Peanuts are annual oilseed legume often produced in rotation or in association with cereal crops in Benin. However, in last years, the yield of peanut has decreased drastically. According to UDA [1], Benin recorded a peanut yield of 0.93 t/ha against 1.2 t/ha in Nigeria, 1.10 t/ha in Ghana and 1.13 t/ha in Côte d’Ivoire. The world average is 1.5 t/ha and more than 3 t in the United States and China [2]. Although this peanuts production yield decrease is imputed to several factors, the lowering soil fertility appears to be non-negligible. According to Igué et al. [3], 68% of the soils in Benin have suffered a severe decrease in its agricultural potential specially the southern and central regions. This is, we according to these authors, to the soil’s low nitrogen, phosphorus and potassium content and its cation exchange capacity.

To improve their peanuts production yields then, producers often make use of chemical fertilizers. However, due to their high cost, the use of chemical fertilizers is restricted to a small number of peanuts producers. Although chemical fertilizer seems to improve their production yield, their impact on the environmental is disastrous at long term.

Actually, the success of groundnut cultivation is mainly due to its ability to fix atmospheric nitrogen thanks to the presence of root nodules hosting colonies of Rhyzobium sp strains, which allow a slight improvement on the production yield on poor soils, and this, with minimal intervention [4]. The success of this crop therefore requires the establishment of an efficient association between the host plant and the bacteria of the genus Rhizobium. Inoculating the peanut with effective rhizobia has been proven a beneficial practice for improving the peanuts productivity [5] [6] in Benin. This practice would also constitute a prospect for promoting a sustainable and environment-friendly farming system.

The present study was initiated, in the same framework, and, conducted in a peasant environment to assess the response of peanut seeds (Arachis hypogaea) grown in Ouèsse- and on the Agonli uplands in the south and center of Benin to inoculation with a rhizobium and to compare its advantage to other fertilization options in these environments.

In general, the highest values for all the parameters observed are recorded on the plants having received the combined contribution of Rhizobium sp and N₁₅P₁₅K₁₅. A higher mean value significantly different to plants having benefited from all the other types of treatment regardless of the experimental site has been recorded.

The results showed that the nutrients provided by the various treatments have a variable and significant influence on the different parameters evaluated.

From the results of our investigation on the growth parameters (dry weights of aerial biomass, root biomass and total biomass), a homogeneity of effects was observed overall on all the plots except for the plots sowed with seeds inoculated and then amended with NPK fertilizer. This would be due to the growing conditions (rainfall) which, overall, were not favorable to a good plant productivity. The non-significant responses thus obtained especially between the three treatments (T1, T2, T3) are similar to the observations made by Ndiaye [16] on the growth of the plant on which the biomass yield also depends. Thus, “the drought occurred in full flowering period probably caused a slowing down of the vegetative development and mostly on the inoculated plots; which must have reduced the differences in growth between treatments (particularly at Ouèssè)”. Indeed, for the flowering-formation of pods phase observed between 30 to 70 days after sowing, Covè seems to have been favored compared to Ouèssè by the drop in rainfall.

The highest values observed for the treatment combining Rhizobium sp + N₁₅P₁₅K₁₅ are in accordance with the observations made by Didagbé et al. [17]. According to their investigation carried in the Center and North Benin, overall, the treatments by Rhizobium strains supplemented with phosphorus produced the highest aerial biomass. This would imply the positive response (p < 0.05) to the addition of NPK at inoculation observed on the peanut plants growth parameters. These same trends were observed for the symbiotic parameters and the production parameters. In reality, in addition to the phosphorus demand from the host plant, nodules require greater amounts of P and energy than other plant tissues [18]. Phosphorus contributes to root growth [19]; nitrogen being a quality of and growth factor [20] [21] and potassium not only improves the plant’s water regime but also increases its tolerance to drought, frost and salinity [20].

Furthermore, the number of nodules observed on controls revealed the presence of Rhizobium specific for peanuts in the soil at each site. Indeed this parcel has been used for peanuts crop cultivation. However, the small quantities of spontaneous nodules obtained could be due to the low strains population density of these soils [22].

On the inoculated plots, the formation of nodules and the symbiotic activities in the different peanut populations were therefore carried out with the native strains of Rhizobium present in the soil as well as the Rhizobium sp strains. The number of nodules formed (transformed value) differ significant to (p < 0.05) from the inoculated plants to the control plants. However, the confident interval of these differences is narrower compared to the number of nodules of the plants fertilized with N₁₅P₁₅K₁₅ only. This could be due to the competitiveness capacity of the strains. The native competitive but efficient less strain [23] can affect the inoculation of a given legume by a selected rhizobium with high efficiency. Nevertheless, the inoculation were important because the native Rhizobium strains would have been infectious but not efficient for the most part. The difference on the number of nodules counted in the control plots and in the inoculated plots is small was statistically different on the two experimental sites and no depressive effect of the inoculation on the nodulation was observed. This would indicate that inoculation has a favorable effect on the nodulation of legumes [16] [24]. Didagbé et al. [17] have shown that Rhizobium strain is one of the best performing exogenous strains of Bradyrhiobium used in their investigation. However, our results on plant nodulation are relatively lower than those previously obtained [17]. The competition between native and introduced strains could be low. As Ndiaye [16] observed on the CB 756 strain, the increases in yield following inoculation seem to indicate that the strain Rhizobium sp has a fixing potential greater than the native soil strains, but the competitiveness of the native strains and the poor survival conditions make it (very) difficult to increase the yields.

According to Tombozara [25], the possibility that the introduced rhizobia have not competed with the microorganisms present in the tested parcels must be considered. Other factors such as the quality of the plant material could also have had an impact on the low performance observed. Notwithstanding the probable competitiveness effect between the strains, the analysis of the dry weights of the nodules indicates that the difference between the average values are insignificant between the controls, the N₁₅P₁₅K₁₅ and Rhizobium sp + N₁₅P₁₅K₁₅ at both Covè and Ouèssè. It is therefore possible that the rainfall regime mentioned above contributed to this reduction in the differences of the effects between the treatments. A water deficiency would induce a significant decrease in the number and yield of nodules [26]. This water deficit is the consequence of pockets of drought occurring at the beginning of the plant development cycle and in full bloom. This could partly explain the fact that the results obtained at Covè are better than at Ouessè.

In addition, the physicochemical properties including among others the phosphorus deficiency affect the multiplication of rhizobia in the soil. It has been established that the effectiveness of endomychorizial symbiosis depends on the level of assimilable phosphorus in the soil [19]. For Mandimba and Djondo [27] phosphorus could increase nodulation in peanuts from 57% to 176% and, by extension, the yield. Phosphorus deficit would result in a reduction in the growth of nodules. Thus, the addition of N-P-K fertilizer has considerably improved the efficiency of the Peanut/Rhizobium symbiosis. N-P-K fertilization alone had more influence on yields than inoculation alone despite the non-significant difference observed. In accordance with our results and those obtained by other researchers [28] [29] [30], like all crops, legumes have nutrient requirements that, depending on their availability, influence their yields. According to these same authors, even nitrogen fertilizer is necessary to improve the yields of legumes. Therefore, the increases in the yield of full pods obtained indicate that the low dose of nitrogen provided by the mineral fertilizer made it possible to increase the groundnut yields from 26% to 36% [30].

The pod yield is one of the first parameters on which all the changes affecting the plant are reflected [31]. Analysis of the average yields in full pods revealed that the local variety “Monto” responded favorably to inoculation Rhizobium sp. This positive effect of inoculation with exogenous Rhizobium on the yields of legumes has been demonstrated by several studies on peanuts, soybeans, cowpeas and other legumes [6] [18] and [26].

However, by including production costs, the use of mineral manure costs more than the inoculum. Therefore, the treatment with Rhizobium sp would be more profitable than the single contribution of the mineral fertilizer especially at Covè. In addition, the use of this type of fertilization also helps to protect the environment by limiting the pollution by nitrites.

The experiment conducted in a peasant environment made it possible to evaluate and compare the response of peasant seed to inoculation with the Rhizobium strain. Compared to Ouessè, the results obtained at Covè were significantly better. It was observed that the nitrogen nutrition of the peanut could be ensured by the inoculation of the seeds and mineral fertilizers could provide the nutritive elements in particular urea, triple superphosphate (TSP) and potassium chloride. However, these two simple factors separately induced a significantly slight improvement in the vegetative parameters (growth, nodulation) compared to the control. The effect of the fertilizer factor was more favorable on the symbiotic activity of the peanut than the growth of the plant thanks to the moderate supply of both N, P and K. As for inoculation alone, despite the increase in nodulation, it had beneficial but not very remarkable effects on the production parameters, certainly due to the poor survival conditions of the bacteria introduced. Regarding the performance of the treatments, the vegetative and productive behaviors of the plants were more influenced by the combined effects of inoculum and mineral fertilizer. The best results in terms of comparative advantages and from the economic point of view were obtained by combining the Rhizobium strain with the manure N₁₅P₁₅K₁₅.

The authors declare no conflicts of interest regarding the publication of this paper.

Adjile, A., Didagbe, O., Ayelo, C., Boko, F., Houngnandan, P. and Mongbo, R. (2020) Inoculation of Rhizobiums sp Strains to Improve Soil Fertility: A Peanut Trial in Covè and Ouessè (Benin). Agricultural Sciences, 11, 440-455. https://doi.org/10.4236/as.2020.114027