**4. Diversity of Rhizobia in Nodules of Legume Trees in the Humid Tropics**

In the humid tropics, more specifically, in the periphery of the Amazon region, legume trees are important for their ability to colonize and supply N in environments with poor soils due to the continuous production of biomass throughout the year, even in long periods of drought, which provides better soil coverage [105].

Legume trees of economic and environmental interest are distributed throughout the world, and some non-native species are widely distributed in the tropics [106]. These legumes can be used to recover degraded areas by providing N-rich biomass for agroforestry systems, produce high-quality forage for livestock, stabilize slopes against erosion, and provide shade for planting crops, fruits, and vegetables for human consumption, among others [33,107]. It should also be noted that the success of the BNF of legume, both introduced and native, depends on the presence of compatible and efficient rhizobia in the native community of the soil [106]. Therefore, the evaluation of the presence or absence of efficient strains in the native community of the area and forecast of the need for inoculation is the first step towards the establishment of legume trees, regardless of the purpose. This review will focus on four legume trees, the leucaena (*Leucaena leucocephala* (Lam.) de Wit), acacia (*Acacia mangium* Willd.), sombreiro (*C. fairchildiana* R. A. Howard), and gliricidia (*G. sepium* (Jacq.) Kunth), common in sustainable agricultural systems in the humid tropics due to their effectiveness in the use of N, mainly by low-income family farmers [10,13].

Leucaena is a fast-growing legume tree belonging to the subfamily Mimosoideae, native to Central America and widely distributed in tropical regions. It has a high BNF (average of 177–247 kg ha−1), high leaf N content (about 1.3%), and high biomass production (about 5 t ha−1) [108]. Leucaena is used to rehabilitate degraded areas by vegetation covering, in agroforestry systems, as forage, human food, firewood, wood, green manure, shade, support for scandent species, and to control wind and soil erosion [109]. It appears that leucaena is non-specific with respect to its symbiotic relationships with NFLNB, since it can form nodules with species of several genera of rhizobia. A survey of several studies carried out in tropical regions indicated that nodulation of leucaena occurs, preferably, with fast-growing rhizobia. Four recent studies have identified species of *Rhizobium* in leucaena nodules [105,109–111], while others have found species belonging to the genus *Mesorhizobium* [109–112]. Only one study reported the occurrence of symbiosis between leucaena and *Bradyrhizobium* [111] and between leucaena and *Cupriavidus* [113].

The BNF capacity of acacia in symbiosis with rhizobia is one of its most advantageous characteristics. This legume establishes symbiosis with species of *Bradyrhizobium*, *Rhizobium*, *Mesorhizobium,* and *Ochrobactrum* [114,115]. However, symbiosis between acacia and strains of *Bradyrhizobium* is more common than with species of other genera of rhizobia, as shown in studies on the identification of rhizobia strains in symbiosis with acacia in several regions [94,116–119]. Some authors have reported that effective nodulation of acacia occurs only with specific strains of *Bradyrhizobium* sp., such as *B. elkanii*, although acacia seedlings show high variability in their ability to fix N2 in symbiosis with their specific strains of *Bradyrhizobium* [114,117,118,120].

Another legume of interest is gliricidia, a medium-sized tree that can reach up to 12 m in height [121]. The species is native to Mesoamerica and is considered the second most important multifunctional legume tree only behind leucaena [122]. Due to the high capacity of climate adaptation, this legume has been transported to most tropical countries, where it is widely distributed. Gliricidia can produce about 5 t ha−<sup>1</sup> of dry matter and 186 kg ha−<sup>1</sup> of N. The capacity for BNF makes the use of this species advantageous, especially for green manure purposes and for use in agroforestry systems. This legume establishes symbiosis, preferably with fast-growing rhizobia, mainly of the genus *Rhizobium* [25,123]. This fact is confirmed by the massive presence of species of the genus *Rhizobium* in a survey carried out with the main studies that addressed symbiosis of gliricidia with rhizobia, identified by a wide variety of molecular or classic methodologies [25,105,109,110,124,125].

The sombreiro is a rustic, medium to large fast-growing legume tree. The species is native to Brazil, and its phytogeographic domain includes the Amazon rainforest. Although it has a clear preference for fertile and moist soils, the sombreiro can also occur in open and altered areas [126]. The BNF capacity of this tree is known, but little studied, despite its high production of biomass (about 8 t ha<sup>−</sup>1) and N (328 kg ha<sup>−</sup>1) [127], with reports of symbiosis with *Rhizobium* strains [114] and *Bradyrhizobium* [25,120].
