Effect of adding earthworms at different time points on the physiological ecology of Vetiveria zizanioides in covering soil of gangue

This study was conducted to reveal the influence of adding earthworms at different time points on the physiological ecology of vetiver grass (Vetiveria zizanioides) in covering soil on the gangue. A soil column experiment was set up for two groups of overburden soil thickness 30 cm (group M) and 50 cm (group N). Each group was set up with four treatments: vetiver grass planting alone (CK), 20 days inoculation with earthworms followed by the addition of vetiver grass (A), inoculation with earthworms and vetiver grass at the same time (B), and 20 days of vetiver inoculation followed by the addition of earthworms (C). The physicochemical properties of the overburden soil and the physiological ecology of the vetiver grass were analyzed for different treatments. The results showed that treatments of A, B, and C all significantly reduced pH towards neutral and increased electrical conductivity (EC), organic matter (OM), total nitrogen (TN) and total phosphorus (TP) compared to CK (P < 0.05). In comparison with CK, inoculation with earthworms at different times all increased vetiver grass germination, total weight and chlorophyll a content; treatment B increased photosynthetic pigment (chlorophyll a, chlorophyll b and carotenoid) content; treatment A increased chlorophyll b content; and treatment C increased root length (P < 0.05). Treatment B was more effective in reducing the soluble protein and proline content of vetiver grass than other treatments. The reduction rate of soluble protein content in treatment B was 9.34% and 4.69% (P < 0.05) in the 30 and 50 cm group, respectively, and proline content in treatment B was 5.55% (P < 0.05) in the 50 cm group, as compared with the CK. While treatment A significantly increased the MDA content of vetiver grass. At the same thickness, treatment A reduced the CAT activity of vetiver grass only in the 30 cm group, but there was no significant difference in SOD and POD activities of vetiver grass in B and C treatments compared to CK (P > 0.05). Correlation and redundancy analysis revealed that, soil OM and TN were the main environmental factors affecting physiological ecology of vetiver grass in treatments A and C, while soil OM and TP were the main environmental factors affecting physiological ecology of vetiver grass in treatment B. This study will provide a theoretical basis and data support for the combined application of earthworm and vetiver grass in reclaimed soil remediation.