P u b l i s h i n g
International Journal of Wildland Fire Scientific Journal of IAWF
Volume 11, 2002 © International Association of Wildland Fire 2002
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International Journal of Wildland Fire, 2002, 11, 107–114
Soil organic matter and aggregates affected by wildfire in a Pinus halepensis forest in a Mediterranean environment J. Mataix-SoleraA, I. Gómez, J. Navarro-Pedreño, C. Guerrero and R. Moral Departamento de Agroquímica y Medio Ambiente, Universidad Miguel Hernández, Avda. del Ferrocarril s/n, E-03202-Elche, Alicante (SPAIN). A Corresponding author: Telephone: +34 96 665 8417; fax: +34 96 665 8532; email:
[email protected] OWFr0ag2n0ic20materandagregatseinbunrtsoils eJ.Matalt. aixS-olrea, I.Góme,zJ. Navro-Pdereoñ, C. GuereorandR. Moral
Abstract. Three Mediterranean soils located in the north of the Province of Alicante (Spain) were studied for a year after a forest fire. The percentage of water-stable aggregates (between 0.2 and 4 mm) and organic matter content were measured. Microaggregates (< 0.2 mm) were observed using electron microscopy. The results showed the importance of type of forest fire on soil organic matter and aggregates. Soil structure was more affected by surface fire (which affects mainly brushwood and soil surface) than crown fire (which burns the tops of trees and some brushwood). Accumulation of organic matter from burnt trees and brushwood in areas affected by crown fire and alterations in organic matter content through the soil profile were observed. Surface forest fire affected soil structure more negatively than crown fire as observed using electron microscopy. Soils affected by surface fire may be more easily eroded and recovery of vegetation may be delayed because of effects on soil structure. Organic matter content through the soil profile comparing burnt and adjacent unburnt soil could be used to determine the type of fire. Additional keywords: Forest fire; calcareous soil; Spain; Mediterranean type-areas. Introduction The Mediterranean forest is an ecosystem which is commonly affected by fire. The climate of Southeast Spain increases the possibility of forest fire (Cerdá 1993). The forest fire is a special concern in arid and semiarid environments because it can accelerate erosion processes (Rubio 1987) and control vegetation composition and dynamics (Brown 1990). For instance, in the Community of Valencia (East Spain), 26 198 ha, 25 978 ha and 113 270 ha were burned in 1992, 1993 and 1994, respectively (data provided by the Conselleria de Medio Ambiente of the Valencian Government). The negative influence on the environment is manifested in the disappearance of vegetation in forest areas, the risk of soil erosion, and alteration of water cycle (DeBano 1971; Sánchez et al. 1994). A rigorous evaluation of fire-damage is necessary to promote management strategies for the regeneration and recovery of the forest. Soil is a basic component of the forest ecosystem. Types of fire, such as crown fire affecting mainly the top of trees and surface fire affecting mainly brushwood and the soil surface, and intensity of fire affect various soil properties (Greene et al. 1990; Imeson 1992). The soil may suffer physical, chemical, and biological alterations such as disaggregation and dispersion of soil particles, loss of nutrients, and reduction in numbers of living organisms. These alterations may delay the growth of new vegetation. Soil structure is strongly related to susceptibility to erosion. Soil structure has important effects on water © IAWF 2002
movement and water reserves, and plays an important role in avoiding losses of nutrients and soil particles, by both leaching and runoff (Ela et al. 1992). How particles aggregate early in soil formation may play an important role in determining which are the dominant soil processes and, thus, the direction of soil evolution (Graham et al. 1995). After a fire, aggregate stability is reduced (Giovannini and Lucchesi 1983). This may be associated with loss of organic matter (Sanroque et al. 1985; Giovannini et al. 1988) and the disappearance of plant cover (Foregeard and Frenot 1987). Organic compounds have a positive influence on soil structure (Golchin et al. 1995). When the fire is of low intensity, increase of aggregate stability has been found (Ibañez et al. 1983; Díaz-Fierros et al. 1989). The objective of this paper is to determine the influence of fire on soil structure in a typical Mediterranean environment. This study is centred on modifications of soil aggregates and organic matter content in calcareous soils as affected by different types of fire after the autumn rainfall period. Materials and methods Sites description Three sites affected by the fire were selected in the north of the Province of Alicante (SE Spain) (sites A, B and C). All are situated in pine forests (Pinus halepensis Mill.) with similar characteristics, including amount of fuel and typical Mediterranean brushwood (Rosmarinus officinalis L.). The three sites are located with 1 km of distance between A and B, and 0.8 km between B and C at: site A: 38º43′26″N, 0º28′55″W; site B: 10.1071/WF02020
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unaffected soil, and these triplicate soil samples were taken from the border between areas affected and not affected by fire at 20 m distance and separated 50 m each one (Fig. 1). The first samples were taken after the autumn rainfall when post-fire soil erosion can be severe because of very high rainfall intensities (Llovet et al. 1994; Cerdá et al. 1995).
SP AI N
Analytical and statistical methods A l i can t e
0
0
10
20
30 km
7 5 15 0 225 km
1000
800
900
8 3 5
U nburned area
1 0 3 9 7 9 7
B urned area 1 1 0 4
9 4 8
Soil organic matter content was determined by rapid dichromate oxidation of organic carbon based on the Walkley-Black method (Nelson and Sommers 1982). Structural stability of the soils was measured as the percentage of stable aggregates (sized between 0.2 and 4 mm) remaining after an artificial rain of known intensity, 270 J/m2 (Benito et al. 1986; Roldan et al. 1994). The ANOVA F-test was used to assess differences between samples of burnt and unburnt soil at the same depth for each site. To determine effects on microaggregates, photographs obtained by scanning electron microscopy (JEOL JSM-840, ×100) were used to observe the microstructure (size and microaggregates) 6 months after the fire. All samples were air-dried and passed through a 0.2 mm sieve before taking the photographs, avoiding as far as possible any mechanical disturbance during hand shaking. However, break-up of some fragile aggregates may have happened. To quantify microaggregate size distribution, we separated into fractions: 0.2–0.08 mm and
