Abundance, Identification and Effectiveness of Indigenous Amf Species Associated With Agroforestry Systems in the Semi-Arid and Humid Agroecologies of Rwanda

Abstract

Arbuscular Mycorrhizal Fungi (AMF) form symbiotic association with the roots of plants. AMF help to facilitate mobilization of nutrients, especially phosphorus, from soil to plant. The study was carried out in sub-humid Rubavu and semi-arid Bugesera districts in Rwanda. We hypothesized that the presence of tree species in farming systems enhances mycorrhizal fungal density. The occurrence and abundance of AMF in the soil around main agroforestry tree species in the two regions were therefore studied. The tree species in Rubavu included Alnus acuminata, Markhamia lutea, Grevillea robusta and Eucalyptus sp. while those in Bugesera were Acacia polyacantha, Senna spectabilis, Grevillea robusta and Eucalyptus sp. AMF spores were isolated from soil samples collected under trees and outside the trees canopy. Results showed that AMF spores abundance in the soil varied with the distance from the tree trunk but not consistently. A significant difference in spore density was recorded between the tree species. Soil samples from under Alnus acuminata of Rubavu gave a significantly greater density of AMF spores compared to the soils collected around Markhamia lutea (p=0.001), Grevillea robusta (p=0.018) and Eucalyptus sp. (p=0.011). In Bugesera, soils collected around Sena spectabilis showed a significantly greater density of AMF spores than soils around Grevillea robusta and Eucalyptus sp. (p=0.0002 and 0.030 respectively); and spores densities around Acacia polyacanta and Eucalyptus sp. were significantly greater than the density around Grevillea robusta (p=0.022 and 0.043, respectively). Based on major differences in spore morphological appearance, four different AMF genera were detected from the agroforestry systems in the semi-arid and humid agroecologies of Rwanda. They were identified as Glomus, Gigaspora, Scutellospora and Acaulospora. All spore types were found in all rhizosphere soil samples and Glomus was the predominant taxonomic group. Genetic characterization was done using nested PCR amplification targeting different regions of the rDNA. The first amplification step was done using the universal primers NS5 and ITS4. The 2nd step of PCR amplification was done with specific primers ARCH1311, GLOM1310 and LETC1670 along with ITS4. Due to the little amount of DNA per single spore, only vi 32.35 % of PCR amplifications gave successful results on agarose gel; three of them were sequenced successfully and only Gigaspora margarita was identified at species level with bioinformatics tools. All the maize roots in the treatments with AMF were colonized by AMF, with morphotype AMF2 performing better than morphotypes AMF1 and AMF3. The mean percentage colonization varied between 40% and 70%. The present work is the initial phase to study the AMF communities associated with some common tree species in agroforestry systems in Rwanda. Further studies are proposed relating mycorrhizal diversity in the agroforestry systems to performance and yields of crops.