Impact of elevated CO₂ concentrations in the soil on soil solarization efficiency

Solarization is a method of heating moist soil by covering it with transparent polyethylene sheets to trap solar radiation. It involves the use of heat as a lethal agent for soil-borne pathogens. Soil temperature under polyethylene sheet cover is a function of incoming radiation and thermal characteristics of the polyethylene sheets and the soil. In order to study the effect of soil CO₂ concentrations on soil solarization efficiency, clay soil samples infested with Verticillium dahliae were exposed to different CO₂ concentrations [350, 700, 1050, 1400, 1750μL CO₂ Lair⁻¹] and incubated in hot water baths at 35, 40, 45, 50 and 55°C. Moreover, field plots were exposed to the same CO₂ levels during soil solarization in three periods [1st of July to 30th of September, 1st of August to 30th of September, and 1st to 30th of September]. Recorded temperatures of 35-55°C during the three soil solarization periods were lethal to V. dahliae. At 35 and 55°C, the exposure time for LD₉₀ was 24 days and 6h respectively for V. dahliae with ambient soil CO₂ content. High CO₂ content in the soil resulted in increasing maximum soil temperatures and soil heat flux while reducing the time required for LD₉₀. The required time for LD₉₀ of V. dahliae in soil heated at 35°C, reduced from 24 days with ambient CO₂ content to 15 days at 1750μLCO₂ Lair⁻¹. Sub-lethal soil temperatures were raised to lethal levels with increasing CO₂ content in the soil. A linear “negative” relationship existed between logarithms of times required to kill 90% of V. dahliae microsclerotia and temperatures for all soil CO₂ enrichment levels. The fungus was killed in field soil solarized for the necessary time periods. It was found that the addition of CO₂ shortens the required time to kill the fungus V. dahliae during solarization and increased the activity of the sub-lethal soil temperatures. Levels of CO₂ and temperature necessary to kill the fungus are useful for evaluating the progress of soil solarization under field conditions.