Research On The Activation Of Inorganic P Sources By Indigenous ECMF In Southwest Forest District Under Acid Al Stress

Acidification and its impact caused by the acid deposits on rhizosphere ecological processes is probably one of the facts that cause forest decline. Al activation by acidification has toxic effects on plants. One of Al toxicity on plants in acidic soils is to lead the imbalances of the transfer and absorption of mineral elements, especially P which is necessary for plant growth. Ectomycorrhizal fungi (ECMF) can form a symbiotic system with a variety of trees, and play a prominent role in terms of resistance to Al and phosphate solubilizing, which is important for maintaining healthy and sustainable development of forest ecosystems. To better understand activation of insoluble inorganic phosphorus sources (Fe-P and Al-P) by ECMF under acid Al stress and its mechanism, Pt715(Pisolithus tinctorius715), and Ld2, Ld3(two strains of Lactarius deliciosus (L. Fr.)Gray) were selected for this study, and effects of different P sources under Al stress on ECMF growth, nutrient absorption and organic acid secretion were studied to provide a theoretical basis for the restoration of forest decline caused by the acid deposits and healthy and sustainable management of forest ecosystems in Southwest Forestry District using liquid pure culture techniques. The main conclusions were as follows:(i) ECMF had strong resistance (tolerance) to Al in the0-1mmol/L range. Resistant ability to Al ranked as Ld2> Ld3> Pt715, and Ld2showed maximum tolerance with respect to growth. Mycelial biomass under Al stress (lmmol/L) ranked as:Fe-P was the largest, and Al-P took second place, and Po (without P treatment) was minimal. These results indicated that insoluble inorganic P source treatment promoted the growth of ECMF, and Ld2growed more quickly under different P source treatment. Ld2is the best choice for anti-Al and phosphate-solubilizing due to its fastest growth.(ii) There were significant differences in absorption and utilization of nutrients by ECMF under different treatments. Both under no Al and Al-processing, P content, P uptake of treated strains, and varying quantity of P both in culture medium and ECMF under Fe-P, Al-P treatments were significantly higher than that under Na-P treatment (P<0.05), while the K content, K absorption of ECMF were significantly lower than Na-P treatment (P<0.05). Al-processing improved the average Al, Mg and Fe content of ECMF. The average Mg content of three strains under Al stress were expressed as Ld3> Ld2> Pt715, and Fe content ranked as Ld2> Ld3=Pt715, namely Lactarius delicious were higher. These suggested that under Al stress ECMF can make good use of insoluble inorganic phosphorus sources Fe-P and Al-P in acidic soils and exhibited characteristics such as rapid growth, more Al absorption, more P absorption, and low K accumulation, which showed strong anti-Al and P-solubilizing ability.(iii) Correlation analysis results showed that P content had significant negative relationship with K content (P<0.05), while had significant positive relationship with Al, Mg and Fe content, suggesting that more P absorption along with low K absorption, and there were synergistic effects between P absorption and absorption of high positive ions including Al, Mg and Fe; while K content was significantly negatively correlated Al and Mg content. There was a significant negative correlation (P<0.01) between Fe and Al content, which may result from competitive interactions between metal cations.(iv) ECMF under each treatment all can secrete H+, oxalate and formate, whose content vary with Al, P treatments and strains. H+secretion under Al stress ranked as Fe-P> Na-P> Al-P, and the average H+secretion of three strains ranked as Pt715> Ld2> Ld3. Oxalate and formate exudation rate of Al-P and Fe-P treatments under Al Stress were expressed as Ld2> Pt715> Ld3. H+from oxalate of each P source treatment under no Al processing conditions was not main source of H+in culture solutions, while the percentage of H+from oxalate of Al-P treatment under Al stress showed that75.17%(Pt715),69.38%(Ld2) and37.22%(Ld3), suggesting that H+under Al-P may be a major source of hydrogen ions.(v) Correlation analysis results showed that oxalate exudation by Pt715and Ld3was significantly associated with P, Fe and K cotent, and oxalate exudation by Ld2was significantly associated with P, Al, Mg and K cotent. Formate exudation by Pt715and Ld3had significant positive relationship with Fe content (P<0.05), while Formate exudation by Ld2was significantly positively correlated to Fe content (P<0.01). H+exudation had significant negative relationship with Al content (P<0.05), while had significant positive relationship with the Fe content(P<0.01). This indicated that oxalate exudation were directly related to the absorption and utilization of P and metal cations, while formate exudation and H+exudation was only directly related to high positive ions such as Al and Fe. Considering correlation between P and cation uptake by ECMF, Formate exudation and H+exudation may be indirect factors of P uptake.