Estimating biological nitrogen fixation potential of tropical legumes grown in acid savannah soils of Venezuela using 15N-isotopic techniques

The main limiting factors of agricultural production in the tropical acid savannah soils of Venezuela are the toxicity of aluminium (Al) and the deficiencies of phosphorus (P) and nitrogen (N). Sustainable and cost-effective N-replenishment strategies, in particular for smallholders can rely on the potential inputs from the biological N2- fixation (BNF) in legumes, thus contributing to the overall N economy of system and reducing the needs for N-fertilizer to be applied to cereals or grasses. Preliminary experiments were carried out to estimate the BNF potential in various tropical legumes grown in a Typic Paleustult of the savannah of Venezuela using 15N-isotopic techniques. In the field experiment pigeon pea (Cajanus cajan) showed high BNF (79% of its total N) with a positive N balance in the soil-plant system. Thus it is a promising grain legume for inclusion in cop rotation systems in the tropical acid savannah soils of Venezuela. In a greenhouse study, the nitrogen fixation estimates in soybean and indigosphera were 68-76% and 62-71% Ndfa respectively indicating their potential for inclusion as N contributors to crop rotation systems. Field experiments should be carried out to confirm this potential, to establish an accurate N balance and assess the N contribution of soybean and indigosphera to the cereal crop grown in rotation or as green manure/cover crop in the savannah areas of Venezuela. (author)