Nodulation of legumes, nitrogenase activity of roots and occurrence of nitrogen-fixing Azospirillum ssp. in representative soils of central Amazonia

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.