Effects Of Fertilization On The Functioning Of AM Fungal Community In Elymus Nutans Roots

Arbuscular mycorrhizal fungi are a kind of important microorganism which can form mutual symbiosis with the most terrestrial plant’s roots, and the basis of mutualistic association is that plants supply AM fungi with carbohydrates, and in return, fungi provide their host plants with nitrogen and phosphorus, AM fungi play key roles in linking the above- and below-ground ecosystem. At present, a large number of studies indicate that the increase of soil nitrogen and phosphorus availability can result in the loss of AM fungal species diversity and the change of AM fungal community structure. However, how does AM fungal community functioning response to the increase of nitrogen and phosphorus levels? Whether the symbiotic relationship between plant and AM fungal community is affected by soil fertility?These problems still don’t get scientific answers. In view of the above problems, the study used the dominant plant Elymus nutans and AM fungal community in an alpine meadow ecosystem under three years different nitrogen and phosphorus fertilization as exprimental materials, we further revealed the effects of fertilization on the functioning of AM fungal community on a basis of researching the effects of fertilization on the relationship between plant and AM fungal community, the mycorrhizal responses of plant and so on, by using an inoculation technology of AM fungal community in greenhouse and the analysis methods of regular plant physiology and ecology. The major results are listed below.(1) Nitrogen supply levels significantly influenced the arbuscular colonization( F = 3.091, P = 0.050), vesicular colonization( F = 4.135, P = 0.020), extraradical hyphal density( F = 6.300, P = 0.003) and plant mycorrhizal dependency( P < 0.050), and high nitrogen had a siginificant inhibitory effect on the above four inddexes, but not spore density( F = 1.319, P = 0.296) and glomalin content( F = 1.224, P = 0.327) in soil. In addition, AM fungi significantly increased the mycorrhizal dependency of plant underground tissue( P < 0.05). Nitrogen supply levels significantly influenced the mycorrhizal responses of plant height, root biomass, aboveground biomass, root : shoot biomass ratio, total biomass, phosphorus nutrition and N:P ratio of plant aboveground part. Under low nitrogen( N5, 5 g/m2 per year) treatment, although AM fungal root length colonization and arbuscular colonization increased slightly( P > 0.05, being compared to CK), the mycorrhizal responses of plant tiller, root biomass, root : shoot biomass ratio, total biomass, shoot N:P ratio were all shown to be significantly positive, on the other hand, significant negative effects were also observed on but the mycorrhizal response of plant height, total nitrogen uptake content, total phosphorus concentration and uptake content; Under middle and highest nitrogen addition treatments, inoculation of AM fungi only significantly increased the root : shoot biomass ratio, and decreased the plant height, shoot biomass and total nitrogen uptake content of shoot. The results of linear-regression analysis found that the vesicular colonization( R2 = 0.218, P = 0.021) and extraradical hyphal density( R2 = 0.190, P = 0.033) were both positively correlated with the second principal component representing the mycorrhizal response of shoot total phosphorus nutrition, but not with mycorrhizal dependency( P > 0.05).(2) Application of phosphorus fertilizer significantly inhibited the AM fungal colonization( P < 0.05), extraradical hyphal density( F = 19.780, P < 0.001) and the mycorrhizal dependency of underground part of the plant, but not significantly on spore density( F = 4.807, P = 0.011) and glomalin content in soil( F = 2.816, P = 0.065). Similarly, AM fungi did not significantly increase the mycorrhizal dependency of underground part of the plant( being compared to the mycorrhizal dependency of downground part of the plant). We also found that phosphorus fertilizer supply significantly influenced the mycorrhizal response of plant height, shoot biomass, root : shoot biomass ratio, total biomass, total concentration of shoot nitrogen and phosphorus, shoot N:P. Under low( P2, 2 g/m2 P per year) and middle( P4, 4 g/m2 P per year) phosphorus treatments, although the mycorrhizal responses of plant growth indexes were not significant, the mycorrhizal responses of the concentration of shoot total nitrogen and phosphorus were significantly negative; Under the highest( P8, 8 g/m2 P per year) phosphorus treatment, significantly positive effects were found on the mycorrhizal responses of plant root biomass, shoot biomass and total biomass, but the mycorrhizal responses of shoot nitrogen nutrition and N:P ratio were significantly negative. The correlation analysis of Pearson found that AM fungal growth characteristics shown significantly positive relationships with the mycorrhizal responses of shoot total nitrogen( r = 0.435, P = 0.034), total phosphorus concentration( r = 0.641, P =0.001), and plant underground part mycorrhizal dependency( r = 0.589, P = 0.002).(3) The effects of nitrogen and phosphorus( NP) fertilizer on AM fungi and mycorrhizal dependency were similar with phosphorus fertilizer treatment. Inoculation with AM fungi increased mycorrhizal dependency of underground part of plant. The functioning of AM fungal community under the highest NP fertilization( N10P8, 10 g/m2 N per year and 8 g/m2 P per year) was the most significance( t = 4.861, P = 0.001, being compared to the mycorrhizal dependency of downground part of the plant). NP fertilizer supply levels significantly influenced the mycorrhizal responses of plant height, tiller numbers, root biomass, root : shoot biomass ratio, total nitrogen nutrition and total phosphorus concentration of shoot, shoot N:P. Under low and middle NP fertilizer treatments, the mycorrhizal responses of plant growth variables and nitrogen phosphorus nutrition were all not significant, but under the highest fertilizer( N10P8) treatment, the mycorrhizal responses of plant tiller numbers, root biomass, root : shoot biomass ratio, total biomass, total nitrogen nutrition of shoot were all significantly positive. The correlation analysis of Spearman indicated that AM fungal growth characteristics had significant positive relationships with the mycorrhizal response of root biomass( rho = 0.550, P = 0.005) and root mycorrhizal dependency( rho = 0.554, P = 0.005), and the relationships with mycorrhizal responses of total nitrogen concentration( rho =- 0.657, P = 0.000) and total nitrogen uptake( rho =- 0.442, P = 0.031) were both significantly negative.Our results suggest that fertilization have a significantly negative effect on AM fungal colonization, but not on spore density and glomalin in soil. Inoculation of AM fungi can promote the mycorrhizal dependency of underground part of plant. Suitable concentration of nitrogen fertilizer( N5) would both promote the growth of AM fungi and plant, indicating the potential favorable environment for AM. The treatments of nitrogen fertilization, phosphorus fertilization and N10P8 indicate that imbalance nitrogen and phosphorus fertilization may be more advantageous to the reciprocal symbiosis between AM fungi and plant. AM fungal growth is not significantly related to all mycorrhizal dependencies, but the mycorrhizal dependency of the root always show the positive relationship.