The objective of this work was to study the soil seed bank of a four-year-old Brachiaria brizantha pasture soil, and in pasture soils of four, 15 and 20 year–old Brachiaria humidicola, in northeastern Pará, Brazil, to evaluate the influence of the forage species and pasture age on weed seed bank size and composition. Each pasture area was divided into five plots of about 500 m², from which 10 soil sub-samples were collected at a depth of 0-10 cm. The soil from each plot was mixed, identified, placed in plastic trays, and put to germinate in a greenhouse during a 15-month period. The seed bank of the four-year-old B. brizantha pasture soil was about 10 times smaller than that in the four-year-old B. humidicola pasture. Among the B. humidicola pasture, the effect of pasture age showed that the weed seed bank was smaller in the 20-year-old pasture (1,247 seeds m-2). No significant difference could be detected between the 15- (11602 seeds m-2) and the four-year-old (9486 seeds m -2) pastures. The botanical families Cyperaceae, Rubiaceae and Labiateae were the most predominant among the weeds infesting all studied treatments.
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