Soil Humin Research Articles

Changes in Soil Humin Macromolecular Structure Resulting from Long-Term Catch Cropping

The aim of this study was to assess the effect of long-term catch crop application on the structural properties of humin, which is considered the most recalcitrant fraction of soil organic matter. Soil samples from a 30-year field experiment on triticale cultivated with and without catch crops were analysed to determine the total organic carbon content and fractional composition of humic substances. Meanwhile, humin isolated from bulk soil was analysed to determine its elemental composition and spectroscopic properties measured with UV-Vis, fluorescence, and 13C-CPMAS-NMR. It was found that catch crop farming enhanced the formation of highly reactive humus substances, like low-molecular-weight fractions and humic acids, while decreasing the humin fraction. The higher H/C and O/C atomic ratios of humin and the UV-Vis, fluorescence, and 13C-CPMAS-NMR results confirmed a higher share of oxygen-containing functional groups in humin isolated from the soil with catch crop rotation, also corroborating its greater aliphatic nature. Under the conditions of our field experiment, the results indicated that organic residues from catch crops quickly undergo the decay process and are transformed mainly into highly reactive humus substances, which can potentially improve soil health, while mineral fertilisation alone without catch crops favours the stabilisation and sequestration of carbon.

Chemical composition of soil humin in an organic soil profile

Humin is identified as the most recalcitrant fraction of soil organic matter, but its synthesis and depth variability in the soil profile are unknown. Here, we suggest that carboxyl-rich alicyclic structures (CRAM) and fused aromatic ring structures (FAR) can process significant components of the humin structure. We conducted molecular-level characterization of humin using advanced solid-state nuclear magnetic resonance spectroscopy (SSNMR) experiments and calculated the proportion of FAR and functionalities of CRAM (OCq, –C(CH)n, –CH3) structures. Dipolar dephased (dd) direct polarization magic angle spinning (DPMAS) NMR experiments provided insights into non-protonated carbon and mobile quaternary carbons. We calculated the proportions of non-protonated aromatic carbon and non-polar aliphatic C in the organic layers of the topsoil (Oa), mid-soil (Om), and mineral subsoil (Cg). It was evident that the total aliphatic carbon constitutes a significant proportion of humin, as is the non-protonated C attached to CH3. The 13C dd/DPMAS NMR spectrum of Cg humin clearly showed the formation and preservation of hydrophobic OCq, which is indicative of CRAM structures. CRAM structures could be attached to FAR, which makes the whole structure relatively more hydrophobic, a critical humin functionality in deeper soil. The findings from our research can be successfully utilized in terrestrial carbon sequestration studies to amplify the potential of humin as a recalcitrant carbon reservoir.

The native SOC increase in woodland and lawn soil amended with biochar surpassed greenhouse — A seven-year field trial

The effects of applying biochar with the same characteristics and at the same dose on the storage, composition, and underlying mechanisms of native organic carbon (n-SOC) dynamics in different ecosystems are still unclear. This study aimed to explore the effects of biochar amendment (7 years) on carbon sequestration and the n-SOC pools of woodland, lawn, and greenhouse soils. The ‘water floating method’ and improved ‘combustion loss method’ were used in this study to quantify residual biochar in soil. The results showed that after 7 years, the amount of biochar left in woodland, lawn, and greenhouse soils was 67.12 %, 87.50 %, and 88.13 % of the initial applied amount, respectively. And the n-SOC content increased approximately 2.07, 3.07, and 0.22 times, respectively, mainly due to increases in the native soil humin (n-HM) content of the soil. Biochar also increased the proportion of large aggregates in woodland and lawn soil and increased the t-SOC content of aggregates in each particle size fraction. Additionally, biochar increased the t-SOC of greenhouse soil aggregates but had no significant effect on the distribution of aggregates. The presence of biochar increased native soil easily oxidizable carbon (n-EOC) and microbial biomass carbon (n-MBC) in all three ecosystems. And increases in n-MBC in woodland and lawn soils occurred, which promoted the depletion of native soil hot water dissolvable organic carbon (n-HWOC) and increased CO2 emissions. Furthermore, the microbial respiration quotient (qCO2) of woodland and greenhouse soils was reduced by biochar, and that of lawn soil was unchanged. The carbon use efficiency (CUE) of lawn soils were reduced, possibly because biochar reduced the abundance of soil fungi/bacteria (F/B). In summary, the 7-year application of biochar significantly enhanced the n-SOC content in woodland and lawn soils, mainly due to an increase in humin, while a weaker enhancement was observed in greenhouse soil.

The Interaction of Pesticides with Humin Fractions and Their Potential Impact on Non-Extractable Residue Formation.

The constant influx of pesticides into soils is a key environmental issue in terms of their potential retention in the soil, thus reducing their negative impact on the environment. Soil organic matter (SOM) is an important factor influencing the environmental fate of these substances. Therefore, the aim of this research was to assess the chemical behavior of pesticides (flufenacet, pendimethalin, α-cypermethrin, metazachlor, acetamiprid) toward stable soil humin fractions (HNs) as a main factor affecting the formation of non-extractable residues of agrochemicals in soil. This research was conducted as a batch experiment according to OECD Guideline 106. For this purpose, HNs were isolated from eight soils with different physicochemical properties (clay content = 16-47%, pHKCl = 5.6-7.7, TOC = 13.3-49.7 g·kg-1, TN = 1.06-2.90 g·kg-1, TOC/TN = 11.4-13.7) to reflect the various processes of their formation. The extraction was carried out through the sequential separation of humic acids with 0.1 M NaOH, and then the digestion of the remaining mineral fraction with 10% HF/HCl. The pesticide concentrations were detected using GC-MS/MS. The pesticides were characterized based on the different sorption rates to HNs, according to the overall trend: metazachlor (95% of absorbed compound) > acetamiprid (94% of absorbed compound) > cypermethrin (63% of partitioning compound) > flufenacet (39% of partitioning compound) > pendimethalin (28% of partitioning compound). Cypermethrin and metazachlor exhibited the highest saturation dynamic, while the other agrochemicals were much more slowly attracted by the HNs. The obtained sorption kinetic data were congruous to the pseudo-first-order and pseudo-second-order models related to the surface adsorption and interparticle diffusion isotherm. The conducted research showed that the processes of pesticide sorption, apart from physicochemical phenomena, are also affected by the properties of the pollutants themselves (polarity, KOC) and the soil properties (SOM content, clay content, and pHKCl).

Effects of tillage practice on fungal and actinobacterial cellulolytic community with different humified particle‐size fractions in a double‐cropping paddy field

AbstractSoil cellulose plays a vital role in keeping or increasing carbon (C) cycling and fertility of rice fields. There is a close relation between soil fulvic acid (FA), humin (HM) fractions, and GH48, cbhI genes. However, the effects of different tillage practices on soil humus C content composition, GH48, and cbhI genes abundances in different soil particle‐size fractions of double‐cropping rice fields still need to be further investigated. Therefore, the impacts of different short‐term tillage practices on soil GH48 and cbhI genes abundances in double‐cropping rice fields were conducted. This experiment set up different tillage practices: rotary tillage with crop straw removed as a control (RTO), conventional tillage with crop straw in put (CT), rotary tillage with crop straw in put (RT), and no‐tillage with crop straw retain (NT). The result indicated that the distribution of soil humus, and GH48, cbhI genes abundances were significantly enhanced under combined applied with tillage and crop straw incorporation conditions. Soil FA, HM, and humic acid (HA) C content in 2000–50 μm and >2000 μm fractions with RTO treatment were significantly lower than that of CT, RT, and NT treatments. This result showed that oalkyl and alkyl of HA and FA in 2000–200, 200–50, 50–2, and 2–0.1 μm fractions with CT, RT, NT, and RTO treatments were lower than that of >2000 μm fraction. There was lower ARO% and more AL% of FA and HA in these fractions under the same tillage condition. The result showed that GH48 and cbhI genes abundances in these fractions with CT, RT, and NT treatments were significantly higher than that of RTO treatment. There had significantly positively correlated with GH48, cbhI genes abundances, and soil HA, FA, HM C contents, o‐alkyl C, and AL% of FA. Therefore, it was positively practiced for increasing GH48 and cbhI genes abundances in the double‐cropping rice fields by applying with tillage and crop straw incorporation.

Compositional changes in the humin fraction resulting from the long-term cultivation of an Irish grassland soil: Evidence from FTIR and multi-NMR spectroscopies

Soil humin (HN), a major long-term sink for carbon in the pedosphere, plays a key role in the global carbon cycle, and has been less extensively studied than the humic and fulvic acids components. There are increasing concerns about the depletions of soil organic matter (SOM) arising from modern soil cultivation practices but there has been little focus on how HN can be altered as the result. This study has compared the HN components in a soil under cultivation for wheat for >30 years with those from an adjacent contiguous soil that had been under long-term grass for all that time. A urea-fortified basic solution isolated additional humic fractions from soils that had been exhaustively extracted in basic media. Then further exhaustive extractions of the residual soil material with dimethyl sulfoxide, amended with sulphuric acid isolated what may be called the “true” HN fraction. The long-term cultivation resulted in a loss of 53 % soil organic carbon in the surface soil. Infrared and multi-NMR spectroscopies showed the “true” HN to be dominated by aliphatic hydrocarbons and carboxylated structures, but with clear evidence for lesser amounts of carbohydrate and peptide materials, and with weaker evidence for lignin-derived substances. These lesser-amount structures can be sorbed on the soil mineral colloid surfaces and/or covered by the hydrophobic HN component or entrained within these which have strong affinities for the mineral colloids. HN from the cultivated site contained less carbohydrate and more carboxyl groups suggesting slow transformations took place resulting from the cultivation, but these were much slower than for the other components of SOM. It is recommended that a study be made of the HN in a soil under long-term cultivation for which the SOM content has reached a steady state and where HN will be expected to dominate the components of SOM.

Effects of Fe3+ and Ca2+ on sorption of phenanthrene by Humin in karst soil, Southwest China

Effects of Fe3+ and Ca2+ on sorption of phenanthrene by Humin in karst soil, Southwest China

Phosphate fertilization and bioactivator influences on fractions of organic matter and soil microbial biomass

Soil organic matter is considered a potential source of phosphorus (P) for plants because of biological cycling. Bioactivators are characterized as tools that can help soil microbial activity and the availability of P demanded by plants. The objective of this study was to evaluate the influence of the use of bioactivators and phosphate fertilization on the activity of soil microbial biomass and humified fractions of organic matter. The experimental design was completely randomized in a factorial scheme (6 × 2), with the first factor consisting of six doses of phosphorus applied to the soil (0, 30, 60, 90, 120, and 150 kg ha-1 of P2O5), using triple superphosphate as the source, and the second factor being the presence or absence of soil and plant bioactivator application, with four replications. The evaluated characteristics were soil microbial biomass carbon (SMBC), soil basal respiration (SBR), microbial biomass nitrogen (MBN), metabolic quotient (qCO2), microbial quotient (qmic), total organic carbon (TOC), fulvic acid fraction carbon (FAC), humic acid fraction carbon (HAC), and humin fraction carbon (HUMC). The use of soil and plant bioactivators and mineral phosphate fertilizer promoted changes in soil microbial activity. Increasing doses of phosphate fertilization promoted a reduction in the carbon characteristics of microbial biomass and basal soil respiration in the presence or absence of soil and plant bioactivators. Moreover, the use of bioactivator promoted higher averages for soil microbial biomass carbon, soil basal respiration, metabolic quotient, humic acid and humin and reduced the levels of microbial quotient, total organic carbon and fulvic acid.

Effect of crop residue and fertilizers on humus fractions and rice (Oryza sativa) yield

An experiment was conducted during 2019–20 at the Research Farm of Dr. BSKKV, Dapoli, Maharashtra to study the effect of crop residues and fertilizers on humus fractions and rice yield in Alfisols. The study was conducted on ex situ incorporation of crop residue along with fertilizers and the results indicated that application of rice straw @2.5 t/ha + Ain leaf residue @2.5 t/ha along with 100% NPK was significantly beneficial for improving the content of soil humin, humic acid, fulvic acid and humic acid : fulvic acid (HA: FA) ratio and yield of rice. Crop residue and fertilizer management significantly improved the soil humin, humic acid, fulvic acid and humic acid: fulvic acid (HA: FA) ratio. Build-up of soil humus was due to the balanced application of crop residue along with fertilizers for developing bio-stability in soil which helps maintain soil health. In contrast, the application of 50%, 75% and 100% NPK alone did not gain optimum humus content in soil and yield of rice because of unbalanced use of fertilizers. Overall improvement in soil humus and yield of rice was observed by the integrated use of crop residue along with fertilizers in Alfisols.

Long-term tea plantation effects on composition and stabilization of soil organic matter in Southwest China

Tea (Camellia sinensis L.) plantation appears to be a sustainable alternative to other crop types because it has the potential to improve nutrient cycling and the productivity in Southwest China. However, the changes of soil organic matter (SOM) quality during the long-term tea planting process have not yet been well documented, and it is difficult to reveal tea plantation effects on the chemical stability of SOM. In this work, the changes of SOM functional groups during tea planting were studied at five sites: four tea plantations with different durations of use (18a, 25a, 33a, 55a), and an adjacent abandoned land (0a), by analyzing the change of soil organic carbon (SOC) and the infrared spectroscopy and element composition characteristics of humic acids (HA), fulvic acids (FA) and humin (HM). Compared with abandoned land (0a), the content of SOC, HA, FA and HM in tea plantation soil increased significantly, and the content of SOC was higher in 25-year-old tea plantation, decreased significantly 33-year-old tea plantation. After tea plantation, the H/C and O/C of HA and HM decreased, and the aliphatic chain hydrocarbons in HA and HM molecules decreased relatively, and the aromatic structure increased relatively. The H/C and O/C in the soil HA, FA and HM were lower in the 25-year-old tea plantation. Tea plantation was beneficial to increase the content of SOC and humic substances, enhance the degree of soil humification and improve the quality of soil humus. The quality of soil humus was better and the effect of carbon sink was stronger in 25-year-old tea plantation. It is necessary to improve fertilization management to increase the exchange sites of SOM, so as to improve nutrient cycling, maintain the structure and fertility of tea plantation soil, and realize the sustainable utilization of regional tea plantation soil.

Revisiting Alice Boer: Site formation processes and dating issues of a supposedly pre‐Clovis site in Southeastern Brazil

AbstractThe Alice Boer site, located in Southeastern Brazil, was considered to be one of the oldest sites in the Americas, and a strong candidate for being a pre‐Clovis site. Ages obtained by researchers during the 1970s and 1980s put the site inside a chronological range between 2 and 14 ka, eventually reaching as old as 30 ka or even 130 ka. Between 2010 and 2012, our research team revisited the site to improve the knowledge of Paleoindian occupation in Southeastern Brazil. Here, we present new data of stratigraphy and chronology (luminescence and radiocarbon dating), pedological and magnetic analysis performed at some of the sites. Results show that the site's oldest occupation took place ca. 8 ka, which provides new insight into human occupation in South America. In addition, we show some issues with radiocarbon contamination due to soil humin fraction mobility.

Free radical property, redox capacity, and the relevant influencing factors of soil humin

Free radical property, redox capacity, and the relevant influencing factors of soil humin

Economics of Hybrid Maize Production Using of Lignite and Poultry Manure Based Humin in an Acid Soil of Eastern Dry Zone of Karnataka

Aims: To study the effect of lignite humin (LH) and poultry manure humin (PMH) application on economics of hybrid maize production in an acid soil of eastern dry zone of Karnataka.
 Study Design: Randomized complete block design (RCBD) comprising ten treatments and three replications.
 Place and Duration of Study: Krishi Vigyana Kendra, Hadonahalli, Bengaluru rural district (Karnataka) during kharif 2018.
 Methodology: A field experiment was laid out in a randomized complete block design with ten treatments, replicated thrice during kharif 2018 at Krishi Vigyana Kendra, Hadonahalli, Bengaluru rural district, Karnataka. The Lignite Humin (LH) and Poultry Manure Humin (PH) were applied at different doses (0, 2.5, 7.5 & 10 t ha-1) in combination with FYM (Farm Yard Manure) applied in such a way that the total quantity of humin and FYM is equivalent to 10 t ha-1.
 Results: The results revealed that significant variation was observed on yield and benefit: Cost ratio due to application of Farm Yard Manure (FYM), LH and PMH. Significantly higher maize kernel (8070 kg ha-1) and stover yield (9948 kg ha-1) were recorded in treatment T2 (100% RDF + FYM @ 10 t ha-1) and which was found on par with treatment T7 (100% RDF + PMH @ 2.5 t ha-1 + FYM @ 7.5 t ha-1) and T3 (100% RDF + LH @ 2.5 t ha-1 + FYM @ 7.5 t ha-1) and T8 (100% RDF + PMH @ 5 t ha-1 + FYM @ 5 t ha-1). Wherein, higher B:C ratio of 2.24 was recorded in treatment T10 receiving 100% RDF + PMH @ 10 t ha-1and it was followed by treatment receiving T2(2.21) and T9 : 100% RDF + PMH @ 7.5 t ha-1 + FYM @ 2.5 t ha-1 (2.20). Whereas the least B:C ratio (1.72) was observed in the absolute control treatment where no manures and fertilizers were given.
 Conclusion: These results suggest that higher B:C ratio in these treatments might be due to lower cost of cultivation and it increases with increased in dose of lignite and poultry manure based humin.

CHEMICAL AND PHYSICAL FRACTIONATION OF THE ORGANIC MATTER OF A CAMBISOL AFTER 21 YEARS UNDER DIFFERENT PREPARATIONS

Soil organic carbon is used as an indicator of soil quality. The objective of this study was to evaluate chemical and physical fractions of the organic carbon in a Humic Cambisol under different tillage for 21 years. The treatments were conventional tillage (CT) and no-tillage (NT). As reference area, a native grassland (NF), next to the experiment, was evaluated. Samples were collected at three depths. Total (TOC) and particulate organic carbon (POC) concentrations and stocks and humic substances (HS) were evaluated. The TOC and POC were higher in the NF than in the crop systems. NT has more carbon than CT in the humin and humic acid fractions in the surface soil. Physical fractions of organic carbon are similar between the management systems. NT has a greater organic carbon stock than CT 15 years after its development and shows greater potential for carbon conservation in the surface soil. Thus, long-term experiments are important to monitor soil behavior under different management conditions.

Impact of tillage practices on soil aggregation and humic substances under double‐cropping paddy field

AbstractThe effects of different tillage practices on soil organic carbon (SOC), soil aggregation, soil fulvic acid (C‐FAF), soil humic acid (C‐HAF), and soil humin (C‐HUM) contents at 0–5, 5–10, and 10–20 cm layers under double‐cropping rice (Oryza sativa L.) fields in southern China were explored. The experiment included four tillage treatments: conventional tillage and crop residues incorporation (CT), rotary tillage and crop residues incorporation (RT), no‐tillage and crop residues incorporation (NT), and rotary tillage without crop residues as a control (RTO). The results showed that SOC content under RT and CT treatments were significantly higher than that of RTO treatment. The percentage of soil aggregates of <0.001 mm, 0.005–0.001 mm, and 0.01–0.005 mm at 0–5, 5–10, and 10–20 cm layers under CT treatment were significantly higher than that of RTO treatment. The C‐HUM, C‐FAF, and C‐HAF contents were increased by combined application of tillage with crop residues practices. The results showed that C‐HUM, C‐FAF, and C‐HAF contents under CT and RT treatments were significantly higher than those under the RTO treatment at 0–5, 5–10, and 10–20 cm layers. As a result, it is an appropriate soil practice for increasing SOC content, soil aggregation, and humic substances under double‐cropping rice fields in southern China under conventional tillage and rotary tillage.

Adsorption of Cu(II) on soil humin: batch and spectroscopy studies

Understanding the interactions of humic substances with metal ions is of great importance in forecasting the behaviors and fates of trace metals in the environment. Humin is the least understood humic substance fraction due to its close association with the mineral matrix in soil. In this study, humin was prepared from three zonal soils (i.e., dark brown soil, yellow brown soil, and latosol) in eastern China by demineralization using HF–HCl solution after removing alkaline-soluble humic substance fractions. The adsorption properties of Cu(II) onto both humin and humic acid were examined using batch and spectroscopy (SEM, EDS, FTIR, XPS, and XAS) techniques. With respect to humic acid, humin was more aliphatic and less polar. The adsorption kinetics of humin and humic acid were best fit with pseudo-second-order kinetics equation, and the adsorption isotherms were well depicted by the Langmuir model. However, the quantity of adsorbed Cu(II) on humin was inferior to that of Cu(II) adsorbed onto humic acid. After adsorption, the surface morphology of humin and humic acid changed. Carboxyl, hydroxyl, aromatic N, and primary amine N on the surfaces of the two humic substance fractions took part in Cu(II) adsorption, and Cu(II) was bound with O/N and C atoms via inner-sphere complexation. Our results suggested that humin and humic acid had different adsorption capacity but similar adsorption mechanisms toward Cu(II).

Sorption Mechanism and Performance of Peat Soil Humin for Methylene Blue and <i>p</i>-Nitrophenol

The responsible mechanism and performance of peat soil humin for the sorption of methylene blue (MB) and p-nitrophenol (p-NP) have been investigated. Humin was obtained from peat soil of Siantan, West Kalimantan, Indonesia, after removing the content of humic and fulvic acids into a NaOH solution using the recommended procedure of International Humic Substances Society (IHSS). The obtained humin was then purified by rigorous stirring in a mixed solution of HCl 0.1 M and HF 0.3 M. Ash content in humin after the purification abruptly decreased from 36.84 to 1.26 wt.% indicating that minerals and other inorganic impurities were mostly removed. Phenolic –OH and carboxyl (–COOH) functional groups contributing to the acidity of humin were in the level of 3.44 and 2.10 mmol/g, respectively. At optimum medium pH of 6.20 for MB and 7.00 for p-NP, –COO– as the deprotonated product of –COOH was the most responsible active site in sorbing MB and p-NP through electrostatic interaction and hydrogen bonding, respectively. The homogeneity of –COO– as the active site for the sorption of MB and p-NP implied that the surface of humin sorbent was energetically uniform and thereby the sorption of both MB and p-NP followed better the Langmuir than the Freundlich isotherm model with sorption capacity of 0.19 and 0.26 mmol/g and sorption energy of 32.92 and 27.27 kJ/mol, respectively.

Altered humin compositions under organic and inorganic fertilization on an intensively cultivated sandy loam soil

Humin is the most recalcitrant fraction of soil organic matter (SOM). However, little is known about quantitative structural information on humin and the roles of soil mircoorganisms involved in the humin formation. We applied advanced solid-state 13C nuclear magnetic resonance (NMR) spectroscopy to provide deep insights into humin structural changes in response to long-term balanced fertilization on a Calcaric Fluvisol in the North China plain. The relationships between humin structure and microbiological properties such as microbial biomass, microbial quotient (qmic) and metabolic quotient (qCO2) were also studied. The humins had a considerable (35–44%) proportion of aromatic C being nonprotonated and the vast majority of O-alkyl and anomeric C being protonated. Alkyl (24–27% of all C), aromatic C (17–28%) and O-alkyl (13–20%) predominated in humins. Long-term fertilization promoted the aliphatic nature of humins, causing increases in O-alkyl, anomeric and NCH functional groups and decreases in aromatic C and aromatic CO groups. All these changes were more prominent for treatments of organic fertilizer (OF) and combined mineral NPK fertilizer with OF (NPKOF) relative to the Control and NPK treatments. Fertilization also decreased the alkyl/O-alkyl ratio, aromaticity and hydrophobic characteristics of humins, suggesting a more decomposed and humified state of humin in the Control soil. Moreover, the soil microbiological properties had strong correlations with functional groups of humins. Particularly, microbial biomass C was a relatively sensitive indicator, having positive correlations with oxygen-containing functional groups, i.e., COO/NCO and protonated O-alkyl C, and negative correlations with nonprotonated aromatic C. The qmic and qCO2 were also significantly positively correlated with NCH and aromatic CO, respectively. Our results deepen our understanding of how long-term fertilization impacts the structure of humin, and highlight a linkage between microbiological properties and recalcitrant fraction of SOM besides labile fraction.

Sorption mechanisms of sulfamethazine to soil humin and its subfractions after sequential treatments

Sorption mechanisms of an antibiotic sulfamethazine (SMT) to humin (HM) isolated from a peat soil and its subfractions after sequential treatments were examined. The treatments of HM included removal of ash, O-alkyl carbon, lipid, and lignin components. The HF/HCl de-ashing treatment removed a large amount of minerals (mainly silicates), releasing a fraction of hydrophobic carbon sorption domains that previously were blocked, increasing the sorption of SMT by 33.3%. The de-O-alkyl carbon treatment through acid hydrolysis greatly reduced polarity of HM samples, thus weakening the interaction between sorbents with water at the interfaces via H-bonding, leaving more effective sorption sites. Sorption of SMT via mechanisms such as van der Waals forces and π-π interactions was enhanced by factors of 2.04–2.50. After removing the lipid/lignin component with the improved Soxhlet extraction/acid hydrolysis, the organic carbon content-normalized sorption enhancement index Eoc was calculated. The results demonstrated that the Eoc-lipid for SMT (16.9%) was higher than Eoc-lignin (10.1%), implying that removal of unit organic carbon mass of lipid led to a higher increase in sorption strength than that of lignin. As each component was progressively removed from HM, the sorption strength and isotherm nonlinearity of the residual HM samples for SMT were gradually enhanced. The Koc values of SMT by HM samples were positively correlated with their aromatic carbon contents, implying that π-π electron donor-acceptor interactions between the benzene ring of sorbate and the aromatic domains in HM played a significant role in their interactions.

Chemical nature of humic substances in two typical Chinese soils (upland vs paddy soil): A comparative advanced solid state NMR study

Knowledge of the structural features of humic substances (HSs) is essential for elucidating the mechanisms of humification in different soil environments and realizing their profound roles in environmental issues. The aim of this work was to investigate the chemical structures of fulvic acid (FA), humic acid (HA) and humin (HM) fractions isolated from an upland soil (Fluvisol) and a paddy soil (Anthrosol) typical in China using advanced solid-state 13C nuclear magnetic resonance (NMR) techniques. The results revealed that there were great structural differences of HSs between the two soils. The two FAs showed distinct quantitative differences in aliphatics with more polysaccharides in the FA from the upland soil than from the paddy soil. The HM from the upland soil differed from the paddy soil HM in having more proteins/peptides (23% vs 20%), total aromatics (21% vs 12%) as well as fewer lipids (24% vs 35%) and polysaccharides (27% vs 31%). The HM fractions represented the most different components of organic matter between the two soils. The degree of difference between the two HAs fell in between that of FAs and HM fractions. In particular, the HA from the upland soil had relatively greater degree of aromaticity. Our study indicated that the upland soil exhibited a higher degree of humification compared with the paddy soil. Among the three humic fractions, the FAs featured COO/N-CO groups, and the HAs were more enriched in protonated aromatic C for both soils. In contrast, the two HM fractions contained more O-alkyl C and fewer aromatics than did the other humic fractions, being closer to the original organic materials in soils. We speculate that the evolutionary route of HSs is likely to be the transformation of original organic materials into HM, followed by increased degradation, further oxidization and conversion into HA, and then into FA.