Arid Laod Rescarch and Management 17: 2b1,2003 C o p m t Q 2W3 Taylor k Fraocis 15324982101 S12.W + .o0 DOI: lo. l080/15324980390169W6

Plant Ash and Heat Intensity Effects on Chemical and Physical Properties of Two Contrasting Soils

DAVID BADÍA CLARA MARTÍ Department of Agriculture Escuela Politécnica Superior Huesca, Spain

Fire p w g e rs accompanied by a heaf wave and ash deposifion afjecfing the upper soil layer. Changes in soii properfies are directly related fo heaf intmsity, the amomis of ashes deposifed, and soil type. We subjecfed two soils (gypsiferous soil and caicareous soil) lo artificial heafing and ash incorporation and compared changes in selecf chemical and physical properties. The two soils srudied were se- lected to provide a wide range of charaeferisfics in soils of the semiarid Ebro Volley (NE Spain). Samples of both soüs were heated for 30 minutes in a m f j e furnnce at temperatures of 25'. l 5P . 25P and 5OPC. Ashes were added only on soil samples heated al 25PC. in a quantity related fo plant b w m s growing on each soil (twice the amount in calcareous soil than in gypsiferous soil). Increasing heat iniensity increased organic matter combustion as well as nufrienf availability. Heating soil lo 250°C cawed a decrease m pH and un increase in elecfrolytic conducfivfty (ECe) and soluble Ca. Heating soil lo 5OO"C c m e d an increase in pH and a decrease in ECe and soIuble Ca. Totd N confent decreased at femperafures greater fhan 25PC. with abart one-fhird being volatilized. Changes in chernicai properties were similar for both soils although quanfitalive diiferences befween soíis were found. Catzon exchange capacify (CEC) was reduced for gypsiferous soil heated lo 5OO"C and fo 25PC for caicareous soii. Heafing increcrred sand-sized particles by f i w n of clay, greafest in soil heated to 50PC. Soil aggregate sfabdity (SAS) of bofh soils wcrr reduced by heating fo 250°C wifh greater reductions at 5 W C , likely due to a re- ducfion in organic matter and clay size particle conteni. A negative correlatwn was observed mong SAS and soil erodibiiity (K-USLE). Bulk density and parficle densify increased in bofh soils when heated to 500°C. Water availabilify increased when soils were heated to 5OO"C. likely due lo texture andsfructural modificafions. Addifwn of the &S increased organic maffer contenf, C/N ratio, andpH in bofh soils and increased nutrient availability. These responses were greater in caicareous than in gypsiferous soil. Physical soil properties were not significanfly modifed by ash additian.

Keywords calcareous soil, fire effects, gypsiferous soil, NE. Spain, short-km effects, shrubland

Received S February 2002; accepted 26 April 2002. This research was part of the project on the Effects of WildFires in Semiarid Ecosystems

(CONSI+D P-48/96), supported by the Aragon govemment. K. Hackett is thanked for English review.

Address correspondence to Dr. D. Badia, Department of Agriculture, Escuela Politécniw. Superior, Crtra. Cuarte s/n, 22071 Huesca, Spain. E-mail: badia@posta.unizar.es

D. Budía and C. Martí

Fire is an important factor affecting the stabitity of various ecosystems. Wild6res in forest systems, including prescribeti buming for reducing the ñre hazard, affect extensive areas of semiarid lands. In agropastoral systems, the burning of shmblands is used to manipulate the vegetative cover and composition so as to favor grazing. In agricultura1 systems ñre is used to reduce weeds and to facilitate seedbed preparation (Chandler et al., 1983). The semiand Mediterranean clirnate of cool winter rains and hot, dry surnrners resulis in rapid fue1 build-up, and high fke hazard in the evergreen sclerophyiious shrubs of the Central Ebro VaUey OiCastri et al., 1981).

Although some beneficia1 responses can be obtained from soil surface burning, many burned soils appear profoundly eroded and degraded. During and immedi- ately after a wildfhe the soil environment is affected by inputs of heat and ash that make it difñcult to dist$guish the causes of chanp in soil propertie-s. Moreover, &re seventy affects both the quality and quantity of ash (Marion et al., 1991). In semiarid Mediterranean climates the soil ecosystem and plant productivity exhibit high spatial and temporal variabitity (DiCastri et al., 1981). For these reasons coníücting results have been reported concerning the effects of fue on semiarid ecosystems (Antolín et al., 1997; Debano & Conrad, 1978; Ferran et al., 1991, Ibaüez et al., 1983, Lázzari et al., 1993; Marcos et al., 1998; Martinez & Diaz, 1994; Rashid, 1987).

The objective of the present research was to determine the effects of heating intensity and ash addition on physical and chemical propertie-s of two contrasting soii types (gypsiferous soii and calcareous soii) in the semiarid Central Ebro Vaiiey (NE Spain). Another article in this issue documents changes in their microbiological properties (Badia & Marti, 2003).

Materials and Methods

Two soils were selected to provide a wide range of characteristics in soils of the semiarid Ebro Valley, northeastern Spain. Soils were classified at the subgroup leve1 (SSS, 1999) as Xeric Haplogypsid, developed on gypsum (gypsiferous soil) and Xeric Tomorthent, developed on marls (calcareous soil). Both soils sarnpled were at an elevation of about 200rn at 41'30' E and 0°18'N. This region has a semiarid Mediterranean climate, with an average annual rainfall ca 350mm and a mean annual temperature of 15°C. The potential annual evaporative demand, estirnated by the Blaney and Criddle method, is ca 1300mm. The soil temperature regime is therrnic, and the soil moisture is aridic ranging to xeric. Pertinent soil formation factors are shown in Table 1. More information about the landscape and the soils has been reported previously (Badia, 1989; Badía & Marti, 2000). On the calcareous soil, dominant plants were Quercus coccfera L., Rharnnus lycioides L, Cistus albidus L., Cktus clusii Dunal, Brachypodium retusurn (Pers) Beauv., and Ephedra nebrodensis Tineo ex Guss. On the gypsiferous soil dominant vegetation consisted of: Cistus clusii Dunal, Ononis rridentafa L., Helianthemum syrianun (Jacq) Dum-Cours, Sfipa offneri Breist, Rosmarinus oficinalis L., and Herniaria fruticosa L.

Fow samples of each soil were collected randomly and analyzed separately. Samples (O-15cm) were air-dried at 25°C and passed through a 2-mm sieve. Samples were then heated for 30min in a mutñe furnace at 2S0, 150°, 250" and 500°C. These temperatures cover the range that is Iiabitually reached in surface soils affected by

26 D. Badia and C. Martí

fires (Chandler et al., 1983; GiovaMini & Lucchesi, 1997; Walker et al., 1986). Soil sample treated at 250eC was selected for adding black ashes. The ashes were denved from fresh plants of Cktus clusii, collected from each soil and burned separately in a fireplace. The quantity of ashes added was related to plant biomass of surface unit present on each soil (Martí, 1998). The amount of ashes added to calcareous soil was twice (10 g kg-') that of gypsiferous soil (5 g kg-').

Soil Analysis

Soil pH was determined potentiometrically in a 1:2.5 ratio in H20 and KCI (McLean, 1982). Total carbonate content was measured volumet~cally (calcimeter) after treating with HCI (Nelson, 1982). Total soil organic C was determined by the method of Walkey & Black (1934), avoiding C1- interferences with 2% AgS04 as needed (MAPA, 1994); organic matter was estimated by using the van Bemmelen factor (1.724). Total N content was determined by Kjeldahl procedure (Bremner & Mulvaney, 1982). Total S was measured by elementary analysis foilowing acid digestion (Tabatabai, 1982). Available P (P-Olsen) was detennined by 0.5 M Na- bicarbonate extraction at a nearly constant pH of 8.5 (Olsen & Sommers, 1982). Cation exchange capacity (CEC) was determined by N ' + retention after leaching with a neutral solution of 1 N NH40Ac (Rhoades, 1982a); exchangeable K+ was detennined in the CEC extract (Thomas, 1982). Soil salinity was evaluated by measuring the electrolytic conductivity ( E a ) of the saturation paste extract at 25OC. The soluble Na+, K+, ca2 + and + were measured in the saturation paste extract (Rhoades, 1982b). Color (value and chroma) was measured by using Munsell tables.

Particle size distnbution was determined by the pipette method using hexam- ethaphosphate as a dispersing agent, with silty and clay fractions being determined after sieving to remove sand size particles (Gee & Bauder, 1986). Soil aggregate stability was assayed by wet sieving (Kemper & Koch, 1966). Soil bulk density was measured with aggregates of 2mm size introduced into a probette and shaken (Gandoy, 1990). Soil particle density was measured by capillary pycnometer method with alcohol (Blake & Hartge, 1986). Soil porosity was calculated using bulk and particle densities. Soil erodibility was evaluated measuring K-USLE (in Mg h MJ-' m - ' ) , according to Wischmeier & Smith (1978). Water available at permanent wilting point (- 1.5 MPa) and field capacity (-0.033 MPa) was measured by volu- metric pressure plate extractor (Richards, 1947).

The differences between mean values of the soil properties were statisticaliy eval- uated by an ANOVA test (Least Significance Differences test; P <0.01) using the StatView statistical package (Abacus Concepts Inc., Berkeley, Cal'ifornia). Regres- sions and correlations between soil properties were also determined (Spearman rank correlation test; P < 0.05 and P < 0.01). A hierarchical cluster analysis using average linkage between groups was made using SPSS program (SPSS, 1997).

Results and Discussion

Soil Reaction (pH) and Gwbonates

Heating the soils had a variable effect on pH (Table 2). Soil heating to 250°C caused a significant decrease in pH in both soils (about 0.6 pH.units). The causes of this

initial decrease upon heating are not well understood. It has been suggested that this decrease could be a result of the oxidation of certain elements, the exposure of new surfaces, the dehydration of coiioids and the consequent decrease of the soil buffer action (Giovannini et al., 1990). in an oak forest in Pisa, Italy, on a Lithic Xer- ochrept, Giovannini & Lucchesi (1997) found similar results. In a semiand area of scrub afforested with Aleppo pine in southern Alacant, Spain, soil pH decreased significantly one day after a simulated fire of low intensity (Mangas et al., 1992). The pH increased greatly at the highest temperature of 5M°C with both soils (1.2 for calcareous soil and 1.0 unit for gypsiferous soii above the original pH value). Increases of a unit in the pH value have been reported for different soils: Petrocalcic Calciustoll (Lázzari et al., 1993), Typic Xerochrept (González et al., 1992), Dystric Xerochrept (Marcos et al., 1998). An increase of soil pH following wildfire is a consistent effect occurring as a result of cation release by the combustion of organic matter and deposition of ashes, particularly in poorly buffered soils (Chandler et al., 1983). The sharp increase in pH at high temperatures may be due to the loss of the hydroxyl groups from the clays but not to the fomation of oxides of several ele- ments derived from the disruption of the carbonates as considered by Giovannini et al. (1990). In our study, total carbonate content was not affected significantly by heating, although a progressive increase was observed. Almendros et al. (1984), at laboratory-controlled conditions, observed a progressive increase of pH from 5.8 at 100°C to 9.5 at 650°C. Also Kutiel & Shaviv (1992) in a study of soil combustion conducted in the laboratory, showed a significant pH increase with heating on a calcareous Lithic Xerochrept under the Aleppo pine forest of Mt. Carmel, Israel. They observed a pH change from 7.6 in the unburned soil to 8.3 in the soil burned at 250°C and to 11.7 in the soil burned at 600°C. In the same experiment Kutiel & Shaviv (1992) reported different responses with different soils: in a highly calcareous Lithic Xerochrept undw Aleppo pine forest the pH at 250°C increased significantly but in a Rhodoxeralf, under an oak shrubland, the pH showed only a slight change. Rashid (1987), in an acid soil under oak forest in the Atlas mountains, found that a wildfire provoked a sharp rise in soil pH in upper layers. By contrast, a sharp fall in soil pH was observed at a depth of 5-15cm in the burned soil where temperatura were lower than upper layers.

Ash addition increased pH but not the total carbonate content in soils. Low carbonate content in plant ashes (Raison & McGarity, 1980) and the high carbonate content in both experimental soils provided a nonsignifxant increase in total carbonates.

Orgairic Matter and @tal N

Soil heating to 150°C did not modify significantly soil organic matter content (Table 2). Heating to 250°C reduced soil organic matter content in calcareous soil but not in gypsiferous soil. At a temperature of 250°C both soils showed a significant decrease of organic matter. At 5W°C, about 1/10 of the initial soil organic matter remained in both soiIs.

Several authors (Giovannini & Lucchesi, 1997; Sanroque et al., 1987) found decreases of soil organic matter in &es of high intensity, although low intensities did not modify this parameter in surface soil layers. Debano & Conrad (1978) showed measurable losses of organic matter in upper layers of soil in a prescribed burn (200°C-370°C) in southern California chaparral. Similar results were found in northeastem Spain, in an Aleppo pine forest (Ferran et al., 1991) and also in northeastern Spain in an acidic heath (Marcos et al., 1998).

Fire ~~ffeecjs on Soil Physical Properties

The dynamics of total N content by heating was different from the soil organic matter pattem. A higher stability of N when exposed to heat was observed, which progressively reduced the C/N ratio (Table 2). In a laboratory study by Kutiel & Shaviv (1992) a decrease of 34% of the total N was obsewed at 250°C, whereas at 600°C the decrease was 86%. Debano & Conrad (1978) found that up to 30% of the total N was lost at temperatures between 200°C and 300°C. Almendros et al. (1984) found that up to 30% of the total N was lost between 160°C and 210°C. At 300°C the loss was 60% and at 650°C 83% of the total N was lost. In our study, about 3/4 of the total N was lost at 500°C in calcareous soil, and about 213 of the total N was lost in gypsiferous soil. At laboratory-controlled conditions, heating promoted mineralization of organic N with increases of NH4+ which is quickly nitrified (Giovannini et al., 1990; Kutiel & Kutiel, 1989; Serrasolsas & Khanna, 1995).

In wildfires on different soils, losses of total C and total N from O-horizons UMiediately after burning have been reported, whereas the underlying A-horizons incorporated them (Ibañez et al., 1983; Lázzari et al., 1993; Mangas et al., 1992; Marcos et al., 1995; Rashid, 1987). Phillips et al. (2000) determined long term (35 yr) changes in the organic matter content in A horizons of a Glossic Fragiudult in response to periodic ( S yr) and annual prescribed fires in an oak forest; they found that the ormnic matter amount was highest on the control plots and decreased with increaseci burning.

Heating up to 150°C produced a significant decrease of C/N ratio for both soils. Also, a signiñcant correlation (Table 3) was found between heating and C/N ratio (R = -0.90; P < 0.01). Organic fractions of Iow humification degree (free organic matter and hereditary hurnin) incleased their relative N content (Almendros et al., 1984). In various wildñres, slight decreases in C/N ratios were found in surface mineral layers of the burned soils in comparison with unburned soils (Ferran et a l , 1991; González et al., 1992; Iglesias et al., 1993; Rashid, 1987). Marcos et al. (1995) did not find a homogeneous evolution of C/N ratios in burned forest soils.

TotalS

Total S represents the gypsum content of these soils, about 55% S in gypsiferous soils and practically absent in calcareous soil. Although a slight increase of total S was observed in gypsiferous soil, both by heating and ash addition treatments, these increases were not significant (Table 2).

Extractable P

Soils from shrublands of the Central Ebro Valley were low in P (Olsen extractable P) (Table 2). In gypsiferous soil, extractable P concentration was 10-fold lower than in calcareous soil and heating sustained these~differences. The extractable P showed a marked increase by heating for both soils (Table 2). This increase was highly sig- nificant from 150°C to 250°C, slmilarly significant was the decrease of organic matter. Similar results have been shown by Kutiel & Kutiel(1989) in an Aleppo pine forest following a hot surnmer wildfire. It is suggested that the extractable P derives from the mineralization process of organic matter. Brown & Mitchell (1986) reported that soil temperatures o£ 200°C-400"C appear to stimulate abiotic miner- alization of P.

From 250°C to 500°C, extractable P content increased while organic matter decreased, however not by the same proportion. It is probable that the combustion of organic P and its mineralization continued but there was an apparent loss of the readily extractable form of P due to its absorption by Ca-carbonate or the pre- cipitation as insoluble Ca compounds in the soils (Harrison, 1982). A negative

Fire ~ffe:ts on Soil Physical Properties

correlation coefficient (R= -0.16), nonsignificant, was obtained between organic matter and extractable P for both soils.

Available P content in soil generally increased following wildfires. I t has been suggested that virtuaiiy all the P in the standing phytomass and some soil organic matter were added to the soil as ash (ibañez et al., 1983; Lazzari et al., 1993; Marcos et al., 1995; Marion et al., 1991; Raison et al., 1985). In our study, elevated ash addition (1%) on calcareous soil increased the P content significantly because of the high P concentration in ashes (Raison et al., 1985). Despite the general pattern of available P increase, exceptions have been described by differential binding of phospate ions in soils and variations in &e intensity (Debano & Conrad, 1978; Kutiel & Shaviv, 1992). In a calcareous clay soil, Kutiel & Shaviv (1992) found less available P immediately after a strong artificial fire of 600°C as wmpared to the available P amount obtained at 250°C. They assumed that it was due to super- saturation of soil solution with respect to Ca-phosphate.

Water-Extractable Cations

The water-extractable caz+ increased as soils were heated to 250°C and decreased at 500°C, significantly at both temperatura (Table 2). This pattern was similar in both soil types, although soluble CaZf content was 4-fold higher in gypsiferous soil than in calcareous soil because the solubility of Ca-sulfate in water is much higher than Ca-carbonate.

The behavior of water-extractable Mg2+ seemed very similar to that of caz+ in calcareous soil. There was a peak at 250°C and then, at 500°C, the content decreased significantly. In gypsiferous soil, however, water-extractable Mg2+ increased pro- gressively with increasing temperatures indicating that gypsiferous soil contained magnesite, dolomite or other sources.

The Na+ and K+ extractability increased continuously with increasing tem- peratures for both soils; data showed a highly significant correlation with heating (Table 3). The Na+ and Kf contents were higher in gypsiferous soil than in cal- careous soil, as we reported previously for soils with similar lithology (Badia & Alcañiz, 1994). Kutiel & Shaviv (1992) found that a sirnulated light ñre (250°C) increased main soil nutrients in a Rhodoxeralf and a Lithic Xerochrept, but a fire of 600°C was followed by a reduction of most nutrients. Combustion of soil organic matter liberates nutrients, which are carried into the mineral soil at a moderate fire intensity (Chandler et al., 1983). A number of authors have shown important increases in soluble and/or exchangeable ions after wildfires (Gonzála et al., 1992; Ibañez et al., 1983; Iglesias et al., 1993; Marcos et al., 1995).

The decrease in caZf extractability in water after a 500°C fire is surely due to Ca-phosphates precipitation. Giovannini et al. (1990) also indicated than the decrease in ion extractability in water could be due to the aggregation of b e r particles into sand-sized particles with a decrease of the surface area in contact with the water. In our study only a ca2+ decrease was observed. It caused an increase of SAR at 500°C for both soils: from 0.2 to 0.4 (mmol L-')''~ in gypsiferous soil, and from 0.1 to 0.5 (mmol L-')''~ in calcareous soil.

Fire mobilized cations into the soluble fraction by rneans of mineralization of organic matter in vegetation and soil. In a silty loam soil from SE Spain Sánchez et al. (1994) found that organic matter, the total N, and available nutnents increased significantly while cation exchange capacity decreased after an experimental fire.

The addition of ash into the soil increased cation content because they are predominant in ashes (Giovannini, 1994; Raison et al., 1985). Also Sánchez et al. (1994) observed a significant increase of available cations in soils affected by fires through enrichment by the ash produced from the burned phytomass.

I Fire Effecis on Soil Physical Properties

Saturated Paste Extract, ECe

The ECe increased after heating, and the increase was significant at 15PC for gypsiferous soil and at 250°C for calcareous soil. ECe increased after buning (Table 2) due to the release of soluble cations. At 500°C ECe decreased significantly because caz+, quantitatively the main cation, showed a sharp decrease. ECe had a significant (P< 0.01) and positive correlation (R=0.99) with total soluble cations (the sum of ca2+, M$+, K+ and Naf) and with every soluble cation with the exception of ca2+.

An imrnediate rise of electrolytic conductivity in burned soils is attnbuted to the increased amount of soluble inorganic ions resulting from the combustion of soil organic matter (Sanroque et al., 1985). Ash addition increased ECe for both soils (by 0.2-0.3 dS m-' units), although it was only significant for calcareous soil as were the soluble cations.

Soil Aggregate Stability

The soil aggregate stability (SAS) of both soils was moderately reduced at 250"C, but notably at 500°C where organic matter was essentially eliminated and clay content was reduced significantly, elements generally considered the most important cementing agents (Table 4). There is no consensus among researchers who have camed out experiments on the changes in the structure of burned soils. Martins et al. (1991) observed the destruction of the surface structure in a sandy loam textured burn soil in Brazil. Antolin et al. (1997) found an increase in soil aggregation besides an increase of soil organic matter after wildfires in different types of Mediterranean soils from Valencia, SE Spain. Giovannini & Lucchesi (1997) observed that soil aggregate stability increased at about 150°C and again at about 500°C. They con- sidered that cementation remained unaltered in spite of the combustion of the soil organic matter because transformations of iron oxides cemented soil aggregates. In this study, soil organic matter combustion caused a structural breakdown and col- lapse with the decrease in soil aggregate stability (Table 4).

Density and Porusity

Bulk density and particle density increased proportionally with heating for both soils (Table 4). The proportional increase of both densities led to relative sustainability of the soil porosity. Both densities were significantly higher at 500°C compared with the control test and moderately heated soils. The increase in densities after heating is attributed to a relative increased contribution of high density materials. The com- bustion of organic matter is essentially complete at 500°C implying a decrease in volume fraction of organic matter and an increase of the volume fraction of mineral constituents. Similar observations were made on bumed surface horizons of sandy loam soils in Brazil (Martins et al., 1991) and sandy soils of coastal plains (Boyer & Miller, 1994). Unburned soils were found more permeable and porous than fre- quently burned soils (Boyer & Miller, 1994), although Bunting and Lundberg (1987) noticed a rapid recovery after burning. However, Mallik & FitzPatrick (1996) con- cluded that soil porosity increased directly after fire, but gradually decreased two to three years after a burn in Scottish heathland soils. Similarily, the data reported by Giovannini & Lucchesi (1997) indicated an increase in bulk density and a decrease of soil porosity related to the decrease of soil organic matter by soil heating. Phillips et al. (2000), in a long term study (35 years), found that bulk density values of forest soils increased with decreasing organic matter content with prescribed burning; bulk

D. Badh and C. Martí

densities of the upper cm of the A h o k n were greater in the amually burned plots than in control and periodicaiiy burned plots. According to Phillips et al. (2000), changes in particle density can indicate damage to the crystallographic and spatiai structure of the mineral part of the soil that cannot in any way be restored.

Avaihbk- Water Capacity

A significant reduction of the permanent wilting point was observed by heating above 250°C, both gypsiferous soii (from 17% in the unheated sample to 12% at the highest temperature) and calcareous soil (from 15% in the unheated sample to 11% at the highest temperature) (Tables 4 and 5). Field capacity decreased about 3% for both soils but only at 250°C, since at 5W°C the control values were reached. In previous studies (Badía et al., 2001), both soii moisture lirnits were correlated posihvely and siflcantly with aggregate stability, clay, porosity, and organic matter content. In this research, however, only the permanent wilting point showed this correlation. Field capacity in calcareous soil decreased proportionaliy to the silt fraction increase and a signiñcant correlation has been found between these para- meters (P <0.01). Different raults could be found in soils affected by wildfires in relation to the effect on moisture limits. Martínez & Diaz (1994) found lower field capacity values in burned plots than in control plots affected by wildñre of moderate intensity; but permanent wilting point vaiues were not shown to be affected by ñre. Different authors found that buming reduced the soil water-holding capacity of sandy-textured tropical soiis (Boyer & Miiler, 1994; Martins et al., 1991) and Mediterranean soils (Debano, 1971), as a consequence of the presence of hydro- phobic substances and a decrease in organic matter content.

Soil Texture

Particle-size distribution showed a continuous increase of the sand fraction with an increase in temperature, corresponding to a simultaneous decrease of the clay frac- tion (Table 4). This process was not very relevant at the beginning of the heating process of the clay loam calcareous soil. We obsewed that the soil texture classifi- cation changed at about 250°C and about 500°C. At 250°C the soil was classified as silty loam, and at 500°C as a sandy loam. Changes in soil texture are related to the transformations of mineral compounds, especially the thermal modifications of the Fe- and Al-siiicates that caused fusion of clay particles into sand sized particles (Giovannini & Lucchesi, 1997). In an experimental burn on siity loam soil in SE Spain, Sánchez et al. (1994) found that clay content decreased significantly one day after fire. Nevertheless, Martínez & Diaz (1994) did not observe changes in texture after a wildñre because of the inherent vanability of this property in a loamy soil from a semiarid part of SE Spain.

Ash addition did not affect soil texture although its characteristics could modify it. Giovannini (1994) observed that ash leachate was able to disperse the soil particles in the hours immediately foliowing their application to the soil; but as the contact time increased he noted a rise in the aggregation of h e r clay particles into silt-sized fraction.

Because particles flocculate in gypsiferous soil, the texture was not detennined and K-USLE values were obtained from calcareous soil only (Table 4). As Giovannini

I Fire Effects on Soil Physical Properties

et al. (1998) found, the erodibility of a burned soil rose as the heat increased and that after an increasingly intense fire an increase in soil erodibility was to be expected. A negative and significant correlation (P< 0.01) was obsewed among SAS and soil erodibility (K-USLE) in calcareous soil. Debano (1971) found that burning reduced inñitration in chaparra1 soils, as a consequence of the presence of hydrophobic substances, which increased surface water movement and soil erosion.

Color

On dry samples, value and chroma of soil color decreased significantly by heating of calcareous soil but not of gypsiferous soil (Table 4). No change was observed on hue notation (10 YR). Heating had a significant and negative correlation (Table 5) with value and chroma for calcareous soil ( P < 0.01) and gypsiferous soil (P < 0.05). It was related to partial combustion of soii organic matter and ash incorporation into the soil. Differences in the organic matter and gypsum content between soils gen- erated the different effects of heating on value and chroma.

With the main physical and chemical properties measured (Tables 2 and 4), the r-values between heating and soil properties were obtained, separately for calcareous and gypsiferous soils (Table 6 ) . For gypsiferous soil, the heating produced a pro- gressive decrease in the r-values but for calcareous soil this decrease was sharp between 250°C and 500°C. Taking into account the parameters significantly corre- lated (P<0.01) with heating, a hierarchical cluster analysis has been elaborated (Figure 1). The correlation matrix as well as the dendrogram indicated that gypsi- ferous and calcareous soils have a different behavior at intermediate heating, the most frequent temperatura in wildfire events (150°C and 250°C). An increased heating temperature provoked similar consequences in both soils, including the combustion of organic matter, the pH increase, the loss of aggregate stability, the fine particles aggregation, and other properties. As a consequence, the dendrogram grouped G500 and C500 treatments separately from other treatments. Also, in the cluster figure the grouping of couples of data, 25-150 and 250-250A treatments is evident, indicating limited differences between them.

Conclusions

The effects of the heating on chemical properties of gypsiferous soil and calcareous soil should be discussed for every parameter. Heating to 500°C produced an increase in sojl reaction @H), although at 150°C a slight decrease was evident. This reaction was similar for both soils. Electrolytic conductivity and caz+ extractability in water increased until 250°C and decreased at 500°C with a similar pattern in both soils; however, the magnitudes of these values were different. In gypsiferous soil both parameters were higher than in calcareous soil. Heating to 500°C caused the com- bustion of the soil organic matter (up to 90% of the initial value) which reduced the chroma and color value. Nutrient elements associated with the organic matter were mineralized and made easily avaiIable to plants. Heating increased P-Olsen content, as well as water soluble M~'+, Kf and Na+ in both soils. Heating up to 250°C decreased availability in calcareous soil and increased it in gypsiferous soil

TABLE 5 R-values Between Heating and Physical Properties for Gypsiferous and Calcareous Soils (n = 30)

Heating S AS Pb PP Porosity FC WP AW Sand Value Chroma

P < 0.05 for R > 0.361 and P < 0.01 for R > 0.463. SAS, Soil AggregateStability; p~>, Bulk density, p,, Real density: FC, Field Capacity; WP, Witing Point, AW, Available Water.

TABLE 6 R-values Between Different Treatments (n =25) for Gypsiferous and Calcareous Soils --

Gypsiferous soil Calcareous soil

Soil treatment G25 G150 G250 G250A G500 C25 C15Q C250 C250A C500 --

25 1 1 150 0.949 1 0.994 1 250 0.615 0.829 1 . 0.924 0.944 1 250A 0.580 0.804 0.999 1 0.894 0.917 0.997 1 500 0.401 0.525 0.660 0.675 1 0.398 0.459 0.612 0.628 1

P < 0.05 for R > 0.396 and P < 0.01 for R > 0.505.

D. Badia and C. Marfí

Rescaled Distance C lus t e r Combine

C A S E O 5 10 15 20 25 Label Num +---------+---------+---------+---------+---------+

FIGURE 1 Dendrogram of experimental soils (G, gypsiferous and C, calcareous) as function of physical and chemical properties affected by heating (25", 150°, 250°, and 500°C) and plant ash addition (A). Numbers O to 25 shows the rescaled distance cluster combine, and 1 to 10 are the case numbers.

because of different soil composition. Plant ash addition increased pH, organic matter, C/N ratio value and some nutrients.

Aggregate stability was reduced by heating above 150°C, because of organic matter combustion that increased soil erodibility. Porosity was not affected by heating because its components (bulk and particle density) were proportionally infiuenced. Water availability, a function of field capacity and permanent wilting point, increased up to 500°C. Particle-size distnbution showed a continuous increase of the sand fraction with increasing temperature, attributed to the fusion of clay particles into sand-sized particles. Ash addition did not affect physical properties.

The overall conclusion in this study is that experimental fire modified the phy- sical and chemical status of gypsiferous and calcareous soils in a similar qualitative pattern, although we did find important quantitative differences between t h h , especially at intermediate temperatura. Low temperature heating has less impact on short-term soil properties than high temperatures which should be considered in the fire management of semiarid ecosystems.

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