Potential of Bioassays to Assess Consequences of Cultivation of Trees on Nitrogen Bioavailability to Trees: Two Case-Studies in Contrasting Tropical Soils.

We hypothesized that the nitrogen-fixing tree could improve the growth and nitrogen nutrition of non-fixing tree species such as . We measured the N-mineralization and respiration rates of soils sampled from plots covered with , or native vegetation at two tropical sites (Itatinga in Brazil and Kissoko in the Congo) in the laboratory. We used a bioassay to assess N bioavailability to eucalypt seedlings grown with and without chemical fertilization for at least 6 months. At each site, seedling growth and N bioavailability followed the same trends as the N-mineralization rates in soil samples. However, despite lower soil N-mineralization rates under in the Congo than in Brazil, seedling growth and N bioavailability were much greater in the Congo, indicating that bioassays in pots are more accurate than N-mineralization rates when predicting the growth of eucalypt seedlings. Hence, in the Congo, planting could be an attractive option to maintain the growth and N bioavailability of the non-fixing species while decreasing chemical fertilization. Plant bioassays could help determine if the introduction of N-fixing trees will improve the growth and mineral nutrition of non-fixing tree species in tropical planted forests.