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Article

Abundance of Indigenous Soybean-Nodulating Rhizobia in Relation to Soil Properties and Cropping Patterns in a Midland Agro-Ecology of Southern Ethiopia

by
Haimanot Beruk
1,2,* and
Tewodros Ayalew
1
1
Schools of Plant and Horticultural Sciences, College of Agriculture, Hawassa University, Hawassa P.O. Box 05, Ethiopia
2
Hawassa Maize Research Sub-Center, Wondo Genet Agricultural Research Center, Ethiopian Institute of Agricultural Research, Hawassa P.O. Box 1793, Ethiopia
*
Author to whom correspondence should be addressed.
Nitrogen 2026, 7(1), 19; https://doi.org/10.3390/nitrogen7010019
Submission received: 12 December 2025 / Revised: 26 January 2026 / Accepted: 29 January 2026 / Published: 2 February 2026
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Abstract

Estimating indigenous rhizobial populations is crucial for understanding soil rhizobia abundance, determining the potential need for inoculation, and evaluating the performance of introduced inoculant strains. However, in southern Ethiopia, information on the population abundance of soybean-nodulating rhizobia is limited. To address this gap, the present study was conducted to evaluate the population abundance of indigenous soybean-nodulating rhizobia and to assess the influence of cropping history and soil properties on rhizobial abundance. The study was conducted across five sites suitable for soybean cultivation in southern Ethiopia: Arsi-Negelle, Boricha, Dore, Hawassa, and Wondo Genet. The study sites represented a range of cropping systems, including sole maize, sole tobacco, sole haricot bean, maize–potato intercropping, and crop rotation. Composite soil samples were collected from a depth of 0–20 cm, and rhizobial abundance was determined using the most probable number (MPN) technique. Indigenous rhizobial populations ranged from 0 to 1.7 × 101 cells g−1 of dry soil. Overall, the population levels were low, suggesting that inoculation with effective rhizobial strains would likely improve nodulation and biological nitrogen fixation. Relatively higher rhizobial population densities were observed at Arsi-Negelle under haricot bean cropping history. Statistically significant positive correlations were found between rhizobial abundance and cation exchange capacity, organic carbon, and organic matter. In general, native rhizobial populations across all study locations were below levels considered sufficient to support effective soybean nodulation and nitrogen fixation, indicating the need for inoculation to enhance soybean productivity in the study areas.

1. Introduction

Declining soil fertility, particularly nutrient depletion resulting from nutrient removal exceeding replenishment, has been identified as a major constraint to agricultural productivity in sub-Saharan Africa [1]. Among essential nutrients, low soil nitrogen (N) availability is the principal yield-limiting factor in sub-Saharan Africa, including Ethiopia [2]. Biological nitrogen fixation (BNF) is a vital source of N in agricultural systems, contributing to productivity, economic efficiency, and long-term sustainability [3,4]. The significance of BNF lies in its ability to convert atmospheric nitrogen into plant-available forms [5], thereby offering a sustainable alternative for agricultural production [6]. Additionally, the effectiveness of Rhizobia in enhancing nitrogen fixation and crop yields largely depends on the population size and efficiency of indigenous rhizobial strains present in the soil [7].
Soybean (Glycine max L.) plays a crucial role in restoring soil fertility, especially in areas dominated by cereal mono-cropping [8], and is widely used in rotation or intercropping with maize [2]. The crop requires approximately 80 kg of N per metric ton of grain yield [9]. On average, BNF supplies 50–60% of soybean’s total N requirements [10], through a symbiotic association with Bradyrhizobium species that fix atmospheric nitrogen [6]. Consequently, soybean-nodulating rhizobia are of considerable importance for enhancing nitrogen fixation, plant growth, and grain yield [11,12,13].
In soils with low or incompatible native rhizobial populations, artificial inoculation is necessary as it can influence rhizosphere microbial activity and promote the establishment of effective rhizobial strains in the root zone [14]. For inoculation to be successful, introduced rhizobia must effectively compete with native strains for nodule occupancy [15] and should be pre-screened for symbiotic effectiveness [16]. However, introduced strains are often outcompeted by native rhizobia. When native populations exceed the threshold of 102 cells g−1 of soil and consist of effective strains, artificial inoculation is unlikely to enhance BNF [17]. Therefore, enumerating the abundance, distribution, and viability of native rhizobial populations is critical for making informed decisions regarding inoculant application [18].
The most probable number (MPN) technique is widely used to estimate rhizobial abundance and assess the need for inoculation [19]. The population size of native rhizobia is a key determinant of BNF efficiency and associated crop yield improvement [7]. Consequently, understanding native rhizobial populations is essential for making informed decisions about inoculant application [20]. Moreover, determining native rhizobial populations is vital for understanding soil biodiversity [21], optimizing the contribution of BNF to crop productivity [22], and maintaining ecosystem functioning through nutrient cycling and plant–microbe interactions [23].
Several studies in Ethiopia have evaluated the effects of inoculants on soybean yield, soil fertility, and plant performance [24,25,26]. Moreover, Beruk, et al. [16] recently evaluated the symbiotic effectiveness of eight soybean infecting Bradyrhizobium strains across five varieties, and found the best mutualism among the strains and soybean varieties currently under commercial production in Ethiopia. Despite evidence confirming the role of soybean–rhizobium symbiosis in enhancing soil fertility and productivity across diverse cropping systems and environments, detailed information on the abundance and dynamics of native soybean-nodulating rhizobia—particularly in relation to cropping history and soil physicochemical properties—in southern Ethiopia remains limited.
Therefore, the present study aimed to quantify the population abundance of indigenous soybean-nodulating rhizobia and to examine their relationships with soil properties and cropping patterns across sites suitable for soybean production.

2. Materials and Methods

2.1. Description of Soil Collection Area and Soil Sampling Pocedures

Soil samples were collected in 2023 from five districts in southern Ethiopia: Arsi Negelle, Boricha, Dore, Hawassa, and Wondo Genet. To ensure that measured rhizobial populations reflected indigenous communities, only fields with no prior history of rhizobial inoculation or recent soybean cultivation were selected. Sampling included fields under both Fabaceae (haricot bean) and non-Fabaceae (maize, teff, potato, sorghum) cropping systems. Although soybean had not been previously cultivated in these fields, including non-Fabaceae systems allowed assessment of the background abundance of native soybean-nodulating rhizobia that may persist in soils regardless of host presence. This approach helps determine whether inoculation would be necessary before introducing soybean to these sites.
Within each selected field, soil samples were collected using a zig-zag sampling pattern at a depth of 0–20 cm. All collected soil samples were placed in labeled plastic bags and transported to the Soil Microbiology Laboratory at Hawassa University for further processing. Information on cropping history was obtained through personal communication with farmers in Boricha and Dore districts, and from farm managers at research and experimental stations in Arsi Negelle, Hawassa, and Wondo Genet.
The soil samples from each site were thoroughly mixed to form composite samples, which were then divided into two subsamples. One subsample was used to determine indigenous rhizobial population abundance, while the other was used for the analysis of soil physicochemical properties. From each composite sample, a representative portion was taken and subjected to the respective analyses.
Key geographic characteristics and cropping histories of the five study locations are summarized in Table 1, and the physical characteristics of the soils collected from these sites are presented in Table 2.

2.2. Description of Soybean Varieties Used as a Test Crop

The soybean varieties Gishama and Awassa-95 used as a test crop for this experiment were sourced from Pawe agricultural research center (Table 3). Gishama and Awassa-95 varieties are medium and early maturing characteristics, respectively, and selected for this experiment based on the best symbiotic performance demonstrated in the former study by the same researchers [16].

2.3. Soil Physicochemical Analysis

Soil physicochemical analyses were conducted at the soil chemistry laboratory, Hawassa University, using standard procedures for each soil parameters. Particle size distribution was determined using the Bouyoucos hydrometer method [27], and soil pH was measured with a pH meter (potentiometric method; van Reeuwijk [28]). Total nitrogen (N) and available phosphorus (P) were determined using the Kjeldahl method [29] and Olsen method [30], respectively. Electrical conductivity, Cation Exchange Capacity, and Soil organic carbon were determined using an Electrical conductivity meter [28], Ammonium acetate method [31], and Wet oxidation method [32], respectively. Percent soil organic matter was obtained by multiplying percent soil organic carbon by a factor of 1.724 following the assumption that organic matter is composed of 58% organic carbon [33,34].

2.4. Soybean Nodulating Rhizobial Population Abundance Enumeration

The most probable number (MPN) plant infection technique [35] was used to estimate the populations of native rhizobia capable of nodulating soybean. Since the size of native populations of rhizobia may vary from field to field within short distances [36], representative soil samples from the field were collected using a zigzag pattern to ensure the consistency of the process. For each site, the subsamples were thoroughly mixed to obtain a composite sample representing a field. Then, the MPN assay was performed to assess the populations of native rhizobia present in each location’s soil.
To establish the experiment on sand culture for rhizobial abundance assessment, modified Leonard jars were constructed from two plastic cups. The upper cups were filled with pre-treated river sand, which had been washed in a quarter-strength solution of 98% H2SO4 to sterilize the medium and support seedling growth. The pH of the sand was raised to nearly 7.0 by washing with tap water, the second cup was connected to the upper cups by cotton. Then the seeds of soybean varieties (Awassa-95 and Gishama) were surface sterilized with 95% ethanol and in a 3% (v/v) solution of sodium hypochlorite and successively rinsed with sterilized distilled water and allowed to air dry. Then the seeds were germinated on Petri dish and pre-germinated one soybean seed for each variety was transferred to acid-treated and sterilized sand in the Modified Leonard jars using forceps and grown in Hawassa University lath house.
Serial tenfold dilution was prepared for the 5 soil samples by diluting 10 g of soil in 90 mL of sterilized distilled water; then, 1 mL of diluents was added in 9 mL of sterile water to obtain the second diluents and the same procedure followed up to the sixth dilution step (10−6). A 1 mL of each dilution (10−1–10−6) was subsequently used to inoculate the soybean seedlings grown in acid-treated and sterilized sand in the modified Leonard Jars and one control pot for each variety of inoculated pots was grown with two replications. Inoculation was undertaken using sterilized a micropipette. The uninoculated controls were used to check for sterile conditions. The experiment was arranged in a factorial design using a completely randomized design (CRD). Then the plants were optimally inspected and watered with an N-free nutrient solution. The N-free nutrient solution was prepared using 0.2 g KH2PO4, 0.2 g MgSO4·7H2O, 1 g CaHPO4, 0.2 g NaCl, 0.1 g FeCl3, and 1 mL of other less-needed elements (2.86 g H3BO3, 2.03 g MnSO4·4H2O, 0.22 g ZnSO4·7H2O, 0.08 g CuSO4·5H2O, 0.14 g Na2MoO4·2H2O) to make up a stock solution in 1 L of distilled water.
After six weeks of planting, the plant was uprooted and separated into shoots and roots; then, the roots of the seedlings were gently washed with tap water and assessed for nodule presence. Nodulation was recorded as “+” (present) or “−” (absent) for each dilution. The total number of nodulated units was then summed for each dilution series.
The number of rhizobia in each soil was estimated using the formula
M P N = m   ×   d v
where m is the most likely number from the MPN table for the lower dilution of the series [37], d is the lowest dilution (the first unit used in the tabulation), and v is the inoculated aliquot volume.

3. Results

3.1. Physicochemical Properties of the Test Sites

The soils across the five sites were categorized as loams based on laboratory study for physicochemical characteristics (Table 2). Soil pH ranged from 6.40 at Boricha to 7.66 at Hawassa (Table 4). Based on the classification of Tadesse, et al. [38], soils at Boricha and Dore (pH 6.6) were slightly acidic, Arsi-Negelle (pH 6.92) and Wondo Genet (pH 7.21) were neutral, while Hawassa soil (pH 7.66) was alkaline.
Available phosphorus (P) ranged from 12.50 mg kg−1 at Dore to 37.34 mg kg−1 at Hawassa (Table 4). According to Cottenie [39], available P levels at Dore, Boricha, and Arsi-Negelle were in the medium range, whereas Hawassa and Wondo Genet exhibited very high available P levels.
Soil organic carbon content varied from 1.37% at Dore to 3.61% at Arsi-Negelle, while organic matter content ranged from 2.36% to 6.22% (Table 4). Based on the rating of Tadesse, et al. [38], Dore soil had low organic matter content, Boricha and Hawassa soils had medium levels, and Wondo Genet and Arsi-Negelle soils had high levels. Overall, 20%, 40%, and 40% of the samples fell into low, medium, and high organic matter categories, respectively.
Cation exchange capacity (CEC) values ranged from 36 cmol(+)/kg at Boricha and Wondo Genet to 47 cmol(+)/kg at Arsi-Negelle, with intermediate values of 38 and 39 cmol(+)/kg at Dore and Hawassa, respectively (Table 4). According to the classification of Hazelton and Murphy [40], all soils exhibited high to very high CEC.

3.2. Abundance of Native Rhizobia Nodulating Soybean

Enumeration of native rhizobia capable of nodulating soybean revealed consistently low population sizes across all test sites. Most Probable Number (MPN) estimates ranged from 0 to 17 cells g−1 of soil (Table 5), with the highest densities observed at Arsi-Negelle for both soybean varieties.
For the soybean variety Awassa-95, no detectable rhizobia were recorded at the Dore and Hawassa sites, whereas the variety Gishama showed detectable rhizobial populations in the same soils. Across all sites, native rhizobial densities remained below the commonly cited threshold of approximately 102 cells g−1 soil required for effective nodulation in the absence of inoculation.

3.3. Relationship of Rhizobial Abundance and Soil Properties

Pearson correlation analysis (Figure 1) revealed significant relationships between selected soil chemical properties and the abundance of native soybean-nodulating rhizobia. Rhizobial population size showed significant positive correlations with soil organic carbon (OC; r = 0.70), organic matter (OM; r = 0.70), and cation exchange capacity (CEC; r = 0.80).
In addition, total soil nitrogen (TN) exhibited strong and significant positive correlations with both organic carbon (r = 0.91) and organic matter (r = 0.90), indicating a close association between soil nitrogen status and soil organic matter pools.
Soil pH, available phosphorus, and total nitrogen showed weak and non-significant correlations with rhizobial abundance (Figure 1). Positive relationships were also observed between CEC and both OM and OC (r = 0.62), although these correlations were not statistically significant (p > 0.05).

3.4. Rhizobial Abundance Association with Cropping History of the Soil Sampling Fields

Native rhizobial abundance varied markedly among sites and was closely associated with the cropping history of the sampled fields (Table 1 and Table 5). Soils from Arsi-Negelle, where haricot bean had been cultivated prior to cereal crops, exhibited relatively higher populations of soybean-nodulating rhizobia for both soybean varieties.
In contrast, soils from Wondo Genet, Boricha, Dore, and Hawassa—where non-legume crops dominated the recent cropping history—showed consistently low rhizobial population densities. In Hawassa, despite the inclusion of haricot bean in the 2021 cropping season, the overall rhizobial population remained low (0.6 × 101 cells g−1 soil for Gishama and undetectable for Awassa-95), likely due to the predominance of non-legume crops such as enset and teff before and after the legume phase (Table 1).
Furthermore, although detectable rhizobial populations were observed for the variety Gishama in Hawassa, Awassa-95 showed no detectable rhizobia, indicating varietal differences in nodulation ability under similar soil and cropping conditions. This suggests that a single legume phase may be insufficient to sustain rhizobial populations capable of nodulating less promiscuous soybean varieties, particularly in systems dominated by non-legume crops.

4. Discussion

4.1. Physicochemical Properties of the Test Sites

The observed soil pH values across all sites fall within ranges generally considered favorable for nutrient availability and biological activity, as soil pH strongly influences nutrient solubility and microbial function [41,42]. Soil pH near neutral enhances microbial processes, including symbiotic nitrogen fixation, while strongly acidic conditions restrict nutrient availability and microbial function. Previous studies have shown that nodulation and nitrogen fixation decline sharply when soil pH drops below 5.5 due to reduced microbial activity and nutrient imbalances [43]. Recent studies further confirm that near-neutral soil pH supports diverse rhizosphere microbial communities essential for nutrient cycling and crop productivity [44].
The variation in available phosphorus among the sites likely reflects differences in soil management practices and fertilizer history. Higher available P levels at Hawassa and Wondo Genet may indicate residual effects of repeated fertilizer applications. Similar spatial variability in available P has been reported in agricultural soils of southern Ethiopia, where land use intensity, fertilizer type, and soil conservation practices strongly influence phosphorus availability [45,46]. In addition, parent material, soil pH, and slope position are known to influence phosphorus fixation and distribution within cultivated landscapes, as these factors affect phosphorus retention, mobility, and spatial heterogeneity in soil profiles [47,48].
Differences in soil organic carbon and organic matter content among the sites may be attributed to climatic conditions and land management practices. Low organic matter levels observed at Dore are consistent with continuous cultivation, high temperatures that accelerate decomposition, complete removal of crop residues, and limited use of organic amendments. Similar findings have been reported in other Ethiopian farming systems, where soils under cultivated fields consistently exhibit lower organic matter and soil organic carbon compared to less-disturbed land uses such as forest and bushland [49,50,51]. Moreover, Negassa and Gebrekidan [52] reported that cultivated fields generally have lower organic matter than other land uses due to continuous cultivation and enhanced organic matter oxidation. Higher organic matter contents at Wondo Genet and Arsi-Negelle may enhance soil structure, nutrient retention, and biological activity, as increased soil organic matter promotes aggregation, water-holding capacity, and provides energy substrates that support greater microbial biomass and nutrient cycling [53,54].
The generally high CEC values observed across the study sites indicate a strong capacity for these soils to retain and supply essential cations. Variations in CEC among sites are likely linked to differences in organic matter content, clay quantity and mineralogy, and cultivation intensity. Studies have shown that intensive cultivation and organic matter depletion reduce soil CEC over time, thereby diminishing the soil’s nutrient buffering capacity [55,56]. Recent research further emphasizes the close relationship between soil organic carbon and CEC in tropical agricultural soils, with higher SOC linked to improved CEC and overall soil fertility under sustainable management practices such as agroforestry and conservation tillage [57,58].

4.2. Abundance of Native Rhizobia Nodulating Soybean

The very low abundance of soybean-nodulating rhizobia observed across the study sites suggests that native rhizobial populations are insufficient to support effective nodulation and biological nitrogen fixation without inoculation. Earlier work established that when native rhizobial populations fall below ~102 cells g−1 soil, inoculation is typically required to ensure adequate nodulation and yield response [59]. More recent studies continue to confirm that low background rhizobial populations severely limit soybean symbiosis, particularly in regions where soybean has not been widely cultivated historically, as evidenced by poor native population counts and strong responses to inoculation in smallholder farming systems [26,60].
The site-specific variability observed in rhizobial abundance aligns with findings from other Ethiopian soils. Temesgen and Assefa [26] reported MPN values ranging from 2.2 × 101 to 6.3 × 103 cells g−1 soil across different locations, highlighting strong spatial heterogeneity in native rhizobial populations. Similarly, Argaw [61] documented the complete absence of soybean-nodulating rhizobia in certain agro-ecological zones, indicating that some soils may inherently lack compatible Bradyrhizobium strains due to historical land use and host absence.
The contrasting nodulation response between soybean varieties further indicates varietal differences in promiscuity. The ability of Gishama to nodulate in soils where Awassa-95 failed suggests that Gishama is more promiscuous, capable of forming symbioses with a broader range of indigenous Bradyrhizobium strains, even at low population densities. Similar varietal differences in soybean promiscuity have been widely reported, where certain genotypes exhibit broader compatibility with indigenous rhizobia, while others require specific symbionts for effective nodulation [62].
The overall scarcity of compatible native rhizobia across the study sites underscores the need for inoculation with effective Bradyrhizobium strains. Field studies in sub-Saharan Africa have consistently demonstrated significant improvements in nodulation, nitrogen fixation, and grain yield following inoculation under conditions of low native rhizobial abundance [13]. However, the effectiveness of inoculation can be influenced by competition between introduced strains and indigenous rhizobia that are well adapted to local soil conditions, even when native strains are less effective symbionts [63]. This highlights the importance of selecting competitive and locally adapted inoculant strains for sustainable soybean production.

4.3. Relationship of Rhizobial Abundance and Soil Properties

The strong positive relationships observed between rhizobial abundance and soil organic carbon, organic matter, and CEC indicate that carbon-related soil fertility attributes play a central role in sustaining native soybean-nodulating rhizobia. Soil organic matter provides essential carbon substrates and microsites for microbial growth, while higher CEC enhances nutrient retention and chemical buffering, thereby improving rhizosphere conditions for rhizobial survival. Similar linkages between soil organic matter, CEC, and microbial abundance have been widely reported in agricultural soils [64,65]. Recent research in Ethiopia also reported similar relationships between soil fertility parameters and native rhizobial populations [22].
The strong and significant correlations between total nitrogen and both organic carbon (r = 0.91) and organic matter (r = 0.90) further emphasize the tight coupling between soil organic matter and nitrogen pools. In most agricultural soils, the majority of total nitrogen is organically bound, and thus closely follows the distribution of soil organic carbon. Recent studies in tropical and subtropical agroecosystems confirm a strong coupling between soil organic carbon and total nitrogen, with carbon-rich soils consistently exhibiting higher total nitrogen contents due to enhanced nitrogen storage and cycling capacity [57,66]. This relationship highlights the importance of organic matter management for maintaining soil nitrogen reserves that indirectly support rhizobial persistence and symbiotic functioning.
Although positive correlations were observed between CEC and both OC and OM, these relationships were not statistically significant (p > 0.05). Nevertheless, the observed trends are consistent with well-established soil science principles whereby organic matter contributes negatively charged functional groups that enhance CEC, particularly in surface soils. Empirical evidence from tropical agricultural systems demonstrates that increases in soil organic carbon can lead to substantial improvements in CEC, even where clay mineral contributions are limited [67,68].
Soil pH and available phosphorus exhibited weak and non-significant relationships with rhizobial abundance, likely because all study soils fell within a pH range (6.4–7.66) generally considered favorable for rhizobial survival and infection. Previous research has shown that rhizobial populations are strongly constrained only under highly acidic conditions (pH < 4.5), whereas near-neutral soils support rhizobial persistence [43,69]. When such threshold conditions are met, organic carbon availability and nutrient retention capacity appear to exert greater control over rhizobial population dynamics.
Overall, the findings indicate that while the soils were broadly suitable for rhizobial survival, management practices that maintain or enhance soil organic matter and associated nitrogen pools are critical for sustaining native soybean-nodulating rhizobia.

4.4. Influence of Cropping History on Native Rhizobial Abundance

The variation in native soybean-nodulating rhizobial abundance observed across the study sites highlights the strong influence of cropping history on the persistence and enrichment of indigenous rhizobia. Soils from Arsi-Negelle, where haricot bean had been cultivated prior to cereal crops, supported relatively higher rhizobial populations for both soybean varieties. This finding is consistent with the well-established role of grain legumes as compatible hosts that maintain and multiply rhizobia through symbiotic nitrogen fixation, thereby enhancing rhizobial carryover effects for subsequent legume crops [60,65].
The observed variation in native rhizobial abundance across sites highlights the strong influence of cropping history on the persistence and enrichment of indigenous rhizobia. Fields with recent legume cultivation, such as those in Arsi-Negelle, supported higher rhizobial populations, consistent with evidence that legumes act as compatible hosts that maintain and multiply rhizobia through symbiotic nitrogen fixation. This enrichment effect can persist for several years after legume cultivation and enhance nodulation potential for subsequent legume crops [65,70].
In contrast, soils from Wondo Genet, Boricha, Dore, and Hawassa—where cropping systems were dominated by non-legume crops—exhibited consistently low rhizobial population densities. In Hawassa, although haricot bean was grown during the 2021 cropping season, the overall rhizobial abundance remained low, with detectable populations only for the more promiscuous soybean variety Gishama. The predominance of non-legume crops such as enset and teff before and after the legume phase likely limited the persistence and multiplication of rhizobia, suggesting that a single or short-term legume phase may be insufficient to sustain rhizobial populations at levels required for effective nodulation of all soybean genotypes. Similar observations have been reported in smallholder systems, where rhizobial populations decline rapidly in the absence of continuous or repeated host presence [70,71].
The contrasting nodulation responses of the two soybean varieties further underscore the importance of host genotype–rhizobia interactions. While Gishama was able to nodulate in soils with very low rhizobial populations, Awassa-95 failed to do so in Dore and Hawassa, indicating a higher degree of specificity in its symbiotic requirements. Genotype-dependent differences in promiscuity have been widely reported in soybean, with some varieties capable of forming symbioses with a broader range of indigenous Bradyrhizobium strains than others [60,61]. These findings suggest that cropping history and host genotype interact to determine nodulation success, particularly in systems characterized by low native rhizobial abundance.

5. Conclusions

The abundance of native rhizobia is a critical determinant of successful soybean nodulation and biological nitrogen fixation, and its assessment provides essential insight into the likely response to rhizobial inoculation. In this study, enumeration of indigenous soybean-nodulating rhizobia revealed consistently low population densities across all investigated locations, indicating limited inherent nodulation potential in the absence of inoculation.
Significant positive relationships between native rhizobial abundance and cation exchange capacity, soil organic carbon, and organic matter content highlight the importance of soil fertility attributes related to carbon availability and nutrient retention in sustaining rhizobial populations. In contrast, soil pH and available phosphorus were not major limiting factors, as all sites fell within ranges generally favorable for rhizobial survival.
The findings further demonstrate that cropping history and soybean variety compatibility strongly influence native rhizobial abundance. Fields with recent legume cultivation, particularly haricot bean, supported relatively higher rhizobial populations, underscoring the role of legumes in maintaining indigenous rhizobia within cereal-based systems. Moreover, varietal differences in nodulation response suggest that host genotype interacts with background rhizobial populations to shape symbiotic outcomes.
Overall, the low abundance of compatible native rhizobia across the study sites indicates that soybean production without inoculation is unlikely to achieve effective nodulation or optimal biological nitrogen fixation. These results emphasize the need for inoculation with effective and competitive Bradyrhizobium strains, to improve soybean productivity and promote sustainable nitrogen management in the study areas.

Author Contributions

H.B. conceptualized and finalized the research methodology, conducted and managed the experiment, collected and analyzed the data, wrote the original manuscript, and revised the manuscript. T.A. initiated the concept, funding acquisition, project administration, and reviewed and edited the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

The research work is supported by Hawassa University (HU) via the thematic research grant ‘Soybean Popularization, Agronomic and Nutritional Evaluation, and Product Development for Agro-processing Industry Linkage in the Sidama Regional State, Ethiopia’. The APC is covered by “Climate Change Effects on Food Security (CLIFOOD)”, a project supported by the DAAD with funds from the Federal Ministry for Economic Cooperation and Development (BMZ) with project ID 57562534.

Data Availability Statement

All data gathered for this study are analyzed and included in this published article. For further information, please contact the corresponding author.

Acknowledgments

We are grateful to HU for supporting this research through its thematic project grand scheme towards TA, as well as to CLIFOOD, a project supported by the DAAD with funds from the Federal Ministry for Economic Cooperation and Development (BMZ), for covering the APC for this paper, along with later providing a competitive Doctoral Bursary to HB.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Correlation between soil physicochemical properties and population of rhizobia.
Figure 1. Correlation between soil physicochemical properties and population of rhizobia.
Nitrogen 07 00019 g001
Table 1. Geographic characteristics and cropping history of the five test locations.
Table 1. Geographic characteristics and cropping history of the five test locations.
GPS CoordinateCropping History
RegionDistrictAltitude (m a.s.l)LatitudeLongitude20222021202020192018
Boricha19606°59′ N38°17′ EM + PMMM + PM
SidamaDore16947°30′ N38°28′ ETMMTM
Hawassa16607°3′ N38°30′ ETHEEE
Wondo Genet17807°19′ N38°38′ EP-TMMMM
OromiaArsi-Ngelle19607°19′ N38°39′ EMSHMS
Abbreviations: M—Maize; P—Potato; T—Teff; H—Haricot bean; S—Sorghum; M + P—Maize potato intercropping; P-T—Potato-Teff rotations.
Table 2. Physical characteristics of the soils sampled across the five test sites.
Table 2. Physical characteristics of the soils sampled across the five test sites.
SitesSoil CharacteristicsTextural Class
Sandy (%)Clay (%)Silt (%)
Dore49.8413.4436.72Loam
Boricha47.8413.4438.72Loam
Wondo Genet51.8414.4433.72Loam
Hawassa47.8412.4439.72Loam
Arsi-Negelle41.8415.4442.72Loam
Table 3. Description of varieties that were used for the experiment.
Table 3. Description of varieties that were used for the experiment.
VarietySourceBreeder/MaintainerAltitude (m.a.s.l.)Year-Released/RegisteredMaturity GroupApprox. Growth Period (Days)
Awassa-95 (G 2261)PARCAwARc/SARI520–18002005Early80–100
Gishama (PR-143-(26))PARCPARC520–18002010Medium110–120
Note: PARC = Pawe Agricultural Research Center; AwARC = Awassa Agricultural Research Center; SARI = South Agricultural Research Institute.
Table 4. Selected chemical characteristics of the soil sampled across the five sites.
Table 4. Selected chemical characteristics of the soil sampled across the five sites.
SitespHOC (%)OM (%)Av. P (mg kg−1)CEC (meq/100 g)Ex. K (cmol/kg)Ex. Na (cmol/kg)TN (%)
Arsi-Negelle6.923.616.2214.04471.750.260.33
Hawassa7.662.634.5337.34391.832.700.32
Boricha6.42.053.5314.64361.890.720.18
Dore6.61.372.3612.50382.140.920.11
Wondo-Genet7.213.025.2133.91361.210.480.26
Table 5. Number of soybean infecting native rhizobia population abundance for the soils across the study sites.
Table 5. Number of soybean infecting native rhizobia population abundance for the soils across the study sites.
VarietiesDistricts for Soil CollectionMPN (Cells g−1 of Soil)
GishamaArsi-Negelle1.7 × 101
Wondo Genet0.6 × 101
Boricha0.6 × 101
Dore0.6 × 101
Hawassa0.6 × 101
Awassa-95Arsi-Negelle1.7 × 101
Wondo Genet0.6 × 101
Boricha0.6 × 101
Dore0
Hawassa0
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Beruk, H.; Ayalew, T. Abundance of Indigenous Soybean-Nodulating Rhizobia in Relation to Soil Properties and Cropping Patterns in a Midland Agro-Ecology of Southern Ethiopia. Nitrogen 2026, 7, 19. https://doi.org/10.3390/nitrogen7010019

AMA Style

Beruk H, Ayalew T. Abundance of Indigenous Soybean-Nodulating Rhizobia in Relation to Soil Properties and Cropping Patterns in a Midland Agro-Ecology of Southern Ethiopia. Nitrogen. 2026; 7(1):19. https://doi.org/10.3390/nitrogen7010019

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Beruk, Haimanot, and Tewodros Ayalew. 2026. "Abundance of Indigenous Soybean-Nodulating Rhizobia in Relation to Soil Properties and Cropping Patterns in a Midland Agro-Ecology of Southern Ethiopia" Nitrogen 7, no. 1: 19. https://doi.org/10.3390/nitrogen7010019

APA Style

Beruk, H., & Ayalew, T. (2026). Abundance of Indigenous Soybean-Nodulating Rhizobia in Relation to Soil Properties and Cropping Patterns in a Midland Agro-Ecology of Southern Ethiopia. Nitrogen, 7(1), 19. https://doi.org/10.3390/nitrogen7010019

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