Mobilization of Inorganic Phosphorus from Soils by Ectomycorrhizal Fungi

Ectomycorrhizal (EM) fungi could form symbiosis with plant roots and participate in nutrient absorption; however, many EM species commonly found in forest soils, where phosphorus (P) concentration and availability are usually very low, particularly in tropical and subtropical areas, have not yet been investigated for their efficiencies to mobilize soil P. In this study, fungal growth, P absorption, efflux of protons and organic acids, and soil P depletion by four isolates of EM fungi isolated either from acidic or calcareous soils were compared in pure liquid culture using soil as a sole P source. Boletus sp. 7 (BO 7), Lactarius deliciosus 3 (Ld 3), and Pisolithus tinctorius 715 (Pt 715) from acidic and P-deficient soils of southwestern China showed higher biomass and P concentration and accumulation than Cenococcum geophilum 4 (Cg 4) from a calcareous soil of Inner Mongolia, northern China, after 4 weeks of liquid culture. Oxalate, malate, succinate, acetate, and citrate concentrations in the culture solutions varied significantly with fungal species, and oxalate accounted for 51.5%-91.4% of the total organic acids. Organic acids, particularly oxalate, in the culture solutions may lead to the solubilization of iron-bound P (Fe-P), aluminum-bound P (Al-P), and occluded P (O-P) from soil phosphates. Fungal species also varied greatly in proton efflux, which decreased the culture solution pH and may dissolve calcium-bound P (Ca-P) in soil. This could be the reason for the increment of both inorganic P in the culture solutions and Olsen P in the soil when EM fungi were present. Total inorganic P, the sum of Al-P, Fe-P, O-P, and Ca-P, in the culture solutions was positively cofrelated with the total concentration of organic acids in the culture solutions (r = 0.918*, n = 5), but negatively with both the total inorganic P in soil (r = 0.970**, n = 5) and the culture solution pH (r = 0.830*, n = 5). These suggested variable efficiencies of EM fungal species to mobilize inorganic P fractions from soil, which could make EM trees to utilize inorganic P in the same way like EM fungi and adapt to the soils with various P concentrations and availabilities.