Background The climate of mining areas in northern China is arid and rainless, and water is the main factor restricting the reconstruction of vegetation. Moreover, the ecological environment of mining area has been greatly destroyed during mining process, the soil erosion is aggravated, and the ecological restoration is more difficult. Medicago sativa is a common soil and water conservation plant in China, and super absorbent polymer (SAP) can effectively promote the restoration of vegetation and prevent soil erosion. Therefore, it is of great significance to study the effect of SAP on the growth of M. sativa in the arid mining area.

Methods In order to obtain the optimal dosage of SAP in the planting of M. sativa in the arid mining area, pot culture method was used to grow M. sativa in the mining area soil, with 6 concentrations of SAP0 (CK), 0.15% (A1), 0.30% (A2), 0.45% (A3), 0.60% (A4), and 0.90% (A5). During the growth period, the soil moisture content were measured with MPM-160B portable soil moisture meter, the net photosynthetic rate (P_n), transpiration rate (T_r), stomatal conductance (G_s), intercellular CO₂ concentration (C_i) were measured with plant photosynthetic system Li-6400P, the leaf water use efficiency (WUE) was calculated, the chlorophyll content was measured by spectrophotometry method and the survival time of plants was measured. After the experiment, the whole plant was harvested, the dry weight of aboveground parts and roots were weighed, and biomass and root shoot ratio were calculated. The soil was sampled and organic matter as well as nutrient content was measured.

Results The growth of plants with SAP was better than the contrast, the survival time prolonged by 6.16%-35.05%, and the maximum appeared at A2 and A3. The application of SAP effectively improved the soil moisture and nutrient status, the soil moisture content increased with the increase of the concentration of SAP, the effect of A2, A3 and A4 treatment on soil nutrient improvement was better and the improvement of available nutrients were better than total nutrients. Compared with the contrast, the moisture content of soil increased 1.04%-8.78%, the total N, total P and total K increased by 13.40%-35.54%, 2.61%~18.93% and 8.53%-15.89%, and the available N, available P and available K increased by 5.88%-47.02%, 17.88%-101.73% and 12.5%-31.55%, respectively. The application of SAP improved the photosynthetic capacity of the plants, and the maximum value of each index appeared in the A2 treatment and decreased with the increase of the concentration of the SAP. P_n, T_r, G_s, C_i, and WUE increased by 8.94%-96.96%, 5.64%-61.12%, 14.17%-135.05%, 14.85%-49.71% and 1.08%-16.48%, respectively, the chlorophyll content increased by 9.48%-30.29%. The biomass increased by 17.01%-61.96%, the maximum one occurred at A2 treatment, and decreased with the increase of the concentration of SAP.

Conclusions The implementation of SAP may increase soil moisture content and preserve soil nutrient, enhance photosynthesis efficiency and water use efficiency of M. sativa, reduce decomposition of chlorophyll, increase biomass and prolong survival time. Considering the improvement effect of SAP on each index, the concentration of 0.30%-0.45% is the optimal concentration for M. sativa planting on mining wasteland.