Nitrogen-fixing trees and nitrogen fixation in sustainable agriculture: Research challenges

Research on nitrogen fixation tends to become more carefully targeted, and covers not only biological but also social and cultural aspects to achieve sustainability. However, many issues remain unsolved, such as improving N2 uptake and the delivery mechanisms, the influence of free nitrogen in the soil and its influence on nodulation. The importance of analysing traditional field practices by farmers is stressed with examples from Mexico and Costa Rica involving Leucaena, Alnus acuminata and Erythrina spp. The case of E. poeppigiana, widely propagated by large cuttings as a shade tree for coffee where it is drastically pollarded twice a year, is described and analysed. Apparently the practice provides considerable benefits to the soil and associated crops. Erythrina spp. also show promise as nurse trees to favour nearby planted timber trees. These and other examples lend themselves to promising research projects.