Urban soils are unlike any others. Excavation, backfilling, sealing, compaction, demolition, and the importation of exogenous materials create intricate mosaics of soil horizons and anthropogenic deposits over very short distances. Physical and chemical properties such as texture, bulk density, porosity, organic matter content, pH, nutrient availability, and contaminant concentrations can change abruptly both laterally and vertically. Trace metals and organic pollutants inherited from former industrial activities, traffic emissions, and construction materials are frequently distributed in localized hotspots, further increasing spatial complexity. This heterogeneity challenges conventional soil sampling strategies, which often fail to adequately represent the spatial organization of soil properties and associated environmental constraints, making accurate characterization essential for land-use planning and avoiding inappropriate uses.

Over the past two decades, near-surface geophysical methods have become valuable tools for investigating soil spatial variability because they provide rapid, non-invasive, and spatially continuous information at resolutions that cannot be achieved through conventional sampling alone. Techniques including electromagnetic induction, ground-penetrating radar, seismic methods, and gamma-ray spectrometry respond to different physical properties of the subsurface, such as electrical conductivity, dielectric permittivity, seismic velocity, or natural radioactivity, which are themselves controlled by soil texture, water content, porosity, bulk density, clay mineralogy, soil structure, and the presence of coarse fragments or anthropogenic materials. Among these approaches, ERT has received particular attention because it provides high-resolution two- and three-dimensional images of the spatial distribution of subsurface electrical resistivity. The method is based on injecting electrical current into the ground and measuring the resulting potential differences, from which the true resistivity distribution is estimated through inversion procedures.

Soil electrical resistivity is governed by several interacting factors, including water content, pore connectivity, texture, clay mineralogy, porosity, and the abundance of coarse fragments or anthropogenic materials. Because electrical current is transmitted predominantly through the pore water rather than the solid mineral phase, bulk resistivity is controlled by the amount and composition of the pore water, pore connectivity, and the geometry of the pore network, all of which are strongly influenced by soil texture, structure, coarse fragment content, compaction, and horizon development. Consequently, resistivity integrates the effects of several key soil attributes and provides a physically meaningful representation of subsurface heterogeneity rather than direct measurements of individual soil properties. This integrative nature is particularly advantageous in urban environments, where soil variability often results from multiple superimposed processes that cannot easily be disentangled using conventional point measurements alone.

The study site was a plot of approximately 1,000 square meters located on the IRD site in Bondy, within the Seine-Saint-Denis department of the Paris metropolitan area. According to the land-use plan of the city of Bondy, the site lies over a succession of alluvial deposits derived from the 1:50,000-scale geological map produced by the French Geological Survey. Historically, the area was known in the mid-18th century for its dense forest, as shown in the 1756 Cassini map, and later became a popular destination for Parisians seeking rural retreats. The site was significantly altered during the construction of the IRD research center in 1947, when many gardens, vegetable plots, and refuse areas were consolidated, and the topography was substantially modified through the import and leveling of various materials, subsequently covered with a layer of forest-origin topsoil that was initially uncontaminated.

The ERT survey was carried out along linear profiles using a SYSCAL Pro resistivity meter operating quadrupoles sequentially selected among seventy-two electrodes. Measurements were performed using two electrode arrays: a reciprocal Wenner-Schlumberger configuration, which is well adapted to detect vertical contrasts in electrical resistivity and offers a better signal-to-noise ratio and greater depth of investigation, and a Dipole-Dipole configuration, which better captures horizontal contrasts and yields more pronounced resistivity contrasts at shallow depths. Short transverse profiles of 26.25 meters were spaced every 6 meters, while long longitudinal profiles of 53.25 meters were spaced every 1.5 meters, with a unit electrode spacing of 0.75 meters for both sets. The total number of measurements collected was 1,870 for the Dipole-Dipole array and 2,282 for the Wenner-Schlumberger array. Data interpretation was performed using RES2DINV and RES3DINV software, and spatial variation maps of calculated resistivity as a function of depth were generated using Surfer software.

The resistivity maps revealed significant variability both laterally and vertically. A linear anomaly observed at x = 5, extending down to approximately 90 centimeters depth, corresponded to a trench, with additional trench-like features visible near the center of the site radiating from a central manhole. At the surface, a first layer with resistivity around 50 ohm-meters corresponded to the topsoil layer distributed across the site. Below this, starting at approximately 25 centimeters depth, a second layer appeared with markedly lower resistivity values, indicative of coarse backfill material. In some areas, high-resistivity materials emerged with values reaching up to 315 ohm-meters, while at greater depths, intermediate resistivity zones of around 102 ohm-meters were detected in the southern and northeastern parts of the site, possibly corresponding to sandy pockets. The three-dimensional inversion enabled the generation of vertical resistivity profiles across the site with a spatial resolution of 37.5 by 37.5 square centimeters.

Two combined statistical analyses, Principal Component Analysis and Hierarchical Cluster Analysis, were used to classify all resistivity profiles by depth into four distinct groups, which served as the basis for the resulting classification map. While resistivity values were relatively similar across classes in the uppermost layer from 0 to 0.5 meters, they became highly discriminant between 0.5 and 2.5 meters depth. Class 1 showed both maximum and median resistivity values below 100 ohm-meters, whereas Class 4 reached a maximum resistivity exceeding 300 ohm-meters with median values around 220 ohm-meters. Classes 2 and 3 were intermediate, and Class 3 was notably distinct, characterized by a near-linear increase in resistivity with depth and high values beyond 2.5 meters. Following this typology, 30 auger surveys were distributed according to a proportionally stratified sampling design considering the surface area of each mapped resistivity unit, and soil samples were collected in each of the four soil categories at four different depths, totaling 120 samples.

The subsequent laboratory characterization showed that the ERT-derived classes corresponded to meaningful differences in measured soil characteristics. Soil texture was primarily controlled by depth, with only a weak effect of soil class, and the overall profiles exhibited a coarse texture with sand contents of 71 to 76 percent across layers. Classes C3 and C4 were relatively enriched in fine silt and clay, and significantly coarser textures occurred in layers B and C of soil class C4, which was also characterized by a relatively high sand content and low clay content in the surface layer. High concentrations of organic carbon and total nitrogen were observed in the surface layer corresponding to the added topsoil, with organic matter content decreasing markedly with depth to approximately tenfold lower organic carbon concentrations in the deepest layer. Both pH in water and pH in potassium chloride varied only with depth, showing an increase of approximately 0.5 pH units from the surface to the deepest layer, while cation exchange capacity, Olsen phosphorus, and most exchangeable cations were mainly driven by depth, positively correlated with soil organic carbon but negatively correlated with pH.

Mid-infrared spectroscopy revealed clear differences in organic matter composition among the ERT-derived classes, particularly for hydrophobic functional groups and polysaccharide-related compounds, suggesting that ERT may be more sensitive to qualitative differences in organic matter than to its total abundance. This interpretation is physically plausible because organic matter exerts a strong indirect control on soil electrical properties through its influence on aggregate stability, pore architecture, wettability, and water retention rather than through its intrinsic electrical conductivity. Hydrophobic organic compounds may reduce soil wettability and modify infiltration patterns, whereas microbial polysaccharides promote aggregate formation and enhance pore connectivity, thereby influencing the spatial distribution of soil water and the propagation of electrical current. Elemental composition determined by ICP-MS showed a strong and significant effect of depth for most elements, with significant differences among soil classes observed for several major elements including aluminum, iron, magnesium, and phosphorus, as well as for trace elements such as chromium and zinc. Trace elements, particularly lead, were detected at the site, but no significant differences among soil types were observed except in deeper layers, and variations in physico-chemical conditions, especially soil moisture, may influence trace element speciation and mobility, potentially affecting biota.

The findings carry broad implications for how cities understand the ground beneath them. Because the geoelectrical classes were established before soil sampling, the subsequent physicochemical measurements provided an independent assessment of whether the geophysical units corresponded to systematic differences in soil characteristics, and the significant differences observed when the physicochemical variables were considered jointly support the relevance of the spatial classification. ERT proved valuable not only for identifying spatial discontinuities but also for confirming the relative homogeneity of the site, thereby reducing uncertainty in soil characterization regardless of the degree of anthropogenic disturbance. The approach extends the use of ERT beyond conventional subsurface characterization by using continuous geophysical information as an operational decision-support tool for subsequent soil sampling, with broad potential applications including the optimization of soil quality and contamination monitoring programs, as well as the design of field experiments that account for pre-existing soil heterogeneity and thereby reduce experimental bias. Future studies should investigate the potential of time-lapse ERT, combined with complementary techniques such as electromagnetic induction, ground-penetrating radar, and digital soil mapping, to better characterize soil functioning, contaminant mobility, and temporal changes in urban environments, potentially opening new opportunities to investigate the links between soil structure, hydrological functioning, and carbon cycling in urban ecosystems.

Subject of Research: Urban soil science and geophysical characterization

Article Title: Exploring spatial variability in the physical and chemical properties of urban soils using electrical resistivity tomography

Article References: Ribler, C., Lerch, T. Z., & Robain, H. (2026). Exploring spatial variability in the physical and chemical properties of urban soils using electrical resistivity tomography. Frontiers in Soil Science, 6, Article 1924576. https://doi.org/10.3389/fsoil.2026.1924576

Image Credits: AI Generated

DOI: 10.3389/fsoil.2026.1924576

Keywords: electrical resistivity tomography, urban soils, soil sampling, organic matter, trace elements, geophysics, Exploring, spatial, variability, physical, chemical, properties

The team collected surface soil samples from 29 sites spanning industrial, urban, and background functional zones, analyzing them for the four metals most closely tied to the city’s ore processing legacy. The results are striking. Zinc concentrations ranged from 5.2 to 1,675 milligrams per kilogram, with a mean of roughly 420 milligrams per kilogram, approximately 23 times higher than the value recorded at a local reference site 20 kilometers southwest of the city. Cadmium, though present at far lower absolute concentrations, averaged 10.4 milligrams per kilogram, about 13 times the reference level. Copper and lead reached maximum values of 57.8 and 59.5 milligrams per kilogram respectively, with coefficients of variation exceeding 110 percent for all four metals, indicating extreme spatial heterogeneity across the urban landscape.

What makes these numbers alarming is not just their magnitude but their distribution. Median concentrations in industrial soils were 704.5 milligrams per kilogram for zinc, 15.8 for cadmium, 11.5 for copper, and 22.1 for lead, roughly one order of magnitude above the medians measured in urban soils. The highest values clustered at sites S16 through S25, within the central industrial sector where the Kazzinc metallurgical complex, the Ust-Kamenogorsk Titanium and Magnesium Plant, and the Ulba Metallurgical Plant operate in close proximity to residential districts and transportation corridors. Site S19 recorded the maximum zinc and lead concentrations, while cadmium peaked at S20 and copper at S25.

To translate raw concentrations into environmental meaning, the researchers applied a battery of pollution indices. The contamination factor, geoaccumulation index, Nemerow pollution index, and potential ecological risk index each quantify different aspects of soil degradation, and their combined application revealed a coherent picture. The Nemerow index, which simultaneously considers average contamination and the worst single-metal exceedance, reached values above 50 at several industrial sites, far beyond the threshold of 3 that defines heavy pollution. The potential ecological risk index, which weights each metal by its toxicity, ranged from 41 to 2,188.9, with roughly one-third of sampling sites exceeding the threshold for very high ecological risk.

Perhaps the most consequential finding concerns cadmium. Although zinc dominated in concentration and spatial extent, cadmium contributed overwhelmingly to total ecological risk because of its high toxic-response coefficient in the Hakanson framework, which assigns cadmium a weighting of 30 compared with 5 for copper and lead and just 1 for zinc. Ecological risk factors for cadmium reached 1,462.5 at individual sites, often surpassing the threshold of 320 that marks very high risk. This means that remediation priorities based solely on total metal concentrations would miss the element posing the greatest environmental threat, a paradox the authors argue should fundamentally reshape how legacy metallurgical cities are monitored and managed.

The geochemical signature of the contamination points unambiguously to its source. Zinc and cadmium showed an exceptionally strong correlation, with a Spearman rank coefficient of 0.90, reflecting the fact that cadmium commonly substitutes for zinc within the mineral sphalerite and is released simultaneously during smelting of sulphide ores. Their spatial distributions were nearly identical in the interpolation maps, forming a coupled enrichment pattern recognized as a characteristic fingerprint of non-ferrous metallurgy worldwide. Copper and lead, by contrast, displayed more localized anomalies, consistent with coarser particles that settle rapidly after emission rather than remaining suspended for long-range atmospheric transport.

Soil properties themselves modulated the accumulation pattern. The investigated soils were predominantly neutral to slightly alkaline, with pH ranging from 5.73 to 7.48, conditions generally favourable for reducing metal mobility through adsorption onto clay minerals and organic matter. Yet even under these relatively buffering conditions, industrial sites remained heavily contaminated, demonstrating that soil chemistry regulates retention rather than the primary source of contamination. Particle-size composition exerted a particularly strong influence: sand content was negatively correlated with all four metals, while silt showed significant positive relationships, indicating that fine-textured soils act as efficient sinks for airborne industrial particulates thanks to their greater specific surface area and cation-exchange capacity.

Multivariate statistics reinforced the anthropogenic interpretation. Principal component analysis separated industrial sites, which clustered along a gradient defined by elevated metals and pollution indices, from urban and reference sites associated with higher pH, finer fractions, and exchangeable base cations. Non-metric multidimensional scaling, with a stress value of just 0.028, confirmed highly significant differences among ecological risk groups, while spatial interpolation using inverse distance weighting identified contamination hotspots surrounding the major metallurgical facilities, with zinc and cadmium showing the broadest footprints and copper and lead more restricted anomalies.

The persistence of these elevated concentrations decades after emission controls improved underscores that contaminated urban soils function as long-term reservoirs, capable of secondary redistribution through dust resuspension, erosion, and surface runoff. The authors argue that environmental management in cities like Ust-Kamenogorsk must address both ongoing emissions and the contaminants already locked in surface horizons, prioritizing cadmium-enriched hotspots for risk-based remediation while considering stabilization approaches using mineral amendments, organic sorbents, or phytostabilization. The study also acknowledges limitations, including reliance on a single local reference site and the measurement of total rather than speciated metal concentrations, leaving questions about bioavailability and human health risk for future investigation.

For a city that has smelted zinc, lead, copper, titanium, and uranium-related materials since the mid-twentieth century, the findings provide a scientific foundation for monitoring, risk-based land management, and sustainable remediation in one of Central Asia’s most intensively industrialized urban environments. They also carry a broader message: in long-established metallurgical cities worldwide, from Poland to China to Spain, the ground beneath our feet retains a geochemical memory of industrial activity that outlasts the factories themselves, and understanding that memory is the first step toward managing it.

Subject of Research: Legacy of non-ferrous metallurgy driving heavy metal-induced geochemical transformation of urban soils

Article Title: Legacy of non-ferrous metallurgy driving heavy metal-induced geochemical transformation of urban soils

Article References: Daurov, D., Abilda, Z., Zhambakin, K., Daurova, A., Sapakhova, Z., Toishimanov, M., Isgandarov, I., Mukhametov, A., Kanat, R., Otynshiev, M., Volkov, D., & Shamekova, M. (2026). Legacy of non-ferrous metallurgy driving heavy metal-induced geochemical transformation of urban soils. Frontiers in Soil Science, 6, Article 1949558. https://doi.org/10.3389/fsoil.2026.1949558

Image Credits: AI Generated

DOI: 10.3389/fsoil.2026.1949558

Keywords: heavy metals, soil pollution, legacy contamination, ecological risk, urban soils, metallurgy, Legacy, non-ferrous, driving, heavy, metal-induced, geochemical

Urbanization is one of the defining land transformations of the modern era. Between 2000 and 2018, more than one million hectares of land were urbanized in Europe, over eight times the area that reverted to agriculture or nature. Urban soils endure simultaneous disturbances, including construction, pollution, trampling and compaction, creating a patchwork of heterogeneous microhabitats. Yet while the effects of urbanization on plants and animals have been studied extensively, the functional consequences for soil microbial communities remain poorly understood, leaving a critical gap in global microbial ecology.

To address this, researchers sampled 135 soils across eight land-use types in Blois, a mid-sized city in central France. The sites ranged from showcase gardens in the historic old town and heavily modified sports fields to parklands, roadsides, residential green spaces, tree-lined paths and unused areas reserved for future development. Each site was assigned a management intensity level from 1 to 4, spanning minimally managed grass mowed once or twice a year to intensively managed turf mowed up to 25 times annually with residue export, watering, scarification and, in the highest category only, fertilizer inputs. Bacterial and fungal communities were characterized using Illumina sequencing, fungal guilds were identified with the FungalTraits database, bacterial taxa were classified by life strategies, enzymatic activities were measured, and co-occurrence networks were constructed.

The results showed that land-use type and management intensity strongly influenced microbial composition, diversity and functioning. Highly managed sites such as gardens and sports fields exhibited lower fungal 18S rRNA gene abundance, higher bacterial 16S rRNA gene copy numbers, reduced diversity and lower nitrogen-related enzyme activities compared to less managed unused areas. Bacterial and fungal richness peaked at the intermediate management intensity, consistent with the intermediate disturbance hypothesis, before declining at the highest level, possibly reflecting soil acidification and nutrient imbalances associated with intensive nitrogen inputs. Sports fields had the lowest fungal richness of any land use, a pattern the authors suggest may stem from fungicide application selectively suppressing non-targeted fungi.

Bacterial life strategies shifted along the management gradient in ways that echo the copiotrophic–oligotrophic continuum. Oligotrophic bacteria, slow-growing taxa adapted to low nutrient availability, were more abundant in unused areas, while copiotrophic Proteobacteria dominated residential soils, likely fueled by heterogeneous organic inputs such as pollutants, pet waste and runoff. Mediation analyses revealed functional consequences of these shifts: copiotrophic Proteobacteria partially mediated the relationship between β-glucosidase activity and soil carbon and nitrogen contents, while oligotrophic Actinobacteria prevalent in unused soils were associated with the decomposition of recalcitrant organic matter and higher carbon-acquisition enzyme ratios.

Fungal communities told a complementary story. Saprotrophic fungi represented the dominant guild in urban areas at 62 percent, followed by plant pathogens at 25 percent. Contrary to expectations based on nitrogen fertilization studies, saprotrophs were not more abundant in fertilized areas; instead, their proportion was higher in less managed soils, likely reflecting greater availability of undecomposed organic materials. Mycorrhizal fungi, unexpectedly, were not more abundant in tree-covered areas than elsewhere. Instead, host-tree identity and symbiotic association type, whether arbuscular or ectomycorrhizal, emerged as stronger determinants of mycorrhizal abundance than land use or soil properties, an insight with direct implications for urban tree planting choices.

Perhaps the most striking findings concerned microbial interaction patterns. Co-occurrence networks constructed from the urban soil communities were consistently less connected than those reported for semi-natural ecosystems such as forests, grasslands and shrublands, indicating more fragmented microbial associations in urban soils. Within the city, unused areas displayed the highest connectivity, with bacterial networks showing large connected components, high clustering coefficients and short average path lengths, while roadsides showed the lowest connectivity. Fungal networks were markedly more interconnected than bacterial ones across all land uses, with positive edge percentages exceeding 97 percent in tree networks.

The authors caution that co-occurrence networks do not provide direct evidence of ecological interactions or resilience, and that network structure is highly sensitive to methodological choices. Still, the reduced connectivity observed in intensively managed sites may indicate shifts in community organization consistent with stronger ecological filtering, potentially reducing functional redundancy and soil ecological resilience. Soil properties, particularly carbon content, nitrogen availability and pH, emerged as primary determinants of microbial richness, diversity and composition, sometimes overshadowing land-use effects, and legacy effects from past land use in this relatively young urban area likely contributed to long-term shifts in microbial communities.

Because all soils were collected during a single season, the dataset does not capture temporal fluctuations driven by temperature, moisture, plant phenology or seasonal management practices, a limitation the authors acknowledge. Future work incorporating repeated seasonal sampling, private gardens, bare soils and engineered soils would help clarify how temporal variability interacts with land use and management intensity to shape microbial communities across the year.

The study’s central message is that management intensity acts as a strong ecological filter shaping microbial functional traits and interaction patterns in urban soils. The reduced connectivity and functional shifts observed in intensively managed sites highlight the need to integrate microbial indicators into urban green-space planning and management. As cities continue to expand and green spaces come under increasing pressure, understanding how microbial communities contribute to nutrient cycling, organic matter decomposition and soil resilience will require moving beyond taxonomic inventories toward functional and interaction-based approaches, ensuring that the invisible ecosystems beneath our feet are not left out of the sustainability conversation.

Subject of Research: Urban soil microbial ecology along a management intensity gradient

Article Title: Urban land use influences microbial functional traits along a management intensity gradient

Article References: Khalfallah, F., Legay, N., Bonthoux, S., Cantarel, A. A. M., Clement, J.-C., Foulquier, A., Joulian, C., Le Roux, X., Richaume, A., Norini, M.-P., & Harris, J. (2026). Urban land use influences microbial functional traits along a management intensity gradient. Frontiers in Microbiomes, 5, Article 1946255. https://doi.org/10.3389/frmbi.2026.1946255

Image Credits: AI Generated

DOI: 10.3389/frmbi.2026.1946255

Keywords: urban soils, microbial communities, management intensity, soil microbiome, co-occurrence networks, urban green spaces, soil enzymes, bacterial life strategies, fungal guilds, urbanization, Urban, land

Soil respiration, the combined output of microbial decomposition and root metabolism, is one of the largest fluxes of carbon dioxide from land to atmosphere. For decades, scientists have modelled it with the Q10 function, an exponential equation in which emissions rise steadily as temperatures climb. The logic seems intuitive: warmer microbes work faster. But the Q10 framework was built largely on data from temperate and mesic ecosystems, and researchers led by Teresa Alía and Sergio González-Ubierna of the Complutense University of Madrid suspected it might fail in the hot, dry, human-altered conditions of a Mediterranean city.

To find out, the team monitored soil CO2 efflux, temperature and moisture at 36 sampling points across three Madrid green spaces: Ciudad Universitaria, La Elipa and Pinar de Conde Orgaz. Measurements were taken fortnightly from April 2023 to April 2024 using an infrared gas analyser with open chambers, covering four dominant ruderal plant communities: annual herbs dominated by Diplotaxis virgata, annual grasslands of Hordeum leporinum, perennial herbs of Malva species, and perennial grasslands of Dactylis glomerata. The visiting order of parks and plots was systematically rotated and randomised across a broad daily window to avoid timing bias, and all soils shared a similar sandy loam texture, isolating vegetation and climate as the key variables.

The results upended the textbook expectation. Instead of rising exponentially with temperature, soil respiration followed a Gaussian, bell-shaped curve, increasing up to a sharply defined breakpoint of 18.4 degrees Celsius and declining above it. The confidence interval around that threshold was narrow, from 17.17 to 19.79 degrees, indicating a robust feature of the system rather than a statistical fluke. Crucially, the same unimodal pattern appeared in all four plant communities, differing only in the intensity of emissions, not in shape.

The explanation is not that heat directly suppresses soil life. Rather, in a Mediterranean climate, temperature and moisture are tightly and negatively coupled: hot months are dry months. When the researchers split the dataset at the 18.4 degree threshold, a clear division emerged. Below it, soil temperature was the main driver, with a positive linear relationship to CO2 emissions, while water variables played only a minor role because moisture was generally plentiful. Above it, the picture flipped entirely: soil moisture and a rewetting index, which quantifies recent rainfall relative to the time elapsed before measurement, became the strongest predictors, and temperature lost its explanatory power once water availability was accounted for.

The rewetting relationship itself carried a twist. Emissions rose logarithmically with the rewetting index, but only up to values of about 20; beyond that, the link weakened, likely because compacted urban soils and altered rainfall partitioning limit effective infiltration, so heavy rain does not translate into sustained soil moisture. This suppression of the classic drying-rewetting CO2 pulse, well documented in natural Mediterranean ecosystems, appears to be a distinctly urban signature, consistent with the emerging concept of the Urban Dry Island, in which impervious surfaces reduce evapotranspiration and infiltration even within unsealed green spaces.

Building on these patterns, the team derived a mechanistic model from first principles, describing soil respiration as the joint outcome of temperature-driven moisture loss and moisture-stimulated CO2 production. Expressed as a pair of coupled differential equations, the model captures the essential constraint of Mediterranean systems: temperature and moisture cannot rise together indefinitely, because warming inevitably dries the soil. When fitted to the data, the model outperformed both the standard Q10 exponential formulation and an existing Gaussian model, achieving the lowest error and significantly reducing deviance in likelihood-ratio tests. A leave-one-plot-out cross-validation confirmed the model generalises well beyond the plots used to fit it, and sensitivity analyses with mixed-effects and autoregressive structures preserved the same ranking of communities and similar optimum temperatures.

Vegetation mattered, but modestly. Only the perennial herb community dominated by Malva species consistently emitted more CO2 than the others, a pattern the authors attribute to greater soil disturbance and labile carbon availability rather than to microclimatic differences, since temperatures and moisture did not differ significantly among communities. More intriguing was a trade-off revealed by the model: communities with lower basal respiration rates tended to reach higher maximum emissions, and those most sensitive to moisture, such as the annual grassland of Hordeum leporinum, also showed the highest temperature optima. That fits the phenology of Mediterranean annual grasses, which concentrate root growth and carbon inputs in the moist, warming spring window when both drivers align.

The practical implications reach well beyond Madrid. Because conventional temperature-based models ignore the simultaneous decline in soil moisture, urban carbon accounting tools may systematically overestimate summer soil CO2 emissions in water-limited Mediterranean cities. The authors argue that keeping urban soils moist, through reduced compaction, organic amendments, improved infiltration and moisture-sensitive irrigation, will do more for the urban soil carbon balance than any attempt to manage temperature alone. As climate change intensifies both the urban heat island and summer drought, the drying role of cities is likely to grow stronger, potentially suppressing average emissions while amplifying their variability through episodic rewetting pulses.

The study’s authors are candid about its limits. The monitoring year saw above-average rainfall, which may have buffered the moisture limitation typical of drier years, and total CO2 efflux was measured without separating microbial from root-derived components. The findings also come from a single city, and validation across other Mediterranean urban contexts and contrasting hydrological years remains an open task. Even so, the core message is hard to ignore: in the parks of a warming, drying Mediterranean city, water writes the carbon budget, and temperature merely sets the stage on which it is written.

Subject of Research: Controls on soil CO2 efflux in Mediterranean urban green spaces

Article Title: Soil moisture as the dominant driver of CO2 efflux in Mediterranean urban green spaces: evidence for a Gaussian temperature response and mechanistic modelling of moisture and temperature interactions

Article References: Alía, T., González-Ubierna, S., Sánchez-Jiménez, A., Abad-Calderón, R., & Casermeiro, M. Á. (2026). Soil moisture as the dominant driver of CO 2 efflux in Mediterranean urban green spaces: evidence for a Gaussian temperature response and mechanistic modelling of moisture and temperature interactions. SOIL, 12(2), 915-935. https://doi.org/10.5194/soil-12-915-2026

Image Credits: AI Generated

DOI: 10.5194/soil-12-915-2026

Keywords: soil respiration, soil moisture, carbon dioxide, urban heat island, Mediterranean climate, urban soils, Q10 model, Madrid, rewetting, carbon cycle, mechanistic modelling, urban green spaces

Salgótarján is a city of roughly 35,000 people whose economic life was built on brown coal. Mining began in the mid-nineteenth century, and over at least 170 years the city developed a heavy industrial portfolio that included a coal-fired power plant, iron and steel smelting, glassworks, and mining machinery production. When the communist system collapsed in the early 1990s, the mines and heavy industries declined, but their waste did not disappear. Coal mine dumps, coal ash heaps, and smelter slag deposits remained scattered across the urban landscape. In several cases, these industrial leftovers were repurposed as filling material during post-industrial redevelopment, meaning that playgrounds, parks, and kindergartens may literally sit on top of the by-products of a vanished industrial era.

The research team focused on strontium because of an unusual geochemical property. Strontium has four naturally occurring stable isotopes, and the proportion of strontium-87 increases over geological time through the radioactive decay of rubidium-87, an element with a half-life of 48.8 billion years. As a result, ancient rocks and the soils derived from them tend to carry elevated ratios of strontium-87 to strontium-86, while industrial by-products such as coal ash typically carry lower ratios. Strontium also substitutes readily for calcium in minerals, which means it concentrates in the limestone additives and carbonate-rich materials used during coal combustion. When coal burns, strontium becomes enriched in the resulting ash: the Salgótarján brown coal contained 52.8 milligrams of strontium per kilogram, while a coal ash sample collected from a cone just half a kilometer from the former power plant contained 113.6 milligrams per kilogram.

To capture these signals, the researchers analyzed fifteen urban soil samples collected from kindergartens, playgrounds, parks, and roadsides in the northeastern part of the city, at depths of five to fifteen centimeters, following the Euro-Geo-Surveys sampling protocol. Samples were digested using microwave-assisted acid treatment, purified through extraction chromatography resin, and measured with thermal ionization mass spectrometry at a laboratory in Fukushima, Japan, using the multi-dynamic technique and the NIST-SRM-987 standard for quality control. The results revealed considerable variation: strontium concentrations ranged from 17.5 to 51.3 milligrams per kilogram, and the strontium-87 to strontium-86 ratios spanned 0.710303 to 0.719559, a spread wide enough to encode multiple distinct sources.

The spatial pattern was striking. More than half of the samples showed a strong positive correlation, with a coefficient of determination of 0.86, between the isotopic ratio and distance from the former coal-fired power plant. Soils closest to the plant, such as a roadside sample 0.6 kilometers away and a park sample 0.9 kilometers away, carried the lowest ratios, consistent with heavy coal ash input, since the local coal ash itself registered a ratio of 0.711652. At intermediate distances the ratios climbed, and beyond three kilometers they approached values typical of the loess deposits that blanket the Pannonian basin, indicating that coal ash influence had faded and local soil dominated. The local coal ash ratio is itself explained by the marine carbonates within the coal seams, which carry the globally uniform seawater signature of roughly 0.7080 to 0.7090, slightly modified by the limestone sorbents added during combustion.

Not every sample fit this tidy distance gradient, and the deviations proved just as informative. A playground sample located near the former iron and steel works recorded the lowest ratio of the entire dataset, 0.710303, even lower than the coal ash itself, despite not being the closest site to the power plant. Two other samples, from a kindergarten and a roadside location, showed similarly depressed values. The researchers attribute these anomalies to smelter slag, a known industrial source in the city that incorporates strontium from carbonate and phosphate minerals with inherently low isotopic ratios, and possibly to unknown technogenic soils transported by humans during land cover changes. Microscopic examination of these samples revealed reddish oxidized layers and slag grains, physical corroboration of the isotopic evidence.

At the opposite extreme, three samples exceeded even the local soil end-member, with the highest ratio reaching 0.719559 at a roadside site. These elevated values point to artificial infills, quartz-rich materials commonly introduced for road construction and soil stabilization during urbanization. Such materials derive from parent rocks rich in rubidium, like granites and metamorphic rocks, in which radiogenic strontium-87 has accumulated over geological timescales. In other words, even the apparently clean end of the isotopic spectrum in Salgótarján tells a story of human landscape engineering rather than untouched nature.

To move from qualitative fingerprints to quantitative estimates, the team applied a two-end-member mixing model, plotting the isotopic ratio against the inverse of strontium concentration. Around eighty percent of the samples fell along a strong linear trend, with a coefficient of determination of 0.89, confirming that local soil and coal ash are the two dominant contributors. The model estimated that coal ash accounts for roughly 77 percent of the strontium in a roadside sample, 71 percent in a park sample, and 67 percent in another park located 4.4 kilometers from the plant, where ash was likely introduced as fill rather than by atmospheric deposition. Conversely, samples from kindergartens showed local soil contributions of 71 to 97 percent, suggesting minimal industrial contamination at those particular sites. The authors emphasize that these figures are semi-quantitative, intended to indicate spatial trends rather than precise apportionment, since the urban environment likely contains additional sources that a two-component model cannot fully resolve.

The physicochemical context reinforced the isotopic story. The soils were predominantly weakly alkaline, with a mean pH of 7.28, and samples rich in coal ash showed elevated pH reflecting the calcium and magnesium content of combustion residues, while samples near the smelter slag hill were more acidic, consistent with the secondary minerals like gypsum and iron oxy-hydroxides that form as slag weathers over decades. Phosphorus and sulfur contents peaked at sites influenced by industrial by-products, and total organic carbon and total nitrogen were highest in vegetated parks, where organic matter appears to help stabilize strontium in the soil matrix. Notably, a previous study by part of the same team had already documented reduced microbial activity at several of these sites, linked to arsenic, lead, mercury, and cadmium from coal ash and slag, underscoring that the strontium signal tracks co-contaminants of direct health concern.

The broader significance of the study lies in its demonstration that strontium isotope geochemistry can serve as a robust, relatively inexpensive tracer for legacy industrial contamination in post-industrial cities worldwide. Comparable isotopic signatures have been reported in urban soils from Kyrgyzstan and in contaminated sediments from Chinese rivers, suggesting the approach travels well across different geological and industrial settings. For city planners and environmental agencies, the message is practical: before repurposing industrial waste as fill, and before siting playgrounds and kindergartens on former industrial land, the isotopic composition of the soil can reveal exactly what lies beneath. As former industrial regions across Central and Eastern Europe continue to be redeveloped, the humble strontium atom may become one of the most valuable witnesses to what the twentieth century left in the ground.

Subject of Research: Strontium isotope tracing of industrial contamination sources in urban soils of a former Hungarian coal and steel city

Article Title: Strontium isotopic signatures and source apportionment in urban soils of Salgótarján: unraveling the impact of industrial emissions

Article References: Maghsoudlou, M., Tserendorj, D., Abbaszade, G., Kavasi, N., Völgyesi, P., Tóth-Bodrogi, E., Sahoo, S. K., Štrok, M., Inoue, K., Kovács, T., & Szabó, C. (2026). Strontium isotopic signatures and source apportionment in urban soils of Salgótarján: unraveling the impact of industrial emissions. Environmental Science and Pollution Research, 33(30), 15829-15848. https://doi.org/10.1007/s11356-026-38225-6

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38225-6

Keywords: strontium isotopes, urban soils, coal ash, coal-fired power plant, smelter slag, soil contamination, source apportionment, Salgótarján, heavy metals, geochemistry, post-industrial cities, environmental forensics