Effects of elevated CO2 and arbuscular mycorrhizal fungi on root exudates of black locust seedlings grown in cadmium-polluted soils by 13C isotope tracer.

Root exudates contribute to heavy metal migration in soils, which are easily influenced by many factors such as arbuscular mycorrhizal fungi (AMF) and atmospheric CO2. To understand the effect of increasing atmospheric CO2 and AMF on root exudates of plants exposed to heavy metals, we investigated the impacts of elevated CO2 (eCO2) and AMF strain Funneliformis mosseae (FM) on root exudates of black locust (Robinia pseudoacacia L.) cultivated in cadmium (Cd)-polluted soils using 13C isotope tracer under sterile conditions. 93 metabolites were identified from root exudates including phenolic acids, amino acids, other organic acids and soluble sugars. 56 differential metabolites were identified under FM colonization using orthogonal partial least squares-discriminant analysis. The eCO2 contributed 11.31, 163.00 and 8.31 μg kg-1 to total phenolic acids, amino acids, and soluble sugars, respectively. The top 10 eCO2-contributed compounds under FM included nine monosaccharides, such as β-d-allopyranose and d-(+)-galactopyranose, and sarosine, and five differential compounds, such as maltose and sarcosine, between eCO2 + FM and eCO2 existed in starch and sucrose metabolism, amino sugar and nucleotide sugar metabolism, and arginine and proline metabolism pathways. Elevated CO2 and FM affected (p < 0.05) root exudates under Cd pollution by regulating plant metabolism. Overall, responses of root Cd and soil C and N to eCO2 and F. mosseae affected (p < 0.05) root exudates. The results will help us understand the combined contribution of increasing CO2 and AMF on phytoremediation of heavy metal-polluted soils, providing the possibility to select bioremediation measures under global change scenarios.