Short-term effect of 915-MHz microwave treatments on soil physicochemical and biological properties.
To control the soil seed bank of invasive plants with microwaves represents an alternative method to chemical treatments, but it could alter soil quality. Microwave effects were investigated on 17 soil physicochemical and biological properties. Four 915-MHz microwave treatments combining power and duration of exposure were applied on alluvial soil from a grassland using Festuca seeds as an internal standard to evaluate the efficiency of sterilization. Two treatments, 2 kW-8 and 4 kW-4 min, completely inhibited Festuca seed germination and caused the soil temperature to reach at least 80°C. Conversely, the other treatments, 2 kW-4 and 4 kW-2 min, failed to affect seed germination while raising the temperature to 50-60°C. Microbial biomass, the density of culturable heterotrophic bacteria and fluorescent Pseudomonads as well as fluorescein di-acetate hydrolysis decreased significantly by 2.7-, 1.1-, 1.4- and 5.1-fold compared with the control, respectively, following the 2 kW-8 and 4 kW-4 min treatments. These treatments also increased dissolved organic carbon and inorganic phosphorus contents by 1.6- and 1.2-fold, respectively. In contrast, nitrate increased only (about 140%) in the soils under the 2 kW-4 and 4 kW-2 min conditions. Furthermore, the growth of Medicago truncatula Gaertn. seedlings transplanted in microwave-treated soils under the 2 kW-8 and 4 kW-4 min conditions was not impaired despite the reduction in total nitrogen and the microbial population and smaller numbers of nodules. These results suggest that soil seed bank sterilization by microwaves that is 100% effective alters soil microbial indicators more strongly than chemical treatments without compromising the early growth stages of transplanted Medicago plants.