Biochar-based fertilizer amendment can improve soil properties partly due to stimulated microbial activities and growths. The karst ecosystem is prone to degradation and accounts for a large proportion of southwest China. Understanding of the response of the microbial community structure to biochar-based fertilizer application is of great significance in karst soil restoration. A field experiment located in southwest China was conducted in typical karst soil, and a high-throughput sequencing approach was used to investigate the effect of biochar-based fertilizer application on microbial community structure in karst soil. Field trials were set up for 24 months using the following treatments: control (CK), compost plus NPK fertilizer (MF), biochar (B), less biochar (half the quantity of biochar in B) plus compost and NPK fertilizer (B1MF), biochar plus compost and NPK fertilizer (BMF), and more biochar (double the quantity of biochar in B) plus compost and NPK fertilizer (B4MF). The results elucidated that BMF and B4MF treatments had higher contents of soil carbon and soil nutrients N, P, and K than the other treatments. Soil microbial abundance and diversity were significantly increased by biochar-based fertilizer amendments (BMF and B4MF), compared to CK (P < 0.05). BMF and B4MF treatments significantly increased the relative abundance of dominant microorganisms, compared to CK (P < 0.05). The difference in the composition of indicator microbes between each treated group indicated that soil amendments altered the microbial community structure. There was a strong correlation between soil properties (soil C-, N-, and P-fractions) and microbial community structure. Furthermore, network analysis revealed that the addition of biochar-based fertilizer increased the scale and complexity of the microbial co-occurrence network. To summarize, the application of biochar-based fertilizer enabled more keystone species in the soil microbial network to participate in soil carbon resource management and soil nutrient cycling, indicating that biochar-based fertilizer is beneficial for the restoration of karst-degraded soils.

The present study used bibliometric methods to analyze the literature regarding the biochar effects on soil that are included in the Web of Science Core Collection database and quantified the annual number of publications in the field and distribution of publications. Using CiteSpace as a visual analytic software for the literature, the distribution of the subject categories, author collaborations, institution collaborations, international (regional) collaborations, and cocitation and keyword clustering were analyzed. The results showed the basic characteristics of the literature related to the effects of biochar on soil. Furthermore, the main research powers in this field were identified. Then, we recognized the main intellectual base in the domain of biochar effects on soil. Meanwhile, this paper revealed the research hotspots and trends of this field. Furthermore, focuses of future research in this field are discussed. The present study quantitatively and objectively describes the research status and trends of biochar effects on soil from the bibliometric perspective to promote in-depth research in this field and provide reference information for scholars in the relevant fields to refine their research directions, address specific scientific issues, and help scholars to seek/establish relevant collaborations in their fields of interests.