Fine-tuning of soil water and nutrient fertilizer levels for the ecological restoration of coal-mined spoils using Elaeagnus angustifolia

Coal mining activities remain of great environmental concern because of several negative impacts on soil ecosystems. Appropriate revegetation interventions of coal-spoiled lands can provide environmental management solutions to restore soil degraded ecosystems. The present study addressed the potential of the pioneer woody species, Elaeagnus angustifolia, in the restoration of coal-mined spoils under a range of different water (W) levels and nitrogen (N) and phosphorus (P) applications. Our results show how moderate applications of N (N₆₀ = 60 mg N kg^-1 soil) and P (P₉₀ = 90 mg P kg^-1 soil) fertilizers led either to maximum or minimum growth performance of E. angustifolia depending on whether W was applied at very high (W₈₀ = 80% field capacity) or very low (W₄₀ = 40% field capacity) levels suggesting that W was the main limiting factor for plant growth. Very low-W regime (W₄₀N₆₀P₉₀) also caused significant reduction of photosynthetic parameters, including net photosynthetic rate, transpiration rate and water use efficiency. The combination of high W-N doses with low P doses (W₇₀N₉₆P₃₆) positively influenced gas-exchange parameters, chlorophyll and carotenoid contents. Seedlings treated with low-W and -N doses (W₅₀N₂₄P₁₄₄) showed highest increases in malondialdehyde content and lowest levels of relative water content (RWC). Decreases in malondialdehyde content and increases in RWC were observed following a gradual increment of W and N doses, indicating that high W and N doses contributed to drought tolerance of E. angustifolia by protecting cell membranes and increasing water status. Low-W and -N applications considerably increased the activities of antioxidant enzymes (superoxide dismutase, catalase, and peroxidase) and the contents of proline and soluble sugars, suggesting that E. angustifolia developed defensive strategies to avoid damage induced by water scarcity. Results from heatmap and principal component analyses confirmed that W and N were the main clustering factors, and both N and P performed well at high-W dose. The optimum growth performance of E. angustifolia was found under a combination of W level at 66.0% of field capacity, N dose of 74.0 mg kg^-1 soil, and P dose of 36.0 mg kg^-1 soil. Our findings demonstrate how optimum growth performance of E. angustifolia can be achieved by fine-tuning doses of W, N, and P resources, and how this in turn could greatly support the ecological restoration of coal-mined degraded environments.

Keywords: Arid lands; Coal spoils; Elaeagnus angustifolia; Growth responses; Restoration; Water shortage.