Grass-knapweed interference involves allelopathic factors associated with ecosystem mineral cycling

Russian knapweed [Acroptilon repens (L.) DC.] is an aggressive invader known to have interference mechanisms that contribute to its robustness and ability to dominate native grassland communities. Observations suggest allelopathy and a vigorous lateral root system contribute to its success in a variety of arid and other stressful environments. During colonization Russian knapweed mineralized Fe, Mn, P, and Zn from lower positions in the soil profile and deposited them in leaves. Due to low herbivory, they accumulated along with organic matter (OM) near the soil surface. Forty years after the invasion by Russian knapweed soil Zn was 3- to 4-fold greater at 0 to 2.5 cm than at 7.5 to 15 cm soil depth and was 4-fold higher than at the 0–2.5 cm depth under native grass. Organic matter levels under Russian knapweed were comparable to Zn in amount of increase when compared to soils under native grass.; In other field experiments in Wyoming, herbicides reduced stand density of Russian knapweed, but the plants gradually regrew unless they were suppressed long enough to allow establishment of improved grass species. Treating tilled areas with picloram followed by seeding Russian wildrye (Psathyrostachys juncea Fisch. Nevski) gave the most consistent long-term control of Russian knapweed. Four field sites ranging from 13 to 23 cm of annual precipitation were evaluated 7 to 9 years after herbicide applications and planting of Russian wildrye. Few knapweed seedlings were noticed indicating most of the re-infestation originated from lateral roots of Russian knapweed that grew 7 to 14 cm below the soil surface. Concentrations of tissue Zn in Russian knapweed were 6- to 12-fold greater than those of Russian wildrye across 7 locations. Killing Russian knapweed and reseeding with grasses did not alter the nutrient profile after 7 or 8 years. Rates of OM and Zn accumulation and degradation under Russian knapweed compared to established grasses indicate it may take as long as 200 years to recover the pre-invasion profile. Complementary lab experiments were conducted to determine if Zn accumulated in plant tissue may contribute to autotoxic and allelopathic properties of Russian knapweed. Water extracts of Russian knapweed (8 g L−1) and solutions of ZnSO ₄ (250 μm) exhibited similar patterns of root growth inhibition of four indicator species including Russian knapweed. This suggests an inherent compromising influence of Zn cycling to the upper soil profile that interferes with seedling growth of other plants. The Zn accumulated in plant tissue of Russian knapweed may be a contributor or stabilizer for other allelopathic chemicals such that Russian knapweed is both autotoxic and allelopathic. Tissue concentrations of Zn alone do not appear to be solely responsible for the observed inhibition.