Arbuscular mycorrhizal fungi are obligate endosymbionts of up to 90% terrestrial plants (1) belonging to the phylum Glomeromycota (2) and sub-phylum Glomeromycotina (3). They are gaining prominence in agriculture for increasing crop production. Therefore, understanding their biogeographical patterns and drivers for maintaining ecosystem services amidst changes in farming systems aimed at agro-ecological adaptability is a prerequisite. Various biotic and abiotic factors and biological processes are reported to impact on microbial biogeographical patterns (4, 5). Several studies have identified different edaphic variables that influence soil bacterial and fungal community compositions (6). Abiotic factors include soil type (7), soil texture (8), soil acidity (9), soil temperature (10), soil moisture (11), soil available P (12) and cropping systems (tillage, crop rotation, fallowing) (13–16). Öpik et al. (17) reported that AMF communities vary in composition due to differences in ecosystems under different disturbance regimes. Other factors include organic matter (18), soil biota (19, 20), and plant communities (21, 22).

Some studies in sub-Saharan Africa have focused on the diversity and spore abundance of AMF (e.g., 23–26) but have not emphasized agricultural zoning and cropping systems moreover in Uganda. Cropping systems influence the biological, physical, and chemical properties of soils, as well as the geographical distribution of plants, which greatly impact on the diversity and spore abundance of AMF (23). Crops such as cereals, legumes, coffee, bananas, cassava, and other root and tuber crops that benefit from AMF associations dominate Africa’s landscape (27). Less frequently cultivated fallow fields always have higher spore abundance than fields under frequent conventional cultivation (23). In the drier areas of the Maasai-Mara Ecosystem in Kenya, maize and wheat monocrops recorded significantly lower AMF diversity, species richness, and spore density in the wet and dry season than the maize-bean intercrops dominated by Scutellospora and Acaulospora species (28). Similarly, the dominance of Scutellospora and Acaulospora had been reported earlier in Western Kenya (29). Sporulation of genus Acaulospora was high in acidic soils (26, 30), increasing their dominance in soils of a pH range of 5.51 to 6.77 (28). Castillo et al. (31) reported high sporulation of Acaulospora spp. Under conventional tillage than no-tillage. On the other hand, Jansa et al. (32) reported higher sporulation of Scutellospora spp. In undisturbed and moderately disturbed soils. The limited occurrence of Gigaspora spp. In various ecosystems compared to other AMF genera has been confirmed by their low density reported by Schalamuk et al. (33) Jefwa et al. (34), and Muchane et al. (28).

In the study of Belay et al. (24) a total of 15 AMF genera (Glomus, Acaulospora, Funneliformis, Gigaspora, Scutellospora, Septoglomus, Claroideoglomus, Entrophospora, Rhizophagus, Paraglomus, Diversispora, Pacispora, Racocetra, Sclerocytis, and Ambispora) were isolated from both field and trap culture soils. A total of 31 species was observed in the study in an irrigated mixed fruit cropping system that received manure followed by 23 species in a crop rotation field with teff, sesame, and sunflower that received 50 kg Urea ha^-1 and 100 kg DAP ha^-1 (24). In the same study, 15 AMF species were noted in a 30-year-old natural forest with acacia, fig, and stinkwood trees, 14 species in an acacia plantation, and up to 11 species in fields of sorghum or maize monocrops receiving 50 to 100 kg Urea ha^-1 and 100 to 150 kg DAP ha^-1 (24). Glomus and Acaulospora AMF were the most diverse groups represented by 9 species each, followed by Funneliformis and Gigaspora. Glomus and Acaulospora spp. Produce more spores in a shorter time than Scutellospora and Gigaspora in the same environment (23, 35, 36). Seven genera comprising Acaulospora, Ambispora, Glomus, Claroideoglomus, Pacispora, Gigaspora, and Scutellospora were isolated from cassava cropping fields in Abengourou, East Côte d’Ivoire of which the genus Glomus was dominant (37). Earlier on in South Africa, studies had shown that the rhizosphere of cassava in Limpopo contained Acaulospora scrobiculata, Glomus rubiforme, and Gigaspora sp. Whereas the Mpumulanga soils had Acaulospora scrobiculata, Acaulospora mellea, Acaulospora racticede, Glomus etunicatum, Glomus rubiforme, Gigaspora sp, and Scutellospora sp (38).

However, no studies have been carried out on the composition and spore abundance of AMF in fields under sweet potato production. Therefore, the objective of the study was to determine the composition and spore abundance of AMF as influenced by different crop types (CTs) in three sweet potato producing AEZs in eastern Uganda.

The high spore abundance of Glomus spp. And Acaulospora spp. Across AEZs and CTs may be attributed to their high frequency of hyphal fusions that plug into compatible extraradical networks and hence immediate access to host plants and subsequent spore formation (58). Glomus and Acaulospora produce more spores in a shorter time than Scutellospora and Gigaspora in the same environment (23, 35, 36). Gigasporaceae species produce large spores (260 to 440 μm for Gigaspora margarita) that require a longer developmental period than small spores (59). Hence, their occurrence can be further ascertained through the study of AMF diversity in the root systems since spore extraction alone may miss species that may have not sporulated. The insignificant response of AMF genera except for Entrophospora to changes in AEZs could be due to quite similar soil properties attributed to similar agronomic practices carried out in the fields ( Table 3 ). The total and mean spore abundance in this study (involving annual crops) were high compared to soils dominated by perennial crops and frequently supplied with inorganic fertilizers and pesticides (23, 34). The high spore abundance can be attributed to increased sporulation caused by frequent soil disturbance in annual crops fields. However, trap cultures were necessary to observe the composition of AMF as evidenced by several authors who reported fewer species from direct isolation from local soil and more species after trapping (26).

Crop type did not significantly affect AMF due to the similar conventional tillage carried out in the fields with the different crops ( Table 3 ). The slight significant difference between fallow and sweet potato in MEHF and LVC was because fallows had been rested for an average of 6 months allowing for the recolonization and sporulation of AMF. Results showed that Gigaspora spp. Abundance was highest in the fallow fields ( Table 3 ) which confirmed the results of Gai et al. (60) that Gigasporaceae spp. Are more often associated with wild plants than open fields. It also confirmed that the 6 months of fallowing was enough time for the development of their large spores (59).

The varying effects of sodium and magnesium on AMF genera in the present study ( Figure 4 ) may be attributed to varying adaptability of AMF genera to varying levels of exchangeable bases (61). The positive effect of nitrogen on the spore abundance of Glomus, Scutellospora, Entrophospora, and Archaeospora might have been indirect through the increased supply of photosynthates from the host-crop to the fungi. Availability of soil N increases mycorrhizal activity (62) and hence its positive effect on the AMF genera.

It was expected that sporulation of all AMF genera is positively affected by organic matter since it is considered a source of energy for the growth and functioning of the fungi. However, the effect of organic matter on AMF growth and sporulation depends on the efficiency of the individual species in acquiring resources from the organic matter (63). Ng et al. (64) also reported that the chemical nature of soil carbon drives the structure and functioning of soil microbial communities which may vary from one AMF type to another. For example, pure cellulose obtained after proper decomposition increases asymbiotic AMF extraradical hyphae growth and root colonization (18, 65).

Soil texture has been reported to alter colonization of AMF (8, 66) depending on the sporulation patterns. The general soil textural classes in the present study were sandy clay loam and sandy loam which are reported to favor mycorrhizal development (67). Gigasporaceae (Gigaspora spp. And Scutellospora spp.) dominate in sandy soils (68) and they are also indicators of soils with lower clay content (69). Glomaraceae and other AMF families with small spores do not show any strong dependence on soil characteristics (70). However, Lekberg et al. (68) also reported that Scutellospora cerradensis (Sc. Rtl) was the only member of Gigasporaceae that occurred predominantly in clayey soil (containing 41:11:48 of clay, silt, and sand). From the same study they reported that Glomaraceae colonized roots well in sandy soil (containing 3:7:90 of clay, silt, and sand), clayey soil (containing 41:11:48 of clay, silt, and sand) and sand/clay mixture (containing 4:1 v/v). There’s a possibility that the difference in biomass allocation and the growth patterns of extraradical hyphae of Gigasporaceae and Glomaraceae are affected by soil texture (71, 72).

Most importantly, the soil pH was within a favorable range (5.85-6.64) for fungi and supported sporulation of especially, Glomus, Scutellospora, Entrophospora, and Archaeospora, and negatively affected the sporulation of Acaulospora and Gigaspora. Earlier, Muchane et al. (28) reported that AMF were favored by pH 5.51 to 6.67, while Dobo et al. (73) reported pH 6.18 to 6.28 to increase AMF sporulation in agricultural soils. However, it was expected that Acaulospora spp. Would be positively affected by the present pH since Acaulosporaceae are tolerant to acidic tropical soils (26, 74) and they highly sporulate under conventional tillage (a common practice in the sampled sites) as compared to other AMF families (31). For example, Acaulospora laevis is predominat in low pH soils and germinates well at pH 4-5 while Gigaspora heterogama isolated from warm climates and maintained in tropical areas showed varying germination rates (8 to 78%) in the same environmental conditions (75).

The six AMF genera identified in this study were present in all the AEZs and CTs but differences in their spore production were not significant. The AMF genera observed in this study were lower than 12 isolated in Sudan (76), 15 in Ethiopia (24), 15 in Southern China (77), 9 in Ethiopia (73) but higher than the 4 to 5 genera isolated in Kenya (23, 28, 34) and 4 in Rwanda (78). This may be due to variances in edaphic conditions and the cropping systems of the study sites. However, Glomus, Acaulospora, Scutellospora, and Gigaspora reported in this study were also observed in the studies carried out in the Sub-Saharan Africa (SSA) countries mentioned above. Incidentally, there were no new AMF genera identified, which alludes to low composition in these AEZs with the mean spore abundance of only Entrophospora spp. In SEKLB being significantly higher than in MEHF.

Most of the commercial mycorrhizal bioinoculants available in SSA are imported, expensive, and majorly contain Glomus species. During efficacy testing of these strains, the environmental conditions of the origin of the strains and where the bioinoculant is to be used may not be compared (79) yet it is important for the adaptability of AMF in the different local SSA edaphic and climatic conditions (80). The population of the introduced strains must build up for their increased competitiveness and effectiveness. In Tchabi et al. (81), AMF from Tropical Africa (Glomus hoi, Acaulospora spinosa, Glomus mosseae, Glomus etunicatum, and Acaulospora scrobiculata) averagely led to increased yam tuber growth by 51, 49, 38, 38 and 31%, respectively. Whereas exotic species from Europe were less efficient except for the three isolates of G. clarum isolates which increased tuber yield by 8.8, 15.6 and 55.6%, respectively compared to non-mycorrhizal control. Additionally, soil (from yam field) increased tuber yield by 40, 33, and 20% compared to exotic G. constrictum, the non-mycorrhizal control, and the exotic G. luteum confirming the existence of superior native AMF species in yam producing regions. Therefore, to overcome the problem of local adaptability of imported species and cost implications, mycorrhizal bioinoculants can be produced locally from the native species observed across different locations in SSA. The physiological characteristics of the species determine to a greater extent their survival and activity in the soil. Hence, different species will show varying responses, in terms of survival and activity. The ability of AMF to enhance root surface area by hyphal growth and provide an extra route for uptake as mycorrhizal pathway (82) depends on the AMF strains colonizing the plant roots. The efficiency of AMF strains is influenced differently by their development and activity of the external hyphae, hyphal transport rates, and solute interchange at the arbuscule-host root cell interface (83, 84). Therefore, the mixing of different compatible AMF strains during bioinoculant production will promote the complementary benefits of AMF strains in crop production.

Plant-host generalist AMF strains should be considered for bioinoculant production as compared to plant-host specialist AMF strains since during fallowing the latter can colonize weeds or new crops which eventually can serve as a source of additional spores (85). The compatibility of the sporogenous Glomus and Acaulospora species with different crops and environmental conditions require further testing for suitable local bioinoculant production. However, Acaulospora spp. Would greatly influence crop production in acidic tropical soils since the Acaulosporaceae species are tolerant to acidic soils (26, 74); highly effective in P-uptake and transfer to the host plant compared to Glomeraceae species (86); and they highly sporulate under conventional tillage (a common practice in sweet potato producing areas) as compared to other AMF families (31).

Glomus and Acaulospora species can be further identified using molecular techniques and their compatibility with different crops and environmental conditions in varying dosage tested for suitable local bioinoculant production. To reduce the cost of soil fertility amending inputs in crop production, bioinoculants containing the most effective species can be integrated with reduced rates of inorganic fertilizers.

A total of six AMF genera comprising of Glomus, Acaulospora, Scutellospora, Entrophospora, Archaeospora, and Gigaspora were isolated from the study sites. The composition and spore abundance of AMF recorded in the AEZs and CTs in the major sweet potato growing areas had limited significant differences due to the similar agricultural practices employed by farmers. The effect of soil parameters on AMF spore abundance varied from genera to genera, however, the strongest influence was by OC, N, pH, silt, Mg and Na. The most dominant AMF i.e., Glomus and Acaulospora species can be isolated for local bioinoculant production.

The original contributions presented in the study are included in the article/supplementary materials. Further inquiries can be directed to the corresponding author.

RM conceived the research idea and conducted the research, collected, and analyzed the data, and co-wrote the manuscript; JT, PE, and CM assisted with conceiving the research and critically reviewed the manuscript. All authors contributed to the article and approved the submitter version.