ALLELOPATHIC EFFECTS OF ANDROPOGON VIRGINICUS AND ITS PERSISTENCE IN OLD FIELDS

Aqueous extracts of fresh roots and shoots of Andropogon virginicus (broomsedge) were found to be inhibitory to the growth of seedlings of Amaranthus palmeri, Bromus japonicus, Aristida oligantha, and Andropogon scoparius. The first two species are often important in the pioneer stage of old-field succession in eastern Oklahoma, Aristida is prominent in the second stage, and Andropogon scoparius is important later in succession including the climax Quercus stellata–Quercus marilandica savanna. Sterile dilute extracts of roots and shoots of broomsedge were inhibitory to two test species of Azotobacter, a free living nitrogen fixer, and to two species of Rhizobium, a symbiotic nitrogen fixer. Small amounts of decaying shoots of broomsedge (1 g per 454 g of soil) were very inhibitory to the growth of the four test species listed above and to Amaranthus retroflexus, another species often important in the first stage of succession. Similar amounts of decaying material in soil also significantly inhibited growth and nodulation of the two most important species of legumes in old-field succession in eastern Oklahoma, Lespedeza stipulacea and Trifolium repens. Broomsedge is known to compete vigorously and grow well on soils of low fertility, so the inhibition of nodulation of legumes could help keep the nitrogen supply low and give broomsedge a selective advantage in competition over species that have higher nitrogen requirements. The combined interference of broomsedge against other species resulting from competition and allelopathy could help explain why it invades old fields in 3–5 yr after abandonment from cultivation and remains so long in almost pure stands.