**5. Final Considerations**

The huge diversity of rhizobia strains and their interactions with legume trees in tropical soils, if used wisely, can contribute to the sustainability of tropical agroecosystems. According to studies carried out in these regions, tropical soils have a high diversity of rhizobia, which interact with different legumes. Furthermore, the efficiency of this symbiosis depends on both the symbionts and external factors. Indeed, BNF is a highly complex process that involves a diverse group of bacteria, currently distributed into seven families and 19 genera identified through several methods based on phenotypic/morphological, biochemical, and molecular characterizations, the latter of which is making major contributions to modern taxonomy. Fortunately, four of the most suitable legumes for use in achieving more sustainable agricultural systems for family farmers in the humid tropics, *L. leucocephala*, *A. mangium*, *G. sepium,* and *C. fairchildina*, perform symbiosis with different groups of rhizobia. Future research must be focused on efficiency of the interaction between the symbionts and external factors, which can lead to higher legume biomass, rich in N. In this scenario, the exploration of BNF as a key ecological service could bring economic, ecological, and agronomic benefits to assist in the process of sustainable intensification of agriculture in the humid tropics, mainly because BNF can increase the efficiency of N use in this environment, where it is a major limiting factor.

**Author Contributions:** Writing—original draft preparation, E.G.M. and K.P.C.; writing—review and editing, C.S.C.; C.P.C.B.; J.L.B.S.; A.C.F.A.; A.S.L.F.J., and C.A.B.; visualization, K.P.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by "NUCLEUS, a virtual joint center to deliver enhanced N-use efficiency via an integrated soil–plant systems approach for the United Kingdom and Brazil", grant number: FAPESP—São Paulo Research Foundation [grant number 2015/50305-8]; FAPEG—Goiás Research Foundation [grant number 2015-10267001479]; and FAPEMA—Maranhão Research Foundation [grant number RCUK-02771/16]; and in the United Kingdom by the Biotechnology and Biological Sciences Research Council [grant number BB/N013201/1] under the Newton Fund scheme.

**Acknowledgments:** The authors thanks the National Council for Scientific and Technological Development (CNPq), the Foundation for the Support of Research and Scientific and Technological Development of Maranhão (FAPEMA), and the Coordination for the Improvement of Higher Education Personnel (CAPES) by research grants from team members.

**Conflicts of Interest:** The authors declare no conflict of interest.
