Phosphorus Requirement for Colonization by Arbuscular Mycorrhizal Fungi (AMF) and Effect of AMF Inoculants on Growth of Perennial Crops and Agroforestry Trees

Abstract: In most tropical soils, phosphorus is deficient and high costs of phosphorus fertilizer made it difficult for smallholder farmers to use it when needed. Arbuscular mycorrhizal fungi is known to improve particularly P in P deficient soils. However, response of plant species to mycorrhizal fungi inoculation and application of different rates of P varies. Therefore, this study was conducted to investigate the effect of phosphorus (P) concentrations on arbuscular mycorrhizal fungi (AMF) colonization and growth of two perennial crops (Catha edulis and Ensete ventricosum) and four multipurpose agroforestry trees (Cordia africana, Croton macrostachyus, Erythrina brucei and Millettia ferruginea). The experiment was conducted in a glasshouse. The treatment consisted of 0, 0.005, 0.01, 0.02, 0.05, and 0.1 mg P/g substrate and three species of AMF. The experiment was laid out in CRD design in a factorial arrangement. The results showed that plant growth parameters (shoot length and dry weight) and P uptake increased significantly after inoculations with AMF, namely Rhizophagus clarus, and Rhizophagus intraradices, and the mixed AMF species. Results on effect of P application on total mycorrhizal dependency (MD) of the studied crops and agroforestry tree species showed that maximum (41.71%) MD value was recorded for Rhizophagus clarus in khat (Catha edulis Forsk.), followed by 34.85 and 34.45% MD values for the same Rhizophagus clarus in Birbira (Millettia ferruginea) and Bisana (Croton macrostachyus), respectively. The next MD values, ranging from 2.57% for Catha edulis to 30.67% in Ensete ventricosum, were recorded for inoculation with the mixed AMF species. The least MD values of 3.51, 16.46, 10.51, 7.71, 4.34, and 14.32 were recorded for treatments with Rhizophagus intraradices for all plant species (Catha edulis, Cordia africana, Croton macrostachyus, Ensete ventricosum, Erythrina brucei and Millettia ferruginea) under the study respectively. Optimum P concentrations for maximum benefits from the AMF symbiosis in the aforementioned six plant species varied from 0.005 to 0.02 mg P g-1 substrate and the corresponding peaks of arbuscules, vesicles, percent colonization, and spore count per 50 cm3 sand were noticed at similar P concentrations. Thus, the current research results revealed that the recorded plant growth peaks were attributed to AMF colonization of the perennial crops and agroforestry trees. Therefore, inoculating plant species with a suitable AMF inoculant could result in a benefit comparable to high P fertilizer input and lead to a significant cost saving from expenditure on inorganic P fertilizer. The information obtained on minimum P requirement for perennial crops and shade trees in Sidama agroforestry can form the basis for further pot/field experiments involving integration of chemical fertilizers with AMFKeywords: Agroforestry; Crops; Inoculation; Phosphorus; Root colonization; Spore density; Trees.