Inhibition of Growth, Nodulation, and Nitrogen Fixation of Legumes by Quackgrass1

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Abstract
Quackgrass (Agropyron repens L. Beauv.) has been previously reported to possess allelopathic potential. Both the shoots and rhizomes of quackgrass have been shown to contain inhibitors of seed germination and crop growth. We recently observed chlorosis and decreases in nodulation in snapbeans that were planted into quackgrass infested sites. The objective of this study was to determine if nodulation, N,2 fixation and legume growth were inhibited in the presence of living or herbicidally treated quackgrass. Soybeans (Glycine max L. Merr.) and navybeans and snapbeans (Phaseolus vulgarls L.) were grown both in greenhouse and field experiments in living or killed quackgrass. Inoculated legumes were seeded in the following regimes: 1) living quackgrass sod which was regularly mowed; 2) glyphosate [N‐(phosphonomethyl) glycine] ‐treated quackgrass sod 3) soil from quackgrass sod from which plant material was removed by sieving or raking; 4) a control soil of similar type and physical structure free from quackgrass infestation. Nodulation and plant growth were measured by recording nodule number and fresh weight and root and shoot weights before flowering. Nitrogen fixation was estimated by the acetylene reduction assay. Legumes grown in mowed quackgrass sod in the greenhouse and in the field exhibited decreased nodule number, nodule fresh weight and N, fixation when compared to legumes grown under similar conditions in screened quackgrass soil or the control soil. Shoot and root weights were also significantly decreased in field and greenhouse experiments when legumes were grown in living quackgrassod. In many cases, legume nodulation and growth were decreased in glyphosate‐treated quackgrassod as compared to screened quackgrass soil or the control soil. Decreases in legume growth and nodulation in the presence of quackgrass may be attributed to allelochemical effects.