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

Molecular Weight Distribution of Humic Acids Isolated from Calcic Cryosol in Central Yakutia, Russia

The transition of soils into fallow state has a significant impact on the accumulation and transformation of soil organic matter (SOM). However, the issue of SOM transformation as a result of soil transition to fallow state in cryolithozone conditions is insufficiently studied. The aim of this study is to investigate the molecular weight (MW) distribution of humic acids (HAs) isolated from soils of central Yakutia. Native, fallow and agricultural soils in the vicinity of Yakutsk city were studied. MW distributions of HA preparations were obtained on an AKTAbasic 10 UPS chromatographic system (Amersam Biosciences, Uppsala, Sweden) using a SuperdexTM 200 10/300 GL column (with cross-linked dextran gel, fractionation range for globular proteins 10-600 kDa). The data on the molecular-mass distribution of HAs of fallow and agricultural soils of Central Yakutia were obtained for the first time. According to the obtained data, it was found that the highest carbon content in the structure of HAs was observed in agricultural soils (52.56%), and is associated with soil cultivation and fertilizer application. Among the HAs of fallow soils, we note that those soils that are in the process of self-vegetation have a relatively high carbon content in the HAs (45.84%), but the highest content was observed in fallow soils used as hayfields (49.98%), indicating that the reinvolvement of agriculture in fallow soils leads to an increase in the carbon content of HAs. According to the data of the MW distribution of HAs, it was found that the highest content of a high MW fraction of HAs was recorded in native soil (18.8%); this is due to the early stages of humification and the low maturity of organic matter. The highest content of a low MW fraction of HAs was recorded in agricultural soil (73.3%); this is due to the formation of molecular complexes of a "secondary" nature, which are more stable in the environment than the primary transformation products of humification precursors. The molecular composition of the HAs of fallow soils in the process of self-overgrowing is characterized by values closer to the HAs of native soils, which indicates their transformation towards HAs of native soils. The obtained results indicate that the reinvolvement of fallow soils leads to the transformation of the molecular composition of HAs towards HAs of agricultural soils, and to an increase in the resistance of SOM to biodegradation.

The impact of maize straw incorporation on arsenic and cadmium availability, transformation and microbial communities in alkaline-contaminated soils

Increasing evidence of the uncertainty of crop straw returning in heavy metal-contaminated soil is a significant concern. The present study investigated the influence of 1 and 2% maize straws (MS) amendment on As and Cd bioavailability in two different alkaline soils (A-industrial and B-irrigation) after 56 days of ageing. Adding MS to the two soils decreased the pH by 1.28 (A soil) and 1.13 (B soil) and increased the concentration of dissolved organic carbon (DOC) by 54.40 mg/kg (A soil) and 100.00 mg/kg (B soil) during the study period. After 56 days of ageing, the overall NaHCO3-As and DTPA-Cd increased by 40% and 33% (A) and 39% and 41% (B) soils, respectively. The MS additions increased the alteration of As and Cd exchangeable and residual fractions, whereas advanced solid-state 13C nuclear magnetic resonance (NMR) revealed that alkyl C and alkyl O-C-O in A soil and alkyl C, Methoxy C/N-alkyl, and alkyl O-C-O in B soil significantly contributed to the As and Cd mobilisation. Collectively, 16 S rRNA analyses revealed Acidobacteria, Firmicutes, Chloroflexi, Actinobacteria and Bacillus promoted the As and Cd mobilisation following the MS addition, while principle component analysis (PCA) demonstrated that bacterial proliferation significantly influenced MS decomposition, resulting in As and Cd mobilisation in the two soils. Overall, the study highlights the implications of applying MS to As- and Cd-contaminated alkaline soil and offers the framework for conditions to be considered during As- and Cd-remediation efforts, especially when MS is the sole remediation component.

Microbial metabolism and humic acid formation in response to enhanced copper and zinc passivation during composting of wine grape pomace and pig manure

Copper (Cu) and zinc (Zn) in piglet feed can lead to heavy metals (HMs) accumulation in pig manure (PM). Composting is crucial for recycling biowaste and decreasing HMs bioavailability. This study aimed to investigate the impact of adding wine grape pomace (WGP) on the bioavailability of HMs during PM composting. WGP facilitated the passivation of HMs through Cytophagales and Saccharibacteria_genera_incertae_sedis which promoted the formation of humic acid (HA). Polysaccharide and aliphatic groups in HA dominated the transformation of chemical forms of HMs. Moreover, adding 60% and 40% WGP enhanced the Cu and Zn passivation effects by 47.24% and 25.82%, respectively. Polyphenol conversion rate and core bacteria were identified as key factors in affecting HMs passivation. These results offered new insights into the fate of HMs during PM composting in response to the addition of WGP, which is helpful to practical application of WGP to inactivate HMs for improving compost quality.

Combined effect of humic acid and vetiver grass on remediation of cadmium-polluted water

Effective treatment of water pollution is an economic and social requirement globally. Humic acid (HA) is a popular mitigator for such waters. However, the combined effect of HA and restorative plants on cadmium (Cd) remediation is not well understood. Therefore, we experimented on Cd remediation using HA along with vetiver grass and HA-vetiver grass. We observed that vetiver grass effectively removed Cd at 15～30 mg/L. The accumulation capacity of the root was significantly higher than the shoots (P < 0.05), and Cd distribution followed the trend: cell wall > organelle > soluble substance (F1 > F2 > F3). The plant's accumulation capacity against 25 mg/L Cd was higher than for other treatments. The root accumulation capacity was much higher (702.3 mg/L) than those without added HA. However, upon adding 200 and 250 mg/L HA, the phytoremediation of Cd in the root and shoot significantly reduced (P < 0.05). Conversely, HA improved the Cd removal efficiency of the plants, notably at a lower HA concentration (150 mg/L). In addition, HA (especially at 150 mg/L) influences Cd distribution in vetiver cells (P < 0.05) and can significantly increase the proportion of Cd in the root cytoplasm. Consequently, a low HA concentration can significantly improve Cd accumulation in the vetiver, shorten the metal's bioremediation cycle, and improve the biological absorption efficiency.

Ecological circular agriculture: A case study evaluating biogas slurry applied to rice in two soils

In the context of carbon peak, neutrality, and circular agricultural economy, the use of renewable resources from agricultural processing for plant cultivation still needs to be explored to clarify material flow and its ecological effects. Paddy-upland rotation is an effective agricultural strategy to improve soil quality. This study evaluated the effects of biogas slurry application against those of chemical fertilisers in these two typical Chinese cropping soils. The application of biogas slurry increased total carbon content in paddy soil by 73.4%, and that in upland soil by 65.8%. Conversely, application of chemical fertiliser reduced total carbon in both soil types. There were significant positive correlations between total carbon and Zn, Cu, and Pb in rice husks grown in paddy soil (R² = 0.95, 0.996, 0.95; p < 0.05). The content of amylose in biogas slurry treatment of paddy soil increased by 35.9%, while that in upland soil decreased by 19.2%. After biogas slurry was applied, the contents of fulvic acid- and humic acid-like substances in paddy soil average increased by 40.9% and 45.6%, while the contents of protein-like components were enhanced by 46.8% in upland soil. This result was consistent with predictions of microbial community function. Microorganisms in paddy soil generally preferred carbon fixation, while those in upland soil preferred hydrocarbon degradation and chemoheterotrophy. Understanding the changes in soil carbon stock and microbial function after biogas slurry application will contribute to sustainable agricultural development and food security.

Unveiling the Potential of Novel Struvite–Humic Acid Composite Extracted from Anaerobic Digestate for Adsorption and Reduction of Chromium

A novel struvite–humic acid composite (S–HA) was derived from an anaerobic digestate and evaluated for the adsorption and reduction of chromium [Cr (VI)] in this study. The results indicated that the struvite–humic acid composite (S–HA) contains higher contents of oxygen-containing and aromatic functional groups (47.05% and 34.13%, respectively) and a higher specific surface area (19.3 m2 g−1). These special characteristics of S–HA contributed to its higher adsorption capacity (207.69 mg g−1 and 254.47 mg g−1 for pseudo-first and second-order kinetic models, respectively) for chromium. Furthermore, XPS analysis showed that a portion of the bonded Cr (VI) was reduced to Cr (III) by carboxyl and hydroxyl functional groups, which oxidized and changed into ketone and phenol functional groups. Based on the findings, it was concluded that the phosphate–humic acid composite has an outstanding chromium adsorptive and reduction capacity. However, more research is needed to fully understand the potential of the struvite–humic acid composite for chromium adsorption and reduction.

Spectral Characteristics Related to Chemical Substructures and Structures Indicative of Organic Precursors from Fulvic Acids in Sediments by NMR and HPLC-ESI-MS

The aim of this work was to determine Fulvic Acids (FAs) in sediments to better know their composition at the molecular level and to propose substructures and structures of organic precursors. The sediment samples were obtained from a priority area for the conservation of ecosystems and biodiversity in Mexico. FAs were extracted and purified using modifications to the International Humic Substances Society method. The characterization was carried out by 1D and 2D nuclear magnetic resonance (NMR) and high-performance liquid chromatography-electrospray ionization-mass spectrometry (HPLC-ESI-MS) in positive (ESI⁺) and negative (ESI⁻) modes. Twelve substructures were proposed by the COSY and HSQC experiments, correlating with compounds likely belonging to lignin derivatives obtained from soils as previously reported. The analysis of spectra obtained by HPLC-ESI-MS indicated likely presence of compounds chemically similar to that of the substructures elucidated by NMR. FAs studied are mainly constituted by carboxylic acids, hydroxyl, esters, vinyls, aliphatics, substituted aromatic rings, and amines, presenting structures related to organic precursors, such as lignin derivatives and polysaccharides.

Humin: No longer inactive natural organic matter

The discovery of the function of humin (HM), an insoluble fraction of humic substances (HSs), as an extracellular electron mediator (EEM) in 2012 has provided insight into the role of HM in nature and its potential for in situ bioremediation of pollutants. The EEM function is thought to enable the energy network of various microorganisms using HM. Recently, a number of studies on the application of HM as EEM in anaerobic microbial cultures have been conducted. Even so, there is a need for developing a holistic view of HM EEM function. In this paper, we summarize all the available information on the properties of HM EEM function, its applications, possible redox-active structures, and the interaction between HM and microbial cells. We also suggest scopes for future HM research.

Fulvic acids from Amazonian anthropogenic soils: Insight into the molecular composition and copper binding properties using fluorescence techniques

Fulvic acids (FA) are one of the components of humic substances and play an important role in the interaction with metallic species and, consequently, the bioavailability, distribution and toxicity of metals. However, only a few studies have investigated these FA properties in specific environment, such as anthropogenic soils. Therefore, knowledge about FA molecular composition as well as the FA-metal interaction is essential to predict their behavior in the soil. For this reason, the aim of this study was to investigate the molecular composition of FA extracted from two sites in an anthropogenic soil (Terra Mulata), from the Amazon region, as well as their interactions with Cu(II) ions as a model. Results from ¹³C NMR, infrared and elemental analysis showed that these FA are composed mostly by alkyl structures and oxygen-functional groups, e.g., hydroxyl, carbonyl and carboxyl. The interaction with Cu(II) ions was evaluated by fluorescence quenching, in which the FA showed both high quantity of complexing sites per gram of carbon and good affinity to interact with the metal when compared with other soil FA. The results showed that the complexation capacity was highly correlated by the content of functional groups, while the binding affinity was largely influenced by structural factors. In addition, through the lifetime decay given by time-resolved fluorescence, it was concluded that static quenching took place in FA and Cu(II) interaction with the formation of a non-fluorescent ground-state complex. Therefore, this fraction of soil organic matter will fully participate in complexation reactions, thereby influencing the mobility and bioavailability of metal in soils. Hence, the importance of the study, and the role of FA in the environment, can be seen especially in the Amazon, which is one of the most important biomes in the world.

Chemical compositional analysis of soil fulvic acids using Fourier transform ion cyclotron resonance mass spectrometry

RATIONALE

Soil fulvic acids (FAs) are considered to be a highly reactive pool of soil organic matter. The functions of FAs are related to their chemical structures, the details of which are largely unidentified. To better understand them, Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) must be a useful but generally unused tool.

METHODS

The structural properties of the components of five FA samples from a variety of soils were determined using FTICR-MS with negative-mode electrospray ionization. The peaks were assigned to molecular formulae, which were categorized into seven compound groups based on the H/C-O/C van Krevelen diagram. Ramp ¹³ C cross polarization/magic angle spinning nuclear magnetic resonance (NMR) spectra with phase-adjusted spinning side bands were also recorded to estimate the C composition.

RESULTS

From FTICR-MS, molecular formulae were assigned to 1746-2605 peaks across the m/z range of 200-700. Those aligned in the lignin-like, tannin-like, and condensed aromatic regions of the van Krevelen diagram accounted for 49-58%, 4-20%, and 18-39% of the total peak magnitude, respectively. The proportion of the summed peak magnitudes that were detected in the lignin-like and condensed aromatic regions correlated positively to the aromatic C% as estimated by ¹³ C NMR. From Kendrick mass defect analysis using a carboxyl group, 94 molecular formulae were assigned to condensed aromatic acids, of which the maximum ring number was 4-7, as potential structures.

CONCLUSIONS

A high proportion of lignin-like formulae and condensed aromatics, including those probably condensed aromatic acids with small ring numbers, as well as the existence of tannin-like formulae, which were generally lacking in soil humic acids, was suggested as a common feature of soil FAs.

Protaetia brevitarsis larvae can feed on and convert spent mushroom substrate from Auricularia auricula and Lentinula edodes cultivation

The edible mushroom industry produces massive amounts of spent mushroom substrate (SMS). Thus, there is an urgent need for high-value utilization technology to process the SMS, especially SMSs originating from woodchips. Protaetia brevitarsis larvae (PBL) can feed on various types of organic matter and can produce organic fertilizer and insect protein. In this study, we investigated the potential of PBL to utilize and convert SMSs from Auricularia auricula (SMS-AA) and Lentinula edodes (SMS-LE) cultivation. The results showed that the PBL were able to feed on SMS-AA and SMS-LE and form nutrient-enriched organic fertilizer with a low phytotoxicity and high humic acid content. Further analysis of the organic carbon dynamics suggested that PBL can efficiently digest and utilize lignin. This study demonstrates a new strategy for the utilization of SMSs originating from woodchips, and provides a new model for further investigations on the mechanism of lignin decomposition.

Molecular Signatures of Humic Acids from Different Sources as Revealed by Ultrahigh Resolution Mass Spectrometry

Humic acid (HA) is extremely important for understanding the geochemical cycle of pollutants in different environments. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has performed molecular-level analysis of two standard HAs from the Suwannee River (SRHA) and leonardite (LEHA) and HA from Jiufeng forest in Beijing (JFHA), which is impossible for other conventional instruments. Regardless of the source of HA, compounds containing more heteroatoms (such as nitrogen and sulfur) have a higher degree of unsaturation and aromaticity. JFHA, SRHA, and LEHA from soil, river, and leonardite, respectively, are arranged in order from the lowest to highest degree of humification, according to molecular unsaturation and aromaticity of HAs. Soil HA is more labile and contains many large molecular weight compounds with low unsaturation. Regardless of unsaturation, molecules of River HA have a homogeneous molecular mass distribution and contain many plant-derived lignin- and tannin-like compounds, which are more stable than lipid and more labile than condensed aromatics. Leonardite HA with a high degree of humification contains a large number of compounds with high aromaticity and more heteroatoms and has low lability. Our results reveal the diversity of humic acid at molecular level because of different degree of humification and the lability. These conclusions are significant for understanding the role of humic acid from different sources in pollutant transformation and the geochemical cycle at the molecular level.

Fluorescence characteristics and biodegradability of dissolved organic matter (DOM) leached from non-point sources in southeastern China

Under the increasingly intensive measures for surface water restoration in China, point source discharge has been strictly regulated; however, for non-point sources, which constitute a large part of surface water pollutants, effective control has been difficult to reach. A comprehensive understanding of the characteristics of non-point source pollutants is essential for surface water improvement programs of cities such as Ningbo, on the southeast coast of China. Ningbo has made tremendous efforts in the past few years to control point source pollutants, but available data and management strategies on the non-point source pollutants are still limited. To this end, leachates of representative non-point source samples from the territory of Ningbo, including cropland and wetland soil, urban channel sediment, and poultry manure, were examined and compared focusing on the fluorescence characteristics and biodegradability of the dissolved organic matter (DOM). Results indicated that biodegradable dissolved organic carbon (BDOC) accounting for the total DOC was 46.7 ± 0.7% for cropland, wetland (56.3 ± 6.8%), non-sewage channel (60.1 ± 0.4%), sewage channel (74.5 ± 1.1%), and poultry manure (62.7 ± 4.5%). The leachates of the studied samples showed significant differences in both the amount and composition of DOM. However, a fluorescence component representing tryptophan-like substances identified by the excitation-emission matrix (EEM) combined with parallel factor (PARAFAC) analysis effectively predicted the BDOC variations among the studied samples. Moreover, under the studied nutrient concentrations, which were equivalent to Grade III water quality in China, nutrient limitation of microbial degradation was not observed. Threats to water quality, especially excessive consumption of dissolved oxygen, could be posed by the non-point source leachates due to their high bioavailability, large distribution, and weak nutrient restraint. Further investigations, including a quantitative evaluation of the non-point source pollution contribution, and pollutant blocking techniques are required.

High-dispersion zero-valent iron particles stabilized by artificial humic acid for lead ion removal

Nano zero-valent iron (nZVI), as a high-efficiency adsorbent for heavy metals, often suffers being oxidized and assembling together due to small size and super reactivity, further decreasing its adsorption performance and limiting application ranges. Herein, we have designed a novel adsorbent with high-dispersion nZVI stabilized by as-prepared artificial humic acid (AHA-nZVI) derived from hydrothermal humification (HTH) technology. Introduction of artificial humic acid (A-HA) can effectively reduce the oxidation and agglomeration of nZVI, leading to superior kinetic removal efficiency of Pb²⁺ (> 99.2%) and huge Langmuir removal capacity of 649.0 mg/g. The combination of nZVI and A-HA (contained abundant functional groups, i.e. -OH and -COOH) via C-O-Fe bonding makes nZVI have good dispersion and oxidation resistance. Multiple interaction mechanisms including reduction reaction, complexation and co-precipitation between heavy metals and AHA-nZVI samples are realized. Overall, AHA-nZVI is a promising material for high-performance heavy metal contaminated water treatment.

The influence of the quantity and quality of sediment organic matter on the potential mobility and toxicity of trace elements in bottom sediment

Knowledge on the fraction of trace elements in the bottom sediments is a key to understand their mobility and ecotoxicological impact. The purpose of this study was to assess the influence of the content of organic matter fractions on the mobility and ecotoxicity of trace elements in sediments from the Rybnik reservoir. The most refractory fraction of organic matter-Cnh (non-hydrolysing carbon)-dominated in the sediments. The content of organic matter fractions are arranged in the following order: Cnh (non-hydrolysing carbon) > Cfa (fulvic acid) > Cha (humic acid) > DOC (dissolved organic carbon). On the other hand, the highest value of correlation coefficients was found for different fractions of trace elements and DOC content in the bottom sediments. A higher content of TOC in the sediments significantly increased the share of elements in the potential mobile fraction and, at the same time, decreased the binding of elements in the mobile fractions. Moreover, in sediments that contain more than 100 g/kg d.m. TOC, no and medium risk of trace element release from sediments was observed. The Cu, Cd and Ni were potentially the most toxic elements for biota in the Rybnik reservoir. However, the correlation between the content of trace elements and the response of bacteria was insignificant. These results suggested that the complexation of trace elements with organic matter makes them less toxic for Vibrio fischeri. The transformation and sources of organic matter play an important role in the behaviour of trace elements in the bottom sediments of the Rybnik reservoir.

Structural characteristics of humic substances in buried ancient paddy soils as revealed by 13C NMR spectroscopy

The study of organic matter in ancient paddy soils is helpful for understanding the influence of human activities on soil carbon sequestration and global climate change. However, little information on the spatial distribution and structural characteristics of the humic substances (HS) in ancient paddy soils is available. The spatial distributions of humic acids (HAs) and fulvic acids (FAs) in ancient paddy soils and modern cultivated paddy soils at the Shanlonggang site on the Liyang Plain were investigated, and the associated structures were characterized by using ¹³C nuclear magnetic resonance (NMR). The ¹³C NMR spectra revealed the following carbon types in HAs and FAs in both types of paddy soil in order of decreasing abundance: O-alkyl carbon (ranging from 39.7 to 51.8% and from 42.6 to 50.9%, respectively) ≥ alkyl carbon (ranging from 16.8 to 23.5% and from 15.7 to 22.4%, respectively) ≈ carboxyl carbon (ranging from 13.3 to 19.3% and from 16.9 to 22.0%, respectively) > aromatic carbon (ranging from 12.8 to 23.5% and from 10.0 to 17.2%, respectively). Moreover, the degree of aromaticity of HA was higher than that of FA in both soil samples. The humic constituents of the buried ancient paddy soils were less aromatic and oxidized than those of the modern cultivated paddy soils. The organic carbon in the ancient paddy soils was also less aromatic and oxidized than that in the modern cultivated paddy soils, suggesting that the structures of the HS in the ancient paddy soils were relatively simple. The results of this study provide new insights into the effect of secondary paddy soil formation on the spatial distribution, structural characteristics, and stability mechanisms of the HS in ancient paddy soils.

Effects of organic wastes on structural characterizations of fulvic acid in semiarid soil under plastic mulched drip irrigation

Poor soil has restricted the crop production in semiarid soil. Fulvic acid (FA) is considered to play an significant role in soil fertility. The amount and structure of FA after application of different organic wastes (OWs) was assessed in this work. Six treatments were involved in this experiment: chemical fertilizer combined with chicken manure (CM), sheep manure (SM), maize straw (MS), fodder grass (FG), and tree leaves (TL), while chemical fertilizer only was used as control (CK). The soil FA content (P < 0.05) after the application of TL was the highest than other OWs. The E₄/E₆ ratios, ΔlogK values, aliphatic C/aromatic C ratios of soil FA after the application of TL were the lowest than other OWs, whereas the C/H ratio was the highest. The specific fluorescence intensities (SFI) of peak A (Ex/Em 260-265/415-430 nm) and peak B (Ex/Em 310-315/415-430 nm) from EEM fluorescence spectrum of FA were the lowest after the application of TL. In conclusion, the application of TL was the most effective for improving FA accumulation, and making FA complex and stability. Thus, TL is the recommended OW for use in semiarid soil under plastic mulched drip irrigation conditions.

Significant contribution of metastable particulate organic matter to natural formation of silver nanoparticles in soils

Particulate organic matter (POM) is distributed worldwide in high abundance. Although insoluble, it could serve as a redox mediator for microbial reductive dehalogenation and mineral transformation. Quantitative information on the role of POM in the natural occurrence of silver nanoparticles (AgNPs) is lacking, but is needed to re-evaluate the sources of AgNPs in soils, which are commonly considered to derive from anthropogenic inputs. Here we demonstrate that POM reduces silver ions to AgNPs under solar irradiation, by producing superoxide radicals from phenol-like groups. The contribution of POM to the naturally occurring AgNPs is estimated to be 11-31%. By providing fresh insight into the sources of AgNPs in soils, our study facilitates unbiased assessments of the fate and impacts of anthropogenic AgNPs. Moreover, the reducing role of POM is likely widespread within surface environments and is expected to significantly influence the biogeochemical cycling of Ag and other contaminants that are reactive towards phenol-like groups.

Demonstration of Chemical Distinction among Soil Humic Fractions Using Quantitative Solid-State 13C NMR

Humic substances (HS) are vital to soil fertility and carbon sequestration. Using multiple cross-polarization/magic-angle spinning (multiCP/MAS) NMR combined with dipolar dephasing, we quantitatively characterized humic fractions, i.e., fulvic acid (FA), humic acid (HA), and humin (HM), isolated from two representative soils (upland and paddy soils) in China under six long-term (>20 years) fertilizer treatments. Results indicate that each humic fraction showed chemical distinction between the upland and paddy soils, especially with much greater aromaticity of upland HMs than of paddy HMs. Fertilizer treatment exerted greater influence on chemical natures of upland HS than of paddy HS, although the effect was less than that of soil type. Organic manure application especially decreased the percentages of aromatic C in the upland HAs and HMs compared with the control. We concluded that humic fractions responded in chemical nature to environmental conditions, i.e., soil type/cropping system/soil aeration and fertilizer treatments.

Characteristic and mechanism of sorption and desorption of benzene on humic acid

The sorption/desorption behaviors of benzene, toluene, ethylbenzene, and xylene (BTEX) on soil organic matter (SOM) have a significant influence on their fate and bioavailability in soil. Humic acid (HA) is a major fraction of SOM. And due to its various structural properties and chemical composition, the sorption/desorption characteristics and mechanisms of HA are diverse for organic contaminants. In this study, batch experiments were conducted to investigate the sorption/desorption behavior of benzene on HA at different conditions (temperature, pH, and ionic strength). The particle size of HA increased at lower initial pH which promoted sorption capacity for benzene, illustrating that HA with larger particle size may develop preferential chemical conformation for benzene sorption at lower pH. Sorption isotherms indicated that the sorption of benzene on HA is an exothermic and spontaneous physical process. And kinetic studies showed that the sorption of benzene on HA is controlled by the diffusion process and the availability of sorption sites. Meanwhile, weak sorbent-sorbate interaction is presented in the desorption experiment. There was no obvious effect of ionic strength on benzene sorption, suggesting that the sorption process is not controlled by ion-exchange or electrostatic interaction. Combined with FTIR analysis, the hydrophobic partitioning and π-π conjugative interaction are the possible sorption mechanisms of benzene on HA.

Organic Carbon Sequestration in Soil Humic Substances As Affected by Application of Different Nitrogen Fertilizers in a Vegetable-Rotation Cropping System

Little is known on the effect of application of different nitrogen (N) fertilizers on soil organic carbon (SOC) sequestration in soil humic substances (HS). We investigated HS molecular characteristics in an Orthic Acrisol, southwestern China, under 2-year field fertilization of a urea (U), a polymer-coated urea (PCU) and a biochar-coated urea (BCU) using ¹³C-CPMAS-NMR spectroscopy. Results showed that N fertilization promoted SOC sequestration into HS and favored alkyl-C and aromatic-C rather than O-alkyl-C and carbonyl-C for humic acids and humin in soil. Application of PCU and BCU may better enhance vegetable yield, SOC sequestration, and HS stability than the U application, which may benefit from longer time of N existence and higher total N in soil. Among the N treatments, BCU application mostly affected the compositions and stability of SOC in the HS for the OC input and prime effect of biochar for SOC transformation.

Humic substances from green waste compost: An effective washing agent for heavy metal (Cd, Ni) removal from contaminated sediments

In this study, humic substances (HS) selected from 8 composting groups (peanut straw, sesame straw, corn straw and deciduous leaves, with or without grape marc) were used to remove Cd and Ni from artificially contaminated sediments. Sesame straw compost appeared to have the highest removal capacity for heavy metals through a series comparison on Cd removal efficiency, yield of HS and fulvic acids (FA), and seed germination index. The selected sesame HS was further used to wash two contaminated sediments of varying properties (a clay type for sediment 1 and a silty loam for sediment 2). Batch desorption experiments were conducted to determine the optimum HS concentration, equilibrium time, pH, solid-to-liquid ratio, and washing frequency. Under optimum conditions, a triple washing removed 74.16% of Cd and 42.91% of Ni from sediment 1, and 86.88% of Cd and 43.84% of Ni from sediment 2, respectively, whereas a commercial FA only achieved half of the efficiency. After washing, both sediments were identified with increased contents of total organic matter total nitrogen and phosphorus. Therefore, HS from the sesame straw compost is a cost-effective, efficient and environmental-friendly washing agent to remove heavy metals from contaminated sediments.

Humic Acids Limit the Precipitation of Cadmium and Arsenate at the Brushite-Fluid Interface

Bioavailability and mobility of cadmium (Cd²⁺) and arsenate (As⁵⁺) in soils can be effectively lowered through the dissolution of brushite (dicalcium phosphate dihydrate, CaHPO₄·2H₂O) coupled with the precipitation of a more stable mineral phase containing both Cd and As. Due to the ubiquitous presence of humic acid (HA) in soil environments, it is more complex to predict the fate of dissolved Cd and As during such sequestration. Here, we used in situ atomic force microscopy (AFM) to image the kinetics of simultaneous precipitation of Cd and As at the brushite-fluid interface in the presence of HA. Results show that HA inhibits the formation of both amorphous and crystalline Cd_{(5- x)}Ca _x(PO₄)_{(3- y)}(AsO₄) _y(OH) on the (010) face of brushite. A combination of X-ray photoelectron spectroscopy (XPS) and real-time surface-enhanced Raman spectroscopy (SERS) reveals that part of As⁵⁺ reduction into As³⁺ with HA and [HA-Cd] complexation occurs, modulating the concentrations of free Cd²⁺ and As⁵⁺ ions to inhibit subsequent precipitation of a Cd_{(5- x)}Ca _x(PO₄)_{(3- y)}(AsO₄) _y(OH) phase on the dissolving brushite surface. A combination of AFM imaging, SERS analyses, and PhreeqC simulations suggests that environmentally relevant humic substances can limit the precipitation of Cd and As at mineral surfaces through a mechanism of oxidation/reduction and aqueous/surface complexation. This may exacerbate the transportation of these contaminants into waters by subsurface fluid flow, and research attempts to weaken the negative effect of HA are needed.

Protaetia brevitarsis larvae can efficiently convert herbaceous and ligneous plant residues to humic acids

Utilization of the organic residues produced after crop harvesting is currently an important issue across the world. The edible insect Protaetia brevitarsis larvae can feed various organic matters. In this paper, we investigated the potential to utilize the insect to convert herbaceous and ligneous plant residues. We feed the insect larvae with maize straw and sawdust and analyzed the produced insect manure. P. brevitarsis larval was found to be able to digest both herbaceous and ligneous straw and insect manure extract shown no phytotoxicity. The mass fractions of humic acids (HAs) in the insect manure derived from maize straw and sawdust digestion were 24.37% and 14.46%, respectively. The ¹³C cross-polarization magic-angle spinning nuclear magnetic resonance (CP-MAS NMR) spectra data indicated that the HAs in the insect manure were similar to those found in the soil. These data suggested that P. brevitarsis larvae can be used to convert agricultural residues and produce organic fertilizers.

Humin as an External Electron Mediator for Microbial Pentachlorophenol Dechlorination: Exploration of Redox Active Structures Influenced by Isolation Methods

Humin (HM) has been reported to function as an external electron mediator (EEM) in various microbial reducing reactions. In this study, the effect of isolation methods on EEM functionality and the chemical/electrochemical structures of HM were examined based on the correlation between dechlorination rates in the anaerobic HM-dependent pentachlorophenol (PCP)-dechlorinating consortium and the chemical/electrochemical structures of HM. A lack of PCP dechlorination activity suggested no EEM function in the HM samples prepared as a soluble fraction in dimethyl sulfoxide and sulfuric acid (which did not contain any electric capacitance). Other HM samples exhibited EEM functionality as shown by the dechlorination activity ranging from 0.55 to 3.48 (µmol Cl⁻) L⁻¹d⁻¹. The comparison of dechlorination activity with chemical structural characteristics suggested that HM with EEM functionalities had predominantly aliphatic and carbohydrate carbons with the partial structures C=O, O=C–N, and O=C–O. EEM functionality positively correlated with the proportion of O=C–N and O=C–O, suggesting an association between peptidoglycan structure and EEM functionality. The lack of detection of a quinone structure in one HM sample with EEM functionality and a negative correlation with aromatic or C=C carbon suggested that the mechanism containing quinone structures is a minor component for the functionality of EEM.

Fungistatic Activity of Multiorigin Humic Acids in Relation to Their Chemical Structure

Humic acid (HA) has an inhibitory effect on phytopathogenic fungi, but the structure-activity relationship remains unclear. HAs were extracted from 14 different materials, and their fungistatic activities and elemental C, N, S, and O contents were measured. Cross-polarization magic-angle spinning ¹³C nuclear magnetic resonance (CPMAS ¹³C NMR) was used to measure the organic carbon composition. The results showed that all HAs suppressed phytopathogenic fungi growth, with Yunnan lignite HAs showing the highest inhibition (85.3%) against Physalospora piricola. The soil and compost HA aromaticity (ARO) was <50%, except for black soil HAs, while the ARO of all coal HAs was >60%. The ARO of meadow and moss peat HAs was <50%, while the ARO of woody peat HAs was 50.61%. Mantel test and redundancy analysis (RDA) were applied to evaluate the structure-activity relationship. The Mantel test revealed that the N, S, O, N/O, carbonyl C, aromatic C-O, and anomeric C contents were significantly correlated with fungistatic activity. The RDA analysis showed that the S content was positively correlated with fungistatic activity, while the O content was negatively correlated. The carbonyl C content had a positive correlation with fungistatic activity, while the anomeric C and aromatic C-O content had a negative correlation. A high S content and an active composition (carbonyl C) in HAs would lead to a high degree of fungistatic activity. Phytotoxicity test indicated all HAs were beneficial to plant growth. This work identified the basic properties of HAs from various raw materials that control their fungistatic activities.

Biochemical stabilization of soil organic matter in straw-amended, anaerobic and aerobic soils

Crop straw incorporation is a useful approach for increasing the quantity and changing the chemical composition of soil organic matter (SOM). This process is influenced by soil aeration. The present study investigated the stability of whole SOM, particulate organic matter (POM) and mineral-associated organic matter (MinOM) fractions with wheat straw amendment under aerobic and anaerobic conditions over a 12-month incubation period. Solid-state nuclear magnetic resonance and Fourier transform infrared spectroscopy were used to analyze the chemical composition of whole SOM, POM and MinOM fractions. The decomposition rate of wheat straw was lower under anaerobic than under aerobic conditions (0.014 vs. 0.020day^-1). Wheat straw incorporation increased the original soil organic carbon content (7.4g kg^-1) under both aerobic (up to 10.2g·kg^-1) and anaerobic (up to 10.3g·kg^-1) conditions, but the content of mineral-associated organic carbon (MinOC) under aerobic condition (7.0g·kg^-1) was significantly larger than that under anaerobic condition (4.9g·kg^-1). The proportion of alkyl carbon (C) in SOM, POM and MinOM fractions was greater under anaerobic than under aerobic conditions, while the opposite was true for the proportion of O-alkyl C of SOM and POM and MinOM fractions. A/O-A indices (i.e., the ratio of alkyl C to O-alkyl C) of whole SOM, POM and MinOM were higher under anaerobic than under aerobic conditions. We conclude that wheat straw incorporation resulted in the enrichment of alkyl C in the POM and MinOM fractions under anaerobic conditions, and thus improved the stability of SOM. In this way, the decomposition of crop residue influenced SOM structural chemistry at the molecular level.

Humic acids facilitated microbial reduction of polymeric Pu(IV) under anaerobic conditions

Flavins and humic substances have been extensively studied with emphasis on their ability to transfer extracellular electrons to insoluble metal oxides. Nevertheless, whether the low-solubility Pu(IV) polymers are microbially reduced to aqueous Pu(III) remains uncertain. Experiments were conducted under anaerobic and slightly alkaline conditions to study the difference between humic acids and flavins to transport extracellular electrons to Pu(IV) polymers. Our study demonstrates that Shewanella putrefaciens was unable to directly reduce polymeric Pu(IV) with a notably low reduction rate (3.4×10^-12mol/L Pu(III)_aq within 144h). The relatively high redox potential of flavins reveals the thermodynamically unfavorable reduction: E_h(PuO₂(am)/Pu³⁺)<E_h^o'(FMN/FMNH₂)≈E_h^o'(RBF/RBFH₂)≈-220mV at pH7.2. The microbially reduced humic acids facilitated the extracellular electron transfer to the polymers and reduced polymeric Pu(IV) (2.1×10^-10mol/L Pu(III)_aq) 62 times more rapidly than the flavins. The driving force for electron transfer explains the observed reduction: E_h(HA_ox/HA_red)<E_h(PuO₂(am)/Pu³⁺) when S. putrefaciens oxidized lactate and respired on the humic acids. In contrast, flavins were able to substantially reduce aqueous Pu(IV)-EDTA (1.9×10^-9mol/L Pu(III)_aq) because of the available driving force for electron transfer: Δ_rG_m=-F[E_h(PuL₂^4-/PuL₂^5-)-E_h^o'(FMN/FMNH₂)]=-33.5kJ/mol is a result of E_h(PuL₂^4-/PuL₂^5-)≫E_h(PuO₂(am)/Pu³⁺), where L is the EDTA ligand. In the presence of humic acids, the reduction of Pu(IV)-EDTA exhibited the most rapid rate (2.2×10^-9mol/L Pu(III)_aq). This result further demonstrates that humic acids facilitated the extracellular electron transfer to polymeric and aqueous Pu(IV). Reductive solubilization of the polymers may enhance Pu mobility in the geosphere and hence increases risks to human health.

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 (q_mic) 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 q_mic 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.

Characterization of transformations of maize residues into soil organic matter

An awareness of the transformation of plant residues returned to cultivated soils is vital for a better understanding of carbon cycles, the maintenance of soil fertility and the practice of a sustainable agriculture. The transformation of maize (Zea mays L) straw residues into soil organic matter (SOM) in a one year incubation experiment was studied in a soil that had been under long term cultivation with wheat (Triticum aestivum L) for >30years. A novel sequential exhaustive extraction and fractionation procedure isolated a series of fractions of SOM. The samples were characterized by elemental and δ¹³C analyses, by amino acids and neutral sugars analyses, by Fourier transformed infrared (FTIR) spectrometry, and by solid state ¹³C nuclear magnetic resonance (NMR) spectroscopy and with chemical shift anisotropy (CSA) -filter and dipolar dephasing (DD) spectral editing NMR techniques. The δ¹³C data indicated that 59% and 38% of the newly transformed organic carbon was in the humic and fulvic acid fractions, respectively, and in general a greater proportion of the transformed carbon was in the fractions isolated at the higher pH values. Results for SOM fractions from the amended soil indicate dominant contributions from carbohydrate and lignin-like material, and that can be clearly identified by FTIR, CP/TOSS, and spectral editing of CSA-filter and DD. The compositions of the fractions from the amended and non-amended soils fractions can be clearly differentiated using principal component analysis (PCA) for the data collected. The sequential extraction procedure showed that the hydrophilicity of humic fractions increased as the result of the maize amendment, and the aromaticity of the fraction decreased. The data may give some indications of transformations that take place during humification processes.