The Addition of Artificial Humic Substances to Artificial Seedling Substrate Can Improve Soil Properties and Rice Quality

The poor quality and scarcity of soil used for raising seedlings are key issues holding back the further development of the rice industry. Artificial humic substances (A-HS) and artificial soils are attracting increasing attention due to their cost-effectiveness and significant potential to improve rice cultivation. This study used native soil (NS), engineered soil (ES) and rice straw to create artificial substrates (AES and ANS) using humification–hydrothermal carbonization technology (24 h treatment of NS and ES with rice straw at 200 °C and 2 MPa). Experiments on cultivation of the rice seedlings were conducted using initial soils (ES and NS) and artificial soils with addition of A-HS (AES+A-HS and ANS+A-HS). This study examined the nutrient content and microbial environment of the seedling substrates as well as the changes in growth and development of the rice seedlings. The combination of rice straw biochar in artificial soils (AES and ANS) with A-HS significantly increased the content of soil organic carbon (SOC) and enhanced the nutrient levels, such as total nitrogen and available phosphorus. Furthermore, it enhanced the microbial diversity, and it increased the abundance of microorganisms such as Actinomycetota, Chloroflexota, and Basidiomycota, thereby improved the soil microbial environment. An enhanced soil nutrient content and improved microbial environment effectively promoted the rice seedling growth. Compared to the original soils (ES and NS), before transplanting to paddy fields, the stem width of the seedlings increased by 5.1% (AES+A-HS) and 10.2% (ANS+A-HS), and their height increased by 18.7% (AES+A-HS) and 4.5% (ANS+A-HS). The rice seedling emergence increased by 6.1% (AES+A-HS) and 3.9% (ANS+A-HS), and the transplant survival rate also increased by 4.1% (AES+A-HS) and 2.9% (ANS+A-HS). This study provides an effective approach to alleviating the scarcity of rice seedling substrates and improving the quality of rice seedlings, and it provides an effective foundation for increasing the yield of rice.