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, 1750 μL CO2 L air−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 6 h 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 1750 μL CO2 L air−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.