Effects on Nonpathogenic Soil Microflora

A broad range of soil microorganisms, apart from major plant pathogens, were found to be affected by soil solarization, as most studies agreed that the heating treatment stimulated marked compositional shifts in composition and richness of soil microbial communities (Chen et al. 1991; Schoenfeld et al. 2003; Palese et al. 2004; Culman et al. 2006; Gelsomino et al. 2007). Alabouvette et al. (1996) hypothesized that heat-induced shifts in soil microbial balance favored saprophytic microbiota, thus increasing competition for nutrients and resulting in a higher soil suppressiveness. Recent greenhouse solarization studies documented negative effects of heating treatment on microbial biomass and enzymatic activities, but a protective role of organic matter against heat detrimental effect (Okur et al. 2006; Scopa and Dumontet 2007).

Some authors described a general reduction of soil total bacterial population by soil solarization (Mahmoud 1996; Patel and Patel 1997; Itoh et al. 2000; Barbour et al. 2002; Sharma et al. 2002), whereas other reports documented a decrease of soil fungal population but no effect on bacteria (Coates-Beckford et al. 1997; Shukla et al. 2000). However, most studies on solarization effects on soil bacterial population focused on microorganisms beneficial to plant growth, antagonistic to plant pathogens, or quick recolonizer of root systems in solarized soil (Stapleton and DeVay 1984; Stevens et al. 1991b; Wadi 1999). Some of these investigations indicated an increase of total bacteria and actinomycetes populations in solarized soil (Kaewruang et al. 1989; Khair and Bakir 1995; Khaleeque et al. 1999). Stevens et al. (2003) reported a shift of bacterial population in solarized soil to Rhizobacteria, Bacillus spp., and fluorescent pseudomonads. Stapleton and DeVay (1982, 1984) found that the population densities of fluorescent pseudomonads, Bacillus spp., Actinomycetes, and Agrobacterium spp. were greatly reduced after solarization. A suppression of various soil bacteria, with a lower effect on Actinomycetes, was found also by Ristaino et al. (1991) and Gamliel and Katan (1991). Fluorescent pseudomonads were stated as quick recolonizer of solarized soil (Stapleton and DeVay 1982; Stapleton and DeVay 1986; Gamliel et al. 1987), providing a degree of protection against fungal root pathogens and stimulating plant growth (Lifshitz et al. 1983; Greenberger et al. 1984; Stapleton and DeVay 1984; Freeman and Katan 1988; Thomashow and Weller 1990; Keel 1992; Chen et al. 2000). A number of studies documented Bacillus species as predominant Gram-positive bacteria surviving soil solarization and playing a major role in disease suppressiveness of solarized soils, due to either their aggressive growth or production of antibiotics (Stapleton and DeVay 1982; Stapleton and DeVay 1984; Katan 1987). Endorhizosphere Bacillus strains selected from tomato root tips after soil solariza-tion were found to be very efficient in inhibiting mycelial growth of V. dahliae in vitro and controlling Verticillium wilt of solanaceous hosts in field trials (Tjamos and Paplomatas 1987; Tjamos et al. 2004). Moreover, population of another antagonist of V. dahliae, Talaromyces flavus (Klocker) Stolk and Samson, was also found to survive solarization and significantly increase in plant rhizosphere (Kim et al. 1988; Tjamos and Fravel 1995).

A number of studies specifically investigated the effect of solarization on population of soil rhizobia, due to their importance as nitrogen-fixing bacteria. Heat treatment was found to reduce soil population of Rhizobium spp. and consequently root nodulation of early-stage plants (Abdel-Rahim et al. 1988; Chauhan et al. 1988; Linke et al. 1991; Mahmoud 1996), though a quick recover of these bacteria occurred after the establishment of a legume crop (Linke et al. 1991). Mauromicale et al. (2005a, b) also reported a delay of root nodulation and a consistent reduction of a number of nodules per plant in faba bean (Vicia faba L.) and chickpea (Cicer arietinum L.), and the adverse effect of soil solarization on native soil rhizobia was also suggested as a potential technique for their replacement with other inoculant beneficial strains (Rupela and Sudarshana 1990). However, Arora and Pandey (1989) and Nair et al. (1990) inversely observed an increase of rhizobial root nodulation in solarized soil.

Few and contradictory studies are available on the side effects of solarization on arbuscular mycorrhizal fungi (AMF), but most reports indicate no damage of soil heating on native AMF and an enhancement of mycorrhizal colonization and plant growth (Pullman et al. 1981; Afek et al. 1991). Daft et al. (1987) found spores of Glomus clarum Nicol. and Schenck as inactivated at 45°C, and Menge et al. (1979) reported thermal death of G. fasciculatum (Thaxter) Gerdeman & Trappe after 10 min at 51.5°C. Soulas et al. (1997) observed that ectomycorrhizal fungi, as suppressed at temperatures above 45°C, were among the soilborne fungi most sensitive to solar heating, and suggested that soil solarization may be an effective disinfection method for a controlled mycorrhization in forest nurseries. No differences in the extent of AMF internal infections were found by Stapleton and DeVay (1984) in roots from solarized or untreated soils, though in other trials indigenous AMF populations were reduced to undetectable levels after 8-week solarization, whereas inoculated Glomus intraradices Schenk and Smith population remained viable (Bendavid-Val et al. 1997). Camprubi et al. (2007) reported that AMF propagules were only reduced, but not completely eliminated, by solarization, and that inoculum of G. intraradices completely lost its mycorrhizal potential when submitted at a 50°C temperature. Schreiner et al. (2001) hypothesized that reduction of AMF following solarization may be indirectly due to the suppression of weeds that would maintain the fungi over the winter.

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