Functional, not Taxonomic, Composition of Soil Fungi Reestablishes to Pre-mining Initial State After 52 Years of Recultivation

Legacy of severe soil degradation hinders the buildup of mineral-associated soil organic carbon

Global efforts target a soil organic carbon (SOC) enhancement rate of 2.4 ‰ y-1 in the upper 30 cm of agricultural soils to address declining soil productivity associated with declining SOC stocks. We explored a unique chronosequence of homogeneous soils formed after mining in Germany, which serve conventional agriculture and exhibit a large margin for SOC storage, but limited SOC accrual, to study SOC protection mechanisms and accrual constraints. We hypothesized that limited SOC accrual is associated to insufficient nitrogen rather than to minerals saturation. Soil samples (0-30 and 30-60 cm) were collected across the chronosequence (0-56 years) and compared to an original non-mined soil (OS) managed similarly. The mean residence time (MRT) of SOC and its protection mechanisms were studied using soil incubation, organic matter density fractionation, and δ13C measurements. After 56 years, total and mineral-associated SOC (MAOC) stocks remained 18 % and 28 % lower than in the OS at 0-30 cm, with estimated replenishment times of 93 and 129 years, respectively. Mineral-associated nitrogen (MAN) stocks stagnated along recultivation time below OS level. Together with significant linear correlation of MAOC with total SOC and MAN stocks, these results indicate that nitrogen rather than saturation of minerals limits SOC accrual. In fact, the MAOC stock deficit to saturation was estimated at 316.4 Mg ha-1. At 30-60 cm, SOC and nitrogen stocks were restored within 30 years, due to comparatively lower initial losses. The initial MRT of SOC at 0-30 and 30-60 cm (15.3 and 27.9 years) declined before finally becoming comparable to OS (11.7 and 7.7 years). This reflected new carbon entering the soil that initially contained predominantly MAOC (78-82 %), followed by its stabilization as MAOC. Due to their susceptibility to nitrogen losses, degraded soils require tailored nitrogen management to restore SOC stocks and comply with European laws requiring agricultural SOC accrual.

Critical steps in the restoration of coal mine soils: Microbial-accelerated soil reconstruction

Soil reconstruction is a critical step in the restoration of environments affected by mining activities. This paper provides a comprehensive review of the significant role that microbial processes play in expediting soil structure formation, particularly within the context of mining environment restoration. Coal gangue and flotation tailings, despite their low carbon content and large production volumes, present potential substrates for soil reclamation. These coal-based solid waste materials can be utilized as substrates to produce high-quality soil and serve as an essential carbon source to enhance poor soil conditions. However, extracting active organic carbon components from coal-based solid waste presents a significant challenge due to its complex mineral composition. This article offers a thorough review of the soilization process of coal-based solid waste under the influence of microorganisms. It begins by briefly introducing the primary role of in situ microbial remediation technology in the soilization process. It then elaborates on various improvements to soil structure under the influence of microorganisms, including the enhancement of soil aggregate structure and soil nutrients. The article concludes with future recommendations aimed at improving the efficiency of soil reconstruction and restoration, reducing environmental risks, and promoting its application in complex environments. This will provide both theoretical and practical support for more effective environmental restoration strategies.

Microbial composition and function in reclaimed mine sites along a reclamation chronosequence become increasingly similar to undisturbed reference sites

Mine reclamation historically focuses on enhancing plant coverage to improve below and aboveground ecology. However, there is a great need to study the role of soil microorganisms in mine reclamation, particularly long-term studies that track the succession of microbial communities. Here, we investigate the trajectory of microbial communities of mining sites reclaimed between three and 26 years. We used high-throughput amplicon sequencing to characterize the bacterial and fungal communities. We quantified how similar the reclaimed sites were to unmined, undisturbed reference sites and explored the trajectory of microbial communities along the reclamation chronosequence. We also examined the ecological processes that shape the assembly of bacterial communities. Finally, we investigated the functional potential of the microbial communities through metagenomic sequencing. Our results reveal that the reclamation age significantly impacted the community compositions of bacterial and fungal communities. As the reclamation age increases, bacterial and fungal communities become similar to the unmined, undisturbed reference site, suggesting a favorable succession in microbial communities. The bacterial community assembly was also significantly impacted by reclamation age and was primarily driven by stochastic processes, indicating a lesser influence of environmental properties on the bacterial community. Furthermore, our read-based metagenomic analysis showed that the microbial communities' functional potential increasingly became similar to the reference sites. Additionally, we found that the plant richness increased with the reclamation age. Overall, our study shows that both above- and belowground ecological properties of reclaimed mine sites trend towards undisturbed sites with increasing reclamation age. Further, it demonstrates the importance of microbial genomics in tracking the trajectory of ecosystem reclamation.

Considerations for the inclusion of metabarcoding data in the plant protection product risk assessment of the soil microbiome

There is increasing interest in further developing the plant protection product (PPP) environmental risk assessment, particularly within the European Union, to include the assessment of soil microbial community composition, as measured by metabarcoding approaches. However, to date, there has been little discussion as to how this could be implemented in a standardized, reliable, and robust manner suitable for regulatory decision-making. Introduction of metabarcoding-based assessments of the soil microbiome into the PPP risk assessment would represent a significant increase in the degree of complexity of the data that needs to be processed and analyzed in comparison to the existing risk assessment on in-soil organisms. The bioinformatics procedures to process DNA sequences into community compositional data sets currently lack standardization, while little information exists on how these data should be used to generate regulatory endpoints and the ways in which these endpoints should be interpreted. Through a thorough and critical review, we explore these challenges. We conclude that currently, we do not have a sufficient degree of standardization or understanding of the required bioinformatics and data analysis procedures to consider their use in an environmental risk assessment context. However, we highlight critical knowledge gaps and the further research required to understand whether metabarcoding-based assessments of the soil microbiome can be utilized in a statistically and ecologically relevant manner within a PPP risk assessment. Only once these challenges are addressed can we consider if and how we should use metabarcoding as a tool for regulatory decision-making to assess and monitor ecotoxicological effects on soil microorganisms within an environmental risk assessment of PPPs. Integr Environ Assess Manag 2024;20:337-358. © 2023 SETAC.