Growth and nitrogen responses to inoculation with arbuscular mycorrhizal fungi, varied soil phosphorus, and water stress by Acacia spp. and Prosopis spp

Tree legumes produce edible pods, fodder, and fuelwood and help maintain soil fertility in agroecosystems or degraded sites through biological nitrogen fixation and litter input. Two tree legume species indigenous to West Africa, Acacia tortilis (Forssk.) Hayne, Prosopis africana (Guill. & Perr.) Taub., and two exotic legume species, Acacia holosericea A. Cunn. and Prosopis juliflora (Swartz) DC were used in nursery and glasshouse trials. The trials incorporated two major factors limiting plant production and nitrogen fixation, soil moisture and phosphorus, to determine influences of soil symbionts on biomass production and nitrogen fixation by these species using {dollar}sp{lcub}15{rcub}{dollar}N isotope enrichment techniques.; Rhizobium and arbuscular mycorrhizal (AM) fungal inoculation, and phosphorus fertilization were used in a Senegalese nursery with unsterilized soil and a reference plant, Balanites aegyptiaca (L.) Del.. A. tortilis and P. juliflora grew well and showed significant growth responses to AM, phosphorus treatments, and Rhizobium. With an infective and effective Rhizobium strain, even in unsterilized soil, both sets of AM aided with P acquisition. Shoot N (mg g{dollar}sp{lcub}-1{rcub}{dollar} dry wt) and atom % {dollar}sp{lcub}15{rcub}{dollar}N contents indicated increased nitrogen fixation with increased soil P level. The effects of AM inoculation were not the same for the two inoculant types and, like Rhizobium, resulted in opposite biomass partitioning responses. AM treatments played a role in greater N as well as P uptake as evidenced by the uptake of the {dollar}sp{lcub}15{rcub}{dollar}N isotope.; For the glasshouse trial, two four week drought cycles were imposed during the four month trial and effects of factor levels on physiology and drought response of Acacia tortilis seedlings were monitored. Two levels of AM inoculation each had three levels of phosphorus fertilization with or without an imposed drought regime. Growth rates of the seedlings differed under both watering regimes due to phosphorus fertilization for the non-AM seedlings. Shoot weights were significantly greater for watered seedlings than water-stressed seedlings. AM significantly increased shoot and root weights in both watered and water-stressed seedlings. Watered seedlings had significantly higher shoot P and N concentrations, and significantly less {dollar}sp{lcub}15{rcub}{dollar}N content than water-stressed seedlings. AM seedlings had significantly higher tissue P and N concentrations, higher rates of acetylene reduction (TAR), and nitrogenase activity than non-AM seedlings.; Pre-dawn xylem pressure potential (XPP) and estimates of leaf conductance were evaluated twice a week. AM seedlings maintained higher transpiration rates and 2{dollar}times{dollar} greater leaf conductance under water stress conditions than non-AM seedlings and recovered more rapidly from stress effects. Watered AM seedlings had the greatest biomass production, highest total P and total N concentrations, highest TAR, lowest atom % {dollar}sp{lcub}15{rcub}{dollar}N content and mycorrhizal dependency ranking. AM increased XPP of drought-stressed plants, and improved P uptake was accompanied by increased transpiration and leaf conductance by the host plants.; The root symbionts associated with tree legumes are important in water and nutrient acquisition. Inoculation with effective root symbionts can aid successful establishment, growth, and survival of seedlings in variable environments such as the Sahel. Information on the diversity and function of soil microbes, their response and effect on tree host productivity under environmental stress, and conditions that affect their survival can be managed to help sustain carrying capacity in the Sahelian zone of West Africa.