Effects of Rare Earth Element-Rich Biochar on Soil Quality and Microbial Community Dynamics of Citrus grandis (L.) Osbeck. cv. Guanximiyou

Rare earth elements (REEs) are key resources of strategic importance, but pollution has increased due to uncontrolled mining. Although heavy metal hyperaccumulating plants are environmentally friendly, they require strict control during post-treatment, or they may cause secondary pollution. Therefore, their safe disposal plays a key role in the ecological restoration of REE mines. In this study, rare earth element (REE)-rich biochar was produced by pyrolyzing the REE hyperaccumulator Dicranopteris pedata. This biochar was then applied to the Citrus grandis (L.) Osbeck. cv. Guanximiyou soil amendment experiment to evaluate its effects on soil physicochemical properties and microbial indicators. Four treatments were established: CK (0% REE-rich biochar), BC1 (1% REE-rich biochar), BC3 (3% REE-rich biochar), and BC5 (5% REE-rich biochar). The BC5 treatment decreased soil REE bioavailability, thereby preventing REE pollution. The BC5 treatment also demonstrated the highest efficacy in improving soil total organic carbon (229.11%), total nitrogen (53.92%), total phosphorus (55.61%), total potassium (55.50%), available nitrogen (14.76%), available phosphorus (46.79%), and available potassium (159.42%) contents compared to CK. Furthermore, soil enzyme activities were significantly increased by BC5 treatment (p < 0.05). At the bacterial phylum level of classification, the bacterial diversity index (Chao1 and Shannon) exhibited elevated levels under BC5 conditions. Furthermore, the Chao1 index of fungal diversity exhibited a substantial augmentation of 55.67% (p < 0.05) in the BC5 treatment in comparison to the CK, and also significantly higher than the other treatments (p < 0.05). Our study showed that the composition of soil microorganisms was altered by REE-rich biochar. Proteobacteria, Acidobacteria, Actinobacteriota, and Chloroflexi are dominant among bacteria, while Ascomycota is dominant among fungi. Mantel and redundancy analyses showed that the most important environmental factor affecting the structure of soil microbial communities was pH, especially in the case of bacteria. In summary, this study showed that the application of 5% REE-rich biochar provided the best improvement in soil physicochemical properties and microbial diversity. These findings highlight its potential for soil remediation and provide new ideas for recycling heavy metal hyperaccumulating plant waste.