Apesar do ciclo efetivamente rápido do feijoeiro, a habilidade no uso dos nutrientes adicionados por ocasião do plantio e cobertura pode ser prejudicada pela presença de determinadas espécies de plantas daninhas. Objetivou-se com este trabalho avaliar os efeitos da competição entre três cultivares de feijão (IPR Colibri, IPR Eldorado e Pérola) e seis espécies de plantas daninhas (Euphorbia heterophylla, Bidens pilosa, Cenchrus echinatus, Amaranthus spinosus, Commelina benghalensis e Brachiaria plantaginea) no acúmulo de N, P e K pelas plantas e na eficiência nutricional do feijoeiro. O experimento foi realizado em condições controladas de temperatura e umidade, em delineamento de blocos casualizados, com quatro repetições. O período de convivência entre os cultivares de feijão e as plantas daninhas foi de 45 dias após emergência da cultura. Os cultivares de feijão apresentaram reduzido acúmulo relativo de N, P e K quando estavam em competição, sendo o sistema radicular o principal órgão afetado negativamente. O cultivar IPR Colibri foi o que menos tolerou a competição com plantas daninhas e E. heterophylla foi a espécie com menor poder de competição, enquanto A. spinosus e B. plantaginea foram as mais competitivas com a cultura do feijão. A eficiência nutricional do feijoeiro variou conforme o genótipo de feijão e a espécie infestante.
The forage grasses are explored for the phytoremediation of heavy metals from contaminated areas. The present study evaluates the tolerance of forages grasses to Pb and the availability of Pb for the Mehlich 1, DTPA, and USEPA 3051 and 3052 extraction methods. The forage grasses cultivars Panicum maximum cultivars Aruana and Tanzânia, Brachiaria decumbens cultivar Basilisk, Brachiaria brizantha cultivar Xaraés and Marandu were assessed. Each cultivar was grown in a greenhouse in Diamantina, Brazil, in Typic Hapludox (Oxisol). The Pb was applied to the soil at doses of 0, 45, 90 and 270 mg kg-1 soil in a randomized design with four replications. The experiment was conducted over a 90-day trial period. Increasing Pb doses in the soil caused a reduction of growth in all varieties studied, and the "Basilisk" forage was generally more tolerant to Pb, while the Xaraés forage was generally more susceptible to the addition of Pb. The lower transfer coefficient of the cultivar "Basilisk", due to the high Pb content and accumulation in its roots, makes it the most suitable for phytoremediation programs, with possible uses in pasture areas. None of the varieties can be considered a Pb hyperaccumulator. The Mehlich 1, DTPA, USEPA 3051 and USEPA 3052 extraction methods indicated different doses of Pb in the soil, and the Mehlich 1 method demonstrated the lowest level of metal extraction from the soil. No correlation was observed between the forage grasses growth and the measurement of Pb in soil by the studied extraction methods, especially for the Mehlich 1 and DTPA methods, suggesting the inefficiency of these techniques for determining the phytoavailability of Pb.
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