**Emanoel G. Moura 1,\*, Cristina S. Carvalho 1, Cassia P. C. Bucher 2, Juliana L. B. Souza 3, Alana C. F. Aguiar 4, Altamiro S. L. Ferraz Junior 1, Carlos A. Bucher <sup>5</sup> and Katia P. Coelho 6,\***


Received: 31 March 2020; Accepted: 30 April 2020; Published: 24 May 2020

**Abstract:** Symbiotic biological nitrogen fixation (BNF) is a complex process that involves rhizobia, a diverse group of α and β-proteobacteria bacteria, and legume species. Benefits provided by BNF associated with legume trees in tropical environments include improvements to efficiency of nitrogen (N) use, increase of soil carbon sequestration, stabilization of soil organic matter, decrease of soil penetration resistance, and improvement of soil fertility. All these benefits make BNF a crucial ecosystem service to the sustainability of tropical agriculture. Due to the importance of this ecological process and the high diversity of rhizobia, these bacteria have been extensively characterized worldwide. Currently, over 400 species of rhizobia are known, distributed into seven families. In the humid tropics, *Leucaena leucocephala*, *Acacia mangium*, *Gliricidia sepium*, and *Clitoria fairchildiana* are four of the most common species used by family farmers to create sustainable agricultural systems. These four legumes perform symbiosis with different groups of rhizobia. Exploring BNF could help to enable sustainable intensification of agriculture in the humid tropics, mainly because it can increase N use efficiency in an environment where N is a limiting factor to plant growth.

**Keywords:** biological nitrogen fixation; nitrogen; *Leucaena leucocephala*; *Acacia mangium*; *Gliricidia sepium*; *Clitoria fairchildiana*
