Background This study aims to explore the soil improvement status of Haizhou opencast coal mine dump acted on different reclamation modes, coupled with the correlation degree between multiple soil enzymes and soil fertility. This study takes acacia woodland, elm woodland and grassland of Haizhou opencast dump in Fuxin as sample plots, and makes a comparison with the bare land. By comparative analysis, the soil enzyme activity and soil fertility status as well as their correlation are obtained.

Methods For the measurement of soil enzyme, volumetric method, TTC colorimetry, ninhydrin colorimetry, disodium phosphate colorimetry, dinitrosalicylic acid colorimetry, and sodium hypochlorite colorimetry was applied to measure the activity of catalase, dehydrogenase, protease, alkaline phosphatase, sucrase, and urease, respectively. For the determination of soil fertility, Kjeldahl boiling method, acid solution-molybdenum antimony colorimetric method, and flame photometry was adapted to determine the total nitrogen content, the total phosphorus content, and available potassium content, respectively. Soil samples from 4 soil layers of 0-5 cm, 5-10 cm, 10-20 cm, and 20-30 cm were collected and combined. The single factor analysis of variance and the canonical correlation analysis were used for their correlation. First, the current situation among the soil enzyme activities (catalase, dehydrogenase, protease, alkaline phosphatase, sucrase, and urease), soil fertility (total nitrogen, total phosphorus, and available potassium) and pH value were set forth in different reclamation modes of dump, and also for which the reasons were investigated. Then, this paper adapted canonical correlation analysis to analyze the correlation between soil enzyme activity and soil fertility under different reclamation modes.

Results Compared with that of the bare land, the soil enzyme activity and soil fertility of the three reclamation lands were significantly improved (P < 0.05). The soil enzyme activity of elm forest at the same depth was significantly higher than that of other plots, and the soil fertility was higher, and the effect of soil improvement was better. Besides, in the same plot, the soil enzyme activity level and soil fertility level gradually decreased with the increase of soil depth. The correlation coefficient of the first pair of canonical variables was 0.998, indicating that protease had significant correlation with total nitrogen, total phosphorus and available potassium. The second correlation coefficient of typical variables was 0.986, showing that the sucrase had significant correlation with organic matter. The third correlation coefficient of the canonical variable was 0.871, indicating that sucrase, protease and urease were closely related to pH value.

Conclusions It is found that planting elm has significant effects on increasing the activity of catalase, protease, alkaline phosphatase, sucrase, as well as and the content of total nitrogen content and organic matter. Meanwhile, planting elm also contributes to stabilize soil pH value. Sucrase, protease and urease in soil enzyme are closely related to total nitrogen, organic matter and pH value in soil fertility. Soil sucrase activity and protease activity can be used to characterize soil fertility.