Kerogen in aquifer material and its strong sorption for nonionic organic pollutants

Sorption of organic pollutants by subsurface materials has been found to not only correlate with the total organic carbon (TOC) content, but also depend on the types of soil and sediment organic matter (SOM). Characterization of geochemically heterogeneous SOM is key to elucidating sorption mechanisms and predicting pollutant transport in ground water systems. In this study, kerogen, a nonextractable organic material, was isolated with an acid demineralization procedure from a sandy aquifer material (Borden, Ontario, Canada) having a TOC content of approximately 0.021% (w/w). Petrographical examinations reveal that the kerogen has three major types of macerals including bituminite (Kerogen Type I and II), vitrinite (Type III), and fusinite (Type IV or charred kerogen). The solid-state 13C nuclear magnetic resonance (NMR) spectrum shows two dominant peaks, aliphatic and aromatic carbons, for the isolated material. Sorption isotherms measured using phenanthrene, naphthalene, 1,3,5-trichlorobenzene (TCB), and 1,2-dichlorobenzene (DCB) as sorbates showed that both the isolated kerogen and the original sand exhibited nonlinear sorption and that the phenanthrene and TCB isotherms measured for the kerogen material are more nonlinear than the respective isotherms for the original sand. The single-point organic carbon--normalized sorption capacity measured for the isolated kerogen can be several times greater than that measured for the original sand for a given sorbate. The study suggests that kerogen plays a major role in overall sorption isotherm nonlinearity and could yield higher-than-predicted sorption capacities for the subsurface material even though the content of this organic material is very low.     

A thermodynamically based method to quantify true sorption hysteresis

Sorption of organic chemicals to soils and sediments often shows true hysteresis (i.e., nonsingularity of the sorption-desorption isotherm not attributable to known experimental artifacts). Since true sorption hysteresis is fundamentally important to contaminant fate, a way to quantify it is desirable. Previously proposed indices of hysteresis are empirical and usually depend on the isotherm model. True sorption hysteresis to synthetic and natural organic solids has been attributed to irreversible alteration of the solid during the sorption-desorption cycle. Given this mechanism, we propose the Thermodynamic Index of Irreversibility (TII) for quantifying hysteresis in soils where natural organic matter dominates the sorption process. The TII is based on the difference in free energy between the real desorption state and the hypothetical fully reversible state. The index is 0 for completely reversible systems and approaches 1 as the process tends toward complete irreversibility. It does not require any assumptions about the physical properties or molecular composition of the solid, and it does not depend on a specific equilibrium model. A sensitivity analysis of measurement errors provides general recommendations for the setup of sorption-desorption experiments. The TII was applied to sorption of 1,4-dichlorobenzene (DCB) to two high-organic soils, Pahokee peat (PP) and Amherst soil (AS), and a low-rank coal reference material, Beulah-Zap lignite (BZL). Common artificial causes of hysteresis were eliminated. Hysteresis was significant in the peat and the coal. The TII was clearly concentration dependent for both solids; it decreased with concentration for the peat, but increased with concentration for the coal. The TII allows quantification of hysteresis as a function of sorbate-sorbent combination, concentration, time, and other variables.     

Nonlinear and competitive sorption of apolar compounds in black carbon-free natural organic materials

Numerous studies have reported a spectrum of sorption phenomena in soils, sediments, and organic matter isolates of those materials that are inconsistent with a partition model proposed in the late 1970s and early 1980s, a model predicated on a hypothesis that sorption is linear and noncompetitive. To explain these nonideal phenomena, prior studies have proposed a hard-soft (glassy-rubbery) model for SOM (soil and sediment organic matter), while others have attributed them singularly to BC (black carbon: soot and charcoal) particles present in topsoils and sediments. In this study, we demonstrated nonideal sorption behavior (isotherm nonlinearity, competitive effects) for a group of apolar compounds in a large set of natural and model organic materials, including a commercial lignin and humic acids from different sources. Complete oxidation of samples by an acidic dichromate method was taken to signify the absence of BC. (However, polymethylene units are stable even if functionalized on both ends, making the technique unreliable for quantifying BC.) Other samples were inferred free of BC by their source and method of preparation. Characterization by thermalanalytical methods indicated the glassy character of the organic materials. The origin of the nonideal behaviors appears to be the glassy character of these materials. Sorption nonlinearity increased or decreased by changing temperature, cosolvent content, or degree of cross-linking by metal ions as predicted for organic solids in a glassy state. We conclude that macromolecular humic substances in the environment may exhibit nonideal sorption behavior in soils and sediments, quite apart from any such behaviors attributable to BC.     

Sorption and desorption of naphthalene by soil organic matter: importance of aromatic and aliphatic components

Nonlinear isotherm behavior has been reported for the sorption of hydrophobic organic compounds (HOCs) in soil organic matter (SOM), but the exact mechanisms are unknown. Our objective was to provide insight into the sorption mechanism of HOCs in SOM by studying the sorption-desorption processes of naphthalene in a mineral soil, its humic fractions, and lignin. Additionally, humin and lignin were used for studying the effects of temperature and cosolvent on HOC sorption. All isotherms were nonlinear. The humin and lignin isotherms became more linear at elevated temperatures and with the addition of methanol indicating a condensed to expanded structural phase transition. Isotherm nonlinearity and hysteresis increased in the following order: soil humic acid (HA) < soil < soil humin. Of the samples, aliphatic-rich humin exhibited the largest degree of nonlinearity and had the highest sorption capacity for naphthalene. High nonlinearity and hysteresis in humin were most likely caused by its condensed structure. A novel aliphatic, amorphous condensed conformation is proposed. This conformation can account for both high sorption capacities and increased nonlinearity observed for aliphatic-rich samples and can explain many sorption disparities discussed in the literature. This study clearly illustrates the importance of both aliphatic and aromatic moieties for HOC sorption in SOM.     

Effect of heat treatment on sorption of polar and nonpolar compounds to montmorillonites and soils

Batch sorption isotherms of 1,3,5-trichlorobenzene, 1,3,5-trinitrobenzene, and tetracycline to organic-free montmorillonites and soils receiving heat treatment (375°C for 24 h) were compared with those to unheated sorbents. Sorption of the nonpolar 1,3,5-trichlorobenzene to soil was lowered after the removal of humus by heating, consistent with the mechanism of hydrophobic partition into organic matter. For 1,3,5-trinitrobenzene, the enhanced sorption to heated soils was attributed to specific interactions with exchangeable cations facilitated by heating-induced irreversible partial dehydration of the clay interlayer. For tetracycline, an additional mechanism for sorption enhancement could be due to increased exposure of strong complexation sites on clay minerals after removal of the humic coating. These hypotheses were supported by the sorption data to heated and unheated Na-, K-, and Cs-saturated montmorillonites. The combustion method is commonly adopted to measure the content of black carbon in soils and sediments. However, findings from the present study indicate that combustion may greatly modify the structural properties of clay minerals, leading to misinterpreted sorption contributions of different soil components to sorption of polar or ionic compounds.     

Phenanthrene sorption to structurally modified humic acids

Several studies emphasize the importance of soil organic matter characteristics in hydrophobic contaminant sorption and outline the strong dependence of sorption on organic matter aromaticity. In this study, the role of organic matter aromaticity in phenanthrene sorption was investigated using humic acids (HAs) from compost, peat, and soil that were structurally modified by bleaching, hydrolysis, oximation, and subcritical water extraction. The HAs were characterized with cross polarization magic angle spinning carbon-13 nuclear magnetic resonance (CPMAS 13C NMR) spectroscopy and used in batch equilibrations with phenanthrene. Bleaching substantially reduced the aromaticity of the samples whereas the other treatments increased the relative aromaticity. Phenanthrene sorption increased, even though there was a substantial reduction in sorbent aromaticity with some samples. The HAs that exhibited comparable CPMAS 13C NMR spectra and aromaticity did not behave similarly with respect to phenanthrene sorption. When the sorption data (K(oc) values) were correlated to sample aromaticity, the correlation coefficients (r2) did not exceed 0.39. Comparisons with the atomic H to C ratio provided slightly better r2 values (up to 0.54). This study demonstrates that macroscopic sorbent characteristics could not explain the observed phenanthrene sorption coefficients, aliphatic structural components of HAs can contribute appreciably to phenanthrene sorption, and organic matter physical conformation may regulate access to organic matter structures. Therefore, the use of only macroscopic sorbent properties, such as aromaticity, to predict and rationalize sorption values cannot solely be used to explain the behavior of organic contaminants in soil environments.     

Sorption and fractionation of dissolved organic matter and associated phosphorus in agricultural soil

Mobility of dissolved organic matter (DOM) strongly affects the export of nitrogen (N) and phosphorus (P) from soils to surface waters. To study the sorption and mobility of dissolved organic C and P (DOC, DOP) in soil, the pH-dependent sorption of DOM to samples from Ap, EB, and Bt horizons from a Danish agricultural Humic Hapludult was investigated and a kinetic model applicable in field-scale models tested. Sorption experiments of 1 to 72 h duration were conducted at two pH levels (pH 5.0 and 7.0) and six initial DOC concentrations (0-4.7 mmol L(-1)). Most sorption/desorption occurred during the first few hours. Dissolved organic carbon and DOP sorption decreased strongly with increased pH and desorption dominated at pH 7, especially for DOC. Due to fractionation during DOM sorption/desorption at DOC concentrations up to 2 mmol L(-1), the solution fraction of DOM was enriched in P indicating preferred leaching of DOP. The kinetics of sorption was expressed as a function of how far the solution DOC or DOP concentrations deviate from "equilibrium." The model was able to simulate the kinetics of DOC and DOP sorption/desorption at all concentrations investigated and at both pH levels making it useful for incorporation in field-scale models for quantifying DOC and DOP dynamics.     

Sorptive and desorptive fractionation of dissolved organic matter by mineral soil matrices

Interactions of dissolved organic matter (DOM) with soil minerals, such as metal oxides and clays, involve various sorption mechanisms and may lead to sorptive fractionation of certain organic moieties. While sorption of DOM to soil minerals typically involves a degree of irreversibility, it is unclear which structural components of DOM correspond to the irreversibly bound fraction and which factors may be considered determinants. To assist in elucidating that, the current study aimed at investigating fractionation of DOM during sorption and desorption processes in soil. Batch DOM sorption and desorption experiments were conducted with organic matter poor, alkaline soils. Fourier-transform infrared (FTIR) and UV-Vis spectroscopy were used to analyze bulk DOM, sorbed DOM, and desorbed DOM fractions. Sorptive fractionation resulted mainly from the preferential uptake of aromatic, carboxylic, and phenolic moieties of DOM. Soil metal-oxide content positively affected DOM sorption and binding of some specific carboxylate and phenolate functional groups. Desorptive fractionation of DOM was expressed by the irreversible-binding nature of some carboxylic moieties, whereas other bound carboxylic moieties were readily desorbed. Inner-sphere, as opposed to outer-sphere, ligand-exchange complexation mechanisms may be responsible for these irreversible, as opposed to reversible, interactions, respectively. The interaction of aliphatic DOM constituents with soil, presumably through weak van der Waals forces, was minor and increased with increasing proportion of clay minerals in the soil. Revealing the nature of DOM-fractionation processes is of great importance to understanding carbon stabilization mechanisms in soils, as well as the overall fate of contaminants that might be associated with DOM.     

Effects of dissolved organic carbon on sorption and mobility of imidacloprid in soil

To evaluate the effects of dissolved organic carbon on sorption and mobility of the insecticide imidacloprid [1-(6-chloro-3-pyridinyl) methyl-N-nitro-2-imidazolidinimine] in soils, adsorption and column experiments were performed by using a typical calcareous soil from southeastern Spain and two different types of dissolved organic carbon, that is, dissolved organic carbon extracts from a commercial peat (DOC-PE) and high-purity tannic acid (DOC-TA). The experiments were carried out from a 0.01 M CaCl2 aqueous medium at 25 degrees C. The results obtained from the sorption experiments show that the presence of both DOC-PE and DOC-TA, over a concentration range of 15 to 100 mg L(-1), produces in all cases a decreasing amount of imidacloprid adsorbed in the soil studied. From the column experiments the retardation coefficients (RC) were calculated for imidacloprid by using either 0.01 M CaCl2 aqueous solution (RC = 2.10), 0.01 M CaCl2 DOC-PE solution (RC = 1.65), or 0.01 M CaCl2 DOC-TA solution (RC = 1.87). The results indicate that mobility of imidacloprid is increased 21.4 and 11.0% in the presence of DOC-PE and DOC-TA solutions, respectively. Dissolved organic carbon reduces imidacloprid sorption by competing with the pesticide molecules for sorption sites on the soil surface, allowing enhanced leaching of imidacloprid and potentially increasing ground water contamination.     

The influence of lipids on the energetics of uptake of polycyclic aromatic hydrocarbons by natural organic matter

Although most of the organic carbon in soils and sediments may be composed of humic substances, their interaction with other compounds, especially their sorption interactions, may be significantly affected by the presence of small amounts of the other components of natural organic matter (NOM). In this investigation, the influence of the lipid fraction of NOM on the sorption thermodynamics of fluorene, phenanthrene, and pyrene to several geosorbent samples was examined before and after extraction of lipids. Batch experiments were performed at the same concentration for all polycyclic aromatic hydrocarbons (PAHs) (0.025 x their solubility in water) at different temperatures (10, 20, 30, and 40 degrees C), and the thermodynamic parameters were calculated. Removal of the lipids increases the sorption capacity of the samples as well as the exothermicity of the process. The free energy change was negative for all the samples and no significant differences were noticed on lipid removal. The entropy changes were small and positive for the whole geosorbent samples, but even smaller or more negative when the lipids were removed. This indicates that the interaction of PAHs with soils and sediments in the absence of extractable lipids is stronger and the mechanisms involved may be different, changing from a partitioning-like mechanism to specific adsorption. Because of the competition between lipids and PAHs for the same sorption sites, the lipids can be viewed as an "implicit sorbate."     

Interactions of carbamazepine in soil: effects of dissolved organic matter

Pharmaceutical compounds (PCs) and dissolved organic matter (DOM) are co-introduced into soils by irrigation with reclaimed wastewater. We targeted carbamazepine (CBZ) as a model compound to study the tertiary interactions between relatively polar PCs, DOM, and soil. Sorption-desorption behavior of CBZ was studied with bulk clay soil and the corresponding clay size fraction in the following systems: (i) without DOM, (ii) co-introduced with DOM, and (iii) pre-adsorption of DOM before CBZ introduction. Sorption of the DOM to both sorbents was irreversible and exhibited pronounced sorption-desorption hysteresis. Carbamazepine exhibited higher sorption affinity and nonlinearity, and a higher degree of desorption hysteresis with the bulk soil than the corresponding clay size fraction. This was probably due to specific interactions with polar soil organic matter fractions that are more common in the bulk soil. Co-introduction of CBZ and DOM to the soil did not significantly affect the sorption behavior of CBZ; however, following pre-adsorption of DOM by the bulk soil, an increase in sorption affinity and decrease in sorption linearity were observed. In this latter treatment, desorption hysteresis of CBZ was significantly increased for both sorbents. We hypothesize that this was due to either strong chemical interactions of CBZ with the adsorbed DOM or physical encapsulation of CBZ in DOM-clay complexes. Based on this study, we suggest that DOM facilitates stronger interactions of polar PCs with the solid surface. This mechanism can reduce PC desorption ability in soils.     

The role of lipids on sorption characteristics of freshwater- and wastewater-irrigated soils

The soil lipid fraction can play an important role in the sorption of organic compounds. In this study, the impact of the lipid fraction of freshwater- and wastewater-irrigated soils on the sorption of non- and relatively polar compounds was assessed. Lipid analyses revealed a clear difference between the two lipid fractions. The lipid extract from the wastewater-irrigated soil was consistent with mainly straight paraffinic chain materials; the lipid extract from freshwater-irrigated soil, on the other hand, exhibited stronger signals of aromatics, double bonds, ester, ether, and methyl, in addition to a smaller contribution from methylene protons. Our data suggest that lipid removal induced a stronger increase in the soil's sorption affinity for solutes capable of polar interactions such as atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) and chlorotoluron (N'-(3-chloro-4-methylphenyl)-N,N-dimethylurea) as compared to phenanthrene. Moreover, the level of increase in sorption affinities due to lipid removal was much higher for the freshwater-irrigated soil than for its wastewater-irrigated counterpart, even though the level of lipids in the freshwater-irrigated soil was half that in the wastewater-irrigated one (6 vs. 11% of the total organic C). The higher level of polar functionalities, such as ether and ester moieties, in the lipid fraction from the freshwater-irrigated soil suggests that these extractable compounds compete successfully with the polar solutes (atrazine and chlorotoluron) for specific binding sites in the soil organic matter (SOM). It appears that the composition of the lipid fraction may be a key consideration in unraveling the sorption of organic molecules in soils.     

Can assessing for potential contribution of soil organic and inorganic components for butachlor sorption be improved?

Sorption of butachlor to various types of common soil components was investigated. Six pure minerals (montmorillonite [Mont], kaolinite [Kaol], Ca homoionic montmorillonite [Ca-Mont] and kaolinite [Ca-Kaol], amorphous hydrated Al and Fe oxides [AHOs-Al, AHOs-Fe]), four soil alkali-extractable pure humic acids (HAs), and the four corresponding HAs originated real unmodified and HO-treated soils were selected as the representative sorbents. Results showed that the HAs played a crucial role, and clay minerals (especially Mont) also showed an important effect in butachlor sorption. The AHOs may likely influence only in a mediator way by enhancing the availability of sorption domains of HAs. By removing 78% (on average) of the total organic carbon (TOC) from the soils with HO, the content ratio of clay to TOC (RCO) increased by an average of 367% and became >60. This change simultaneously decreased the sorption capacity of soils (40%, on average). Considering that the surface sorption domain on clay minerals may be highly exposed and more competitive after the partial removal of soil organic matter (SOM), this reaffirmed the potential contribution from clay minerals. It can thus be inferred that in the real soil where SOM and clay minerals are associated, the coating of clay minerals may have weakened the partition function of SOM or blocked some sorption domain within SOM, resulting in a decreased sorption of butachlor. Therefore, clay minerals, especially 2:1 type expanding minerals, may play a dual function vs. SOM content for the sorption of butachlor in soil.     

Sorption and desorption of cadmium by different fractions of biosolids-amended soils

To evaluate the importance of both the inorganic and organic fractions in biosolids on Cd chemistry, a series of Cd sorption and desorption batch experiments (at pH 5.5) were conducted on different fractions of soils from a long-term field experimental site. The slope of the Cd sorption isotherm increased with rate of biosolids and was different for the different biosolids. Removal of organic carbon (OC) reduced the slope of the Cd sorption isotherm but did not account for the observed differences between biosolids-amended soils and a control soil, indicating that the increased adsorption associated with biosolids application was not limited to the increased OC from the addition of biosolids. Removal of both OC and Fe/Mn further reduced the slopes of Cd sorption isotherms and the sorption isotherm of the biosolids-amended soil was the same as that of the control, indicating both OC and Fe/Mn fractions added by the biosolids were important to the increased sorption observed for the biosolids-amended soil samples. Desorption experiments failed to remove from 60 to 90% of the sorbed Cd. This "apparent hysteresis" was higher for biosolids-amended soil than the control soil. Removal of both OC and Fe/Mn fractions was more effective in removing the observed differences between the biosolids-amended soil and the control than either alone. Results show that Cd added to biosolids-amended soil behaves differently than Cd added to soils without biosolids and support the hypothesis that the addition of Fe and Mn in the biosolids increased the retention of Cd in biosolids-amended soils.     

Dissolved organic carbon and disinfection by-product precursor release from managed peat soils

A wetland restoration demonstration project examined the effects of a permanently flooded wetland on subsidence of peat soils. The project, started in 1997, was done on Twitchell Island, in the Sacramento-San Joaquin Delta of California. Conversion of agricultural land to a wetland has changed many of the biogeochemical processes controlling dissolved organic carbon (DOC) release from the peat soils, relative to the previous land use. Dissolved organic C in delta waters is a concern because it reacts with chlorine, added as a disinfectant in municipal drinking waters, to form carcinogenic disinfection byproducts (DBPs), including trihalomethanes (THMs) and haloacetic acids (HAAs). This study explores the effects of peat soil biogeochemistry on DOC and DBP release under agricultural and wetland management. Results indicate that organic matter source, extent of soil organic matter decomposition, and decomposition pathways all are factors in THM formation. The results show that historical management practices dominate the release of DOC and THM precursors. However, within-site differences indicate that recent management decisions can contribute to changes in DOC quality and THM precursor formation. Not all aromatic forms of carbon are highly reactive and certain environmental conditions produce the specific carbon structures that form THMs. Both HAA and THM precursors are elevated in the DOC released under wetland conditions. The findings of this study emphasize the need to further investigate the roles of organic matter sources, microbial decomposition pathways, and decomposition status of soil organic matter in the release of DOC and DBP precursors from delta soils under varying land-use practices.     

Studying organic matter molecular assemblage within a whole organic soil by nuclear magnetic resonance

This work shows the applicability of two-dimensional (2D) (1)H-(13)C heteronuclear correlation (HETCOR) nuclear magnetic resonance (NMR) spectroscopy to the characterization of whole soils. A combination of different mixing times and cross polarization (CP) methods, namely Lee-Goldberg (LG)-CP and Ramp-CP are shown to afford, for the first time, intra- and inter- molecular connectivities, allowing for molecular assemblage information to be obtained on a whole soil. Our results show that, for the brackish marsh histosol under study, two isolated domains could be detected. The first domain consists of O-alkyl and aromatic moieties (lignocellulose material), while the second domain is comprised of alkyl type moieties (cuticular material). The role of these domains is discussed in terms of hydrophobic organic compound sorption within soil organic matter (SOM), including the possible effects of wetting and drying cycles.     

Modeling concentration-dependent sorption-desorption hysteresis of atrazine in a loam soil

Nonequilibrium sorption plays an active role in the transport of organic contaminants in soil. We applied a two-stage, one-rate model (2S1R) and a new, nonlinear variant (2S1RN) of this model to examine the effects of wastewater irrigation on the sorption kinetics of atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) in soil. The models were applied to previously published sorption-desorption data sets, which showed pronounced deviations between sorption curves and desorption curves (sorption-desorption hysteresis). Moreover, the slopes of the desorption curves decreased with decreasing concentration. Different treatments had been used, and two experimental time steps (2 and 14 d) were used. Treatments considered were lipid removal, fulvic and humic acid removal, and untreated soil. The 2S1R model was unable to reproduce the observed type of hysteresis, but the 2S1RN model, which assumes that the sorption-desorption process follows a power function relationship, was able to reproduce the observed type of hysteresis. Visually, applying the new model improved the model fits in all test cases. Statistically, as tested by an extra sum of squares analysis, the new model performed significantly better in 50% of all test cases. According to an example simulation, the choice of the sorption model has a considerable impact on the prediction of atrazine transport in soil.     

On the interaction mechanisms of atrazine and hydroxyatrazine with humic substances

Atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) is retained against leaching losses in soils principally by sorption to organic matter, but the mechanism of sorption has been a matter of controversy. Conflicting evidence exists for proton transfer, electron transfer, and hydrophobic interactions between atrazine and soil humus, but no data are conclusive. In this paper we add to the database by investigating the role of (i) hydroxyatrazine (6-hydroxy-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) and (ii) hydrophobicity in the sorption of atrazine by Brazilian soil humic substances. We demonstrate, apparently for the first time, that hydroxyatrazine readily forms electron-transfer complexes with humic substances. These complexes probably are the cause of the well-known strong adsorption by humic acids and they may be the undetected cause of apparent electron-transfer complexes between soil organic matter and atrazine, whose transformation to the hydroxy form is facile. We also present evidence that supports the important contribution of hydrophobic interactions to the pH-dependent sorption of atrazine by humic substances.     

Role of nitrogen fertilization in sustaining organic matter in cultivated soils

Soil organic matter (SOM) is essential for sustaining food production and maintaining ecosystem services and is a vital resource base for storing C and N. The impact of long-term use of synthetic fertilizer N on SOM, however, has been questioned recently. Here we tested the hypothesis that long-term application of N results in a decrease in SOM. We used data from 135 studies of 114 long-term experiments located at 100 sites throughout the world over time scales of decades under a range of land-management and climate regimes to quantify changes in soil organic carbon (SOC) and soil organic nitrogen (SON). Published data of a total of 917 and 580 observations for SOC and SON, respectively, from control (unfertilized or zero N) and N-fertilized treatments (synthetic, organic, and combination) were analyzed using the SAS mixed model and by meta-analysis. Results demonstrate declines of 7 to 16% in SOC and 7 to 11% in SON with no N amendments. In soils receiving synthetic fertilizer N, the rate of SOM loss decreased. The time-fertilizer response ratio, which is based on changes in the paired comparisons, showed average increases of 8 and 12% for SOC and SON, respectively, following the application of synthetic fertilizer N. Addition of organic matter (i.e., manure) increased SOM, on average, by 37%. When cropping systems fluctuated between flooding and drying, SOM decreased more than in continuous dryland or flooded systems. Flooded rice ( L.) soils show net accumulations of SOC and SON. This work shows a general decline in SOM for all long-term sites, with and without synthetic fertilizer N. However, our analysis also demonstrates that in addition to its role in improving crop productivity, synthetic fertilizer N significantly reduces the rate at which SOM is declining in agricultural soils, worldwide.     

A new way to use solid-state carbon-13 nuclear magnetic resonance spectroscopy to study the sorption of organic compounds to soil organic matter

Several solid-state 13C nuclear magnetic resonance (NMR) techniques were used to characterize soil organic matter spiked with 13C-labeled organic compounds spanning a range of hydrophobicities (benzoic acid, benzophenone, naphthalene, phenanthrene, and palmitic acid). The chemical shifts of NMR resonances of the sorbed species were shifted by up to 3 ppm relative to those of the neat compounds. Sorption also resulted in increased resonance linewidth for the compounds containing a single 13C label, indicating the presence of a range of different chemical environments at the sites of sorption. On the other hand, sorption decreased the linewidth of the resonance of naphthalene, which was uniformly 13C-labeled. This was attributed to the removal of intermolecular 13C-13C dipolar coupling. Heterogeneity of the organic matter was demonstrated using the spectral editing technique proton spin relaxation editing (PSRE), which enabled the identification and quantification of charcoal-rich domains characterized by rapid rates of proton spin-lattice relaxation in the static frame (T1H), and humic domains characterized by slow rates of T1H relaxation. Furthermore it was demonstrated that the sorbed 13C-labeled molecules "inherit" the T1H "signature" of the organic matrix in their immediate vicinity. Thus PSRE on the spiked soils enabled evaluation of the relative affinity of the two domain types for the sorbate molecules. The charcoal-rich domains were shown to have a twofold to tenfold greater affinity for the organic compounds, with greater differences found for the more hydrophobic compounds.