基于三维荧光光谱的土壤溶解性有机质的提取

Soil dissolved organic matter(DOM) consists of many organic compounds and plays an important role in contaminant transport in the ecosystem. However, the effects of different extraction conditions on contents of different DOM components are poorly understood.We extracted DOM from three soils using different extraction times, solid to liquid ratios(SLR), and extraction solution(KCl)concentrations, and evaluated the extractions using three-dimensional excitation-emission matrix(EEM) fluorescence spectroscopy.The sum of the contents of the main DOM components(protein-, fulvic-, and humic-like compounds) increased by 0.6–3.5, 2.5–3.9,and 0.11–0.37 times(P 0.05) when SLR decreased from 1:2 to 1:10, the extraction solution concentration increased from 0.01 to 1.5 mol L~(-1), and extraction time increased from 10 to 300 min, respectively. The three-dimensional EEM fluorescence spectroscopy is a useful tool to characterize the components of DOM and evaluate DOM extraction in soils.     

Fractionation of dissolved organic carbon from soil solution with immobilized metal ion affinity chromatography

The presence and identity of Cu‐complexing ligands in soil solution strongly affects biogeochemistry, bioavailability and the fate of Cu in soils. In this study, we compared the influence of heavy metal pollution, vegetation and soil type on the amount and characterization of ligands able to form ternary complexes with Cu in soil solution. For separation and characterization, we applied immobilized metal ion affinity chromatography (IMAC) combined with fluorescence spectroscopy. All separated IMAC‐fractions exhibited excitation‐emission wavelengths of humic‐like fluorescence (240–285/365–434 nm). Protein‐type fluorescence (270–280/295–365 nm) and fluorescence at 330–340/375–385 nm were detected only in the retained fraction whereas carboxylate‐type fluorescence (300–310/420–430 nm) was observed only in the non‐retained fraction. These findings are in agreement with the behaviour of model ligands. The IMAC‐retained ligands represented between 5 and 30% of dissolved organic carbon. The soil type and the vegetation had the largest influence on the quality and quantity of Cu ligands able to form ternary complexes. In the topsoil, the IMAC retained fraction was greater in soil without vegetation (16%) compared with soil with vegetation (12%). A larger amount (75%) of the protein‐type ligands able to form ternary complexes with Cu was found in soil with vegetation compared with plant‐free soil (69%). Metal pollution also affected the composition of the extracted ligands; the fraction with protein‐type ligands decreased from 75% in unpolluted to 65% in the polluted topsoil. The results show that IMAC‐retained ligands are related to the biological activity in soils.     

不同来源有机肥释放的溶解有机质粒径分布与光谱特征

  【目的】  研究不同来源有机肥释放的溶解有机质 (DOM) 的粒径分布与光谱特征,为有机肥在农业生产中的应用及DOM后续环境行为的研究提供理论指导。  【方法】  本研究选择海藻、羊粪、虾肽以及小麦秸秆生物炭4种有机肥,提取有机肥中的DOM (<0.7 μm)。利用超滤分级技术对提取的DOM进一步区分为 <1 kDa、1～100 kDa、100 kDa～0.2 μm和0.2～0.7 μm 4个粒级,使用总有机碳 (TOC) 分析仪测定各粒径DOM的含量并使用傅里叶变换红外光谱 (FTIR)、紫外?可见吸收光谱 (UV-Vis) 和三维荧光光谱 (3D-EEM) 进行光谱表征。  【结果】  从全量 (粒径<0.7 μm) 溶解有机碳(DOC)来看,小麦秸秆生物炭 (308 mg/kg)<虾肽 (1060 mg/kg)<海藻 (1266 mg/kg)<羊粪 (2989 mg/kg)。供试有机肥中不同粒径的DOC所占比例和含量差异明显,均以最小粒径 (<1 kDa) 所占比例最高,除海藻为47%外,其余有机肥处理皆达到50%及以上。4种不同来源有机肥DOM的紫外和荧光特征值表明,4种有机肥的荧光指数 (FI) 和自生源指数(BIX)随着DOM粒径的减小而增大,而SUVA₂₅₄、SUVA₂₆₀和腐殖化指数 (HIX)随着DOM粒径的减小而减小。虾肽DOM各粒径的类蛋白组分含量高且主要为内源DOM,自生来源有机质丰富,生物可利用性高;羊粪DOM各粒径受人类活动影响较大;而小麦秸秆生物炭的DOM大粒径(>100 kDa)组分的FI<1.4,表明其大粒径DOM主要为外源性的,自身生产和微生物活动贡献相对较低。此外,尽管海藻、羊粪和虾肽各粒径的DOM的HIX值随着粒径的减小而逐渐减小,除虾肽DOM的<1 kDa组分外,其腐殖化程度依旧较高 (HIX>10),而小麦秸秆生物炭小粒径DOM的HIX<4,表明小麦秸秆生物炭的小粒径DOM疏水组分含量高,腐殖化程度相对较低。荧光组分和红外光谱表明了4种不同来源有机肥DOM以类腐殖质物质为主,且含有大量氨基酸N—H键、O—H键和C—O键等官能团。  【结论】  依据有机肥释放的DOM的粒径分布和光谱特征,海藻、羊粪、虾肽有机肥中的DOM主要以小粒径为主,其腐殖化程度高,蛋白组分含量较低。小麦秸秆生物炭DOM的生物稳定性要高于其他有机肥,生物可利用性较低,因此,施加过量的生物炭不利于微生物对土壤DOM的降解利用;而虾肽来源有机肥的DOM类蛋白组分贡献最大,生物可利用性高,施用虾肽有机肥可能有利于微生物对土壤DOM的降解利用。     

Dissolved organic matter: Fractional composition and sorbability by the soil solid phase (Review of literature)

The behavior of dissolved organic matter (DOM) in soils under varying environmental conditions represents a poorly studied aspect of the problem of organic matter loss from soils. The equilibrium and sustainable development of ecosystems in the northern latitudes are largely determined by the balance between the formation of DOM, its accumulation in the lower soil horizons, and its input with runoff into surface waters. The residence time, retention strength in the soil, and thermodynamic and biochemical stabilities depend on the localization of DOM in the pore space and its chemical structure. Amphiphilic properties represent a valuable diagnostic parameter, which can be used to predict the behavior of DOM in the soil. Acidic components of hydrophobic and hydrophilic nature constitute the major portion of DOM in forest soils of the temperate zone. The hydrophilic fraction includes short-chain aliphatic carboxylic acids, hydrocarbons, and amino acids and is poorly sorbed by the solid phase. However, the existence of this fraction in soil solution is also limited both in space (in the finest pores) and time because of higher accessibility to microbial degradation. The hydrophilic fraction composes the major portion of labile DOM in soils. The hydrophobic fraction consists of soluble degradation products of lignin; it is enriched in structural ortho-hydroxybenzene fragments, which ensure its selective sorption and strong retention in soils. Sorption is favored by low pH values (3.5–5), the high ionic strength of solution, the heavy texture and fine porous structure of soil, the high contents of oxalate- and dithionite-soluble iron (and aluminum) compounds, and hydrological conditions characterized by slow water movement. The adsorbed DOM is chemically and biochemically recalcitrant and significantly contributes to the humus reserves in the low mineral horizons of soils.     

Experimental nitrogen deposition alters the quantity and quality of soil dissolved organic carbon in an alpine meadow on the Qinghai-Tibetan Plateau

Dissolved organic matter (DOM) plays a central role in driving biogeochemical processes in soils, but little information is available on the relation of soil DOM dynamics to microbial activity. The effects of NO₃⁻ and NH₄⁺ deposition in grasslands on the amount and composition of soil DOM also remain largely unclear. In this study, a multi-form, low-dose N addition experiment was conducted in an alpine meadow on the Qinghai–Tibetan Plateau in 2007. Three N fertilizers, NH₄Cl, (NH₄)₂SO₄ and KNO₃, were applied at four rates: 0, 10, 20 and 40 kg N ha⁻¹ yr⁻¹. Soil samples from surface (0–10 cm) and subsurface layers (10–20 cm) were collected in 2011. Excitation/emission matrix fluorescence spectroscopy (EEM) was used to assess the composition and stability of soil DOM. Community-level physiological profile (CLPP, basing on the BIOLOG Ecoplate technique) was measured to evaluate the relationship between soil DOC dynamics and microbial utilization of C resources. Nitrogen (N) dose rather than N form significantly increased soil DOC contents in surface layer by 23.5%–35.1%, whereas it significantly decreased soil DOC contents in subsurface layer by 10.4%–23.8%. Continuous five-year N addition significantly increased the labile components and decreased recalcitrant components of soil DOM in surface layer, while an opposite pattern was observed in subsurface layer; however, the humification indices (HIX) of soil DOM was unaltered by various N treatments. Furthermore, N addition changed the amount and biodegradability of soil DOM through stimulating microbial metabolic activity and preferentially utilizing organic acids. These results suggest that microbial metabolic processes dominate the dynamics of soil DOC, and increasing atmospheric N deposition could be adverse to the accumulation of soil organic carbon pool in the alpine meadow on the Qinghai-Tibetan Plateau.     

Adsorption of “real” dissolved organic matter on the clay and fine silt fractions of a forested Stagnic Gleysol

Dissolved organic matter (DOM) in soils is partially adsorbed when passing through a soil profile. In most adsorption studies, water soluble organic matter extracted by water or dilute salt solutions is used instead of real DOM gained in situ by lysimeters or ceramic suction cups. We investigated the adsorption of DOM gained in situ from three compartments (forest floor leachate and soil solution from 20 cm (Bg horizon) and 60 cm depth (2Bg horizon)) on the corresponding clay and fine silt fractions (< 6.3 μm, separated together from the bulk soil) of the horizons Ah, Bg, and 2Bg of a forested Stagnic Gleysol by batch experiments. An aliquot of each clay and fine silt fraction was treated with H₂O₂ to destroy soil organic matter. Before and after the experiments, the solutions were characterized by ultra‐violet and fluorescence spectroscopy and analyzed for sulfate, chloride, nitrate, and fluoride. The highest affinity for DOM was found for the Ah samples, and the affinity decreased in the sequence Ah > Bg > 2Bg. Dissolved organic matter in the 2Bg horizon can be regarded as slightly reactive, because adsorption was low. Desorption of DOM from the subsoil samples was reflected more realistically with a non‐linear regression approach than with initial mass isotherms. The results show that the extent of DOM adsorption especially in subsoils is controlled by the composition and by the origin of the DOM used as adsorptive rather than by the mineralogical composition of the soil or by contents of soil organic matter. We recommend to use DOM gained in situ when investigating the fate of DOM in subsoils.     

Einfluß der Bodeneigenschaften auf die Freisetzung der gelösten organischen Substanz (DOM) aus dem Oberboden

Influence of Soil Properties on the Release of Dissolved Organic Matter (DOM) from the Topsoil A percolation experiment over a period of three month with small monoliths from forest and grassland soils varying in their anthropogenic changes was carried out to identify, to weigh and to quantify important soil properties for DOM release from the topsoil. Quality of soil organic matter determines the amount of DOM released from the topsoil if the soils have a low ability to adsorb and to precipitate DOM. Favorable conditions for high DOC concentrations in the soil solution are wide C/N ratios in the soil and in the hot water soluble fraction, a high soil content of hot water soluble organic carbon and a high portion of hot water soluble organic carbon in the total organic carbon content. Anthropogenic changes of the soils are effective to DOM release via changing quality of soil organic matter. Long dry periods and large water fluxes can further increase DOM release. The effects of soil organic matter can be disguised in soils with a high DOM retention capacity (high CEC, high content of exchangeable bases, Fe_ox and Fe_d). Then adsorption and precipitation determine DOM release from the topsoil and contribute to a decrease of DOM release.     

Earthworms (Eisenia foetida, Savigny) mucus as complexing ligand for imidacloprid

Earthworms can excrete copious amounts of mucus that may affect the fraction, transport fate, and bioavailability of contaminants in soil. However, interaction of mucus with organic contaminants is still not well-known. In the present study, complexation properties of surface mucus (from the earthworm species Eisenia foetida, Savigny) with imidacloprid were investigated using fluorescence excitation emission matrix (EEM) spectroscopy. It was found that carbohydrates and proteins are major components in mucus of this species. Two fluorescent peaks belonging to protein-like substances were identified in the EEM spectrum of mucus. The protein-like fluorescence was clearly quenched by imidacloprid, indicating that the protein-like substances reacted strongly with imidacloprid. The fluorescence quenching processes was governed by a static process. The values of effective quenching constant (logK _a) for these two peaks were 11.46 and 7.96, respectively, indicating that there is a strong interaction between mucus and imidacloprid and mucus–imidacloprid complexes are formed. Higher binding constants (logK _b?=?25.6 and 14.0) than those for heavy metals binding to dissolved organic matter or organic pollutants binding to proteins confirm the strong complexation between mucus and imidacloprid. Our study implies that earthworm surface mucus may significantly affect the fraction, toxicity, and bioavailability of organic contaminants in the soil due to its high affinity for organic contaminants.     

Effect of gamma-sterilization and autoclaving on soil organic matter structure as studied by solid state NMR, UV and fluorescence spectroscopy

Sterilized soil is often used, for example in degradation studies, sorption experiments, microbiological tests and plant test systems, to distinguish between microbial processes and abiotic reactions. The most commonly used technique for sterilization is autoclaving of the soil. Another technique is irradiation with high‐level gamma radiation (γ‐radiation). One major drawback of sterilization procedures is the possible alteration of the structure of soil components, for example the organic matter. A change in the chemical structure of the soil organic matter can cause different reactions in the above‐mentioned experiments and hence interfere with the aim of clearly distinguishing between biotic and abiotic processes. Two soils (Gleyic Cambisol and Orthic Luvisol) were sterilized by two γ‐irradiation procedures (4 kGy hour^?1 for 9 hours and 1.3 kGy hour^?1 for 27 hours) and repeated autoclaving at 121°C. Gentle physical aggregate fractionation of the sterilized soils revealed a decrease in the aggregation of the soil, which was reflected in an increase of the clay fraction. Subsequent analysis of the aqueous phase revealed much more dissolved organic matter (DOM) in the γ‐sterilized and autoclaved soils than in the untreated soils. Ultraviolet (UV) and fluorescence spectra of the DOM showed a decrease in the aromaticity and polycondensation of the dissolved organic carbon (DOC). ¹³C cross‐polarization/magic‐angle spinning nuclear magnetic resonance (¹³C‐CP/MAS NMR) spectra of the unfractionated soils and their respective soil fractions before and after sterilization showed that the most important change occurred in the carbohydrate and N‐alkyl region, the main components of microorganisms. In general, the impact of the sterilization method was stronger for autoclaving. The γ‐sterilized soils and fractions displayed both fewer and smaller changes in the soil organic matter.     

Contribution of dissolved organic matter to carbon storage in forest mineral soils

Dissolved organic matter (DOM) is often considered the most labile portion of organic matter in soil and to be negligible with respect to the accumulation of soil C. In this short review, we present recent evidence that this view is invalid. The stability of DOM from forest floor horizons, peats, and topsoils against microbial degradation increases with advanced decomposition of the parent organic matter (OM). Aromatic compounds, deriving from lignin, likely are the most stable components of DOM while plant‐derived carbohydrates seem easily degradable. Carbohydrates and N‐rich compounds of microbial origin produced during the degradation of DOM can be relatively stable. Such components contribute much to DOM in the mineral subsoil. Sorption of DOM to soil minerals and (co‐)precipitation with Al (and probably also with Fe), especially of the inherently stable aromatic moieties, result in distinct stabilization. In laboratory incubation experiments, the mean residence time of DOM from the Oa horizon of a Haplic Podzol increased from <30 y in solution to >90 y after sorption to a subsoil. We combined DOM fluxes and mineralization rate constants for DOM sorbed to minerals and a subsoil horizon, and (co‐)precipitated with Al to estimate the potential contribution of DOM to total C in the mineral soil of a Haplic Podzol in Germany. The contribution of roots to DOM was not considered because of lack of data. The DOM‐derived soil C ranges from 20 to 55 Mg ha^–1 in the mineral soil, which represents 19%–50% of the total soil C. The variation of the estimate reflects the variation in mineralization rate constants obtained for sorbed and (co‐)precipitated DOM. Nevertheless, the estimates indicate that DOM contributes significantly to the accumulation of stable OM in soil. A more precise estimation of DOM‐derived C in soils requires mineralization rate constants for DOM sorbed to all relevant minerals or (co‐)precipitated with Fe. Additionally, we need information on the contribution of sorption to distinct minerals as well as of (co‐)precipitation with Al and Fe to DOM retention.     

Dissolved organic carbon and nitrogen in precipitation,throughfall, soil solution,and stream water of the tropical highlands in northern Thailand

Dissolved organic matter (DOM) is important for the cycling and transport of carbon (C) and nitrogen (N) in soil. In temperate forest soils, dissolved organic N (DON) partly escapes mineralization and is mobile, promoting loss of N via leaching. Little information is available comparing DOC and DON dynamics under tropical conditions. Here, mineralization is more rapid, and the demand of the vegetation for nutrients is larger, thus, leaching of DON could be small. We studied concentrations of DOC and DON during the rainy seasons 1998–2001 in precipitation, canopy throughfall, pore water in the mineral soil at 5, 15, 30, and 80 cm depth, and stream water under different land‐use systems representative of the highlands of northern Thailand. In addition, we determined the distribution of organic C (OC) and N (ON) between two operationally defined fractions of DOM. Samples were collected in small water catchments including a cultivated cabbage field, a pine plantation, a secondary forest, and a primary forest. The mean concentrations of DOC and DON in bulk precipitation were 1.7 ± 0.2 and 0.2 ± 0.1 mg L^–1, respectively, dominated by the hydrophilic fraction. The throughfall of the three forest sites became enriched up to three times in DOC in the hydrophobic fraction, but not in DON. Maximum concentrations of DOC and DON (7.9–13.9 mg C L^–1 and 0.9–1.2 mg N L^–1, respectively) were found in samples from lysimeters at 5 cm soil depth. Hydrophobic OC and hydrophilic ON compounds were released from the O layer and the upper mineral soil. Concentrations of OC and ON in mineral‐soil solutions under the cabbage cultivation were elevated when compared with those under the forests. Similar to most temperate soils, the concentrations in the soil solution decreased with soil depth. The reduction of OC with depth was mainly due to the decrease of hydrophobic compounds. The changes in OC indicated the release of hydrophobic compounds poor in N in the forest canopy and the organic layers. These substances were removed from solution during passage through the mineral soil. In contrast, organic N related more to labile microbial‐derived hydrophilic compounds. At least at the cabbage‐cultivation site, mineralization seemed to contribute largely to the decrease of DOC and DON with depth, possibly because of increased microbial activity stimulated by the inorganic‐N fertilization. Similar concentrations and compositions of OC and ON in subsoils and streams draining the forested catchments suggest soil control on stream DOM. The contribution of DON to total dissolved N in those streams ranged between 50% and 73%, underscoring the importance of DOM for the leaching of nutrients from forested areas. In summary, OC and ON showed differences in their dynamics in forest as well as in agricultural ecosystems. This was mainly due to the differing distribution of OC and ON between the more immobile hydrophobic and the more easily degradable hydrophilic fraction.     

A comparison of extraction procedures for water‐extractable organic matter in soils

The characteristics of dissolved organic matter (DOM) in soils are often determined through laboratory experiments. Many different protocols can be used to extract organic matter from soil. In this study, we used five air‐dried soils to compare three extraction methods for water‐extractable organic matter (WEOM) as follows: (i) pressurised hot‐water‐extractable organic carbon (PH‐WEOC), a percolation at high pressure and temperature; (ii) water‐extractable organic carbon (WEOC), a 1‐hour end‐over shaking; and (iii) leaching‐extractable organic carbon (LEOC), a leaching of soil columns at ambient conditions. We quantified the extraction yield of organic carbon; the quality of WEOM was characterized by UV absorbance, potential biodegradability (48‐day incubation) and parallel factor analysis (PARAFAC) modelling of fluorescence excitation emission matrices (FEEMs). Biodegradation of dissolved organic carbon (DOC) was described by two pools of organic C. The proportions of labile and stable DOC pools differed only slightly between the WEOC and LEOC methods, while PH‐WEOC contains more stable DOC. The mineralization rate constants of both labile and stable DOC pools were similar for the three methods. The FEEMs were decomposed into three components: two humic‐like fluorophores and a tryptophan‐like fluorophore. The effect of extraction method was poorly discriminant and the most similar procedures were PH‐WEOC and LEOC while WEOC extracts were depleted in humic‐like fluorophores. This study demonstrates that WEOM quality is primarily determined by soil characteristics and that the extraction method has a smaller, but still significant, impact on WEOM quality. Furthermore, we observed considerable interaction between extraction procedure and soil type, showing that method‐induced differences in WEOM quality vary with soil characteristics.     

Dynamics and origin of the non-hydrolysable organic fraction in a forest and a cultivated temperate soil, as determined by isotopic and microscopic studies

We isolated the non‐hydrolysable macromolecular organic fraction (insoluble fraction resistant to drastic laboratory hydrolyses) from a temperate, loamy, forest soil (Lacadée, France) and from the soil of an adjacent plot cleared 35 years ago and continuously cropped with maize. The quantitative, morphological (light, scanning and transmission electron microscopy) and isotopic (bulk δ¹³C, individual compound δ¹³C and radiocarbon dating) features of these two non‐hydrolysable fractions were determined and compared. It appeared that: (i) extensive degradation of the non‐hydrolysable material inherited from the forest soil occurred upon cropping, revealing that its resistance to drastic laboratory hydrolyses is not paralleled by a great resistance to natural biodegradation triggered by change in land use; (ii) only small inputs of maize‐derived compounds occurred in the non‐hydrolysable fraction of the cultivated soil so that, in spite of extensive degradation, the forest‐inherited carbon still predominates; (iii) the non‐hydrolysable fractions of both soils comprise the same components (wood debris, spores, pollen, and, predominantly, granular organic aggregates), which correlate with the previously identified chemical components (charcoal, aliphatic lipid components and melanoidin‐like components); (iv) the non‐hydrolysable fraction of the cropped soil shows a greater contribution of aliphatic moieties, reflecting differential degradation of the components of the non‐hydrolysable material inherited from the forest soil; (v) this degradation resulted in enrichment in the oldest components; and (vi) no relationship is observed, in the two Lacadée soils, between resistance to drastic laboratory hydrolyses, on the one hand, and stability towards biodegradation in situ, on the other. These observations, added to recent ones on other types of soils, suggest that such a conspicuous uncoupling between non‐hydrolysable and stable carbon is probably a general feature of organic matter in soil as opposed to sedimentary organic matter.     

Influence of origin and properties of dissolved organic matter on the partition of polycyclic aromatic hydrocarbons (PAHs)

We investigated dissolved organic matter (DOM) from soil, sewage sludges, water from waste disposal sites, and composts as sorbents and potential carriers for hydrophobic polycyclic aromatic hydrocarbons (PAHs) in soil. Partition coefficients (expressed log K_DOC) for two 5-ring compounds were 4·8–4·9 for DOM from soil, 4·5–47 from composts, and 4·3–4·4 from sewage sludges. The DOM from compost and sewage sludge can influence the transport of non-ionic organic contaminants because of the large concentrations of dissolved organic carbon (DOC) released from these materials. Leachates from waste disposal sites did not sorb PAHs. The DOM from compost contained a large percentage of organic molecules > 14 000 Da (32–46%), whereas DOM from waste disposal leachates contained only 7-lo%, and so bound less PAHs. The percentage of total hydrophobic components, as characterized by XAD-8 chromatography, was 50 ± 9% for most of the DOM solutions and did not express the differences in affinity of the organic sorbents to PAHs in the same way as the K_DOC values. Isolated molecular-weight fractions of DOM from composts sorbed benzo(k)fluoranthene in each fraction. The log K_DOC values were 4·1–4·3 for both fractions, < 1000 and 1000–14 000 Da, and 4·8–5·0 for the fraction > 14 000 Da. The interaction of PAHs with DOM < 1000 Da cannot be explained by partitioning within intramolecular nonpolar environments of dissolved macromolecules; rather it seems to be due to the amphoteric properties of DOM. This type of interaction of PAHs with small DOM molecules might affect the mobility of hydrophobic organic chemicals in soils.     

SYNOPSIS Polycyclic Aromatic Hydrocarbons (PAHs) in Soil — a Review

PAHs are mainly produced by combustion processes and consist of a number of toxic compounds. While the concentrations of individual PAHs in soil produced by natural processes (e.g., vegetation fires, volcanic exhalations) are estimated to be around 1—10 μg kg⁻¹, recently measured lowest concentrations are frequently 10 times higher. Organic horizons of forest soils and urban soils may even reach individual PAH concentrations of several 100 μg kg⁻¹. The PAH mixture in temperate soils is often dominated by benzofluoranthenes, chrysene, and fluoranthene. The few existing studies on tropical soils indicate that the PAH concentrations are relatively lower than in temperate soils for most compounds except for naphthalene, phenanthrene, and perylene suggesting the presence of unidentified PAH sources. PAHs accumulate in C‐rich topsoils, in the stemfoot area, at aggregate surfaces, and in the fine‐textured particle fractions, particularly the silt fraction. PAHs are mainly associated with soil organic matter (SOM) and soot‐like C. Although the water‐solubility of PAHs is low, they are encountered in the subsoil suggesting that they are transported in association with dissolved organic matter (DOM). The uptake of PAHs by plants is small. Most PAHs detected in plant tissue are from atmospheric deposition. However, earthworms bioaccumulate considerable amounts of PAHs in short periods. The reviewed work illustrates that there is a paucity of data on the global distribution of PAHs, particularly with respect to tropical and southern hemispheric regions. Reliable methods to characterize bioavailable PAH pools in soil still need to be developed.     

Characterization of fresh and decomposed dissolved organic matter using excitation-emission matrix fluorescence spectroscopy and multiway analysis

Fresh and decomposed dissolved organic matter (DOM) derived from 13 plant biomass and animal manure sources was characterized using multidimensional fluorescence spectroscopy with parallel factor analysis (PARAFAC), high-performance size-exclusion chromatography, and UV-vis spectroscopy. The PARAFAC analysis modeled seven fluorescence components: tryptophan-like, tyrosine-like, and five humic substance-like components. For most of the plant-derived DOM solutions, decomposition significantly affected the concentration of three humic substance-like-associated components, increasing two and decreasing one. The effect of decomposition upon DOM derived from animal manures was dependent on the manure source. For a majority of the DOM extracts, the ratio of fluorescence intensity to absorptivity at 254 nm increased following decomposition, indicating that fluorescing DOM compounds were generally more resistant to biodegradation than nonfluorescing UV-absorbing compounds. Molar absorptivity, humification index (HIX), and apparent molecular weight (MWAP) increased by 38.0, 38.8, and 370%, respectively, following decomposition. Spearman correlation analysis showed a strong positive relationship between the humic substance-like components and the DOM MWAP, absorptivity, and HIX. The results of this study support the use of multidimensional fluorescence spectroscopy with PARAFAC as a method to monitor the decomposition of carbon-rich soil amendments such as crop residues, green manures, and animal manures.     

Soil structural indices' dependence on irrigation water quality and their association with chromophoric components in dissolved organic matter

Irrigation with treated wastewater (TWW) may affect soil structure and stability and the characteristics of dissolved organic matter (DOM) of the soil solution. The objectives of our study were (i) to evaluate the impact of TWW irrigation, as compared with fresh water (FW) irrigation, on aggregate stability and saturated hydraulic conductivity (indices of soil structure stability) and (ii) to determine whether these indices can be associated with the chromophoric indicators of water‐extractable DOM in TWW‐ and FW‐irrigated soils. We studied aggregate stability and soil hydraulic conductivity (HC) of four different soil types irrigated with either TWW (for at least 5 years) or FW. The results were linked to earlier published data on the concentration scores of fluorescent chromophoric DOM components (obtained from excitation‐emission matrices of flouorescence coupled with parallel factor analysis), dissolved organic carbon (DOC) concentration and absorbance at 254 nm (Abs254). These were all obtained from water extracts of the same soils as those used in the current study. Irrigation with TWW decreased aggregate stability, in comparison to irrigation with FW, in the sandy clay and clay soils, while in the loamy sand TWW increased aggregate stability. The apparent steady state HCs in the TWW‐irrigated samples in the loamy sand, sandy clay and clay soils were similar to, or significantly less than, those obtained in the FW‐irrigated samples. In the sandy loam the opposite trend was noted. Results of principal component and classification analyses showed that the aggregate stability indices were directly associated with soil organic matter and DOM attributes in the coarse‐textured soils, while in the fine‐textured soils inverse associations were noted. Only in the fine‐textured soils were the HC attributes associated (directly) with some of the DOM characteristics. Our results suggest that structural indices of fine‐textured soils are more sensitive than those of coarse‐textured soils to the composition of water extractable DOM.     

Characterization of zinc in contaminated soils: complementary insights from isotopic exchange,batch extractions and XAFS spectroscopy

Isotopic exchange (IE) of trace metals is an established method for characterizing metal reactivity in soils, but it is still unclear which metal species are isotopically exchangeable. In this study, we used IE to quantify ‘labile’ zinc (Zn) in 51 contaminated soils that were previously studied by Zn K‐edge X‐ray absorption fine structure (XAFS) spectroscopy and sequential extraction (SE). All soils had been contaminated by runoff water from 17‐ to 74‐year‐old galvanized power‐line towers. They covered a wide range in pH (4.0–7.7), organic carbon (0.9–10.2%), clay (3.8–45.1%) and Zn concentrations (251–30 090 mg kg^?1). Isotopic exchange was also performed on selected Zn minerals used as references for linear combination fitting of XAFS spectra. The isotopically exchangeable fraction (%E) of Zn generally decreased with increasing pH, but small %E values were also observed for acidic soils with a large fraction of Zn in hydroxy‐interlayered minerals (Zn‐HIM). The fraction of Zn identified by XAFS spectroscopy as (tetrahedrally and octahedrally coordinated) ‘sorbed Zn’ agreed reasonably well with the isotopically exchangeable fraction but was in many cases larger than the %E, indicating that some ‘sorbed Zn’ may be isotopically non‐exchangeable, such as Zn sorbed in micropores of Fe oxyhydroxides. Zinc identified by XAFS spectroscopy as Zn precipitates (Zn phyllosilicates, Zn‐layered double hydroxide (Zn‐LDH) or hydrozincite) or as Zn‐HIM was largely isotopically non‐exchangeable (‘non‐labile’). Comparison between IE and extraction results suggested that the isotopically exchangeable Zn was mainly extracted in the first two fractions of the SE. However, non‐labile Zn was also extracted in these first two fractions for some soils, including a hydrozincite‐containing soil. Despite the presence of Zn‐LDH and/or Zn phyllosilicates in almost all soils, the Zn concentrations in solution and labile Zn increased with increasing soil total Zn at a given pH, which contradicts the concept of precipitation control by a single phase. Solution Zn was well predicted from the labile Zn following a sorption model.     

Transformation of organic matter from maize residues into labile and humic fractions of three European soils as revealed by 13C distribution and CPMAS-NMR spectra

The dynamics of incorporation of fresh organic residues into the various fractions of soil organic matter have yet to be clarified in terms of chemical structures and mechanisms involved. We studied by ¹³C‐dilution analysis and CPMAS‐¹³C‐NMR spectroscopy the distribution of organic carbon from mixed or mulched maize residues into specific defined fractions such as carbohydrates and humic fractions isolated by selective extractants in a year‐long incubation of three European soils. The contents of carbohydrates in soil particle size fractions and relative δ¹³C values showed no retention of carbohydrates from maize but rather decomposition of those from native organic matter in the soil. By contrast, CPMAS‐¹³C‐NMR spectra of humic (HA) and fulvic acids (FA) extracted by alkaline solution generally indicated the transfer of maize C (mostly carbohydrates and peptides) into humic materials, whereas spectra of organic matter extracted with an acetone solution (HE) indicated solubilization of an aliphatic‐rich, hydrophobic fraction that seemed not to contain any C from maize. The abundance of ¹³C showed that all humic fractions behaved as a sink for C from maize residues but the FA fraction was related to the turnover of fresh organic matter more than the HA. Removal of hydrophobic components from incubated soils by acetone solution allowed a subsequent extraction of HA and, especially, FA still containing much C from maize. The combination of isotopic measurements and NMR spectra indicated that while hydrophilic compounds from maize were retained in HA and FA, hydrophobic components in the HE fraction had chemical features similar to those of humin. Our results show that the organic compounds released in soils by mineralization of fresh plant residues are stored mainly in the hydrophilic fraction of humic substances which are, in turn, stabilized against microbial degradation by the most hydrophobic humic matter. Our findings suggest that native soil humic substances contribute to the accumulation of new organic matter in soils.