In order to investigate the effects of plant diversity and nutrient resource on weed composition, density and dry matter, an experiment was conducted as split plot based on complete randomized block design with 3 replications at the Agricultural Research Station, Ferdowsi University of Mashhad, Iran, during 2006 and 2007. Treatments included manure and chemical fertilizers as main plots and intercropping of 3 soybean varieties (Wiliams, Sahar and Gorgan3), intercropping of 3 Millet species (Common millet, Foxtail millet and Pearl millet), intercropping of Millet, Soybean, Sesame (Sesamum indicum) and intercropping of Millet, Sesame, Fenugreek (Trigonella foenum-graecum), Ajowan (Trachyspermum ammi) as sub plots. Result indicated that nutrient resource affected weed dry matter and density. Weed dry matter and density was respectively, 1.3 and 1.8 folds higher in chemical fertilizer compared to manure in first year. In the second year, weed dry matter in manure and chemical fertilizers was 173.2 and 300.2 g.m-2 and weed density was 98.6 and 84.9 plants per square meter. With increasing crop diversity, weed dry matter and density decreased and intercropping systems had the lowest weed dry matter. Crop species affected weed dry matter in monocultures. There was a negative correlation between diversity and weed dry matter. In the first year Shannon diversity index was highest in sesame and Ajowan monocultures (0.75 and 0.72, respectively). Different intercropping systems had the lowest Shannon index. In the second year, Shannon index was highest in soybean (Sahar variety) monoculture (0.72) and 3 Millet species intercropping (0.71). More researches on the effects of crop diversity on weed population are needed in mixed intercropping. With increasing crop diversity, soil microbial respiration was increased. Also, there was positive significant regression between weed Shannon index and soil microbial respiration.

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