effects of crop factors and soil characteristics on weed composition and diversity in wheat (triticum aestivum l.) fields in city of jajarm

the study was performed for assess the environmental factors affecting weed species diversity and composition of wheat (triticum aestivum l.) fields in jajarm, north-khorasan of iran, in 2009. the crop management (herbicide, amount of nitrogen applied and preceding crop type) and edaphic factors (ph, texture and content of phosphorus) of 16 fields were collected. shannon’s diversity index, species richness and simpson’s dominance index were used as measures of species diversity. means comparison showed that use of herbicide versus non-herbicide application, silt texture than loam texture and planting after fallow versus planting after melon significantly decreased species richness (4.92 versus 7.12, 5 versus 6.5 and 5.12 versus 6.75 for herbicide, soil texture and preceding crop type, respectively) and shannon’s diversity index (1.08 versus 1.6, 1.1 versus 1.48 and 1.19 versus 1.49 for herbicide, soil texture and preceding crop type, respectively), but increased simpson’s dominance index (0.43 versus 0.25, 0.41 versus 0.3 and 0.39 versus 0.29 for herbicide, soil texture and preceding crop type, respectively). regression analysis revealed negative linear relationship between applied nitrogen and diversity components (r2= 0.45). the relationship between applied nitrogen and simpson’s dominance index was positively linear (r2= 0.46). there wasn’t relationship between ph and phosphorus of soil with shannon’s diversity index, species richness and simpson’s dominance index. redundancy analysis (rda) resulted in patterns per weed community composition (f= 4.03, p-value= 0.001). major changes in weed species composition in the study area were associated with application of herbicide. using of 2, 4- d and nitrogen fertilizers lead to domination of grass in these fields. the method of variation partitioning out showed that crop factors on changes in arable weed species composition are more important edaphic variables.