites (G1–G12) reveal basically the same numbers of fire-induced increases and decreases in aggregate stability (6 vs. 5). Furthermore, their median aggregate stability decreases with fire but only marginally from 56 to 50%. The immediate fire-induced changes in topsoil organic carbon content and water repellency are shown in Figs. 3 and 4, respectively. The results obtained for both parameters bear considerable resemblance to the above-mentioned results of aggregate stability. First, the median values for the 18 burned and unburned soils differ little, i.e. 70.7 and 68.6 g kg− 1 organic carbon, and 16.0 and 16.9% ethanol related to water repellency, respectively. Second, the wildfires at the produce widely different changes at the individual study locations, ranging from decreases of 51.6 g kg− 1 organic carbon and 16.3% ethanol values related to water repellency, to increases of 62.2 g kg− 1 organic carbon and 13.4% ethanol related to water repellency. Third, the neighbouring burned and unburned soils do not differ statistically in organic carbon content or water repellency (Wilcoxon Signed Ranks Test: Z = 0.07; p = 0.95; Z = 1.13; p = 0.26, respectively). Fourth, there exist contrasting tendencies for median and individual site values of carbon content and water repellency to increase at study locations S1 to S6 and to decrease at study locations G1 to G12. Spearman's rank correlation coefficient was used to quantify how well the fire-induced changes in aggregate stability can be explained by the concurrent changes in organic carbon content or soil water repellency. This was not only done for the absolute differences but also for the relative differences (in % of the unburned value). Differences in aggregate stability between the pairs of burned and unburned soils are more closely associated with differences in organic carbon content than with differences in ethanol concentration related to water repellency. Furthermore, relative differences in aggregate stability can be explained better than absolute differences. Only one of these four correlations is significantly different from zero, namely that relating the relative changes in aggregate stability and organic carbon

Fig. 2. Comparison of the percentages of stable aggregate for 18 pairs of neighbouring long unburned and recently burned pine forest soils.

Fig. 3. Comparison of the topsoil organic carbon content (g kg− 1) for 18 pairs of neighbouring long unburned and recently burned pine forest soils.

The between-site variability in aggregate stability of the unburned soils cannot be explained very well by any of the other soil parameters included in this study. Clay content reveals the closest association with aggregates stability but the corresponding Spearman's rank correlation coefficient of 0.45 does not differ significantly from zero, albeit only marginally (p = 0.06). The relationship of aggregate stability with organic carbon content is remarkably poor (Spearman's rho = 0.05; p = 0.85), given the important role that is generally attributed to organic matter in aggregation of Galician forest soils (Díaz-Fierros et al., 1994). Also water repellency appears to be of practically no importance for the aggregate stability of the unburned soils (Spearman's rho = −0.01; p = 0.95). This clearly deviates from the positive correlation that aggregate stability is commonly found to have with water repellency (Chenu et al., 2000; Hallett et al., 2001; Mataix-Solera and Doerr, 2004; Arcenegui et al., 2008). 3.2. Immediate wildfire effects

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M.E. Varela et al. / Catena 83 (2010) 127–134 Table 4 Percentage of stable aggregates, organic carbon content (g kg− 1) and water repellency (% ethanol) of soil samples subjected to various temperatures. Values are mean ± s.d. Within each column, the mean values designated with different letters in superscript are significantly different at α b 0.05. Ta Stable aggregates

Organic carbon

Water repellency

25 °C 170 °C 220 °C 380 °C 460 °C 25 °C 170 °C 220 °C 380 °C 460 °C 25 °C 170 °C 220 °C 380 °C 460 °C

G2

G3 a

52 ± 4 89 ± 5b 84 ± 14b 8 ± 8c 1 ± 2c 53.1 ± 6.6a 48.3 ± 10.4a 52.3 ± 9.8a 23.8 ± 1.9b 4.5 ± 3.2c 19.1 ± 2.3a 20.8 ± 1.6a 23.5 ± 0.8b 0.0 ± 0.0c 0.0 ± 0.0c

S3 a

69 ± 13 52 ± 7ab 64 ± 24a 19 ± 18b n.d. 101.9 ± 3.2a 103.1 ± 15.6a 104.5 ± 11.1a 25.5 ± 5.0b 5.9 ± 1.1c 21.8 ± 1.5a 21.0 ± 1.3a 23.0 ± 3.1a 0.0 ± 0.0b 0.0 ± 0.0b

31 ± 12a 47 ± 19ab 56 ± 4b 9 ± 10c 0 ± 0c 87.7 ± 17.5a 86.5 ± 18.8a 72.6 ± 9.2a 32.3 ± 2.5b 15.0 ± 5.8b 10.9 ± 2.0a 21.4 ± 0.8b 22.2 ± 0.9b 0.0 ± 0.0c 0.0 ± 0.0c

n.d.: not determined, i.e. no aggregates of 4–5 mm diameter were present following heating at 460 °C in any of the three samples.

Fig. 4. Comparison of the topsoil water repellency (% ethanol) for 18 pairs of neighbouring long unburned and recently burned pine forest soils.

content (Spearman's rho = 0.53; p = 0.02). Nonetheless, Spearman's rho for the absolute changes in aggregate stability and organic carbon content is not widely different (rho = 0.42; p = 0.08), and the same is true for Spearman's rho for the relative changes in aggregate stability and water repellency (rho = 0.40; p = 0.10). The fact that relative changes in aggregate stability are somewhat easier explained than absolute changes could suggest that the effect of wildfire depends in part on the degree of aggregation prior to the burning. The significant correlation between pre- and post-fire aggregation stability points in that same direction (Spearman's rho = 0.51; p = 0.03). 3.3. Effects of laboratory soil heating Three sites were selected for the laboratory heating experiments to represent contrasting responses to wildfire, in particular of soils with representative levels of aggregate stability. Pre-fire aggregate stability at sites G2 (52%) and G3 (69%) is similar to the median value for the unburned soils overlying granite and affine materials (56%). The same is true, mutatis mutandis, for site S3 (31%) and the unburned soils overlying metamorphic rocks (34%). The wildfire-induced changes in aggregate stability at these three study locations encompass: (i) a decrease of the pre-fire value with about 75% in the case of G3; (ii) no detectable change in the case of G2; (iii) an increase with nearly 55% in the case of S3. The changes in aggregate stability at G3 and S3 are paralleled by marked changes in organic carbon content as well as water repellency. In the case of G3, organic carbon content and water repellency decrease with 45 and 75%, respectively; in the case of S3, they increase with 30 and 60%, respectively. In the case of G2, water repellency remains basically unaltered with the passage of the wildfire, whilst organic carbon content increases only slightly (about 10%). Table 4 resumes the results of aggregate stability, organic carbon content and soil water repellency for the individual study locations as obtained by laboratory heating of three unburned soil samples at the five different temperatures. The significant differences amongst the five temperatures according to one-way ANOVA are also indicated. There exists a clear agreement amongst the three sites in that heating at temperatures of 380 °C and 460 °C substantially and significantly decreases aggregate stability compared to heating at

lower temperatures. The results for the three sites also coincide in that heating at 460 °C basically destroys all stable aggregates and in the case of G3 even all aggregates. By contrast, the results obtained at 170 °C and 220 °C suggest a discrepancy between sites G2 and S3, on the one hand, and, on the other, site G3. Whilst aggregate stability is significantly higher following heating at 170 and/or 220 °C than at 25 °C in the case of G2 and S3, in the case of G3 it is either significantly lower or not significantly different. This discrepancy in controlled heating results agrees well with the results of the field data, equally suggesting a greater susceptibility to wildfire-induced reduction in aggregate stability in the case of G3 than G2 and S3. Heating-induced increases in aggregate stability were reported earlier for Galician forest soils, albeit controlled heating was done at somewhat different temperatures (Soto et al., 1991). The heating-induced changes in organic carbon content are analogous to those in aggregate stability in two respects. First, organic carbon contents also markedly and significantly reduced by heating at the two highest temperatures compared to at the three lowest temperatures. Second, organic carbon content is noticeably, albeit not necessarily significantly, lower following heating at 460 than 380 °C. The results for organic carbon content diverge from those for aggregate stability in that the three lowest heating temperatures not produce significant differences for any of the sites. Furthermore, in the instances of sites G2 and S3 organic carbon content reveals a tendency to decrease with heating at 170 and 220 °C compared to at 25 °C, whilst the opposite is true for aggregate stability. The minor increases in organic carbon content in the case of site G3 must be attributed to sample heterogeneity and/or measurement errors. Namely, there are no additional sources of organic carbon in the heating experiments, contrary to under field conditions where lowintensity fires have been found to increase topsoil organic carbon contents (Úbeda, 2001; Knicker et al., 2006; Varela et al., 2010). The laboratory heating results for water repellency agree well with those for aggregate stability as well as organic carbon content. Namely, heating at 380 and 460 °C produces a marked and significant decrease compared to heating at 25, 170 and 220 °C, turning the soil fully wettable. The agreement with the aggregate stability results extends to the above-mentioned contrast between sites G2 and S3, on the one hand, and, on the other, site G3. Compared to heating at 25 °C, heating at 170 and/or 220 °C increases water repellency in a statistically significant manner in the cases of G2 and S3 but not in that of G3. Nonetheless, the differences in water repellency between these three heating temperatures tend to be minor compared to the differences in aggregate stability. There is one noticeable exception,

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though, with heating at 170 °C roughly doubling the % ethanol related to water repellency following heating 25 °C in the case of S3. This distinct response of the soil of site S3 fits in well with the abovementioned differences between the three sites with respect to the wildfire-induced repellency changes as suggested by the field data. The present heating results for water repellency are also in good agreement with those from prior studies. In general, heating at temperatures below 175 °C is found to leave water repellency unaltered, whereas heating at temperatures between 175 and 250 °C is reported to increase it and heating at higher temperatures to decrease and ultimately destroy it (Savage, 1974; DeBano et al., 1976; DeBano, 1981; Varela et al., 2005; 2010). The changes in aggregate stability produced by the different heating temperatures can be accounted for very well by the corresponding changes in organic carbon content (Table 5). They can, however, be explained about equally well by the heating-induced alterations in water repellency, reflecting a strong inter-correlation between the changes in both explanatory variables. The Pearson correlation coefficients tend to be highest for the separate sites but do not decrease substantially when the three sites are analysed together. This attests to the general rather than site-specific nature of the relationships emerging from the heating experiments. Furthermore, the above does not only apply to the relative changes in the three soil parameters but also to their absolute changes compared to the results of heating at 25 °C. The corresponding Pearson coefficients are basically the same as those given in Table 5. 3.4. Comparison of the effects of wildfires and laboratory heating Comparison of recently burned soils with neighbouring unburned soils suggests that the passage of a wildfire in itself does not produce clear-cut effects on aggregate stability of pine forest soils. Fireinduced increases and decreases occur in roughly similar proportions, and the extent of these changes varies widely. Whilst a highly variable response to wildfire also emerges from prior field studies (e.g. DíazFierros et al., 1994; Mataix-Solera and Guerrero, 2007), this variability can to a large extent be attributed to differences in soil heating and, thus, wildfire intensity. Namely, for three soils with contrasting responses to wildfire the heating experiments invariably resulted in massive destruction of the stable aggregates at the highest temperatures. Furthermore, heating at the intermediate temperatures significantly enhanced aggregation of two of these three soils, i.e. producing the opposite changes to heating at the highest temperatures. At the same time, however, the lack of a significant increase in aggregate stability in the case of the third soil indicates that, besides fire intensity, also other factors can play a relevant role in the response to less extreme heating in particular. This argument is reinforced by the consistency of the laboratory results with the field observations, suggesting a reduction in aggregate stability by wildfire in the case of this third soil as opposed to an increase or lack of change in the case of the other two soils. Whilst the importance of other factors is also indicated by other findings of this study, further research is needed to untangle the relative importance of wildfire severity and factors like, Table 5 Pearson correlation coefficients (*p b 0.05; **p b 0.01) between the heating-induced relative changes (“delta”) in aggregate stability, organic carbon and MED for the three sites separately as well as together. The deltas are computed in relation to the values following heating at 25 °C.

Delta stable aggregates (%) vs. delta organic C (g kg− 1) Delta estable aggregates (%) vs. delta MED (% ethanol) Delta organic C (g kg− 1) vs. delta MED (% ethanol)

G2

G3

S3

all sites

0.90**

0.92**

0.91**

0.81**

0.96**

0.82**

0.86**

0.84**

0.91**

0.93**

0.87**

0.78**

133

for example, pre-fire aggregate stability, textural composition and soil type. A crucial aspect would be to obtain quantitative field estimates of wildfire intensity, with Near-Infrared spectroscopy of burned soils offering especially promising perspectives (Lewis et al., 2006; Guerrero et al., 2007). The laboratory heating experiments revealed, as expected based on earlier field and laboratory studies (e.g. Díaz-Fierros et al., 1994; Roldán et al., 1994; Amézketa, 1999; Neary et al., 1999), a close relationship of aggregate stability with soil organic matter or, to be more exact, between the heating-induced changes in these two parameters. The field data, however, evidenced a clearly less pronounced—albeit still statistically significant—association. This difference between laboratory and field results can involve various factors that limit the comparability of the laboratory experiments with wildfires. First, the laboratory experiments do not mimic the increase in organic carbon content that especially low-intensity fires are known to produce (Úbeda, 2001; Knicker et al., 2006). Second, the field results concern considerably more study sites and are thus likely to suffer a greater influence of other factors than heating intensity. This effect is also suggested by the laboratory results themselves, i.e. by the lowering of the correlation coefficient when the results for the three soils are analysed together instead of separately. Third, very few if any of the wildfires appear to have involved soil heating at temperatures producing such pronounced effects on aggregate stability as well as organic carbon content as the 380 °C and especially 460 °C of the most extreme heating experiments. The laboratory results of these experiments, however, have a marked influence on the correlations presented in Table 5. Eliminating two out of every three 380 °C experiments and all 460 °C experiments, for example, lowers the correlation coefficient for the three sites together to 0.50, i.e. basically the same value as reported for the field data. The significant increases in aggregate stability observed following moderate heating in the laboratory deserve special mention, since they cannot be explained by changes in organic carbon content. They could, however, be due to alterations in the composition of the soil organic matter. According to Almendros et al. (1984), soil organic matter reaches its maximum stability and maturity between 100 and 160 °C. A possible way in which these compositional changes could affect aggregate stability is through water repellency and, in particular, its increase by moderate heating as reported here and in various prior studies (e.g. DeBano, 1981; Varela et al., 2010). The role of repellency is supposedly an indirect one, with a thin film of hydrophobic substances providing a protective cover of aggregates against the direct impact of water drops (i.e. the principle of the WDI test applied here) and/or against disaggregation by wetting (e.g. García-Corona et al., 2004; Goebel et al., 2005; Arcenegui et al., 2008). Although the close association between aggregate stability and water repellency emerging from the present laboratory experiments agrees well with the findings of other studies (Chenu et al., 2000; Hallett et al., 2001; Mataix-Solera and Doerr, 2004), it would deserve further research attention. For example, it remains unclear how the widely different increases in water repellency in the cases of sites G2 and S3 can equally bring about significant increased in aggregate stability. 4. Conclusions The main conclusions of this combined field and laboratory study on the effects of wildfire and controlled heating on aggregate stability of pine forest soils Galicia, NW Spain, are as follows: (i) aggregate stability of long unburned soils is highly variable and, unlike expected, lacks a clear relationship with the underlying lithology; (ii) wildfire-induced changes in aggregate stability are highly variable, as well as extent, and this can partly be attributed to the above-mentioned variability in the pre-fire values;

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