Effect of chemical control on weed seed bank size and composition in corn-barley rotation system

Changes in weed seed bank due to crop production practices are an important determinant of subsequent weed problems. To study the effect of chemical control on agricultural rotation systems, a study was conducted at the University of Tehran research fields, Tehran, Iran during 2004 and 2005. Corn-barley was the selected rotation with and without applying of herbicide. Method of sampling was systematic (zigzag) and the time of the sampling stages was in two dates; before sowing and after harvesting of barley. To compare the diversity between farms, Shannon-Weiner diversity index was calculated. Based on results, weed seed bank densities in chemical managed farms (CMFs) was generally higher than those farms without chemical control (NMFs). At first sampling, average weed seed bank populations in CMFs, were 49 and 31 seeds kg-1 of soil, and for NMFs 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 NMFs were 210 and 254 seeds kg-1 soil in 2004 and 2005, respectively. Seed bank density decreased over sampling times (growing season) for CMFs in 2004 as compared to NMFs. In 2005, the NMFs variation trend of the seed bank densities for managed farms, was constant between the two stages of sampling. But the trend of variation in NMFs was similar to the previous year. Shannon-Weiner diversity index in CMFs was higher than those of NMFs. Probably, 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.