The Responses of C, N, P and Stoichiometric Ratios to Biochar and Vermicompost Additions Differ from Alfalfa and a Mine Soil

The use of ecological stoichiometry is quite effective for exploring the nutrient dynamics and relationships between plants and soils. However, the way that the plant and soil stoichiometry changes with soil remediation in mining ecosystems remains unclear. Biochar and vermicompost are generally applied to remediate contaminated soil. In this study, a pot experiment was conducted with a mine soil planted with alfalfa. Biochar (B) and vermicompost (V) were added to the soil separately in three different proportions, equivalent to application rates ( w / w ) of 0% (control, CT), 2.5% (low rate, l), and 5% (high rate, h). This resulted in nine treatments, including control (CT), B l , B h , V l , V h , B l V l , B l V h , B h V l , and B h V h . The carbon (C), nitrogen (N), and phosphorus (P) concentrations and stoichiometric characteristics of the alfalfa aboveground parts (plant) and soil were investigated. The results showed that biochar application significantly increased the concentrations of soil organic C (SOC), soil total N (TN), soil total P (TP), soil C:N, and plant P concentration, but decreased plant N concentration, and plant C:P and N:P ratios. The effects of vermicompost addition on SOC, soil TN, TP, and stoichiometric characteristics depended on the biochar addition rates, but it increased plant N concentration and N:P, and decreased plant C:N under the condition of low biochar addition. Additionally, the plant N concentration was negatively correlated with soil N and total manganese (Mn) concentrations, whereas there was a positive correlation between plant and soil P concentrations. The soil total and available cadmium (Cd) were positively correlated with plant N concentration but negatively correlated with plant P concentration. The results indicated that the stoichiometric characteristics of plants and soil had diverse responses to biochar and vermicompost additions, and different soil heavy metal elements. Biochar and vermicompost application improved external P and N utilization by plants, respectively. Vermicompost addition enhanced biological N fixation in alfalfa. These findings suggest that vermicompost addition could be an optimal method by which to promote vegetation restoration in mine soils with poor N levels, and that biochar could be applied to low-P soils. The effects of heavy metals on plant and soil stoichiometric characteristics should be taken into consideration. Consequently, this study will provide scientific references for biochar and vermicompost applications in alfalfa planting and management, and vegetation restoration in mining areas.