## 4 Discussion
Arbuscular mycorrhizal fungi are well known to improve plant growth, while seedlings usually benefit more from the symbiosis than do adult plants [45]. The study also partly supported this point of view. Continuous cropping results in an imbalance of nutrient availability, and affects the growth of plant. [PERSON] et al. reported banana yield decline was accompanied with continuous cropping deteriorated soil quality, which was evidenced by an increase of soil acidity; a decrease of total organic C; accumulation of N, P, K, Ca and Cu; and deficiency of Mg, S, Fe, Mn, and Zn [46]. The main effect of AM fungi in improving plant growth is through improved uptake of nutrients, both macroelements and minor elements, which is due to exploration by the external hyphae of the soil beyond the root hair [47]. Our results also showed inoculation with _G. moseae_ significantly increased K concentration by 24.3%, Mn concentration in the shoots (by 93.4%) and roots (by 65.1%), and Fe concentration in the roots (by 75%) of _S. miltiorrhiza._ Therefore, we speculated that inoculation with _G. moseae_ improved growth by improved nutrient uptake of the plant. AMF colonization promotes the synthesis of the secondary compounds (phenolics, cyclohexenor derivatives, and terpenes) in the roots and/or shoots of many economically important crops [48; 49; 50]. We found that inoculation with _G. moseae_ could enhance the accumulation of salvianolic acid B in _S. miltiorrhiza._ Salvianolic acid B belongs to phenolic compounds, which occur naturally in plant system and, owing to their antimicrobial properties, inhibit fungal spore germination and toxin production by pathogens [51]. On pea (_Pisum sativum_) plants, _G. moseae_ colonization and total phenolic accumulation were closely correlated with powdery mildew disease (_Erysiphe pisi_ infection) intensity [52]. The resistance of the mycorrhizal plants is enhanced through increased of phenolic acid synthesis in the roots [53]. Thus, we deduced that the accumulation of salvianolic acid B in mycorrhizal plants may also contribute to the tolerance of _S. miltiorrhiza_ to continuous cropping. Our results also showed that AMF inoculation significantly increased the Mn concentration in the plant roots and shoots (Table 3), during continuous cropping of _S. miltiorrhiza_. According to [PERSON] et al., an optimal amount of exogenous Mn promoted the growth of the shoots and roots [54], whereas Mn deficiency increased the susceptibility of wheat plants to infections compared with plants adequately supplied with all essential nutrients [55]. The application of Mn was shown to decrease the intensities of a spectrum of diseases in different crops, including root rot, take-all, powdery mildew, and leaf and stem rust in cereals, damping-off and wilt in cotton, scalp and late blight in potato, leaf spots in soybean, and black leaf mold on tomato, to name a few; excessive Mn above the optimum level for plant growth could contribute to the control of _Pseudocerospora fuligena_ in tomato plants [56]. We therefore assume that AMF-mediated biocontrol of soil-borne diseases in _S. miltiorrhiza_ is associated with increased Mn concentrations in the roots and shoots, which, in mycorrhizal plants, may serve as a mechanism for resisting the harmful effects of continuous cropping. We found that inoculation with _G. moseae_ reduced Fe concentration in the shoots, which might also be attributed to an increased biomass of the host plant. On the other hand, the inoculation significantly increased Fe concentration in the roots of _S. miltiorrhiza_ by 75%, a finding consistent with the results of a previous study. In that study, [PERSON] et al. reported that _Glonus versiforme_ had a significant effect on the contents of Fe in _Poncirus trifoliate_; the AMF-treated seedlings presented significantly higher contents of Fe compared with the control seedlings [57]. Fe is an essential micronutrient for plants and their associated microorganisms.