Nitrogen fixation in shade trees (Erythrina poeppigiana) in cacao plantations in north Venezuela

Abstract

Symbiotic nitrogen fixation in nodules of Erythrina poeppigiana, a shade tree used traditionally in Venezuelanean cacao plantations, was studied during 10 months in a 32 year old plantation which had been abandoned and in a 12 year old plantation fertilized regularly. Nitrogenase acitivity (C₂H₂) in soil squares containing nodules was very low during the dry season when most nodules fell off, and increased rapidly with the beginning rains. The largest nodule density was closer to the cacao trees in the fertilized area (0.4 m), then in the not fertilized area (3.8 m).The mean nitrogen content in nodules was 22 kg/ha of N without significant difference between treatments. Estimates from the loss of nodules mainly during the dry season amounted in 57 and 66 kg/ha of N in the not fertilized and fertilized areas respectively. These data indicate that the nitrogen exported by the fruits can be easily replaced by nitrogen fixation associated with the Erythrina shading trees.