A long-term study on the soil reconstruction process of reclaimed land by coal gangue filling

The main problem hindering the land reclamation process is the lower quality and long recovery period of reclaimed soil. Soil is a complex ecosystem and it is difficult to describe the state of soil reconstruction state. In this paper, we applied complex network theory to characterize the development and the mutual influence of soil quality variables in the 0-20-cm soil layer at 1, 3, 5, and 10 years after reclamation. In each study period, five samples were collected from the area reclaimed with coal-gangue fill and from the neighbour undamaged land. Fifteen soil quality-related variables of each sample, including soil physical, chemical, and biological parameters were determined. Five networks were constructed for 1-, 3-, 5-, and 10-year reconstructed soil and the control group. Each network had 15 soil quality-related variables as nodes, correlation coefficients of the variables as the criterion of the edge, and absolute values of the coefficients as the edge weight. The topological parameters were calculated and compared among these five networks. Compaction is the key indicator that can be used as a sensitive indicator for evaluating the quality of reclaimed soil. After 10 years of reconstruction, the soil variables have almost returned to the undamaged state, but the reclaimed soil is more fragile due to the stronger correlation between variables. The variables that have the greatest direct, indirect, and most rapid impact on soil quality varied with the reconstruction process, but the soil organic matter is the major direct impact indicator, soil texture (sand and clay) is the major indirect impact indicator, and silt content and soil nutrients (available phosphorus and nitrogen) are the major rapid impact indicators. The results of this study may help to provide an improved understanding of the soil reconstruction mechanism and provide a scientific basis for appropriate artificial interventions to accelerate the recovery process.