The objective of this study was to compare the ex situ and in situ floristic diversity of the soil weed seed bank of a rice field in northeastern Brazil. In a rice field in the county of Bacabal, located in the state of Maranhão, thirty 25-m 2 plots were laid out. From 15 plots, soil samples (6/plot; n = 90) were taken with a soil probe (25 × 16 × 3 cm) and placed in aluminum trays in the greenhouse. From the remaining 15 plots, weed samples (6/plot; n = 90) were taken with the same soil probe. The number of seeds was estimated by germination. We evaluated the numbers of species and individuals, as well as the density, frequency, abundance and importance value (IV) for each species. Diversity was computed by the Shannon index (H'). We recorded 13,892 individuals (among 20 families, 40 genera and 60 species), of which 11,530 (among 50 species) germinated ex situ and 2,362 (among 34 species) germinated in situ. The family Cyperaceae had the highest number of species (16), followed by Poaceae (10). The dominant species, in situ and ex situ, were Schoenoplectus juncoides (IV=47.4%) and Ludwigia octovalvis (IV=34.8%), respectively. Floristic diversity was higher ex situ (H'=2.66). The information obtained here could help determine the infestation potential of these species, which could lead to improved management strategies.

RESUMO -Os objetivos desta pesquisa foram identificar e avaliar a estrutura das comunidades e a diversidade das espécies herbáceas invasoras presentes no banco de sementes do solo, em áreas de cultivo de subsistência nos . The results may be used to forecast invasive species infestations in municipalities of the studied region. SOIL SEED BANK IN SUBSISTENCE CROPPING AREAS IN THE OMBROPHILOUS OPEN FOREST WITH BABASSU (Orbygnia phalerata Mart.) IN MARANHÃO STATE, NORTHEASTERN BRAZIL

The weed seedbank in the soil is the major source of weeds in rice fields. Therefore, information on ecological aspects of weeds occurring in rice, including their potential seed production, is crucial for weed management. The size of the weed seedbank in rice fields is highly variable depending on the climate, relief position, soil moisture content, depth of sampling, history of the areas and management practices used by farmers. As a survival strategy, colonization and persistence in the communities, most common weeds in rice fields produce huge number of seeds and vegetative propagules with physical and physiological dormancy mechanisms, insuring seed viability in the soil for long periods. A large proportion of weed seedbank remains generally on or close to the soil surface after seed rain. Sampling protocols involve the use soil cores at variable soil depths. Determination of the size of the weed seedbank can be made by seed direct extraction and germination methods. The latter is more precise with respect to enumeration of viable seeds in the soil. Weed management in rice fields should focus on methods suitable to decrease the weed population in the soil seedbank.

ABSTRACT. The objective of this research was to assess the density, floristic composition, phytosociology and diversity of a soil weed seed bank ex situ by germination in a greenhouse and in situ by weed sampling on a smallholder corn field located in Lago Verde County, Maranhão State. Fifteen pairs of 25 m 2 plots were designated. In half of these plots, 90 soil samples were collected with an open metal template measuring 25 x 16 x 3 cm and placed in a greenhouse to germinate. In the other half, 90 weed samples were collected using the same metal template. We recorded a total of 1,998 individuals from 40 species, 31 genera and 16 families, from which 659 individuals germinated in situ and 1,339 ex situ. Density was higher ex situ, with 372 plants m -2 . The Cyperaceae family had the highest floristic richness with nine species, followed by the Poaceae with six. The dominant species based on the Importance Value Index were Lindernia crustacea (IVI 27.7%) in situ and Scleria lithosperma (IVI 37.0%) ex situ. Floristic diversity was higher ex situ, with H' = 2.66 nats ind -1. These results could help predict infestation potential and could lead to improved weed management strategies in corn-growing areas on smallholdings in Maranhão State, northeastern Brazil.Keywords: cyperaceae, competition, biological invasion, phytosociology, smallholder farmers.Banco de sementes de plantas invasoras no solo in situ e ex situ em área de agricultura de subsistência no Estado do Maranhão, Nordeste do Brasil RESUMO. O objetivo desta pesquisa foi avaliar a densidade, composição florística, fitossociologia e a diversidade no banco de sementes do solo ex situ pelo método de germinação em casa de vegetação e in situ em área de cultivo de subsistência de milho no município de Lago Verde, Estado do Maranhão. Quinze pares de parcelas de 25 m 2 foram alocadas. Na metade destas parcelas, 90 amostras de solos foram retiradas com um gabarito de metal vazado de 25 x 16 x 3 cm e colocadas para germinar em casa de vegetação. Na outra metade, 90 amostras de plantas invasoras foram retiradas com o mesmo gabarito. Foi registrado o total de 1.998 indivíduos de 40 espécies, 31 gêneros e 16 famílias, dos quais 659 indivíduos de 29 espécies germinaram in situ e 1.339 individuos de 29 espécies ex situ. A família Cyperaceae teve a maior riqueza florística com 9 espécies, seguida pela família Poaceae com seis. As espécies dominantes baseado no índice de valor de importância foram Lindernia crustacea (IVI 27,7%) in situ e Scleria lithosperma (IVI 37,0%) ex situ. A diversidade florística foi maior no banco de sementes do solo ex situ com H' = 2,66 nats ind -1 . Estes resultados podem ajudar na previsão de infestações e contribuir para melhorar estratégias de manejo de plantas invasoras em areas de cultivo de subsistência no Maranhão.

-Knowledge of weed floristic composition and phytosociology are key factors for improving weed management in pastures. Information on weed species that occur in pastures in Northeast Brazil, particularly in Maranhão State is very limited. It is, therefore, important, to search for information to help farmers to control weeds in livestock farming. This paper describes the weed flora diversity and community structure parameters, including density, frequency, abundance and importance value for each weed species found in five pastures of same age and management in Maranhão State, Northeast Brazil. The weed survey was carried out using a wooden frame (80 cm x 30 cm) placed randomly on the soil surface 30 times in each pasture (n = 150). Weeds were pulled out, separated by species and counted. The