**1. Rhizobia Strains and Legume Tree Interactions: Importance and Use in Tropical Agricultural Systems**

In the humid tropics, the edaphoclimatic conditions, which involve soils with fragile structure subjected to high temperatures, rainfall, and insolation, are unfavorable for organic matter accumulation and thus N availability and nitrogen use efficiency. The adoption of inappropriate agricultural practices for local conditions reduces nutrient availability and results in the depletion of soil fertility. Small family farmers in the humid tropics, especially in the pre-Amazon region, practice itinerant agriculture that is

associated with the slashing and burning of natural vegetation. These small farmers mainly cultivate food crops such as rice, maize, cassava, and beans, using low technology. According to Moura et al. [1], this system has negative effects on the local and global environment and no longer provides social benefits to rural communities. The environmental impact of agriculture in tropical regions can include deforestation and loss of wildlife habitat, soil nutrient depletion, increased greenhouse gas emissions, and loss of biodiversity [2]. In contrast, there are excellent opportunities to avoid environmental damage through the use of ecosystem services to achieve ecological and economic benefits [3].

No-tillage alley cropping can be an alternative for the family farmers for maintaining productivity in the low-fertility soils of humid tropical regions. In this system, legume trees or shrubs are planted in two or more sets of single or multiple rows. Before and during the cropping period, the leguminous branches are periodically pruned and laid down on the soil surface between the leguminous tree sets, where other crops are planted. The use of legume trees can provide ecosystem services such as biomass production, recycling of nutrients, biological nitrogen fixation, and carbon sequestration [4].

Following the work of Fischer et al. [5], we define ecosystem services as the ecological processes of ecosystems that are used to produce human well-being. In this context, nitrogen (N) fixation by legume trees, in which bacteria living in root nodules convert atmospheric dinitrogen (N2) gas to a plant-available form of N, is crucial to the sustainability of tropical agriculture [6]. Fixation of N in this manner can provide much of the N needed to drive crop growth and biomass creation [7]. In addition, proper management of legume tree biomass can increase soil organic matter, sequestered carbon, and recycled nutrients, which increases soil fertility and agricultural productivity [8]. In the tropics, the high biodiversity in rhizobia and legume trees, combined with long growing seasons and adequate soil moisture, provides the ideal conditions for making use of the ecosystem services provided by legume trees [9].

These phenomena offer a potential solution to agricultural sustainability challenges in the Amazon region, which are mainly associated with depletion of soil fertility [10]. The region's low-activity clay soils present difficulties for agriculture due to their reduced capacity to retain nutrients under high rainfall intensity, making agriculture an unfeasible activity [11]. Nutrient retention in the root zone can be enhanced by adding nutrients in slow-release forms and through biologically mediated processes like N-fixing symbiosis [12]. In the agroecosystems of the humid tropics, these approaches may be more sustainable than saturating the soil with soluble nutrients [13].

Biologically mediated processes are one of the most important targets in modifying and improving N uptake efficiency by roots [14]. This assertion is particularly true in humid tropical conditions, where N use efficiency is usually very low. High temperatures increase the volatilization rates of fertilizers [15], while high rainfall rates increase nitrate (NO<sup>−</sup>3) concentration and leaching [16]. Inefficient use of N, beyond its negative economic impact, contributes to agriculture disservices through greenhouse gas emissions and groundwater pollution [17]. Indeed, in the humid tropics, N uptake is impaired by N leaching due to high rainfalls, and by reduced root growth in hard-setting soil when rainfall is low [18]. The poor agronomic efficiency of N, reaching as low as 14 kg/kg (maize grain/applied N) has been the main reason why many family farmers have resisted changing from the traditional slash-and-burn systems to conventional tillage systems, despite recommendations to switch.

Some authors have reported the positive effect of the symbiosis between rhizobia and legume trees on agroecosystem services, one of the main results of which is an increase in tropical agriculture sustainability. Sena et al. [10] showed that the total N content of maize was increased by around 50% when adding biomass of gliricidia (*Gliricidia sepium* (Jacq.)) on its own and with urea. Aguiar et al. [13] reported that a combination of high- and low-quality residues from legume trees increased N concentrations in maize. A combination of gliricidia with sombreiro (*Clitoria fairchildiana* (R. A. Howard)) resulted in a nearly threefold increase in N accumulation compared to bare soil with urea (148.4 vs 54.1 kg ha<sup>−</sup>1). Treatments involving legume residues plus urea led to higher N accumulation in comparison to the treatments with residues but without urea [13].

N fixation by legume trees also has a positive effect on soil carbon sequestration. N-fixing trees sequester CO2 directly through their growth or indirectly via the turnover of their N-rich tissues, whose decomposition increases plant growth due to higher soil N [7]. However, some authors have highlighted the influence of quality of the applied residue (i.e., residues with high N contents and low to medium cellulose contents) on the stabilization of the soil organic matter [19,20]. According to Bradford et al. [21], high-quality leaf litter, which increases microbial activity, results in more organic matter that can be physicochemically stabilized. Therefore, the higher the symbiotic efficiency between rhizobia and legume trees, the higher the quality and amount of stabilized organic matter or sequestered carbon. According to Sena et al. [10], total carbon stock was 30% higher in treatments with high-quality legume tree residue than in treatments with bare soil. Thus, compared with the control treatment, the treatment with gliricidia, when added to the accumulated organic content, resulted in the carbon stock increasing by 5.0 g kg<sup>−</sup>1.

Improvement in soil fertility from legume tree biomass can be physical (decreased soil penetration resistance), or chemical (increased base cations due to recycled nutrients) [18]. After four rainless days, the effect of biomass application on penetration resistance in a hard-setting soil extended to a depth of 17.5 cm, due less to soil moisture conservation than to the increase in soil organic matter [22]. This biomass may also capture a considerable amount of recycled base cations that were previously out of the reach of crops, improving the environment of the root zone [9,23].

Since the Forest Code in Brazil specifies that only 20% of a farmer's land can be used for agriculture in the Amazon, local research and policy agendas must consider N-fixing symbiosis between rhizobia and legume trees. This would allow sustainable intensification of agriculture in this region, for which increased N availability and use efficiency is crucial [24]. However, strategy to meet the challenges of the systematic use of ecosystem services provided by interactions of leguminous trees and rhizobia will require the mastery of a vast knowledge base [25]. First, we need to know how symbiotic relationships between different species could work through the ecological gradient, like the one that makes up the Amazon and its periphery. Such a process can determine the efficiency of N input in tropical family farm systems [26], increasing the ease of adoption by farmers. Second, it will be necessary to identify and select rhizobia communities which have greater efficiency to produce high quality biomass if the aim is to take advantage of the large tropical biodiversity [24].

A large number of texts have recently appeared in the scientific literature on these different issues, but often with an orientation less applied to the tropical environment and its agrosystems. This paper provides an overview of recent developments in the diversity of rhizobia for N fixation but is mostly concerned with contributing to meet the challenges of feasibility and sustainability of the agrosystems in tropical family farms.
