Agro-Ecosystems最新文献

Details are presented on crop associations and relative densities of crops in food crop fields. Assessment of yield potential is based on available yield data from various sources and on soil analyses. A fertility budget is developed and limiting factors are discussed.

Leguminosae do not predominate in the Brazilian Amazon rain forest, although they are among the five best represented families. Plant roots from various soils were examined for the presence of nodules, acetylene-reducing activity and N₂-fixing Azospirillum spp.

Abundant nodulation was found in black earth (“terra preta dos índios”) and in one case on sandy soil under campinarana vegetation along the tributary of the upper Rio Negro. Nodules were not found on plants from many other soils and sites in this area, nor in primary or secondary forest on clayey latosol near Manaus. In sandy latosol some nodules occured in secondary forest and fewer in primary forest. Legumes in distributed clayey or sandy latosol showed more frequent nodulation. Primary forest on alluvial (“várzea”) soil, and in Bahia coastal rain forest on sandy latosol and Erythrina glauca used for shading cacao plantation were abundantly nodulated.

Acetylene reduction assays showed no, or very little, nitrogenase activity of roots from forest on sandy latosol showed acetylene-reducing activity. High rates of acetylene reduction were observed in nodulated roots of primary on alluvial “várzea” soil. Root samples showed ethylene absorption in controls without acetylene which migh interface with the results of acetylene reduction tests.

The incidence of Azospirillum was also higher in black earth than the other soils examined, and in soils with higher pH. The hypothesis that Azospitillum is associated with Trema micrantha roots was refuted. Roots and soils collected under cultivated grasses showed a higher incidence of Azospirillum when fertilized with phosphorus and line.

Results indicate the nitrogen fixation did occur in association with roots in some soils, but not with roots of primary or secondary forest on clayey latosol in the vicinity of Manaus, which is the most common soil in Central Amazonia. The possible reasons for this are discussed.

A marginal agricultural area (soil pH = 4.0, sandy moraine) in Mols Bjerge, Jutland, Denmark, was grazed by a herd of nursing cows for five years. The amount of exchangeable Ca, Mg, K, Mn, Fe and P together with the pH, bulk density and organic material in the upper 10 centimetres of the soil as well as the amounts of the above elements in the above ground biomass were measured over the period.

Grazing did not cause any significant decline in the amounts of available nutrients in the soil even though no fertilizer was added, but the exchangeable amounts of Ca, Mg and to some extent Mn and P were increased as a result of grazing. No changes were detected in the pH of the soil, but the bulk density and the content of organic material in the upper soil layer were increased. Apart from P, the deposition of nutrients from precipitation was greater than the amounts removed from the area with the carcasses.

Two diphenyl ether pre-emergence herbicides, oxyfluorfen and nitrofluorfen, have been compared with manual weeding as a means of establishing Centrosema pubescens and Pueraria phaseoloides cover crops under Elaeis guineensis. Oxyfluorfen at 0.25 kg a.i. ha⁻¹ (a.i. = active ingredient) was the most effective treatment. Application of oxyfluorfen led to faster rates of legume establishment and higher yields of legumes. Twenty-six months after legume planting, the amount of legumes in each plot exceeded 95% of the sown area. Mineral composition of both legume shoots and litter in the oxyfluorfen treated plots was at least comparable, and usually better than that found in covers established by hand weeding.

Data on nutrient acquisition and nutrient re-cycling were obtained by comparison with plots containing naturally regenerating vegetation, fastidiously maintained bare soil, or hand-weeded legume plots in which the oil palms received extra fertilizer. These data show that legumes established by herbicide application contributed greater quantities of nutrients to the developing oil palms. This improved nutrient status led to significantly enhanced relative growth rates of the oil palms. All these benefits accrued at lower cost than the normal estate practice of hand-weeding. Thus oxyfluorfen appears to have great potential in legume cover establishment.