Short-term arsenic mobilization, labilization, and microbiological aspects after gasoline and diesel addition in tropical soils

The effect of the presence of gasoline and diesel on the speciation and mobility of inorganic arsenic species in tropical topsoils was investigated. Topsoil samples (n = 25) were contaminated with gasoline and diesel (500 mg kg^-1) in laboratory and were incubated under unsaturated conditions and regular aeration for 21 days. Speciation analysis and chemical fractionation were performed in the pore water from control, gasoline, and diesel-contaminated soil samples. Arsenic concentrations were compared to microbiological parameters (microbial metabolic quotient and soil basal breathing) and the presence of ArsM-harboring bacteria. The spike of gasoline and diesel to the topsoils increased pore water As³⁺ (H₃AsO₃) concentration. Arsenic mobilization was lower compared to previously reported data for other sources of organic matter (biochar, litter, and a mixture of sphagnum peat moss and composted poultry manure). However, gasoline or diesel addition mobilized As fractions that were adsorbed to the solid phase, in approximately 60% of the soils. Methylation presented an important role in the As³⁺ regulation in control soils, which was no longer observed after gasoline or diesel addition. The quantification of the labile fractions sampled by the diffusive gradients in thin films technique showed that the increased As concentration in the gasoline or diesel-contaminated soils mostly included inert species. Dissolved organic carbon content seems to be an important control mechanism of the labile As concentration. The increase in As mobility seems to pose a more concerning scenario due to As leaching than to plant uptake.

Dissolved Organic Matter (DOM) in a Warm-Temperate Forested Watershed—A Possibility of Ultraviolet Absorbance as an Indicator of DOM

We investigated changes in the quantity and quality of dissolved organic matter (DOM) from rain to stream water in a forested watershed at Yamashiro Experimental Forest (YMS) in southern Kyoto prefecture. The dissolved organic carbon (DOC) concentration and specific UV absorbance at 254 nm (SUVA254) in rainwater increased in the order of bulk rain, throughfall, stemflow, and O layer leachate because of the DOM supply from tree tissue and O layer. Decreases in DOC concentration and SUVA254 with soil depth were not observed in the soil-percolating water. This finding may have been caused by the low free oxide content of the soil and the collection of soil water with a tension-free lysimeter. The DOC concentration was very low in both seepage and stream waters; seasonal variation with a high concentration in summer was observed in the stream water. An increase in K+ concentration in summer was also observed in the stream water; thus, we presumed that DOC seasonal variation was caused by the DOM supply with the accumulated decomposition of litter in the streambed. The significant correlation between DOC concentration and absorbance at 254 nm (UV254) was observed for all sample types of observation target in the watershed; the ratio of DOC concentration to UV254 was different, while the correlation coefficient between DOC concentration and UV254 value differed among sample types in the watershed. We concluded that UV254 which can be measured by simply and easily is a good indicator for estimating DOC concentration in liquid samples in forested watersheds.

Interaction between polychlorinated biphenyls and dissolved organic matter of different molecular weights from natural and anthropic sources

Polychlorinated biphenyls (PCBs) are compounds of significant interest due to high toxicity, persistence, long-range atmospheric transport, and bioaccumulation. These compounds can interact with components present in the environment, including dissolved organic matter (DOM) in soils and waters, thereby modifying its availability and movement. In this study, DOM was fractionated by ultrafiltration and characterized according to its hydrophobicity and hydrophilicity, then the interaction of a series of PCBs and different DOM fractions was evaluated. The DOM was collected from the surface waters of three sectors located along a river in the southern part of America. These sectors are subject to different anthropic activities, thus the DOM of sector 1, with the least anthropic influence, was mainly hydrophobic and with a high content of aromatic structures. In contrast, the DOM collected from sectors 2 and 3, where anthropic activity is highest, was slightly hydrophobic and hydrophilic, respectively. The DOM of these two sectors was mainly composed of low molecular weight macromolecules. These results revealed that more hydrophobic PCBs (i.e., 101, 118, 138, and 180) have a greater affinity to DOM with a higher molecular weight (i.e., >1 kDa). In turn, PCBs with lesser chlorination and hydrophobicity presented a greater affinity to DOM with a lower molecular weight. In conclusion, our study shows that the high molecular weight DOM is responsible for mobilizing PCBs with a high degree of chlorination.

Dissolution of copper oxide nanoparticles is controlled by soil solution pH, dissolved organic matter, and particle specific surface area

Dissolution is the primary process affecting the bioavailability and toxicity of nanoscale copper oxide (nano-CuO) to plants and soil organisms. In this study, particle morphology, organic acid, and soil properties were considered to understand the dissolution characteristics of nano-CuO in soil solutions. The results showed that the copper ions (Cu²⁺) released from spherical nano-CuO (CuO NPs), tubular nano-CuO (CuO NTs), and spherical microsized CuO (CuO MPs) in the ten soil solutions were 26.6-4194.0 μg/L, 4.90-217.1 μg/L, and 10.8-326.0 μg/L, respectively. The concentration of Cu²⁺ was negatively correlated with the pH of the soil solution and positively correlated with the contents of dissolved organic carbon (DOC), aluminum, and manganese. Multivariate stepwise regression analysis indicated that the dissolution of CuO NPs could be well predicted by pH and DOC contents of the soil solutions. In the GD soil solution (acidic), 4- and 8-fold of the DOC content amendments significantly promoted the dissolution of the three sizes of CuOs, resulting in an increase of Cu²⁺ 4.55-11.3 and 5.67-16.2 times, respectively. In the CQ soil solution (neutral), 8-fold DOC amendments increase the release of Cu²⁺ 2.13-16.6 times. While in the SD soil solution (alkaline), promoting effect on the dissolution was only observed for nano-CuOs, with Cu²⁺ elevated by factors of 1.56-4.64 and 1.38-4.48. The amendments of Al³⁺ and Mn²⁺ in soil solution increased the amounts of Cu²⁺ 1.13-4.80 and 1.02-1.46 times in the GD soil solution. In comparison, no significant promoting effects were observed in CQ and SD soil solutions due to their stronger buffering capacities. These findings offer insight into the dissolution behavior of nano-CuOs in soils and be helpful to evaluate their environmental risks.

Simple Derivatization-Gas Chromatography-Mass Spectrometry for Fatty Acids Profiling in Soil Dissolved Organic Matter

Dissolved organic matter is an important component of the global carbon cycle that allows the distribution of carbon and nutrients. Therefore, analysis of soil dissolved organic matter helps us to better understand climate change impacts as it is the most dynamic and reactive fraction in terrestrial ecosystems. Its characterization at the molecular level is still challenging due to complex mixtures of hundreds of compounds at low concentration levels in percolating water. This work presents simple methods, such as thermochemolysis– or derivatization–gas chromatography, as an alternative for the analysis of fatty acids in dissolved organic matter without any purification step. The variables of the protocols were examined to optimize the processing conditions for the C₉–C₁₈ range. As a proof of concept, fatty acid distributions of soil percolating water samples from a long-term field experiment were successfully assessed. The variability of dissolved organic acid distributions was pronounced through depth profile and soil treatment but no major change in composition was observed. However, although the optimization was done from C₉ to C₁₈, detection within the C₆-C₃₂ fatty acids range was performed for all samples.

Water extractable organic matter and iron in relation to land use and seasonal changes

The soil carbon pool is an essential part of the global carbon cycle although it is sensitive to climatic changes and the local environment. Terrestrial areas are important sources of organic matter for aquatic ecosystems and the fluctuation of different soil minerals and elements is largely influenced by land use and season changes. We studied water-extractable organic matter (WEOM) properties including iron (Fe), water extractable organic carbon (WEOC) and spectral characteristics from forests and arable soils to evaluate the effects of land use and seasonal change on WEOM and Fe in terrestrial areas. We collected soil samples randomly from arable land (AR), broadleaf (BL) and aged needle leaf forests (NL) and extracted WEOM for analysis using rainwater. Results of WEOC and Fe showed similar trend seasonally and were higher in forest sample than in AR. WEOC was high in the upper layer while Fe was independent of the depth and higher in AR. On the other hand, specific ultra-violet absorbance at 254 nm (SUVA₂₅₄) and a proxy for aromaticity significantly varied with both land use and season and was on average two times higher in arable land than forests during spring and summer. Humic-like components significantly varied between the studied sites seasonally while tyrosine-like was affected by season only. The relative abundance of both humic-like and tryptophan-like components were significantly affected by land use while [Fe]: [WEOC] ratio was also high in arable land and negatively correlated with humic-like components in forest sites. As observed from ratio and Fe oxidation rate constant, summer presented ideal conditions for WEOM interactions. The synchronized seasonal WEOC and Fe changes indicate an enhancement of Fe mobility by DOM whereas the differences observed between sites especially from the high humic- and protein-like components in NL and BL reflects the effect of land use.

Variation in quantity and chemical composition of soil dissolved organic matter in a peri-urban critical zone observatory watershed in Eastern China

Soil organic matter plays a fundamental role in terrestrial ecosystems. However, little is known about the spectral characteristics and fluorescent components of soil dissolved organic matter (DOM) in peri-urban ecosystems and their influencing factors. In this study, we used the fluorescence excitation-emission matrix and parallel factor analysis (EEM-PARAFAC) to characterize soil DOM in a peri-urban Critical Zone Observatory watershed in Eastern China. Soil samples were collected at depths of 0-10 cm, 10-20 cm, and 20-40 cm from 82 sites (29 forest, 11 orchard, and 42 cropland). The results showed that soil organic carbon and water-soluble organic carbon content varied with land use type, and both were significantly higher in forest than cropland and orchard samples. Three fluorescent components, humic-, fulfic-, and protein-like substances, were derived from soil DOM using the PARAFAC model. All these components were distributed differently in the three land use types, as were their UV and fluorescence indices. The spatial distribution of soil DOM showed that the quantity and chemical composition of DOM were affected by environmental variables and human disturbance, among which pH and altitude significantly affected DOM content. Anthropogenic parameters such as distance to road and distance to town strongly influenced the chemical composition and characteristics of soil DOM. Negative correlations were observed between humic-like substances and distance to road or town, while positive correlations were observed between protein-like substances and these two parameters. These results demonstrate the pronounced influence of human activities on DOM composition and characteristics in the watershed area. The findings highlight the value of soil DOM characterization to better understand the origin, composition, and fate of DOM in soils in peri-urban critical zones.

Strength of Humic Acid Aggregates: Effects of Divalent Cations and Solution pH

Aggregation-dispersion, charging, and aggregate strength of Leonardite humic acid (LHA) were investigated in CaCl₂ and MgCl₂ solutions as a function of pH and ionic strength (I). The strength or the withstanding force of aggregates of humic substances (HSs) against breakage is important because this force influences the transport and distribution of pollutants and nutrients along with HSs through the change in the size of HS aggregates as a transport unit. We observed the dominancy of aggregation of LHA at high pH than at low pH in every case of CaCl₂ and MgCl₂ solutions. This observation suggests the higher binding efficiency of these divalent ions at high pH, though there was no obvious relation with electrophoretic mobility and aggregation of LHA. Further, we first revealed the numerical value of the strength of HS aggregates by using a simple experimental setup of aggregate breakup under laminar converging flow through a capillary tube. The obtained values of the strength of LHA aggregates were higher in the presence of CaCl₂ solution than MgCl₂ solution, and the strength increased with pH. The highest strengths of LHA aggregates in 30 mM (I) CaCl₂ and MgCl₂ solutions were around 5.8 and 2.4 nN, respectively, at pH around 9.

The Case for Digging Deeper: Soil Organic Carbon Storage, Dynamics, and Controls in Our Changing World

Most of our terrestrial carbon (C) storage occurs in soils as organic C derived from living organisms. Therefore, the fate of soil organic C (SOC) in response to changes in climate, land use, and management is of great concern. Here we provide a unified conceptual model for SOC cycling by gathering the available information on SOC sources, dissolved organic C (DOC) dynamics, and soil biogeochemical processes. The evidence suggests that belowground C inputs (from roots and microorganisms) are the dominant source of both SOC and DOC in most ecosystems. Considering our emerging understanding of SOC protection mechanisms and long-term storage, we highlight the present need to sample (often ignored) deeper soil layers. Contrary to long-held biases, deep SOC—which contains most of the global amount and is often hundreds to thousands of years old—is susceptible to decomposition on decadal timescales when the environmental conditions under which it accumulated change. Finally, we discuss the vulnerability of SOC in different soil types and ecosystems globally, as well as identify the need for methodological standardization of SOC quality and quantity analyses. Further study of SOC protection mechanisms and the deep soil biogeochemical environment will provide valuable information about controls on SOC cycling, which in turn may help prioritize C sequestration initiatives and provide key insights into climate-carbon feedbacks.

Composition of dissolved organic matter (DOM) from periodically submerged soils in the Three Gorges Reservoir areas as determined by elemental and optical analysis, infrared spectroscopy, pyrolysis-GC-MS and thermally assisted hydrolysis and methylation

Soil-derived dissolved organic matter (DOM) has a major influence in biogeochemical processes related to contaminant dynamics and greenhouse gas emissions, due to its reactivity and its bridging role between the soil and aquatic systems. Within the Three Gorges Reservoir (TGR, China) area, an extensive water-fluctuation zone periodically submerges the surrounding soils. Here we report a characterization study of soil-derived DOM across the TGR areas, using elemental and optical analysis, infrared spectroscopy (FTIR), pyrolysis-GC-MS (Py-GC-MS) and thermally assisted hydrolysis and methylation (THM-GC-MS). The results showed that the soil DOM from the TGR area is a mixture of "allochthonous" (i.e., plant-derived/terrigenous) and "autochthonous" (i.e., microbial) origins. The terrigenous DOM is composed primarily of phenolic and aliphatic structures from lignin and aliphatic biopolymers (i.e. cutin, suberin), respectively. Multivariate statistics differentiated between two fractions of the microbial DOM, i.e. chitin-derived, perhaps from fungi and arthropods in soil, and protein-derived, partially sourced from algal or aquatic organisms. Molecular proxies of source and degradation state were in good agreement with optical parameters such as SUVA₂₅₄, the fluorescence index (FI) and the humification index (HIX). The combined use of elemental analysis, fluorescence spectroscopy, and Py-GC-MS provides rigorous and detailed DOM characterization, whereas THM-GC-MS is useful for more precise but qualitative identification of the different phenolic (cinnamyl, p-hydroxyphenyl, guaiacyl, syringyl and tannin-derived) and aliphatic materials. With the multi-methodological approach used in this study, FTIR was the least informative, in part, because of the interference of inorganic matter in the soil DOM samples. The soil DOM from the TGR's water fluctuation zone exhibited considerable compositional diversity, mainly related to the balance between DOM source (microbial- or plant-derived), local vegetation and anthropogenic activities (e.g., agriculture). Finally, the relationship between DOM composition and its potential reactivity with substances of environmental concerns in the TGR area are also discussed.

Colloidal crystals and water: Perspectives on liquid-solid nanoscale phenomena in wet particulate media

Solid colloidal ensembles inherently contain water adsorbed from the ambient moisture. This water, confined in the porous network formed by the building submicron spheres, greatly affects the ensemble properties. Inversely, one can benefit from such influence on collective features to explore the water behavior in such nanoconfinements. Recently, novel approaches have been developed to investigate in-depth where and how water is placed in the nanometric pores of self-assembled colloidal crystals. Here, we summarize these advances, along with new ones, that are linked to general interfacial water phenomena like adsorption, capillary forces, and flow. Water-dependent structural properties of the colloidal crystal give clues to the interplay between nanoconfined water and solid fine particles that determines the behavior of ensembles. We elaborate on how the knowledge gained on water in colloidal crystals provides new opportunities for multidisciplinary study of interfacial and nanoconfined liquids and their essential role in the physics of utmost important systems such as particulate media.

Organic nitrogen storage in mineral soil: Implications for policy and management

Nitrogen is one of the most important ecosystem nutrients and often its availability limits net primary production as well as stabilization of soil organic matter. The long-term storage of nitrogen-containing organic matter in soils was classically attributed to chemical complexity of plant and microbial residues that retarded microbial degradation. Recent advances have revised this framework, with the understanding that persistent soil organic matter consists largely of chemically labile, microbially processed organic compounds. Chemical bonding to minerals and physical protection in aggregates are more important to long-term (i.e., centuries to millennia) preservation of these organic compounds that contain the bulk of soil nitrogen rather than molecular complexity, with the exception of nitrogen in pyrogenic organic matter. This review examines for the first time the factors and mechanisms at each stage of movement into long-term storage that influence the sequestration of organic nitrogen in the mineral soil of natural temperate ecosystems. Because the factors which govern persistence are different under this newly accepted paradigm we examine the policy and management implications that are altered, such as critical load considerations, nitrogen saturation and mitigation consequences. Finally, it emphasizes how essential it is for this important but underappreciated pool to be better quantified and incorporated into policy and management decisions, especially given the lack of evidence for many soils having a finite capacity to sequester nitrogen.