Abstract
Soybean has the potential to bring significant benefits in diversified cropping systems, which could help restructuring soil fertility and allow smallholders to increase grain yield. Rhizobium inoculation improves the biological nitrogen fixation (BNF) in legume crops and assists resource-poor farmers to increase grain yield at lower financial costs. The efficacy of symbiotic bacteria on legumes can also be improved through supplementation of phosphorus fertilizer. In this work, a meta-analysis of 29 peer-reviewed studies was performed to understand the effects of various Rhizobium strains and phosphate fertilizer application on soybean nodules. Results showed that Rhizobium inoculation was highly effective in increasing the number of soybean nodules, nodule dry weight, and shoot dry weight. Application of phosphorus fertilizer increased the overall nodule number due to improved BNF processes by Rhizobia. The main effects of both Rhizobium inoculation and phosphate fertilizer resulted in moving grain yields to 1.67 t ha-1 and 1.95 t ha-1, respectively. Furthermore, the interaction of Rhizobium inoculants and phosphorus led to relatively higher grain yield (2.51 t ha-1). Therefore, African smallholders were advised to adopt Rhizobium inoculation in soybean fields concomitantly to phosphate fertilizer application, to improve soybean productivity at lower costs. Key words: Phosphorus application, nodule number, nodule dry weight, shoots dry weight, grain yield. 
Highlights
The African population was expected to double in the 40 years (Cleland, 2013), raising food insecurity especially in the sub-Saharan region where 239 million people are experiencing dire undernourishment (FAO, 2020)
The results of the meta-analysis confirmed that the inoculation of soybean with Rhizobia strains in African soils has a highly significant influence on nodule number, nodule dry weight, shoot dry weight, and yield
standardized mean differences (SMD) [95%−confidence interval (CI)] −0.23 [ −1.07, 0.61] −0.61 [ −1.34, 0.12] −2.25 [ −2.77, −1.74] −3.11 [ −4.57, −1.65] −1.27 [ −2.34, −0.19] −0.12 [ −0.74, 0.50] −0.58 [ −1.47, 0.32] −1.46 [ −2.86, −0.07] −8.85 [−14.11, −3.59] −0.23 [ −0.74, 0.28] −6.63 [−11.62, −1.63] −1.20 [ −2.54, 0.15] −3.40 [ −5.17, −1.63] −1.54 [ −2.65, −0.42] −0.72 [ −1.32, −0.12] −0.32 [ −2.29, 1.66] −0.58 [ −1.43, 0.27] −0.29 [ −1.90, 1.32] −0.54 [ −1.27, 0.19] −2.54 [ −4.40, −0.67] −1.20 [ −2.43, 0.03] −0.77 [ −1.95, 0.40] −1.25 [ −2.49, −0.02] −0.72 [ −1.28, −0.16] −0.80 [ −2.24, 0.64] −1.05 [ −1.39, −0.72] Favors control that the efficacy of inoculants (Bradyrhizobium and Sinorhizobium)for nodule number varied from -28 to +178 nodules in contrast to the non-inoculated controls
Summary
The African population was expected to double in the 40 years (Cleland, 2013), raising food insecurity especially in the sub-Saharan region where 239 million people are experiencing dire undernourishment (FAO, 2020). Kanomanyanga et al 207 agricultural productivity of smallholders, improve household food security, and reduce poverty at the country level (Peoples et al, 1995; McNamara, 2009). Integrating legume crops, especially soybean, is an important approach in many cropping systems, as they can perform biological N2 fixation (BNF), reducing N fertilizer requirements and improving grain yield (Peoples et al, 1995; Giller, 2001). Soybean was first domesticated in China and has been grown in other Asian countries like Japan and Korea for more than 3000 years as a primary source of vital proteins and vegetable oil (Giller, 2001; Herridge et al, 2008; Nishinari et al, 2014). There is no clear evidence of when soybean was first introduced to Africa (Mpepereki et al, 2000), but its nodulation with indigenous Rhizobia in African soils was first ascertained by Corby (1967)
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