Effects of different nitrogen levels on phytotoxicity of some allelopathic crops

Intensive usage of herbicides can result in the serious negative impacts on environment. Allelopathy by reducing seed germination and early seedling growth can play a fundamental role in suppressing weeds in crop fields. The effectiveness of allelochemicals is governed by different factors such as soil nutrient status, pH and microorganisms. Outdoor pot experiments were conducted at the Faculty of Agriculture and Natural Resources of Razi University, Kermanshah, Iran, in 2013, to evaluate the effects of different levels of N fertilizer (0, 150, 300 kg ha^-1) on the suppressing effects of alfalfa (Medicago sativa L.), sorghum (Sorghum bicolor L.), and tobacco (Nicotiana tabacum L.) plant materials on emergence and growth parameters of some weed species including Johnson grass (Sorghum halepense (L.) Pers.), barnyard grass (Echinochloa crus-galli (L.) Beauv.) and redroot pigweed (Amaranthus retroflexus L.). Results indicated that adding plant materials of tobacco, sorghum, and alfalfa substantially reduced seed germination and early growth of the tested weeds. However, the weed species responded differently to the presence of the allelopathic plant materials. The use of N fertilizer had significant effects on the inhibitory potentials of the allelopathic plants. However, we didn t find consistent trends regarding the responses of the allelopathic crops to elevated N fertilizer levels in related to the traits under study.