Utilization Of Phosphorus And Potassium From Acid Yellow Soil By Ectomycorrhizal Fungi

In acidic soils, aluminum (Al3+) toxicity and nutrient deficiency could be one of most important reasons for the limitation of plant growth. At present, global environmental pollution increases and intensifies soil acidification, which seriously threatens the stability and security of plant ecological systems. Ectomycorrhizal fungi are an important component of forest ecological systems, and much research showed that ectomycorrhizal fungi were aluminum resistant. But there were scanty reports on the characteristics of aluminum resistance of mycorrhizal fungi under nutrient stress and the mechanism of nutrient mobilization by mycorrhizal fungi under aluminum stress. Therefore, Suillus luteus (Sl13), Pisolithus tinctorius (Pt 715) and Suillus subluteus (Ss 00) were grown in pure liquid culture medium with soil as sole K and P sources and with variable Al3+concentrations and then fungal growth, nutrient uptake, organic acid efflux, activity of key enzymes related to oxalate efflux, and unitization phosphorus (P) and potassium (K) were studied. The objectives of the experiment were to realize mechanism of aluminum resistance of ectomycorrhizal fungi and to select fungal strains insensitive to aluminum. Following are the main results obtained:(1) The fungal biomass reached highest, respectively, in culture medium with K-deficient acidic yellow soil as K source together with 1.6 mmol Al3+·L-1(Pt 715), 0.8 mmol Al3+·L-1(Sl13), and 0.4 mmol Al3+·L-1(Ss 00). This seems reasonable to suggest all of the three fungi were able to adapt environment with Al3+. Fungal species also varied significantly in Al3+resistance, with Pt715 from Southern China showing highest resistance (3 times higher than Ss 00 from Northern China), followed by Sl 13 also from Southern China (twice as much of Ss 00) and Ss 00 with lowest resistance.(2) Fungal P uptake was highest in solution with P-deficient acidic yellow soil as P source together with 1.6 mmol·L-1 (Pt 715),0.8 mmol·L-1 (Sl13), and 0.8 mmol·L-1 (Ss 00). Maximum P absorption by fungi coincided with highest biomass and the mean P uptake by Pt 715 was twice as much as other two fungal isolates, indicating that P uptake was beneficial to increase the ability of Al resistance and Pt 715 was resistant to both high Al and low P.(3) Fungal K uptake was highest in solution with K-deficient acidic yellow soil as K source together with 1.2 mmol·L-1 (Pt 715) and 0.8 mmol·L-1 (Sl13 and Ss00). The mean K absorption by Pt 715 was significantly higher than other two fungi. This suggests that Pt 715 resistant to Al had high ability to absorb K in contrast to SI 13 also from Southern China which was sensitive to low K.(4) Fungal strains varied in mobilization and utilization of insoluble P from nutrient-deficient acidic yellow soil. For example, more O-P, Ca-P and Al-P were mobilized from soil by Pt 715 than other two fungi under medium Al or high Al stress and Sl 13 had high ability to absorb Fe-P, all three fungal strains could utilize O-P varying degrees.(5) Fungal strains varied in mobilization and utilization of structural K from nutrient-deficient acidic yellow soil. The mineral structural K mobilization by Pt 715 was significantly higher than other two fungi, and that could be the reason for high K absorption by Pt 715.(6) Fungal proton efflux was highest in solution with nutrient-deficient acidic yellow soil with 1.6 mmol·L-1 (Pt 715),0.8 mmol·L-1 (Sl13), and 0.4 mmol·L-1 (Ss 00). Maximum proton efflux by fungi coincided with highest biomass, indicating that protons efflux by fungal strains with high resistance to Al was to mobilize soil P and K and high P absorption was beneficial to increase the ability of Al resistance. Three fungal strains varied in the types and amount of organic acids and protons, could be the mainspring of significant differences in insoluble P and K.(7) The mean oxolate accumulation by Pt 715 was significantly higher than other two fungi. This suggests that malate dehydrogenase activity coincided with oxolate accumulation by fungi. Fungalmalate dehydrogenase activity was highest while oxolate accumulation had significantly decreased with 2.4 mmol·L-1, could be due to the strains were restrained.(8) Fungal socitrate lyase activity was highest in solution with P-deficient acidic yellow soil as P source together with 0.8 mmol·L-1. This suggests that socitrate lyase activity coincided with oxolate accumulation by fungi.