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2022

Sammendrag

Established invasive alien plant species make it difficult and costly to move and make use of infested soil in public and private construction work. Stationary soil steaming as a non-chemical control method has the potential to disinfect soil masses contaminated with seeds and other propagative plant materials. The outcome can vary depending on steaming temperature and duration. Higher temperatures and longer durations are relatively more efficient but may also have side-effects including change in soil physical and chemical characteristics. Barnyard grass (Echinochloa crus-galli) is among troublesome invasive species in Norway. We have tested different steam duration at 99°C to find the possible lowest effective exposure duration for killing seeds of this annual grass species. Four replications of 40 barnyard grass dry seeds of one population were placed in polypropylene-fleece bags (9*7 cm), moistened for 12 hours, and covered by the soil at a depth of 7 cm in 60*40*20 cm boxes. The boxes with soil and bags were steamed at 99°C for 1.5, 3 and 9 min. The bags including steamed seeds were taken out, opened, placed on soil surface in pots and covered by a thin layer of soil. The pots were placed in greenhouse and watered from below and seed germination was followed for a month. Moistened non-steamed seeds were used as control. It was shown that steaming at 99°C gave 0% germination indicating that 100% of the seeds were killed regardless of exposure duration while in the control seed germination was 100%. Consequently, to achieve an efficacy of 100%, exposure duration of 1.5 min would be enough. Finding the lowest possible steam temperature and exposure duration to get the highest possible seed killing in a seed mixture of different plant species as well as other factors to increase the heat transferability are under investigation. Keywords: Echinochloa crus-galli; Resource recovery; Steaming temperature and duration; Thermal soil disinfection

Sammendrag

Invasive plant propagative material can be introduced to new regions as contaminants in soil. Therefore, moving soil should be done only when the soil has been verified to be free of invasive species. Stationary soil steaming as a non-chemical control method has the potential to disinfect soil masses contaminated with invasive species. We investigated the possibility of thermal control of propagative material of Bohemian knotweed (Reynoutria × bohemica) in two experiments using a prototype of a soil steaming device. Five soil temperatures of 60, 70, 80, 90 and 99 °C with an exposure duration of 3 min were tested. In each replicate and target temperature, rhizome cuttings containing at least two buds and shoot clumps were placed at the bottom of a plastic perforated basket and covered by a 7-cm soil layer. Each basket was placed in the steaming container and steam was released from the top and vacuumed from the bottom. Soil temperature was monitored by 10 thermocouples and steaming was stopped when 5 of the thermocouples had reached the target temperature. The basket was then removed from the steaming container after 3 min. Plant materials were taken out and planted in pots. Buds sprouting was followed for 8 weeks. Non-steamed plant materials were used as controls. Results showed 100% rhizome death at soil temperatures of ≥70 and 99 °C in the first and second experiments, respectively. Shoot clumps death was obtained at ≥90 °C in both experiments. These results showed that steaming at 99 °C for 3 min can guarantee control of Bohemian knotweed in infested soils supporting the steam treatment as a potential method of disinfecting soil against invasive species. However, depending on the intended re-use of the soil, further studies are needed on the effect of potential negative impacts of high temperatures on the soil quality.