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Controlling weed populations by manipulating their seed banks is an important weed management option. To assist such efforts, we investigated relationships between fertilization treatments and depth-related characteristics of the weed seed bank (density, species composition and diversity) under a wheat-soybean rotation after long-term (16 years) fertilization. Numbers of weed species present and the Shannon-Wiener index were significantly lower under NPK, NP, NK, and PK fertilization treatments than under the fertilization-free control treatment (CK), and the vertical distribution of dominant species differed under the treatments. Generally, the species richness and Shannon-Wiener index decreased and the Pielou index increased with increases in soil depth, but the relationship of the Simpson index with depth was complex and unclear. The results show that effects of considered fertilization treatments on weeds warrant careful attention, and that PK fertilization would be optimal for suppressing weeds in the wheat-soybean rotation system studied.
Weed seed return and seedbank composition, with particular reference to common lambsquarters, were studied in four tillage systems established on a site near Fingal, Ontario. The tillage treatments were moldboard plow, chisel plow, ridge-till, and no-till. The cropping system was a cornsoybean rotation. Tillage effects on weed population composition were assessed after all weed control measures had been implemented. More than 60% of the weed seedbank was concentrated in the upper 5 cm of soil in chisel plow and no-till. The seedbank of the moldboard plow system was more uniformly distributed over depth and larger than the other systems. Common lambsquarters comprised more than 50% of the seedbank in all systems except ridge-till, but only dominated the aboveground weed population in chisel plow. Seedbank populations of common lambsquarters with moldboard plowing were greater than those with ridge-till and no-till, and chisel plow seedbank populations were greater than those in ridge-till. Chisel and moldboard plow systems generally had higher aboveground plant populations of common lambsquarters than the other two systems. Seed production per plant by common lambsquarters was equivalent among the four systems, but estimated seed production per unit area was higher in moldboard plow and chisel plow systems than in the other systems. Populations of common lambsquarters and similar species may produce more seeds and persist in moldboard plow and chisel plow systems; these weeds may produce fewer seeds per unit area and be easier to manage in no-till and ridge-till systems.