Iron and zinc deficiencies are two major malnutrition problems in women and children in sub-Saharan Africa. In Senegal, the prevalence of iron and zinc deficiency is estimated to be 66% and 18%, respectively, in children under the age of 5 years. In addition, more than half of the women in the population have a high iron deficiency (Haddad et al., 2015; ANSD/ICF, 2018). These deficiencies are due to low micronutrient food intake (Krebs et al., 2014; Harika et al., 2017). The government of Senegal is investigating how to develop pearl millet (Pennisetum glaucum L.) production to improve food security and reduce malnutrition (Vilakati et al., 2016; Govindaraj et al., 2019). Pearl millet is the second most common source of caloric food in this country and the least expensive source of iron and zinc. The average consumption of this cereal is 30.2 kg/person/year in Senegal, of which the average consumption is 53.3 kg/person/year for residents of rural areas and 23.1 kg/person/year for those in urban areas (IPAR, 2018). However, the yield of pearl millet remains low (296 kg/ha), while the potential is 3000 kg/ha (Bamba et al., 2019). The production of this cereal is facing many constraints, such as poor soil fertility management (Diallo et al., 2017), progressive soil salinization, high soil acidification (Dasylva et al., 2019) and erratic rainfall patterns (Faye et al., 2018). Therefore, appropriate and sustainable ways to improve the fertility of soils and increase crop yields are urgently needed to improve nutritional quality. A better alternative would be to use agroecological and environmentally friendly practices based on the joint production of knowledge, combining science and traditional and indigenous knowledge of producers (Food and Agriculture Organization (FAO), 2018). Several approaches have been developed, including the application of organic residual products (ORPs) (Kaboré et al., 2011), the use of nitrogen-fixing green manure (Akhtar et al., 2020), and the use of beneficial rhizospheric microorganisms such as plant growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF) (Diedhiou et al., 2016).

Mycorrhizal symbiosis is a beneficial relationship between most terrestrial plants and AM microorganisms that play an important role in plant hydromineral nutrition (Rodriguez and Redman, 2008). Fungi promote better plant growth and improve nutrient acquisition by forming a large and complex mycelial network in the soil that can absorb phosphorus beyond the depletion zones around the host plant roots (Smith and Read, 2008). AMF improve the uptake of major nutrients such as nitrogen and phosphorus under stress conditions (Jansa et al., 2019; Song et al., 2020), as well as micronutrients such as zinc, iron, and copper (Begum et al., 2019) in the plant. Several studies have demonstrated the positive effect of AMF on pearl millet (Diop et al., 2013; Beggi et al., 2016; Oruru et al., 2018; Mofini et al., 2020). The development of arbuscular mycorrhizal symbioses and the responsiveness of plants depend on the environmental conditions and the specific combinations of plants and fungi (Davison et al., 2015; Diedhiou et al., 2016; Rúa et al., 2016). Exploiting the potential of AMF symbiosis in crop production requires the selection of an appropriate combination of cultivar, fungus, and farming practices.

In addition, ORPs are potential organic fertilizers for agriculture (Kaboré et al., 2011) because their use improves the soil nutrient content (Alburquerque et al., 2012; Mohamed et al., 2018) and, consequently, the yield and nutritional quality of cultivated plants. Nutrients in ORPs can therefore reduce mineral fertilizer inputs and effectively correct soil micronutrient deficiencies (Tambone et al., 2010). The positive effect of combining AMF and ORP on improving plant growth and nutritional quality has been demonstrated in many studies (Abdullahi et al., 2014; Cobb et al., 2018; Pelealu et al., 2019). These studies from different areas have shown that the synergistic effects of soil amendments and AMF reduce agricultural costs while maintaining or/and improving crop yields and quality. It is also important to highlight that the positive response of plants to organic fertilization in combination with AMF differs from the type of organic fertilizer used as well as the AMF strain (Sadiq et al., 2012; Abdullahi et al., 2014; Jiang et al., 2021). Hence, in this context, the objective of this study was to improve pearl millet yield by using AMF biofertilizers and organic residual product combinations.

All the control and inoculated pearl millet varieties were mycorrhized since the soil was not sterilized. Mycorrhization intensity was low (ranging from 3.7% to 15.14%), which corroborates the results of Mofini et al. (2020), who showed that cultivated and wild millet species contract mycorrhizal symbiosis but with low mycorrhization rates. The soil collected from Nioro used in this study contained a significant number of spores of the genera Glomus and Gigaspora, indicating the presence of an indigenous AMF community in the soil used, which explains the mycorrhization of noninoculated plants. The lack of difference between the treatments that were inoculated and those that were not inoculated for most of the growth parameters could be explained by the fact that the native AMF strains were able to provide the plant with nutrients capable of photosynthesis in the absence of the inoculation of the selected AMF strain (Duponnois et al., 2001). The AMF strain Glomus mosseae was therefore not effective in the presence of the existing indigenous AMF strains for root colonization of the pearl millet varieties since a significant effect was not obtained on root mycorrhization. The lack of a significant effect of mycorrhization on growth in the GB8735, SL23, and THIALACK_II varieties shows that pearl millet is not dependent on AMF, although these fungi can stimulate its growth (Plenchette et al., 2000; Mofini et al., 2020).

Organic fertilization affected the indigenous AMF microbial community in the presence or absence of the selected AMF strain. Cow dung, alone or in combination with the Glomus mosseae strain, enhanced plant growth, while horse manure improved the mycorrhization of the roots of all the varieties. These results confirm those of Sadiq et al. (2012), who recorded greater growth and yield of pearl millet treated with cow dung than poultry manure. Abdullahi et al. (2014), on the other hand, showed that poultry manure promoted pearl millet growth more than cow manure did. These discrepancies could be explained by the differences in the mineral composition of the different types of organic fertilizers used. In this study, horse manure was much richer in nutrients (especially N, P, and K) than was cow dung, and the highest mycorrhization intensity was observed with horse manure. The additional mineral fertilizer (urea) applied in the cow dung treatment could also explain the difference in growth between the pearl millet varieties and the horse manure treatment because the nutrients provided by the mineral fertilizers are immediately available to the plants. This difference in results between the two types of ORP could also be due to their mineralization over time (Morvan et al., 2006). The effects of AMF inoculation combined with organic fertilization were variety dependent because the results showed different responses for each variety.

The results also revealed an increase in the mineral content of the leaves due to AMF inoculation combined with organic fertilization. Cow dung and AMF promoted an increase in the zinc and phosphorus contents of SL23 leaves. On the other hand, horse manure with AMF increased the potassium content of the SL23 leaves and the iron content of the SL423 leaves. In addition, the mycorrhization intensity was positively correlated with the phosphorus and potassium contents of the leaves. This means that the more AMF colonize plant roots, the greater the P and K leaf tissue content. These results confirm the work of Abdullahi et al. (2014), who showed the positive impact of AMF inoculation combined with organic fertilization on nitrogen, phosphorus, and potassium uptake in pearl millet plants. According to Coccina et al. (2019), AMF play a significant role in the uptake of Zn by cereals, especially wheat and barley, depending on the plant species and available soil Zn. The important role of AMF in improving the uptake of inorganic phosphorus (Pi), nitrogen, potassium, and magnesium in shoots has also been demonstrated in maize (Zhao et al., 2015; Garcés-Ruiz et al., 2017).

Several studies have shown the positive effect of AMF on the bioavailability of nutrients, particularly phosphorus, potassium, zinc, and iron, in plants (Marschner and Dell, 1994; Clark and Zeto, 2000; Ortas and Bilgili, 2022). However, although the results showed an improvement in nutrient uptake by plants, the discrepancies observed can be justified by the type of organic manure applied and the presence of indigenous fungi, which, in the presence of the inoculated fungus, did not provide a microbial balance favorable for the optimal absorption of some nutrients by the plant. Indeed, the contribution of AMF to nutrient acquisition depends on fungal-specific effects on the activity of the plant pathway and on the efficiency with which both partners interact and exchange nutrients across the mycorrhizal interface (Bücking et al., 2012). Jiang et al. (2021) argued that organic fertilizer increases AMF biomass and is less harmful to AMF richness than mineral fertilizer is. The same authors highlighted that AMF generally responded positively to organic fertilizer when AMF and host plants had strong mutualistic symbioses, such as in phosphorus-deficient soil, drought and semidown areas; at low latitudes; and at testing sites that contained two or more plant species or included legumes (Jiang et al., 2021).

This research study evaluated the effect of the combination of organic fertilization and mycorrhizal inoculation on the growth of four pearl millet varieties. The results revealed that Glomus mosseae alone did not stimulate the growth of pearl millet. Organic fertilization alone or in combination with Glomus mosseae promoted optimal growth; high mycorrhization; and zinc, iron, potassium, and phosphorus uptake in all the pearl millet varieties. Cow dung alone or combined with Glomus mosseae significantly improved plant growth, while horse manure improved root mycorrhization in all the varieties. However, a field study in which Glomus mosseae would be tested in combination with organic fertilizer is recommended for evaluating the effects of Glomus mosseae on pearl millet and other cereals and legumes because pot trials might have affected the results of the study.

The original contributions presented in the study are included in the article/supplementary files, further inquiries can be directed to the corresponding author/s.

HF: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Supervision, Validation, Visualization, Writing – review & editing. BD: Conceptualization, Methodology, Supervision, Writing – review & editing. RA: Conceptualization, Data curation, Investigation, Methodology, Writing – original draft, Writing – review & editing. AK: Conceptualization, Methodology, Supervision, Writing – review & editing. AFF: Conceptualization, Formal analysis, Methodology, Validation, Writing – original draft, Writing – review & editing.