The effect of temperature and exposure time on redroot pigweed (Amaranthus retroflexus) and yellow foxtail (Setaria pumila) seed mortality in the natural soil seed bank

Heat disinfection of soil can be used to reduce the content of the soil seed bank. However, species differ in the lethal temperature needed for seed destruction and mortality. Laboratory research was conducted on the seeds of two weed species, redroot pigweed (Amaranthus retroflexus L.) and yellow foxtail [Setaria pumila (Poir.) Roem. et Schult]. The soil samples were collected at the experimental station Šašinovečki Lug, Zagreb (45.850289°N; 16.180465°W), Croatia and exposed to linearly increasing constant temperatures of 40, 50, 60, 80, 100 and 120 C and exposure times of 30, 60 and 90 min in a laboratory oven. Weed seeds were then extracted from the soil using the sieve separation method and survival measured by germinating seeds on the filter paper. Germination counts were converted into percentages of mortality compared to untreated seeds. The results show that both temperature and exposure time significantly affected seed mortality of both weed species. A. retroflexus shows a greater susceptibility to high temperatures than S. pumila. Fitted three-parameter sigmoid model was used to define the relationship between temperature and exposure time needed for 50% (LT50) and 90% (LT90) seed mortality. The estimated LT50 values for A. retroflexus are 58.89 – 46.08 C over the 30 to 90 min exposure times; the estimated LT90 113.36 to 65.72 C for the same durations. The estimated LT50 values for S. pumila over the 30 to 90 min exposure times ranged from 91.33 to 75.15 C; the estimated LT90 ranged from 98.79 to 90.32 C over the same durations. The research results contribute to the knowledge about the thermal sensitivity of seeds. Estimating efficacy of soil heating treatments is essential when comparing the environmental, economic and social costs of alternatives to conventional weed control methods.