We compiled quantitative estimates on symbiotic N-2 fixation by trees in agroforestry systems (AFS) in order to evaluate the critical environmental and management factors that affect the benefit from N-2 fixation to system N economy. The so-called "N-2-fixing tree" is a tripartite symbiotic system composed of the plant, N-2-fixing bacteria, and mycorrhizae-forming fungi. Almost 100 recognised rhizobial species associated with legumes do not form an evolutionary homologous clade and are functionally diverse. The global bacterial diversity is still unknown. Actinorrhizal symbioses in AFS remain almost unstudied. Dinitrogen fixation in AFS should be quantified using N isotopic methods or long-term system N balances. The general average +/- A standard deviation of tree dependency on N-2 fixation (%Ndfa) in 38 cases using N isotopic analyses was 59 +/- A 16.6 %. Under humid and sub-humid conditions, the percentage was higher in young (69 +/- A 10.7 %) and periodically pruned trees (63 +/- A 11.8 %) than in free-growing trees (54 +/- A 11.7 %). High variability was observed in drylands (range 10-84 %) indicating need for careful species and provenance selection in these areas. Annual N-2 fixation was the highest in improved fallow and protein bank systems, 300-650 kg [N] ha(-1). General average for 16 very variable AFS was 246 kg [N] ha(-1), which is enough for fulfilling crop N needs for sustained or increasing yield in low-input agriculture and reducing N-fertiliser use in large-scale agribusiness. Leaf litter and green mulch applications release N slowly to the soil and mostly benefit the crop through long-term soil improvement. Root and nodule turnover and N rhizodeposition from N-2-fixing trees are sources of easily available N for the crop yet they have been largely ignored in agroforestry research. There is also increasing evidence on direct N transfer from N-2-fixing trees to crops, e.g. via common mycelial networks of mycorrhizal fungi or absorption of tree root exudates by the crop. Research on the below-ground tree-crop-microbia interactions is needed for fully understanding and managing N-2 fixation in AFS.
