Allelopathic effect of common weeds on soybean growth and soybean-Bradyrhizobium symbiosis

Water extracts of shoot of common lambsquarters (Chenopodium album), yellow nutsedge (Cyperus esculentus) and sunflower (Helianthus annuus) at 1% level significantly reduced soybean seed germination. Soybean seedlings inoculated with broth culture of nodule bacterium were grown for 25 days in N-free nutrient solution amended with cold water extracts of weed residues at 1 and 2% levels. At both levels extracts from residues of all weeds except that of lambsquarters enhanced growth of soybean. Nodulation was generally stimulated by the extracts of five weeds at 1% level except that of lambsquarters. Extracts from lambsquarters at 2% level completely suppressed and at 1% level reduced nodulation by 60%. Extracts from green foxtail (Setaria viridis), Pennsylvania smartweed (Polygonium pensylvanicum) and sunflower at 2% level reduced and at 1% level enhanced nodulation. The residues of lambsquarters shoot incorporated with soil at 0.5 and 1% levels caused 85 and 96% reduction respectively in seed germination and those of Pennsylvania smartweed and sunflower at 1% reduced seed germination by 40–70% but not at 0.5% level. The residues of foxtail and smartweed at both levels enhanced growth and nodulation. Under similar conditions nutsedge at 1% level stimulated nodulation but not growth. The residues of lambsquarters at both levels were inhibitory to nodulation but stimulated growth at 0.5% level.