Effect of Soil Solarization on Downy Brome)Bromus tectorum L.) Control in Birjand Area

Weeds are important components of agricultural ecosystems that compete with agricultural crops to capture resources and caused irreparable damage to the crops. By producing herbicides, a significant change was created in the management of weeds. But now days some new problems such as resistance of weeds to herbicides, loss of useful species and environmental pollutions have forced farmers to employ non-chemical weed management methods. An important non-chemical method for seed control of weeds is soil solarization, which has been used as a way for controlling weeds, pests and diseases. South Khorasan enjoys plenty of solar radiation during the summer which can be used as a useful tool for weed control.
To evaluate the effect of soil solarization on downy brome )Bromus tectorum L.( control, an experiment was conducted at the research farm of College of Agriculture, University of Birjand during the summer of 2010. The factorial experiment was performed as a randomized complete block design with three factors including the number of clear plastic layers (uncovered as the control, one-layer and two–layer plastic sheets(, duration of coverage (15, 30 and 45 days) and seed burial depth (0, 5, 10 and 15 cm) with three replications. Seeds were buried at different soil depths in the field and experimental plots were covered with the corresponding plastic layers. The seed samples alternatively were pulled out of different depths of soil and transported to the research laboratory. The seeds which were germinated in the field were counted, and the rest of them were incubated in a germinator set at 25/15°Cfor day/night. During the germination period, some index such as; germination rate, percentage of decayed and germinated seeds were measured. Tetrazolium test was used to examine the viability of not germinated seeds.
Results showed that solarization significantly increased soil temperature (as much as 6.3 to 15.1°C) compared to the control. The percentage of germination in the field decreased by increasing the layers of plastic coverage, as well asby increasing burial depths. The maximum percentage of decayed seeds were observed at two-layer plastic, one-layer plastic and control treatments, respectively. The percentage of decayed seeds increased by increasing solarization duration and it decreased by increasing burial depth. During the whole period of solarization, the lowest decay percentage was observed in the control treatment, while the greatest corresponding values were obtained from the seeds placed under oneand two-layer plastic sheets, indicating that accumulated temperatures under these sheets caused an increased decay percentage. At the soil surface and 5 cm burial depth, none of the downy brome seeds in 1- and 2-layer plastic sheet could germinate in the laboratory. It seems that the high temperatures generated by soil solarization have destroyed downy brome seeds under plastic covered treatments. A soil solarization period of 15 days was sufficient for destroying the seeds on the soil surface and those buried at 5 cm depth, while a solarization period of 45 days was required to destroy the seeds buried at 10 and 15 cm depths. Seeds that had been exposed to high temperatures caused by solarization for a longer period, had a lower germination rate. The greatest germination rate was observed in the control treatment and on the soil surface. The results of Tetrazolium test showed that all none germinated downy brome seeds which were subjected to Tetrazolium test had lost their viability, or in other words, were decayed. After finishing the solarization experiment we found that the concentration of sodium, calcium, and magnesium and sodium uptake ratio and soil pH decreased decreased under solarized plots. Moreover, electrical conductivity (EC) increased.
Overall, the results showed that solarization reduced the downy brome seed germinability and ultimately led to an effective method to control of this weed. It appears that solarization can be used as a non-chemical method for downy brome control especially in arid regions of the country. According to the results of this study, covering the soil with two-layer plastic sheet was the most effective and the best method of solarization.