Comparative evaluation of the effects of salinity and drought on germination and seedling growth of Barnyardgrass (Echinochloa crus-galli L.) and Rice (Oryza sativa L.), and its relationship with their competition under stress conditions.

An experiment was conducted at Pakdasht barley fields in 2004 & 2005 growing seasons to study the effect of chemical control on weed seed bank in rotation systems. Corn-barely rotation with and without chemical application was implemented. Method of sampling was systematic (zigzag) and the time of the sampling stages were in two dates; before sowing and after harvesting of barley. To compare the diversity between farms, Shannon-Weiner diversity index was calculated. Weed seed bank densities in chemical managed farms (CMFs) was generally lower than those without chemical control (NCMFs). At first sampling, average weed seed bank populations in CMFs, were 49 and 31 seeds kg-1 of soil, and for NCMFs were 136 and 177 seeds kg-1 soil in 2004 and 2005, respectively. The weed seed bank density in second sampling date (post harvesting of barley) for CMFs were 33 and 30.5 seeds kg-1 soil, and for NCMFs were 210 and 254 seeds kg-1 of soil in 2004 and 2005, respectively. Seed bank density decreased over sampling times (growing season) for CMFs in 2004 as compared with NCMFs. In 2005, the NCMFs variation trend of the seed bank densities for managed farms was constant between the two stages of sampling. But the trend of variation in NCMFs was similar to that of previous year. Shannon-Weiner diversity index in CMFs was higher than those of NCMFs. Herbicide application had reduced the seed production of weeds and the ultimate seed rain into the soil seed bank. Results of this study demonstrate the importance of weed control practices in reducing weed seed bank size.