The seed bank in the soil was estimated in November/92 at 0-10 cm depth in some agro-ecosystems: arable land, meadow land, citrus orchard and a Brachiaria brizantha pasture in Distrito Federal, Brazil. These estimates were carried out by the evaluation of seedling emergence in incubated soil samples, with and without pre-washing through sieve (0,297 mm mesh). The pre-washed samples were incubated with potassium nitrate, while water was added to the others. The pre-washing plus potassium nitrate reduced the amount of seedlings in all samples, and thus is not recommended. The average numbers of viable seeds per square meter in the non-washed samples were: 22313 in the marsh land, 6 768 in the arable land, 3 595 in the projection of the plants in the orchard and 529 in the pasture. The established plants in the field were evaluated once in May/93, on normal crop conditions. They represented, in relation to the initial seed bank: 0.34% in the pasture, 0,71% in the meadow land, 1,48% in the orchard and 1.56% in the arable land. These plants would contribute to the maintenance of the seed bank in the soil. There was a direct correlation between the plant population and the number of species with the size of seed bank. The species that predominated in the disturbed areas were: Ageratum conyzoides, Bidens pilosa, Cenchrus echinatus, Commelina benhhalensis, Emilia sonchifolia, Euphorbia heterophylla and Richardia brasiliensis. Brachiaria spp predominated between the plants in the orchard, while the few weeds that occurred in the pasture belonged to the natural cerrado vegetation of the region. The similarity of agro-ecosystems was higher in disturbed areas, such as arable land, meadow land and orchard.
Population dynamics; weed seeds; soil