NITROGEN-FIXATION IN THE EPIPHYLLAE AND ROOT-NODULES OF TREES IN THE LOWLAND TROPICAL RAIN-FOREST OF COSTA-RICA

Nitrogen fixation, measured using the acetylene reduction technique and N-15(2), was quantified in the epiphyllae of the common understory palm Welfia georgii and in root nodules of saplings of the trees Pentaclethra macroloba and Inga spp. at Finca La Selva, Costa Rica. Nitrogenase activity in W. georgii appeared to be associated with both filamentous cyanobacteria which were loosely intertwined with leafy liverworts, and a thin layer of coccoid cyanobacteria lying immediately above the leaf cuticle. Rates of N2 fixation, measured using N-15(2), in direct sunlight, were high, averaging 37.5-mu-g N cm-2 h-1, while rates at light values typically found in the understory (ca.12-mu-E m-2 sec-1) averaged 1.8-mu-g cm2 h-1 suggesting that autotrophs (cyanobacteria) were the principal diazotrophs. Contribution of epiphyllae N2 fixation on W.georgii at La Selva, Costa Rica, is conservatively estimated to be 270 mg N ha-1 day-1. Since there undoubtedly are diazotrophic epiphyllae on other trees in this area, this estimate should be considered as a minimal value. In the legumes P.macroloba and Inga spp., mean rates of N2 fixation in root nodules of saplings were 33 and 183-mu-g N g nodule dry wt-1 h-1, respectively. These two species are abundant trees at La Selva, and nitrogen fixation by bacteria in root nodules of saplings may play a role in their success.