The study of the soil seed bank in pasture areas is essential to understand the structural complexity of the predominant plant species, placing greater emphasis on investigations of the ecological and morphological behavior of local vegetation in the ecosystem. Soil seed bank is called the stock of viable seeds in the soil of a given area, at a given time, functioning as a dynamic system, with seeds that germinate in up to one year, or persistent, with seeds that remain in the soil for several years. The objective of this work was to study qualitatively and quantitatively the soil seed bank of pasture areas with the occurrence of buttonweed (Spermacoce verticillata L.), in the municipality of Itacoatiara-AM. The collection of soil samples for this research was carried out in three pasture areas, being two on dry land (area 1 and 2), whose frequency of occurrence of buttonweed was higher than 70%, and the third area on lowland soil (area 3), visually without the occurrence of a buttonweed. In each study area, 20 soil samples were collected, with the aid of iron templates 25 cm long, 16 cm wide and 3 cm deep. The soil samples, after being collected, were placed in plastic bags and transported to the greenhouse, where they were torn apart, uniformed, placed in trays with the same dimensions as the templates and were placed on benches in the greenhouse. The count and identification of seedlings started with the emission of cotyledonary leaves above the ground, and continued until the moment when there was no more seed germination in the trays. The characteristics evaluated were: emergence curve, floristic composition, phytosociological diversity and structure. In total, 24,716 seeds germinated, among these, 6,571 in area 1; 5,245 in area 2 and 12,900 in area 3. Buttonweed, Cyperus distans and Fimbristyles miliacea were the most important species in the study areas. The floristic diversity index of areas 1, 2 and 3 was 1.52, 1.31 and 1.19, respectively. The evenness indices for areas 1 and 2 were 0.173 and 0.153, respectively, and for area 3 it was 0.173. The average seed bank density in pasture areas 1 and 2 was 8,214 and 6,556 seeds per m², respectively, while in area 3 it was 16,125 seeds per m². The most abundant species in area 1 was buttonweed, in area 2 it was C. distans, and in area 3 the species F. dichotoma and F. miliacea. In the seed bank in area 1, the most important species was buttonweed, in area 2 it was C. distans and in area 3, F. miliacea. Buttonweed had a great influence on the dynamics and composition of the seed bank in areas 1 and 2, apparently decreasing the herbaceous diversity of these areas. Buttonweed, C. distans, and F. miliacea were the most important species in the study areas. Lowland areas present unfavorable conditions for the growth of buttonweed. Buttonweed negatively affected the dynamics and composition of the seed bank, visibly decreasing the species diversity in the infested areas.
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