SOYBEAN NODULATION AND NITROGEN-FIXATION ON SOIL AMENDED WITH PLANT RESIDUES

Residues from some tree species may contain allelopathic chemicals tha t have the potential to inhibit plant growth and symbiotic N-2-fixing microorganisms. Soybean [Glycine max (L.) Merr] was grown in pots to c ompare nodulation and N-2-fixation responses of the following soil ame ndments: control soil, leaf compost, red oak (Quercus rubra L.) leaves , sugar maple (Acer saccharztm Marsh) leaves, sycamore (Platanus occid entalis L.) leaves, black walnut (Juglans nigra L.) leaves, rye (Secal e cereale L.) straw, and corn (Zea mays L.) stover. Freshly fallen lea ves were collected from urban shade trees. Soil was amended with 20 g kg(-1) air-dried, ground plant materials. Nodulating and nonnodulating isolines of ''Clark'' soybean were grown to the R2 stage to determine N-2-fixation by the difference method. Although nodulation was not ad versely affected, soybean grown on leaf-amended soil exhibited tempora ry N deficiency until nodulation. Nodule number was increased by more than 40% for soybean grown on amended soil, but nodule dry matter per plant generally was not changed compared with control soil. Nonnodulat ing plants were severely N deficient and stunted as a consequence of N immobilization. Nodulating soybean plants grown on leaf or crop resid ue amended soil were more dependent on symbiotically fixed N and had l ower dry matter yields than the controls. When leaves were composted, the problem of N immobilization was avoided and dry matter yield was n ot reduced. No indication of an allelopathic inhibition on nodulation or N-2-fixation from heavy applications of oak, maple, sycamore, or wa lnut leaves to soil was observed.