Previous studies have showed that continuous monoculture can severely inhibit the growth medicinal plants and reduce the yield and medicinal active ingredients (Sui et al., 2019; Song et al., 2018; Zhang et al., 2018). Continuous monoculture significantly inhibited the growth of Salvia miltiorrhiza or even led to the failure of harvest, as well as the dramatic reductions (up to 35%) in the active component of dihydrotanshinone (Liu et al., 2013). Salvia miltiorrhiza with four years continuous monoculture results showed that both the yields and medicinal active ingredients were decreased year by year with the extension of continuous monoculture time (Liu et al., 2020). Raising medicinal plant seedlings has a further negative impact on the yield and quality of typical geo-authentic medicinal herb. Our observations showed that the seedling weight and seedling number, which reflected the health quality and effectiveness of A. sinensis seedlings, were significantly reduced by 195.4% and 36.7% under one-year continuous monoculture land (Fig. 1A and B); correspondingly, the high-quality seedlings declined sharply (Fig. 1C), which obviously affected the efficiency of raising A. sinensis seedlings. Our findings were consistent with previous research that number and health index of surviving seedlings were significantly decreased under continuous monoculture of Panax quinquefolius L. (Chen et al., 2012). These results demonstrated that the practice of continuous monoculture reduced the quality and efficiency of typical geo-authentic medicinal herb seedlings.
Soil microenvironment, such as disorder of soil physiochemical propertiesy and microbial imbalance, is the major cause for continuous cropping obstacles (Zhao et al., 2018). Previous studies suggested that the function of microbial populations were changed by continuous monoculture from "bacteria-dominated type" to "fungi-dominated type", with the raise of fungi count and the decline of bacteria and actinomyces counts (Song et al., 2018; Xiong et al., 2014). Our investigations were in part consistent with previous results that continuous monoculture significantly increased fungi count, and significantly decreased actinomycetes, ammonifier and azotobacteria counts (Fig. 2). Compared with RW, rhizosphere soil nutrients, SOM, total N, available P and K of CC were significantly decreased (Table 1), and the correlation of microbial population and nutrients in CC were dramatically altered (Table 2). The results further illustrated that continuous monoculture changed the function of microbial populations from "bacteria-dominated " to "fungi-dominated ".
What’s more, the dominant genera Pseudomonas with high abundance of 17.92% in RW, which is common soil beneficial bacteria with the function of alleviating diseases in plants (Liu et al., 2020; Peng et al., 2014; Qiu et al., 2021), and promote the growth and quality of A. sinensis (Xie et al., 2022), sharply decreased to 0.87% in CC (Fig. 3C); while Flavobacterium, which is usually regarded as plant pathogen (Mou et al., 2021), dramatically increased from 0.98% in RW to 8.10% in CC (Fig. 3C). The results suggested that both diversity and abundance of beneficial bacteria were drove to degenerate by continuous monoculture. Alternaria is known as probiotic fungal with the functions of inhibiting pathogenic microbes and secreting metabolites to promote plant growth (Qiu et al., 2021), Mortierella has the functions of dissolving soil phosphorus and providing nitrogen nutrition to promote plant growth and disease resistances (Tan et al., 2022; Kanse et al., 2015), and Ceratubasidium not only increases the plant chlorophyll contents but also secretes IAA to improve plant growth (Chen et al., 2011). However, the abundance of Alternaria, Mortierella and Ceratubasidium sharply decreased by continuous monoculture. On the other hand, Lasiosphaeriaceae (Meng et al., 2021), Vishniacozyma (Sun et al., 2022), Myrmecridium (Yu et al., 2022); Sa et al., 2017) and hypocreales (Bao et al., 2021) are considered pathogen fungi with the very low abundance in RW, but greatly high in CC. These findings were in consistent with previous results (Tan et al., 2022; Liu et al., 2020). The bacterial and fungal diversity analysis provided critical evidence that continuous monoculture reduced the richness and diversity of bacteria, but increased the richness and diversity of fungi. And the composition and structure of bacteria and fungi tended from beneficial type to harmful type with plants, which supported the conclusion of continuous monoculture driving the soil microbes from "bacteria-dominated type" to "fungi-dominated type".
Soil degradation usually accompanied by over-exploitation of nutrients such as SOM, nitrogen and phosphorus, changes in soil pH and imbalance of microbial populations (Huang et al., 2019; Liu et al., 2020). Our results showed that continuous monoculture deteriorated soil quality by increasing pH value and reducing the nutrients (Table 1). Elevated pH not only modulated rhizosphere soil microbial population, but also altered the functions of interplay between nutrients and microbial populations (Tables 2 and 3). With the deteriorating microbial community diversity and function, soil quality under continuous monoculture became degradated and unhealthy (Sun et al., 2021), resulting in the reduction of plant output and quality (Zhang and Zhang, 2008; Li et al., 2021).
As reported, glomalin produced by AMF plays an important role in improving soil physicochemical properties, carbon sequestration and microbial activities (Cabral et al., 2016; He et al., 2017; Singh et al., 2020). Some existing research mainly focused on the interplay between AMF and plants or the improvement of continuous cropping obstacles (Sun et al., 2021; Wang et al., 2014). However, little was reported about glomalin changes derived by continuous monoculture. In this study, we observed that even though AMF spore density could not directly affect EEG and TG content, which was far from PC1 (Fig. 5) and no significant correlation between AMF spore density and glomalin (Table 2), it was shown that root colonization rate almost overlapped with SOM in PCA (Fig. 5), suggesting that AMF and its product glomalin had potential applications in maintaining soil health and restoring degraded land (Singh et al., 2020). Furthermore, our study showed that continuous monoculture significantly decreased TG and EEG content, not only because it reduced the AMF spore density and root colonization (Fig. 4). These results demonstrated that continuous monoculture caused the degradation of A. sinensis seedling production fields, partly by inhibiting the activity of AMF, reducing the symbiotic relationship between AMF and the host, and affecting soil physicochemical properties.