Effect of biochar on soil nutrient effectiveness
Biochar is produced from agricultural organic waste through high temperature charring. Most biochar is alkaline, has a loose texture, and is rich in N, phosphorus, potassium and many other elements, as well as many mineral nutrients. Such as calcium, magnesium, zinc and other trace elements. When biochar is applied to the soil, it can not only change the physical properties of the soil, improve soil porosity, but also raise the nutrient content of the soil. It is favorable to the balanced supply of multiple nutrients (Zhang et al., 2017; Zhang et al., 2019). The results of this paper showed that short-term application of lees biochar can significantly improve the pH, SOM, TN, AN, NN, AP and AK content of the yellow soil in Guizhou, which is similar to the results of previous studies (Liu et al., 2019; Zhang et al., 2019). Maotai lees are a by-product of sorghum and other by-products formed after many high temperature fermentations. The pH of biochar was weakly acidic, rich in nutrients, and contains a large number of residual fats, proteins, cellulose and vitamins. trace elements and N-free leachables (Dai et al., 2020). The lees were prepared into biochar after high temperature charring, and its own pH changed to alkaline (8.78). The content of SOM, TN and AK was significantly increased, thus the fertility of yellow soil could be improved after application to the soil. However, short-term application of lees biochar reduced the microbial mass carbon (MBC) and microbial mass N (MBN) content of the yellow soil. One reason may be due to the high C/N of biochar, which disrupted the microbial C/N balance when applied to the soil, thus suppressed the number and activity of microorganisms (Xu et al., 2014; Zhu et al., 2917). Another reason, the application of organic materials generally increases the C and N content of soil microbial mass (Foster et al., 2016). However, biochar is rich in inactive organic C that is stable in nature. Although the application of biochar directly increases the soil organic C content, it decreases the proportion of activated C available to the microorganisms, resulting in the soil microbial quantity C/N decreased (Johannes et al., 2006; Zwieten et al., 2010).
MBC/MBN can be used to reflect structural information about the soil microbial community. Bacteria have C/N ratios ranging from 3 to 6, while fungi have C/N ratios ranging from 7 to 12 (Vries et al., 2006). In this study, MB0, MB0.5, MB1.0, MB2.0, MB4.0 treated MBC/MBN 4.50, 5.65, 10.07, 8.23, and 7.01, respectively. This indicates that the yellow soil microorganisms were predominantly bacterial under no biochar (MB0) or low application (MB0.5) conditions. At high application rates (MB1.0, MB2.0, MB4.0), the fungus was predominant. It has been shown that N fertilizer paired with biochar can increase the proportion of bacteria in the microbial community. When the total amount of soil flora remains constant, the rate of bacterial growth and reproduction increases, which leads to a decrease in soil microbial C/N (Wang et al., 2010). In the present study, MBC/MBN tended to decrease when biochar was applied above 1.0%. Although the soil microorganisms were still dominated by fungal communities, the bacterial growth and reproduction rate gradually increased, resulting in a soil microbial C/N decrease. Soil microbial communities are gradually changing from "fungal" to "bacterial", which means that biochar application can improve the quality of soil microorganisms. Soil microhabitat environment to alleviate soil continuum barriers (Chen et al., 2012).
Effect of biochar on soil nutrient effectiveness
Fungi are important members of the soil microbial community that drive energy flow and material cycling in soil-vegetation ecosystems. It plays an important role in the ecosystem. The results of this study found that short-term application of lees biochar significantly increased the fungal Simpson index, but Ace, Chao1 and Shannon indices had no effect. This indicates that the application of lees biochar can reduce the diversity of yellow soil fungal communities, but has no effect on species diversity, which is consistent with the results of previous studies (Hu et al., 2014). It has been shown that soil fungal richness and diversity indices were significantly reduced after charcoal-based fertilization, which may be related to the different pathways of soil microorganisms to decompose soil organic matter. Biochar application can not only affect soil fungal community diversity, but also improve the fungal community structure and reduce the species with pathogenic potential, which in turn can lead to the development of soil fungi towards beneficial flora (Bai et al., 2019). In the present study, application of lees biochar significantly reduced the relative abundance of Fusarium and increased the relative abundance of Mortierella, which is similar to the results of a previous study (Yao et al., 2017). Fusarium is the main causative agent of soil-borne diseases of crops, and application of biochar can significantly reduce the gene copy number and relative abundance of Fusarium in the soil, which may be related to the increase in soil pH and decrease in the effectiveness of some phenolic acids in the soil after application of biochar (Jaiswal et al., 2017; Wu et al., 2020). Mortierella promotes an increase in soil organic matter and nutrient content, thereby promoting crop root growth and development (Curran et al., 2000). In addition, the application of lees biochar also increased the relative abundance of some functional genera of bacteria such as Chaetomium. It has been found that the increase in Chaetomium not only promotes the uptake of active substances from the soil, but also produces antibiotics and cell wall-degrading enzymes, thus acting to alleviate soil continuity disorders and inhibit the occurrence of soil-borne diseases (Ingrid et al., 2015).
Relationship between fungal community structure and soil environmental factors
Soil microbial community structure is not only influenced by physical factors such as soil moisture and aeration conditions, but also soil chemical factors such as pH, SOM and TN are key factors influencing the changes in soil microbial community structure (Liu et al., 2019). Results of RDA redundancy analysis in this study showed that SOM, AN, and NN were important factors influencing the structural changes of fungal communities in yellow soil. It has been noted that the fungi prefer to grow in acidic environments. The application of biochar increased the soil pH and therefore was associated with other soil influences (such as TC content, TN content, electrical conductivity) compared to pH as the main factor influencing changes in soil microbial communities (Chen et al., 2015; Nielsen et al., 2014). However, it has also been found that soil pH is the dominant factor influencing changes in soil fungal community structure, which is influenced by soil nutrients much more than soil pH (Dai et al., 2016), and some studies have confirmed the significant correlation between the effective nutrient content of soil and fungal community composition (Zhang et al., 2019). Thus, the response of environmental factors and fungal community structure differed according to soil type, crop type, and biochar type. In addition, there are few studies on the effect of Maotai lees biochar on the fungal diversity of yellow soil, and the soil microbial environment will certainly change with the time of biochar application and the aging process of biochar itself. Therefore, more experiments need to be conducted to explore the short- and long-term effects of lees biochar on the diversity of fungal communities, so as to provide theoretical reference for the improvement of loess geology and the rational utilization of wine lees in Guizhou.