Study of diversity and density of weed seed bank species in onion production cropping systems

By identifying the weed seed bank can be ascertained to the composition and density of weed seed bank species and flora during the growing season and in choosing the best time and method for effective control of weed seeds in the soil was careful. Since continuous cultivation of a plant in the farm increased the seed bank of weeds, therefore, in order to describe the plant communities of the farm surface and evaluate any changes in them, it is necessary to know and study the soil seed bank. In fact, knowing the seed bank can help control future weeds on the farm and focus on better management operations. This study was conducted in order to evaluate the effect of onion monoculture continuing on weed density and diversity in the conventional agroecosystems of Jiroft region.

Weed seed bank sampling was performed from two soil depths of 0-20 and 20-40 cm by systematic random method. Density and diversity of weed seed bank were determined by germination method in greenhouse conditions. The study population were includes soil samples from four onion monoculture crops with a duration of less than three years (S1), between three to five years (S2), between five to eight years (S3) and more than eight years (S4) and long period fallow system (F). Analysis of variance of data related to different indices and comparison of different indices of diversity and richness in different onion monoculture systems were calculated by Duncan test.

At both 0-20 and 20-40 cm depths, the highest density was related to chameleon weeds. The total number of germinated seeds in greenhouse conditions in fallow and onion monoculture systems at depths of 0 to 20 was about 4140 seeds in m3 of soil that at this depth, F and S4 cropping systems had the lowest and highest number of weeds with an average of 221 and 1704 seeds in m3, respectively, and the majority of seeds were observed at this depth. At a depth of 20-40 cm, 2049 seeds in m3 of soil were observed. At this depth, S4 and F cropping systems with the average of 830 and 143 seeds had the highest and lowest number of weeds, respectively. Also, the results showed that continuous onion monoculture at both depths had a significant effect on weed diversity and species richness indices and S4 and F cropping systems had the highest and lowest diversity and richness, respectively. In fact, weed density increased with increasing continuous onion monoculture.

Because the density of weed seeds in fields under continuous cultivation is higher than fallow and a higher proportion of seeds accumulate in the surface layer of the soil. So, seed density can be reduced in the seed bank with proper rotation. Therefore, it is recommended to use crop rotations with a diverse collection of winter and spring plants instead of continuous cultivation of a crop that by increasing diversity, it provides suitable conditions for better control and interruption of weed life cycle.