Key Takeaways & Executive Findings
- •• • MS and CMS application increased soil organic matter from 20.38 g/kg (Level 3) to 38.52 g/kg (Level 2), a 89% relative increase, directly improving soil fertility for vegetation establishment in mine waste rock. • • Available nitrogen, phosphorus, and potassium levels improved from Level 4, 6, and 2 to Level 1, 4, and 1 respectively, indicating a significant nutrient boost that supports plant growth and reduces the need for synthetic fertilizers. • • Ryegrass growth metrics (fresh weight, aboveground height, root length, and stem diameter) increased significantly, demonstrating the agronomic efficacy of MS and CMS as soil amendments. • • Microbial community richness and diversity were enhanced only at application rates below 1.5 kg/m²; higher rates suppressed microbial diversity, highlighting a critical threshold for safe application.
Abstract
The rapid population growth and accelerating urban development have made the comprehensive utilization of municipal sludge (MS) an urgent challenge. MS contains substantial organic matter and essential nutrients for crop growth, making it a promising soil amendment for the ecological restoration of mine waste rock. However, research evaluating the impact of MS application on soil health and ecological safety from a soil microbiology perspective remains understudied. Therefore, this study investigated the effects of MS and composted municipal sludge (CMS) on the ecological restoration of mine waste rock soil through pot experiments. High-throughput sequencing technology was employed to analyze changes in soil microbial community structure and diversity. Finally, network analysis and correlation heatmaps were utilized to elucidate the microbial driving mechanisms. The results indicated that after MS and CMS application, organic matter content increased from 20.38 g/kg (Level 3) to 38.52 g/kg (Level 2). The levels of available nitrogen, phosphorus, and potassium rose from Level 4, 6, 2, to Level 1, 4, 1, respectively. Fresh weight, aboveground height, root length, and stem diameter of ryegrass all increased significantly. Venn diagram and heatmap analyses indicated that lower application rates (<1.5 kg/m²) enhanced microbial community richness and diversity. This study confirms municipal sludge as an effective amendment for mine waste rock soil. It is recommended to limit application rates below 1.5 kg/m² in practical mine ecological restoration projects, with particular attention to long-term dynamics of heavy metals and salinity to ensure safe and sustainable land reuse.
1. Introduction
Mine waste rock, characterized by irregular particle shapes, poor water retention, and deficiency in carbon, nitrogen, and phosphorus, severely limits vegetation recovery. Additionally, heavy metals such as Cd, Pb, and Hg in mine groundwater can damage plant chloroplasts and disrupt photosynthesis, further complicating ecological restoration. Conventional physical and chemical remediation methods, including topsoil transfer, backfilling, and electrokinetic techniques, are often costly and environmentally intrusive. Biological approaches, particularly plant-microbe combined systems, offer a sustainable alternative, yet their effectiveness is constrained by the poor soil quality of mine waste rock.
Municipal sludge (MS) is rich in organic matter and essential nutrients, presenting a potential low-cost soil amendment for mine waste rock. However, its application raises concerns about heavy metal contamination and microbial safety. This study systematically evaluates the effects of MS and composted municipal sludge (CMS) on soil physicochemical properties, plant growth, and microbial community dynamics in mine waste rock soil. By employing high-throughput sequencing and network analysis, the research identifies key microbial taxa that drive soil health, providing a scientific basis for optimizing application rates and ensuring ecological safety in mine restoration projects.
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ZHANG Bo, WANG Jiawei, YANG Chaofeng, LU Jiafei, LI Li, HU Xiaomin (2026). Effects of Municipal Sludge Application on Composition and Microbial Communities of Mine Waste Rock Soil. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202607023
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Frequently Asked Questions
What is the optimal application rate of municipal sludge to avoid microbial diversity loss while achieving soil fertility improvement?
The study indicates that application rates below 1.5 kg/m² enhance microbial richness and diversity, while higher rates suppress them. At this threshold, organic matter increased from 20.38 to 38.52 g/kg, and available nutrients improved significantly, supporting ryegrass growth. Therefore, 1.5 kg/m² is the recommended upper limit for balancing benefits and risks.
How does municipal sludge application affect the abundance of pathogenic bacteria such as Ralstonia?
The study found that MS application significantly suppressed the abundance of the pathogen Ralstonia. This is likely due to the competitive effects of beneficial microbial taxa enriched by sludge, such as Sphingomonas and Thiobacillus, which may outcompete pathogens and enhance soil suppressiveness.
What are the long-term risks of heavy metal accumulation and salinity from sludge application, and how should they be monitored?
The study emphasizes the need for long-term monitoring of heavy metals and salinity. While immediate benefits are observed, the accumulation of metals and salts over repeated applications could pose risks to soil health and groundwater. It is recommended to conduct periodic soil tests and adjust application rates accordingly to ensure sustainable land reuse.
Which microbial taxa are key drivers of soil health after sludge application, and what are their functional roles?
Network analysis identified Bacteroidota, Myxococcota, Acidobacteriota, Patescibacteria, and Gemmatimonadota as key taxa after MS application, and Planctomycetota, Verrucomicrobiota, Patescibacteria, and Nitrospirota after CMS application. These taxa are likely involved in organic matter decomposition, nutrient cycling, and soil structure formation, contributing to improved soil fertility and plant growth.
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