Effects on Soil Chemical and Physical Properties

Solar heating was normally reported to increase soil content of soluble nutrients, and particularly of dissolved organic matter, inorganic nitrogen forms, and available cations, either under field-scale or in growth chamber simulated solarization (Stapleton et al. 1985; Stevens et al. 1991a; Grunzweig et al. 1999; Chen et al. 2000; Salerno et al. 2000; Ghini et al. 2003). Chen and Katan (1980) observed increased concentrations of dissolved organic matter in saturated extracts of solarized soils, and Gelsomino et al. (2006) recently hypothesized this increase of soluble organic matter as caused by a mild hydrolysis or depolymerization of soil organic matter under the effect of the solarization-induced high temperatures. Short-term availability of soluble forms of nitrogen, particularly NH4+ and NO3- fractions, was usually found increased after solarization, due to the higher decomposition rates of organic matter and the mineralization of microbial biomass killed by heat (Chen and Katan 1980; Stapleton et al. 1985; Kaewruang et al. 1989a, b; Ahmad et al. 1996; Grunzweig et al. 1998; Freitas et al. 2000; Mauromicale et al. 2005a, b). Relative concentration of different nitrogen forms was described as a function of soil pH and nitrifying microorganisms density, as thermal death of nitrifying bacteria during soil solarization favors the accumulation of soluble ammonium nitrogen, whereas the occurrence of lower temperatures and poor organic matter content allow the survival of nitrifying microorganisms and consequent nitrogen loss due to the easy leaching of NO3- (Hasson et al. 1977; Kaewruang et al. 1989a).

Most authors reported as uncommon an increase of soil phosphorus content after solar heating (Chen and Katan 1980; Stapleton et al. 1985; Kaewruang et al. 1989b; Chen et al. 1991), though few reports indicated an increased availability of total or water-soluble phosphorous as following the solarization treatment (Kaewruang et al. 1989a; Gelsomino et al. 2006). Potassium, calcium, magnesium, and sodium availability was generally found to increase in soil after solarization (Chen and Katan 1980; Stapleton et al. 1985; Kaewruang et al. 1989b; Gamliel and Katan 1991; Ahmad et al. 1996; Grunzweig et al. 1998).

The increased growth response documented in almost all the solarization studies is mainly due to the above-cited higher levels of macronutrients or the improved uptake of micronutrients solubilized by humic substances (Chen and Aviad 1990; Chen et al. 1991). As a consequence of the enhancing effect of solarization on soil nutrients, Flores et al. (2007) suggested the application of low rates of mineral fertilizers before heating soil, in order to avoid an increased vegetative growth of the plants at the expense of crop yield.

A number of solarization studies reported an increase in electrical conductivity of soil solution (Chen and Katan 1980; Stapleton et al. 1985; Kaewruang et al. 1989a; Ahmad et al. 1996), which was hypothesized to be related to the higher content of ions, released by decomposed and mineralized organic matter migrating in soil solution from deeper to the upper heated soil layers (Chen and Katan 1980). Similarly, diurnal downward movement of soil moisture and solubilized salts was suggested to explain the reduction in soil salinity reported as following soil solar-ization (Abdel-Rahim et al. 1988; Al-Kaysi et al. 1989). Contrasting effects of heat treatment were described for soil hydraulic conductivity (Chen and Katan 1980; Al-Kaysi et al. 1989). Soil physical properties were generally found limitedly and inconsistently affected by soil solarization (Chen et al. 1991), though Melero-Vara et al., (1989) reported an improvement of soil structure and aggregation as following a solarization treatment.

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