Editorial for the Special Issue on “New Horizons for Acid Sulfate Soils Research”

The recognition and study of acid sulfate soils (ASS) have evolved remarkably over the past five decades, moving from a niche concern to a global research priority (Fanning et al. 2017). Often described as the “nastiest soils on Earth,” ASS pose significant environmental risks through the liberation of sulfuric acid and toxic metal-laden waters, which can lead to severe impacts on aquatic ecosystems, vegetation mortality, loss of agricultural productivity, and infrastructure corrosion. This Special Issue brings together 28 papers on the theme of “New Horizons for Acid Sulfate Soil Research.” Together, they provide a cross-disciplinary insight into recent advancements in ASS classification, process understanding, and management strategies across diverse global climates and ecosystems. The collection of papers in the Special Issue describes the research presented primarily at the 9th International Acid Sulfate Soils Conference (9th IASSC), held in Adelaide, Australia from 26 to 31 March 2023. The international acid sulfate soils conferences are the premier interdisciplinary forum for the presentation of new advances and research results in the fields of ASS research, policy and management practices. The 9th IASSC brought together 89 leading academic scientists, researchers and managers in this domain of interest from around the world under the auspices of the International Union of Soil Sciences (IUSS) Working Group on Acid Sulfate Soils. The full meeting program and the 64 peer-reviewed extended conference abstracts presented at the 9th IASSC are directly available on-line (9th IASSC Abstract Book 2023). The conference's opening presentation was delivered by Professor Delvin Fanning in recognition of his receipt of the Pons Medal at the 8th IASSC in 2016 and provided a global perspective of the importance of ASS in his talk titled “Historical developments in the understanding of Acid Sulfate Soils”. Since the 8th IASSC near Washington, D.C. USA in 2016, and the accompanying Special Issue in 2017 (Rabenhorst et al. 2017), there have been numerous insights and advances to our understanding of ASS around the world. Three whole day field tours were held in conjunction with the 9th IASSC. A comprehensive Field Tours Guidebook (Fitzpatrick et al. 2023) includes maps, diagrams, sketches, photographs and land-use recommendations to better illustrate and represent huge volumes of data, text and conceptual model representations of each site visited. This has also been updated with additional explanations, references and photographs taken during the field trips to better clarify questions asked by delegates at several field sites. For example, we included a photograph of Professor Delvin Fanning demonstrating the Pons “index of squishiness” test or “n-value” test (developed by Pons and Zonneveld 1965 to define the degree of physical ripening of soft sediments) on hypersulfidic material from the Mangrove swamp during the mid-Conference field trip (Figure 1). The pre-conference tour visited the Lower Lakes and River Murray highlighting the enormous negative impacts ASS had during the Millennium Drought, a severe and prolonged drought in southern Australia between 2001 and 2009 (van Dijk et al. 2013). Another mid-conference tour visited the coastal Barker Inlet estuary, Garden Island, Gillman, then St Kilda, highlighting mangrove deaths and adjacent abandoned salt evaporation ponds. A post-conference tour to the Adelaide Hills showed Inland ASS, which included land degradation, mineral exploration and the site where the mineral Schwertmannite was first discovered in natural inland ASS; finally, the Brukunga pyrite mine, where participants viewed acid mine drainage and rehabilitation. The Field Tours Guidebook (Fitzpatrick et al. 2023) has been extensively used as a teaching aid for student field excursions and self-guided field tours. The 28 papers included in this Special Issue are synthesized below under five headings: (i) Advancements in Classification and Mapping, (ii) Biogeochemical Dynamics and Mineralogical Transformations, (iii) Geochemical Assessment and Elemental Mobilization, (iv) Microbial Ecology and Process Drivers, and (v) Innovative Management and Technical Refinements. A key theme of this Special Issue is the refinement of soil classification systems to better reflect the geochemical realities of ASS. Rabenhorst, Fitzpatrick, and Boman (2025) provide a comprehensive review of how international systems, such as the US Soil Taxonomy (ST), the World Reference Base (WRB), and the Australian Soil Classification (ASC), have evolved to accommodate concepts like hypersulfidic, hyposulfidic, and monosulfidic materials. A significant development is the re-introduction of the “para-acid sulfate soil” (para-ASS) category to identify soils that pose environmental risks despite not meeting strict pH < 4 criteria (Boman et al. 2023; Estévez et al. 2025; Rabenhorst, Fitzpatrick, and Boman 2025). The geographical and technical scope of identification continues to expand. Andersson and Hansen (2024) document the first characterized Norwegian ASS, highlighting risks associated with deep drainage. Fitzpatrick et al. (2025a) also provide the first characterization of how ASS from diverse inland wetlands on Norfolk Island in the South Pacific sequentially changed during prolonged drying from 1970 to 2020, followed by wetting in 2021 and flooding in 2022. To improve hazard identification, Nystrand et al. (2025) propose an accelerated incubation method, significantly reducing diagnostic periods. Furthermore, Estévez et al. (2025) demonstrate the power of machine learning for multiclass mapping of ASS types, providing essential spatial data for land-use planning. Understanding the dynamic response of ASS to climate variability and anthropogenic factors is a critical emerging theme. Fitzpatrick et al. (2025a, 2025b) highlight irreversible soil property changes during extreme drying-wetting cycles on Norfolk Island, where prolonged drought exposed hypersulfidic materials to oxygen, forming sulfuric horizons. Hulisz et al. (2025) explore environmental controls along the Baltic Sea, noting how topographic position and sedimentary history drive site-specific variability in acidification potential. Research in Brazil's Doce River Delta by Firmino, Ker, Fontes, and Mesquita (2025) has introduced new insights into mineralogical shifts within thionic horizons, specifically how sulfurization leads to the selective dissolution of poorly crystalline kaolinite, increasing the overall crystallinity of the remaining minerals. They also document a unique anthropogenic impact: heat from surface fires can transform iron oxides into magnetic minerals like maghemite and hematite within acidic profiles (Firmino, Ker, Fontes, and Mesquita 2025). The impact of these soils on aquatic health is also detailed (Firmino, Ker, Fontes, Melo, et al. 2025b), showing that re-flooding triggers the reductive dissolution of iron oxyhydroxy-sulfate minerals like jarosite, releasing associated potentially toxic (e.g., Al, Mn, Ni, and Zn) metals into surface waters. The Special Issue highlights a need to shift toward more precise measurement techniques. Fundamental pH methodological accuracy is addressed in the review by Mosley et al. (2024), who emphasize how measurement conditions dramatically influence measurements, particularly in ASS. Firmino et al. (2025a) evaluate sequential extraction schemes, finding that while useful for partitioning toxic elements, these methods face challenges due to rapid metal reservoir transformations under extreme pH fluctuations. Firmino, Ker, Fontes, Melo, et al. (2025) further evaluate contamination indices, noting that element mobilization from ASS can significantly influence environmental risk assessments. The interplay between iron, sulfur, and carbon (Fe–S–C) is also recognized as a driver of ecosystem health. Tan et al. (2025) review these dynamics in coastal vegetated “blue carbon” environments, suggesting that ASS influence C sequestration rates. Yli-Halla and Sullivan (2024) examine the characteristics of remnant vegetation pipestems as indicators of Fe and S cycling. Rabenhorst, Dallam, and Kim (2025) elucidated how water halinity influenced the presence of hypersulfidic materials in estuarine tidal marsh soils, demonstrating that even low-halinity environments (< 2 ppt) can contain ASS with significant acid-producing potential. With global sea level rise, this suggests that some soil map units should be renamed to better identify these hazards. Furthermore, Wang et al. (2025) and Xu et al. (2024) examined how vegetation gradients and brackish water inundation alter acidification risks. Nyman et al. (2024) provide critical data on the most mobile, water-soluble fractions of trace elements across Swedish coastal plains. Dalhem et al. (2025) show that acid drainage loads in the past 100 years from Finnish ASS catchments is reflected in estuarine sediments, with large enrichments of metals (Al, Cd, Co, Ni, and Zn) observed. Microorganisms catalyze the conversion of hypersulfidic soils to sulfuric (pH < 4) soils. Johnson et al. (2025a, 2025b) demonstrate that regional variation in microbial communities in boreal systems is primarily driven by temperature rather than just soil geochemistry, a finding with significant implications for a warming climate. In montane peatlands, Carroll et al. (2025) find that microbial biodiversity is regulated by depth and specific geochemical zones, with sulfur-reducing bacteria playing key roles at depth. Kononova et al. (2024) showed that limestone and peat treatments shifted microbial diversity and reduced acidification of ASS materials. However, microbial communities that developed within the limestone treatments sustained iron sulfide oxidation, despite the higher pH. The role of ASS in greenhouse gas emissions is explored by Mäkelä et al. (2025) via field measurements of a cultivated peatland underlain by black schist in Finland. They found that soil wetness driven by high groundwater levels, rather than cold temperature, is a prevalent limiting factor to microbial gas production. The Special Issue highlights advanced mitigation and management. Kronberg et al. (2026) review the stabilization of sulfide-bearing clays using industrial side streams to create stable building grounds while immobilizing harmful elements. However, remediation can be complex and hampered by soil physical properties. Kölbl et al. (2024) caution that sorption of dissolved organic carbon (DOC) to clay minerals can limit electron donor availability for microbial reduction. Agricultural management is addressed by Quirk (2024), who presents a “win-win” model for sugarcane productivity through modified drainage and a focus on building soil organic matter content. Virtanen et al. (2025) utilize a dual-porosity model to demonstrate that the ripening of ASS horizons significantly increases saturated hydraulic conductivity and decreases air entry values, indicating that these evolving physical properties must be incorporated into environmental models for these systems. The wide range of topics represented in the contributions to the 9th IASSC as well as to this Special Issue provide an excellent representation of progress made in ASS research over recent years. Together, the contributions lead to an increased holistic understanding of ASS classification, properties and processes, their links to innovative management and technical refinements, as well as the importance of ASS to society. We hope the 28 papers in this Special Issue will inspire further publication of research on ASS, particularly following the 10th International Acid Sulfate Soils conference that was held in 2025 in Luleå, Sweden. Luke M. Mosley: conceptualization, writing – original draft. Robert W. Fitzpatrick: conceptualization, writing – original draft. Anton Boman: writing – review and editing. Emily Leyden: writing – review and editing. Miriam I. Nystrand: writing – review and editing. Martin C. Rabenhorst: writing – review and editing. Vanessa N. L. Wong: writing – review and editing. We thank all those who gave their time to provide anonymous reviews of manuscripts for this Special Issue—your constructive criticism has contributed greatly to the scientific quality of the published papers. We express our gratitude to the European Journal of Soil Science for having trust in the intention of publishing this Special Issue and assisting us to develop it, especially the previous Editor-in-Chief Professor Jennifer Dungait and Deputy Editor Dr. Mark Farrell. Finally, we also want to thank all the authors of the articles for their hard work to produce this Special Issue. A great many individuals contributed to the very successful 9th International Acid Sulfate Soils Conference, especially representatives from Adelaide University. We gratefully acknowledge their contributions. We would also like to acknowledge the help provided by the international organizing committee for the conference. Luke Mosley declared a conflict of interest that he is an Associate Editor of this journal. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.