Previous studies have demonstrated that emergence and growth of Powell amaranth is inhibited in soils where buckwheat has been grown and incorporated. The primary objectives of this research were to (1) evaluate the possible role of allelopathy in explaining that suppression; (2) distinguish between suppression caused by incorporation of fresh buckwheat residues from suppression caused by changes in soil during buckwheat growth; and (3) quantify the relative importance of buckwheat root vs. shoot tissues in suppression. When all buckwheat plant parts were removed from soil in which buckwheat was grown, Powell amaranth emergence was not suppressed, but growth was reduced 70% compared to bare soil. Addition of buckwheat shoots, but not roots to these soils reduced emergence by 80%, and contributed to additional reduction in growth. Addition of chemically activated carbon did not increase emergence or growth in buckwheat-amended soil. However, thermally activated carbon resulted in greater adsorption of phenolics than chemically activated carbon and alleviated suppression of Powell amaranth in buckwheat-amended, high organic-matter soils. However, suppression was not overcome on mineral soils. In addition to adsorbing phenolics, activated carbon changed the nitrogen (N) content and electrical conductivity of soil extracts. Aqueous shoot extracts of buckwheat stimulated Powell amaranth germination slightly, but inhibited radicle growth. Aqueous soil extracts from buckwheat-amended soil inhibited germination of Powell amaranth compared with extracts from unamended soil. Results suggest that emergence suppression of Powell amaranth by buckwheat residues might be due to allelopathic compounds concentrated in the shoot tissues. However, these inhibitory effects appear to depend on interactions of buckwheat residues with soils. In contrast, suppression of growth of Powell amaranth appears to be associated primarily with lower N availability in buckwheat-grown soils.

Nomenclature: Powell amaranth (= green pigweed), Amaranthus powellii S. Wats. AMAPO; buckwheat, Fagopyrum esculentum Moench