Black carbon in urban topsoils of Xuzhou (China): environmental implication and magnetic proxy

Black carbon in urban soils: land use and climate drive variation at the surface

BACKGROUND

Black carbon (BC) encompasses a range of carbonaceous materials--including soot, char, and charcoal--derived from the incomplete combustion of fossil fuels and biomass. Urban soils can become enriched in BC due to proximity to these combustion sources. We conducted a literature review of BC in urban soils globally and found 26 studies reporting BC and total organic carbon (TOC) content collected to a maximum of 578 cm depth in urban soils across 35 cities and 10 countries. We recorded data on city, climate, and land use/land cover characteristics to examine drivers of BC content and contribution to TOC in soil.

RESULTS

All studies were conducted in the northern hemisphere, with 68% of the data points collected in China and the United States. Surface samples (0-20 cm) accounted for 62% of samples in the dataset. Therefore, we focused our analysis on 0-10 cm and 10-20 cm depths. Urban soil BC content ranged from 0-124 mg/g (median = 3 mg/g) at 0-10 cm and from 0-53 mg/g (median = 2.8 mg/g) at 10-20 cm depth. The median proportional contribution of BC to TOC was 23% and 15% at 0-10 cm and 10-20 cm, respectively. Surface soils sampled in industrial land use and near roads had the highest BC contents and proportions, whereas samples from residential sites had among the lowest. Soil BC content decreased with mean annual soil temperature.

CONCLUSIONS

Our review indicates that BC comprises a major fraction (nearly one quarter) of the TOC in urban surface soils, yet sampling bias towards the surface could hide the potential for BC storage at depth. Land use emerged as an importer driver of soil BC contents and proportions, whereas land cover effects remain uncertain. Warmer and wetter soils were found to have lower soil BC than cooler and drier soils, differences that likely reflect soil BC loss mechanisms. Additional research on urban soil BC at depth and from diverse climates is critical to better understand the role of cities in the global carbon cycle.

Black carbon, soil organic matter molecular signatures under different land uses in Shenyang, China and relationship with PAHs

The content, composition and molecular signatures of soil organic matter (SOM) have important influences on the cycle of soil organic carbon (SOC) and the partitioning of polycyclic aromatic hydrocarbons (PAHs) in soil. Seventy-nine soil samples from farmland, forest and urban areas were collected in Shenyang, China to investigate black carbon (BC) content, SOM molecular signatures varied with land use patterns, as well as the relationship with PAHs. The content of BC in urban soils was significantly higher than that of farmland and forest. BC was a key contributor of urban SOM which accounted for 0.35 ± 0.31 of SOC in urban soil. Based on BC/SOC ratio, the main sources of BC were identified as fossil fuel combustion for urban soils, while for farmland and forest soils, it is the mixed results of fossil fuel combustion and biomass burning. All categories of PAHs in urban soils showed the highest level compared to farmland and forest soils. Pearson's correlation analysis results showed there were significant positive correlations between BC and PAHs categories in urban soils, indicating the important role of BC in the accumulation of PAHs in soil. SOM from each of the two different land use patterns can be distinguished by molecular signatures. Urban SOM had abundant molecular markers derived from condensed organic carbon inputs, which was consistent with the BC/SOC value. Farmland SOM had abundant carbon from vegetation and microorganisms, and forest SOM was rich in organic carbon from fresh plant materials. The markers enriched in urban SOM showed significant correlations with most PAHs categories, highlighting the affinity of urban SOM for PAHs at the molecular level. This study contributed to understanding the impact of land management methods on SOM molecular composition signatures and its influence on PAHs occurrence in soil, providing a theoretical basis for regional soil pollution management.

Black carbon flux in terrestrial and aquatic environments of Kodaikanal in the Western Ghats, South India: Estimation, source identification, and implication

Evolving Anthropocene epoch wields significant influence in altering atmospheric carbon, which affects the carbon cycle, leading to climate change. Understanding the carbon stock, fate, and transport across ecosystems are essential in determining India's carbon budget, hitherto, unavailable. In this study, we have analysed the stock, source, distribution, flux, and the relationship between terrestrial and aquatic black carbon over a high-altitude mountainous area in the Western Ghats region using the data collected from September 2019 to February 2021. Soil Organic Carbon (SOC) and Black Carbon (BC) are the highest in the forest region (SOC:23 ± 3 g of C/kg (dry weight (dw)), BC:6 ± 3 g/kg) and are the lowest in the urban region (SOC: 13 ± 2 g of C/kg (dw), BC:2 ± 1 g/kg). SOC is labile, whereas BC is non-labile. The BC/SOC ratio represents soil carbon lability. Topsoil BC/SOC ratios vary by land use and land cover, with urban areas having greater labile carbon pools than the forests. Dissolved BC (DBC) concentrations were most strongly correlated with bulk Dissolved Organic Carbon (DOC) concentrations in midstream (R = 0.6, p < 0.05), headwater streams (R = 0.3, p < 0.05) and to the soil bulk DBC (R = 0.3, p < 0.05), indicating the presence of transfer mechanism of soil to streams. The molecular associations revealed the presence of biolabile autochthonous compounds suggesting the crucial role land use and land cover play on watersheds. A positive relationship between DOC with seasonal hydrology and gradient significantly influences the DBC flux across regional streams. Intercomparison of observed terrestrial and aquatic carbon stocks with globally modelled data indicates an overestimation of regional-scale stock. These new findings have repercussions to policy framework on regional climate change. Further, the results suggest that a consistent quantification of BC and integration of regional, and global source-to-sink process are needed in order to understand and better quantify biogeochemical process cycles and associated climatic impacts.

Effects of Urbanization Intensity on the Distribution of Black Carbon in Urban Surface Soil in South China

Rapid urbanization causes the accumulation of large amounts of pollutants, including heavy metals, organic pollutants, and black carbon (BC). BC is the carbonaceous residue generated from the incomplete combustion of fossil fuels and biomass. It plays an important role on the migration of heavy metals and organic pollutants, as well as soil carbon sequestration. BC accumulation due to human activities greatly affects the global carbon budget, helps to drive climate change, and damages human health. To date, few studies have examined how the intensity of urbanization affects the distribution of BC in soils in urban areas. Therefore, the objective of this study is to determine the effects of urbanization intensity on the spatial distribution and content of BC in urban surface soil. We collected samples from 55 sites in South China and used a multi-scale geographical regression model to evaluate the impact of the interference intensity of urbanization on the amount and distribution of BC. Our results showed that the BC content was significantly higher in urban areas (9.74 ± 1.18 g kg−1) than in rural areas (2.94 ± 0.89 g kg−1) and that several urban parks with a higher interference intensity were hotspots of BC accumulation, suggesting that urbanization promoted BC accumulation. Our model revealed that road density was significantly and positively correlated with BC accumulation. Because there are more cars driving in areas with high road density, vehicle emissions may be one of the causes of BC accumulation. Our results also indicated that the impact of urbanization intensity on the BC distribution was sensitive to sampling density.

Changes in Soil Chemistry After 17 Years in Urban and Rural Forest Patches

Cycling of carbon (C), nitrogen (N), calcium (Ca), phosphorus (P), and sulfur (S) is an important ecosystem service that forest soils provide. Humans influence these biogeochemical processes through the deposition of atmospheric pollutants and site disturbances. One way to study these potential anthropogenic trajectories is through long-term monitoring in association with human-caused environmental gradients such as urban-rural gradients. The objective of this study was to characterize changes in surface soil chemistry of urban, suburban and rural forest patches in the Baltimore Metropolitan area. Soil composite samples (0–10 cm) were analyzed for macro- and micronutrients, pH, and C. A total of 12 sites in forest patches dominated by white oak (Quercus alba) and tulip poplar (Liriodendron tulipifera) were established in 2001, and resampled in 2018. We hypothesized that after almost two decades (1) concentrations of N, Ca, and P, as well as soil pH would be higher, especially in urban forest patches due to local deposition; (2) S levels would be lower due to decreased regional atmospheric deposition and; (3) total soil C would increase overall, but the rate of increase would be higher in the urban end of the gradient due to increased NPP. Overall, means of Ca concentration, pH, and C:N ratios significantly changed from 2001 to 2018. Calcium increased by 35% from 622 to 844 mg kg–1, pH increased from 4.1 to 4.5, and C:N ratios decreased from 17.8 to 16.7. Along the gradient, Ca, N, P, and S were statistically significant with Ca concentration higher in the urban sites; S and N higher in the suburban sites; and P lower in the urban sites. Confounding factors, such as different geologic parent material may have affected these results. However, despite the unique site conditions, patterns of surface soil chemistry in space and time implies that local and regional factors jointly affect soil development in these forest patches. The increase in pH and Ca is especially notable because other long-term studies demonstrated changes in the opposite direction.

Black Carbon in Surface Soil and Its Sources in Three Central Asian Countries

Black carbon (BC) aerosol impacts the air quality, public health, agricultural productivity, weather, monsoon, cryosphere, and climate system from the local to the global scale. However, its distribution over vast Central Asia is poorly known, because it is one of the poorly sampled regions of the world. BC in the soil can be resuspended into the atmosphere and transported to downwind regions with sensitive ecosystems and vulnerable populations, such as from Central Asian countries to the cryospheric regions in the Tianshan Mountain and the Tibetan Plateau, which could accelerate the melting of the snowfields and glaciers. We report the distribution of BC and total organic carbon (TOC) in surface soil with samples collected at multiple sites, for the first time, over three countries in Central Asia (Uzbekistan, Tajikistan, and Kyrgyzstan). The mean BC (TOC) concentrations over three countries were 0.06 ± 0.06 (11.86 ± 4.84) mg g^-1, 0.15 ± 0.21 (20.35 ± 10.96) mg g^-1, and 0.32 ± 0.29 (26.45 ± 20.38) mg g^-1, respectively. They were found to be originated from the same or similar sources, at least over Tajikistan and Kyrgyzstan, as indicated by their high and significant correlation (R² > 0.6, p < 0.001). The char/soot ratio indicated the diesel and gasoline combustion as dominant BC sources over this region. To gain further insights into the soil BC and its implications to air quality, climate, and cryosphere, future studies should include a wider area over Central Asia with different land-use types and other soil parameters combined with atmospheric simulations for this important yet relatively less studied region of the world.

Distribution of Black Carbon in Topsoils of the Northeastern Qinghai-Tibet Plateau Under Natural and Anthropogenic Influences

Black carbon (BC), ubiquitous in soils, plays an important role in global carbon cycles, the radiative heat balance of the Earth, pollutant fate, emissions of greenhouse gas, soil fertility, soil microbial community, and ecosystem stability. However, information on BC in topsoils of the northeastern Qinghai-Tibet Plateau is limited. Therefore, this study performed field sampling and analyzed contents of total BC and soot BC in topsoils. The results indicated that the contents of total BC in all soil samples ranged from 0.504 to 74.381 g kg^-1 with an average value of 5.152 g kg^-1, whereas those of soot BC were in the range of 0.400-15.200 g kg^-1 with a mean value of 1.719 g kg^-1. Contents of BC were significantly correlated with those of total carbon and total organic carbon. Soil types affected the distribution of soil BC. The contents of total BC in the loam soils were larger than those in the clay soils, whereas soot BC was more easily enriched in the clay soils. Total BC was negatively correlated with Ca, and soot BC was negatively correlated with Ti. The contents of soil BC in functional areas, such as agricultural and pastoral areas, industrial areas, and mining areas, were significantly higher than those in other areas, illustrating that anthropogenic activities drastically affected the distribution of soil BC. This study exhibits the fundamental information on soil BC in the northeastern Qinghai-Tibet Plateau to provide important knowledge on global soil carbon sink.

Biochar accelerates PAHs biodegradation in petroleum-polluted soil by biostimulation strategy

Sawdust and wheat straw biochars prepared at 300°C and 500°C were applied to petroleum-polluted soil for an 84-day incubation to estimate their effectiveness on polycyclic aromatic hydrocarbons (PAHs) removal. Biochars alone were most effective at reducing PAHs contents. However, adding biochar to soils in company with NaN₃ solution resulted in a decreasing trend in terms of PAHs removal, which was even lower than treatment CK without biochar. Moreover, it was discovered by PCR-DGGE files and sequencing analysis that the predominant bacterial diversity slightly decreased but the abundance of some specific taxa, including PAHs degraders, was promoted with biochar input. These results highlighted the potential of biochar application on accelerating PAHs biodegradation, which could be attributed to the properties of biochars that benefit for making the amended soil a better habitat for microbes. The impacts of biochar preparation and pollutants nature on PAHs removal were also determined. Significant reduction in the PAHs contents was detected when adding biochar prepared at a high temperature (500°C), while the feedstocks of biochar showed little effect on PAHs removal. Due to the high hydrophobicity of aromatic rings, high-molecular weight PAHs were found much more resistant to microbial degradation in comparison with low-molecular weight PAHs.

Pyrogenic carbon in Australian soils

Pyrogenic carbon (PyC), the combustion residues of fossil fuel and biomass, is a versatile soil fraction active in biogeochemical processes. In this study, the chemo-thermal oxidation method (CTO-375) was applied to investigate the content and distribution of PyC in 30 Australian agricultural, pastoral, bushland and parkland soil with various soil types. Soils were sampled incrementally to 50cm in 6 locations and at another 7 locations at 0-10cm. Results showed that PyC in Australian soils typically ranged from 0.27-5.62mg/g, with three Dermosol soils ranging within 2.58-5.62mg/g. Soil PyC contributed 2.0-11% (N=29) to the total organic carbon (TOC), with one Ferrosol as high as 26%. PyC was concentrated either in the top (0-10cm) or bottom (30-50cm) soil layers, with the highest PyC:TOC ratio in the bottom (30-50cm) soil horizon in all soils. Principal component analysis - multiple linear regression (PCA-MLR) suggested the silt-associated organic C factor accounted for 38.5% of the variation in PyC. Our findings suggest that PyC is an important fraction of the TOC (2.0-11%, N=18) and chemically recalcitrant organic C (ROC) obtained by chemical C fractionation method accounts for a significant proportion of soil TOC (47.3-84.9%, N=18). This is the first study comparing these two methods, and it indicates both CTO-375 and C speciation methods can determine a fraction of recalcitrant organic C. However, estimated chemically recalcitrant organic carbon pool (ROC) was approximately an order of magnitude greater than that of thermally stable organic carbon (PyC).

Black carbon (BC) of urban topsoil of steel industrial city (Anshan), Northeastern China: Concentration, source identification and environmental implication

Black carbon (BC) and total carbon (TC) concentrations in urban topsoils and vertical profiles from steel industrial city, Anshan, Northeastern China, were determined. A total of 115 topsoil samples and 4 soil profiles were collected, in which the BC concentrations were determined using chemical oxidation technique. The BC concentrations in urban topsoils are in the range of 1.86 to 246.46gkg(-1) with an average of 33.86gkg(-1). Both BC and TC concentrations decrease sharply with soil depth, whereas BC/TC ratio shows a little variation with depth. The spatial distribution of BC in urban topsoils reveals that the BC concentration is much higher in the northern part of the city, which is consistent with the steel production. The distribution factors (DF) of BC are the highest in 1000-500 and 500-250μm size fractions, while the lowest in 50-2μm fraction. The mass loading of BC in 250-50 and 50-2μm size fractions accounts for 76.2% of bulk soil, indicating these two size fractions responsible for BC accumulation in soils. Enrichment factor (EF) of BC in urban topsoils ranges from 0.92 to 122.01 with an average of 16.76, indicating that the urban topsoils studied are moderately or severely accumulated by the BC. Strong correlation is found between BC and pollution load index (PLI) of heavy metals, indicating the possibility of similar sources of BC and heavy metals in soils. The BC/TC ratio in soils ranges from 0.45 to 0.97, with an average of 0.75. The BC/TC ratio shows the mixed sources of BC derived from fossil fuel combustion and vehicle emissions. The BC concentration and BC/TC ratio may reflect the degree of industrial activities and pollution sources in urban soils. The study demonstrated that BC is an effective indicator of degree and "hotspots" of heavy metals pollution in urban soils.

Roadside soils show low plant available zinc and copper concentrations

Vehicle combustion and component wear are a major source of metal contamination in the environment, which could be especially concerning where road ditches are actively farmed. The objective of this study was to assess how site variables, namely age, traffic (vehicles day(-1)), and percent carbon (%C) affect metal accumulation in roadside soils. A soil chronosequence was established with sites ranging from 3 to 37 years old and bioavailable, or mobile, concentrations of Zinc (Zn) and Copper (Cu) were measured along major highways in North Carolina using a Mehlich III extraction. Mobile Zn and Cu concentrations were low overall, and when results were scaled via literature values to "total metal", the results were still generally lower than previous roadside studies. This could indicate farming on lands near roads would pose a low plant toxicity risk. Zinc and Cu were not correlated with annual average traffic count, but were positively correlated with lifetime traffic load (the product of site age and traffic count). This study shows an often overlooked variable, site age, should be included when considering roadside pollution accumulation. Zinc and Cu were more strongly associated with %C, than traffic load. Because vehicle combustion is also a carbon source, it is not obvious whether the metals and carbon are simply co-accumulating or whether the soil carbon in roadside soils may facilitate previously overlooked roles in sequestering metals on-site.

Stronger association of polycyclic aromatic hydrocarbons with soot than with char in soils and sediments

The knowledge of the association of polycyclic aromatic hydrocarbons (PAHs) with organic matter and carbonaceous materials is critical for a better understanding of their environmental transport, fate, and toxicological effects. Extensive studies have been done with regard to the relationship of PAHs with total organic carbon (TOC) and elemental carbon (EC) in different environmental matrices. The relationship between PAHs and the two subtypes of EC, char (combustion residues) and soot (produced via gas-to-particle conversion) also has been tested in field and laboratory experiments using reference materials. However, a direct comparison of associations of PAHs between with char and with soot in real environmental matrices has to our knowledge not yet been reported because of a lack of methodology to differentiate them. In this study, char and soot were measured using the IMPROVE method to test their associations with 12 EPA priority PAHs measured in topsoil samples (N=22, top 10 cm) collected from the Guanzhong Plain and in surface sediment samples (N=32, top 5 cm) from the Wei River (central China). In both soils and sediments, ∑12PAHs were more strongly associated with soot than with char, mainly due to the fact that soot and PAHs were produced in the same gas phase during combustion, had a strong affinity for each other, and were transported and deposited together, while char, the combustion residue, was transported differently to PAHs due to its large particle size. Stronger correlations between PAHs and the different carbon fractions (TOC, soot, and char) in sediments than in soils were observed, which is associated with the redistribution of PAHs among the organic matter pools in water because of the processes during soil erosion and sedimentation in the river.

Biochar: an effective amendment for remediating contaminated soil

Biochar is a carbon-rich material derived from incomplete combustion of biomass.Applying biochar as an amendment to treat contaminated soils is receiving increasing attention, and is a promising way to improve soil quality. Heavy metals are persistent and are not environmentally biodegradable. However, they can be stabilized in soil by adding biochar. Moreover, biochar is considered to be a predominant sorptive agent for organic pollutants, having a removal efficiency of about 1 order of magnitude higher than does soil/sediment organic matter or their precursor substances alone.When trying to stabilize organic and inorganic pollutants in soil, several features of biochar' s sorption capacity should be considered, viz., the nature of the pollutants to be remediated, how the biochar is prepared, and the complexity of the soil systemin which biochar may be used. In addition, a significant portion of the biochar or some of its components that are used to remediate soils do change over time through abiotic oxidation and microbial decomposition. This change process is commonly referred to as "aging:" Biochar "aging" in nature is inevitable, and aged biochar exhibits an effect that is totally different than non-aged biochar on stabilizing heavy metals and organic contaminants in soils.Studies that have been performed to date on the use of biochar to remediate contaminated soil are insufficient to allow its use for wide-scale field application.Therefore, considerable new data are necessary to expand both our understanding of how biochar performs in the field, and where it can be best used in the future for soil remediation. For example, how biochar and soil biota (microbial and faunal communities)interact in soils is still poorly understood. Moreover, studies are needed on how to best remove new species of heavy metals, and on how biochar aging affects sorption capacity are also needed.

Artifact weathering, anthropogenic microparticles and lead contamination in urban soils at former demolition sites, Detroit, Michigan

A chronological sequence of urban soils 3-92 years old was studied to determine the effects of time on morphogenesis, artifact weathering, and the geochemical partitioning of Pb. Key chronofunctions determined are an increase in ˆA horizon Development Index (defined herein based on soil color) and water-soluble Pb, and a decrease in pH and C/N, with increasing soil age. Key artifact weathering reactions are: 1) portlandite in mortar altered to calcite, 2) ferrite in wrought-iron altered to ferrihydrite and goethite, and 3) carbonaceous materials altered to water-soluble organic substances. Mortar and wrought-iron were found to be Pb-bearing, but weather to produce immobilizing agents. Hence, they are both a source and a sink for Pb. The origin and mobilization of water-soluble Pb is complex and probably includes microbial extracellular polymeric substances, biodegraded soil organic matter, and solubilized organic substances derived from carbonaceous anthropogenic microparticles (soot, char and coal-related wastes).

Testing the indicative value of magnetic susceptibility measurements for concluding on site potentials and risks provoked by fly ash deposition

The article presents results of testing the indicative value of magnetic susceptibility for fly ash deposition and its effects on forest site properties. Base saturation and concentrations of Ca and Mg were used as indicators for nutrient pools resulting from fly ash deposition. Concentrations of Fe, Al, Mn, Cd and Black Carbon were used as indicators for risks of leaching. The correlation of magnetic susceptibility with concentrations of nutrient, acidic cations, heavy metals, base saturation and Black Carbon was calculated. Additionally, we tested the suitability of magnetic susceptibility as a parameter in a linear regression based model to predict the concentrations of Ca, Mg, Fe, Al, Mn, Cd and Black Carbon. We were able to show a positive correlation between magnetic susceptibility and the selected indicators. In contrast to previous studies, we were also able to demonstrate the suitability of magnetic susceptibility to predict the size of fly ash deposition influenced nutrient pools mainly for humus layers, especially for Oa horizons. The spatial distribution of magnetic susceptibility showed also a positive correlation with regionalized base saturation. However, because of the data base and other factors impacting the measurement and modeling results, some shortcomings of using a linear regression model must be noted. From these results, we concluded that magnetic susceptibility might be a valuable parameter in a multiple regression based approach, but should not be used alone for predicting effects of fly ash deposition.