Labile Organic Compounds Research Articles

Soil Texture Alters the Impact of Salinity on Carbon Mineralization

Soil salinization typically inhibits the ability of decomposer organisms to utilize soil organic matter, and an increase in soil clay content can mediate the negative effect of salinity on carbon (C) mineralization. However, the interactive effects of soil salt concentrations and properties on C mineralization remain uncertain. In this study, a laboratory experiment was performed to investigate the interactive effects of soil salt content (0.1%, 0.3%, 0.6% and 1.0%) and texture (sandy loam, sandy clay loam and silty clay soil with 6.0%, 23.9% and 40.6% clay content, respectively) on C mineralization and microbial community composition after cotton straw addition. With increasing soil salinity, carbon dioxide (CO2) emissions from the three soils decreased, but the effect of soil salinity on the decomposition of soil organic carbon varied with soil texture. Cumulative CO2 emissions in the coarse-textured (sandy loam and sandy clay loam) soils were more affected by salinity than those in the fine-textured (silty clay) soil. This difference was probably due to the differing responses of labile and resistant organic compounds to salinity across different soil texture. Increased salinity decreased the decomposition of the stable C pool in the coarse-textured soil, by reducing the proportion of fungi to bacteria, whereas it decreased the mineralization of the active C pool in the fine-textured soil through decreasing the Gram-positive bacterial population. Overall, our results suggest that soil texture controlled the negative effect of salinity on C mineralization through regulating the soil microbial community composition.

Динаміка лабільних органічних сполук ясно-сірих лісових поверхнево оглеєних ґрунтів за тривалого агрогенного впливу

Goal. To study the dynamics of labile organic compounds of light gray forest surface gleyed soil in the process of spring barley vegetation depending on the long-term application of different fertilizer systems and periodic liming. Methods. Field research, physicochemical and comparative analytical. Results. The content of labile humus in the arable layer of acidic light gray forest soil is insignificant and does not exceed 0.73%. Mineral fertilizer systems with long-term use cause the lowest content of labile humus compounds, which does not exceed 0.50% during the growing season. Application of organomineral fertilizer systems with application per 1 ha of crop rotation area of one norm of mineral fertilizers (N65P68K68), 10 t/ha as manure on the background of liming with a dose of CaCO3, calculated on hydrolytic acidity and acid-based buffering provides during the growing season almost the same contents of labile (0.49 – 0.60%) and water-soluble organic (0.025 – 0.042%) matters. The lack of fertilizer during the spring barley vegetation caused the lowest indicators of water-soluble organic matter (0.013–0.02%) in the control variant. Conclusions. The supply of light gray forest surface gleyed soils with labile forms of organic matter under conditions of long-term agrogenic exposure varies depending on the fertilizer systems and their use.

The Influence of Spent Mushroom Substrate Fertilization on The Selected Properties of Arable Soil

Abstract In the last decades, Poland has become the leader in mushroom Agaricus bisporus production in Europe. Mushroom production generates large amounts of spent substrate, which as a waste material must be recycled. Significant in this macronutrient content, inducing nitrogen, encourages the use of this organic waste material to fertilize, especially light soils. The aim of the study was to assess the effect of spent mushroom substrate fertilization on the properties of A horizon of the Stagnic Luvisol, especially for the variation in the nitrogen fraction, in the two-year cultivation period. The following laboratory analyses were performed: soil pH, organic carbon content, total content of nitrogen and sorption properties. The sequential extraction of nitrogen compounds was performed with a 0.25 M KCl solution (for the extraction of mineral nitrogen forms and the most labile organic nitrogen compounds) and with 0.25 and 2.5 M H2SO4 (hot hydrolysis for sequencing of organic nitrogen compounds that are easily hydrolysing and difficult to hydrolyse). The application of spent mushroom substrate to the A horizons of the experimental objects had an effect on differentiation of soil properties and increased their value (pH, Corg, N-tot, Hh, CEC) in comparison to the control object to a greater degree after the first, then after the second year of cultivation. As the content of nitrogen mineral forms was differentiated, more nitrogen was found in an ammonium form (N-NH4) than in a nitrate form (N-NOx). The organic nitrogen content formed the following series of increasing amounts: soluble organic nitrogen NOS < NOEH < NODH < NONH.

Regulation of Microbial Activity Rates by Organic Matter in the Ross Sea during the Austral Summer 2017.

The active prokaryotic communities proliferate in the ecosystems of the Antarctic Ocean, participating in biogeochemical cycles and supporting higher trophic levels. They are regulated by several environmental and ecological forcing, such as the characteristics of the water masses subjected to global warming and particulate organic matter (POM). During summer 2017, two polynyas in the Ross Sea were studied to evaluate key-microbiological parameters (the proteasic, glucosidasic, and phosphatasic activities, the microbial respiratory rates, the prokaryotic abundance and biomass) in relation to quantitative and qualitative characteristics of POM. Results showed significant differences in the epipelagic layer between two macro-areas (Terra Nova Bay and Ross Sea offshore area). Proteins and carbohydrates were metabolized rapidly in the offshore area (as shown by turnover times), due to high enzymatic activities in this zone, indicating fresh and labile organic compounds. The lower quality of POM in Terra Nova Bay, as shown by the higher refractory fraction, led to an increase in the turnover times of proteins and carbohydrates. Salinity was the physical constraint that played a major role in the distribution of POM and microbial activities in both areas.

Influence of Algal Production and Decomposition on the Carbon Isotope Signature of Labile Particulate Organic Matter on a Productive Continental Shelf Under the Stress of Coastal Hypoxia

To better understand the sources and recycling of labile organic matter in coastal water, we studied the carbon isotope signature of particulate organic carbon (POC) and particulate amino acids, a major group of labile organic compounds, on the shelf of the northern South China Sea. In addition, we were able to compare effects of hypoxia on labile organic matter at one station on the shelf. We found that carbon-weighted average d13C values of particulate amino acids (wAAd13C) were generally higher at more productive offshore stations than nearshore stations. This amino acid d13C distribution suggests that the labile fraction of organic matter in coastal water originated primarily from local primary production. In contrast, bulk POC d13C values showed little spatial variation on the NSCS shelf. The different patterns of wAAd13C and PO13C distributions indicate a combined effect of POM lability and physical processes on the isotopic distribution of organic materials on this productive shelf. Moreover, the amino acid d13C in coastal hypoxic water suggests local primary production as the most likely source of labile organic matter that drives hypoxia on the NSCS shelf. No clear difference was observed in particulate amino acid d13C distribution between hypoxic and oxic waters,

Soils and Nitrogen Nutrition of Plants in Alpine Ecosystems of the Northwest Caucasus under Long-Term Increase in Availability of Biogenic Elements

An experiment with the enrichment of acid organic-rich mountain-meadow soils (Umbric Leptosols) of alpine ecosystems with mineral nutrients has demonstrated that the contents of organic carbon, total nitrogen, and labile organic compounds are stable and tolerant towards long-term (20 years) application of mineral fertilizers. Only the following direct effects are well pronounced: an increase in the content of inorganic nitrogen and phosphorus after the application of corresponding fertilizers, as well as a rise in pH as a result of liming. Plants are more sensitive indicators of changes in the conditions of nitrogen nutrition. They are characterized by active absorption of additional nitrogen, and the isotopic composition of nitrogen in them becomes heavier. The degree of these effects depends on the initial nitrogen availability. In addition to these direct effects, changes in the nitrogen status of plants also reflect changes in the transformation of nitrogen-containing compounds in soil and in the nitrogen nutrition of plants taking place due to an increased availability of phosphorus and lower acidity under the most phosphorus-depleted and most acid conditions, respectively. These impacts lead to the mobilization of soil organic nitrogen and are likely to reduce the role of mycorrhiza in plant nitrogen nutrition. As a result, a heavier isotopic composition of nitrogen is formed in some plant species.

The Conversion of Abandoned Chestnut Forests to Managed Ones Does Not Affect the Soil Chemical Properties and Improves the Soil Microbial Biomass Activity

Recently, several hectares of abandoned chestnut forests (ACF) were recovered into chestnut stands for nut or timber production; however, the effects of such practice on soil mineral horizon properties are unknown. This work aimed to (1) identify the better chestnut forest management to maintain or to improve the soil properties during the ACF recovery, and (2) give an insight into the effect of unmanaged to managed forest conversion on soil properties, taking in consideration sweet chestnut (Castanea sativa Mill.) forest ecosystems. The investigation was conducted in an experimental chestnut (Castanea sativa Mill.) forest located in the northern part of the Apennine chain (Italy). We identified an ACF, a chestnut forest for wood production (WCF), and chestnut forests for nut production with a tree density of 98 and 120 plants ha−1 (NCFL and NCFH, respectively). WCF, NCFL and NCFH stands are the result of the ACF recovery carried out in 2004. After 15 years since the ACF recovery, generally, the effects on the main soil chemical properties were negligible. Some differences occurred for the water-soluble organic carbon (WSOC) and microbial biomass and its activity. NCFL showed the highest WSOC content in the uppermost soil horizon likely due to higher amount of roots which are source of labile organic compounds. The higher WSOC amount might explain the greatest amount of microbial biomass in the A horizon of NCFL. Furthermore, the microbial biomass harboring in the A horizon of NCFL has also shown both a better C use efficiency and a larger soil organic carbon immobilization in the microbial biomass itself. Our data would indicate that the ACF recovery into pure chestnut forests did not have negative impacts on soil chemical and biochemical properties, though chestnut stands for nut production with a low plant density are the most suitable ones.

Biotic and abiotic nitrogen immobilization in soil incorporated with crop residue

Incorporating crop residue favors biotic and abiotic nitrogen (N) immobilization, which can effectively conserve active N in soil. Here we summarize the occurrence characteristics of biotic and abiotic N immobilization and affecting factors, and then identify a general pattern of the relative importance of the two processes in soil incorporated with crop residue. When microorganisms decompose crop residue for a source of energy and carbon (C) to support their metabolism, they need N to build cellular components, resulting in N immobilization in biomass. Alternatively, N can be incorporated into soil organic matter (SOM) through the following known mechanisms: ammonium (NH4+) fixation by clay minerals, condensation of ammonia (NH3) with phenol or quinone rings, and nitrosation of nitrite (NO2-) with phenolic compounds. The biotic and abiotic contribution to N immobilization is significantly controlled by soil properties and crop residual quality. When a large fraction of C is in the form of labile organic compounds, biotic N immobilization may be important. In contrast, high fraction of lignin-derived phenolic compounds or other recalcitrant organic compounds facilitates abiotic N immobilization. In the N-limited soil, the increase in N availability contributes to microbial activity, which would stimulate biotic processes. However, when microbial metabolism switches from N-limited to C-limited due to the increase in N availability, abiotic processes become important. Abiotic processes even predominate over biotic processes under N-enriched conditions. The relative roles of biotic or abiotic N immobilization in soil incorporated with crop residue are dependent on the availability of C relative to N and soil N availability. Carbon and N availability in turn is related to their forms. Moreover, N availability is affected by soil pH. The mechanisms by which N is immobilized determine the fate of immobilized N and are of vital importance for the management of N availability and losses through combining N fertilizer with crop residue. More investigation is necessary to quantify the contribution of biotic and abiotic processes during the decomposition of crop residue and to determine the factors affecting N immobilization in the field.

Molecular Evidence of Arsenic Mobility Linked to Biodegradable Organic Matter.

Molecular characteristics of natural organic matter (NOM) and their potential connections to arsenic enrichment processes remain poorly understood. Here, we examine dissolved organic matter (DOM) in groundwater and water-soluble organic matter (WSOM) in aquifer sediments being depth-matched with groundwater samples from a typical arid-semiarid basin (Hetao Basin, China) hosting high arsenic groundwater. We used Fourier transform ion cyclotron resonance mass spectrometry to determine molecular characteristics of DOM and WSOM and evaluate potential roles of biodegradable compounds in microbially mediated arsenic mobility at the molecular level. High-arsenic groundwater DOM was generally enriched in recalcitrant molecules (including lignins and aromatic structures). Although potential contribution of recalcitrant compounds to arsenic enrichment cannot be ruled out, preferential degradation of the labile molecules coupled with reduction of Fe(III) (oxyhydr)oxides seemed to dominate arsenic mobilization. Both the number and the intensity of biodegradable compounds (including aliphatic/proteins and carbohydrates) were higher in WSOM than those in DOM in depth-matched high-arsenic groundwater (arsenic >0.67 μmol/L or 50 μg/L). Groundwater arsenic concentration generally increased with the increase in the number and the intensity of unique biodegradable compounds (especially N-containing compounds) in WSOM at matched depths. Anoxic incubations of sediments and deionized water show that more arsenic and Fe(II) were released from aquifer sediments with greater numbers and intensities of consumed biodegradable compounds in WSOM (especially N-containing compounds), with a higher proportion of microbially derived compounds produced. These observations indicate that the biodegradation of aliphatic/proteins and carbohydrates (especially CHON formulas) in WSOM fueling the reductive dissolution of Fe(III) (oxyhydr)oxides predominantly promotes arsenic release from aquifer solids. Our unique data present a better understanding of arsenic mobilization shaped by microbial degradation of labile organic compounds in anoxic aquifers at the molecular level.

Applicability of conventional and non-conventional parameters for municipal landfill leachate characterization

The disposal of municipal solid waste (MSW) in landfills generates leachate, a highly polluting liquid to the aquatic environment. Leachate composition become a challenge to choose the best treatment process. Then, detailed techniques to determine the organic content, in terms of refractability, composition, sources and biodegradability in landfill leachate can help to choose the appropriate treatment and improve landfill management. In this sense, the aim of this study is to apply conventional and non-conventional parameters through inert chemical oxygen demand (COD) analyses and spectroscopic techniques of fluorescence and UV–vis absorbance for the characterization of municipal landfill leachate. Results indicated that physicochemical characterization cannot provided enough detailed information about leachate composition, which becomes the treatment process fragile. Inert COD, besides have high time to execution (∼30 days), presented additional information on potential of biological treatability in anaerobic conditions. Dissolved organic matter (DOM) characterization showed transitions between labile and refractory organic matter compounds. Moreover, the cost estimated showed that non-conventional parameters analysis have lower investment than conventional, being their implementation feasible. In conclusion, the synergy between conventional and non-conventional parameters, and the detailed information provided by inert COD and DOM characterization, shown a useful tool to the landfill management and, consequently, improving treatment process and its efficiency.

C:N stoichiometry of stable and labile organic compounds determine priming patterns

Priming effects (PE) can greatly influence global carbon (C) storage in soil and lead to climate feedbacks by accelerating the decomposition of organic matter (OM). Although nitrogen (N) availability can alter the magnitude and direction of priming (stoichiometric constrains), it remains unclear whether additions of NO3− (nitrate) and NH4+ (ammonium) have distinct effects on the decomposition of various OM. Thus, the aims of this study were to investigate the responses of OM decomposition along a decay continuum (i.e. decreasing decomposition degree) to labile C and N inputs and determine the PE induced by the two N forms. Four OM forms, namely leaf litter, wood litter, organic soil horizon, and mineral soil, with a broad range of C:N ratios were collected along a decay continuum in a typical subtropical forest and incubated for 38 days with labile C (13C labeled glucose) and N (NO3−) additions. Based on the very broad range of C:N ratios in OM in soil and inputs of labile C and N, we demonstrated the OM decomposition within a decay continuum as well as PE intensities and the thresholds for the switch of PE directions. In contrast to NH4+ additions, NO3− generally accelerated the decomposition of all OM. Priming of plant litter was dependent on the C:N ratios of the labile inputs. However, leaf litter decomposition was more controlled by N addition than wood litter. Glucose addition greatly increased the priming of OM decomposition in soils, demonstrating energy limitation for microorganisms. Distinct priming patterns were observed between NO3− and NH4+ additions, both for the individual OM types and for all four types of OM. On a basis of C balance between primed C and the remaining added C, PE induced by labile C and N inputs can increase or reduce C sequestration depending on C:N stoichiometric ratios of labile inputs. Our findings provide important insights into the specific role of NO3− or NH4+ together with labile C inputs (e.g. from root exudation), and thus changes in the composition of deposited N (atmospheric deposition and fertilization) may induce distinct climate feedbacks.

Comparable effects of manure and its biochar on reducing soil Cr bioavailability and narrowing the rhizosphere extent of enzyme activities

Chromium (Cr) contamination is especially hazardous to soil biota. Application of manure and biochar has been frequently proposed to remediate Cr-contaminated soil. However, the understanding of mechanisms behind manure and biochar impacts on soil enzyme activities requires advanced visualization technologies. For the first time, we compared manure and its biochar influence on the spatial distribution of β-glucosidase, N-acetyl-glucosaminidase and phosphomonoesterase activities in Cr-contaminated soil using direct zymography. Maize was planted for 45 days in (a) soil mixed with manure, (b) soil mixed with manure-derived biochar and (c) soil without any addition. Soil pH decreased over 45 days, inducing an increase in acid soluble Cr. The concomitant decrease in β-glucosidase and N-acetyl-glucosaminidase activities explained the narrowing rhizosphere extent of enzyme activities by 13–44%, indicating that increased Cr bioavailability decreases microbial activities. A larger maize performance index and the greatest plant shoot/root ratio after biochar application suggested enhanced maize growth (p < 0.05). In contrast, manure induced the narrowest extent of β-glucosidase and phosphomonoesterase activities due to the addition of labile organic compounds and nutrients following its application. Our study emphasizes the importance of pH on Cr bioavailability and enzyme activities and demonstrates that biochar application is more ideally suited for remediating Cr-contaminated soil.

Authigenic metastable iron sulfide minerals preserve microbial organic carbon in anoxic environments

The burial of organic carbon (OC) in sedimentary environments promotes long-term carbon sequestration, which allows the release of oxygen in the atmosphere. Organo-mineral interactions that form between terrigenous minerals and OC during transport to and deposition on the seabed enhance OC preservation. Here, we propose an authigenic mechanism for the coupled preservation of labile OC and metastable iron sulfide minerals under anoxic conditions. Sulfate-reducing microorganisms (SRM) are ubiquitous in anoxic environments and produce the majority of free sulfide in marine sediments, leading to the formation of iron sulfide minerals in situ. Using high spatial resolution microscopy, spectroscopy and spectro-microscopy, we show that iron sulfide biominerals precipitated in the presence of SRM incorporate and adsorb organic molecules, leading to the formation of stable organo-mineral aggregates that could persist for years in anoxic environments. OC/iron sulfide assemblages consist of the metastable iron sulfide mineral phases mackinawite and/or greigite, along with labile organic compounds derived from microbial biomass or from organic molecules released extracellularly by SRM. Together these results underscore the role that a major group of anoxic microbes play in OC preservation and illustrate the value of the resulting authigenic metastable iron sulfide minerals mackinawite and greigite in protecting labile organic molecules from degradation over time.

Labile organic matter intensifies phosphorous mobilization in paddy soils by microbial iron (III) reduction

Iron (Fe) is one of the most abundant elements in the earth's crust and intensively interferes with the biogeochemical cycles of carbon (C) and phosphorous (P), especially in highly weathered tropical and subtropical soils. The strong affinity of phosphate to Fe oxides and hydroxides limits the P availability in paddy soils. Reductive dissolution of Fe(III) can release occluded and adsorbed P into solution and make it available for plant uptake. Such effects remain elusive, especially in highly weathered subtropical paddy soils with oscillating redox (Eh) conditions. Under submerged conditions, incomplete litter decomposition occurs and rice roots release large amounts of labile organic compounds as exudates. We investigated the role of acetate, formate, oxalate, and propionate on Fe(III) reduction, P mobilization, and CO2 efflux in two paddy soils (i.e. oxisol and ultisol) of varying organic C (OC) and Fe(II) contents. Microbial mineralization of added labile C decreased soil Eh and increased the rates of Fe(III) reduction followed by P mobilization. This indicated that microbially-mediated Fe(III) reduction was intensified by labile OC compounds, which acted as energy sources and electron donors. The release of available P via Fe reduction was accompanied by peaks in Fe(II), dissolved OC, and pH and was followed by a decrease in iron-bound P (FeP). This implied that FeP was the main source of available P in the paddy soils. The faster release of available P in the oxisol than the ultisol indicated that the higher OC and Fe(II) contents in the oxisol allowed a fast Eh decrease, leading to rapid microbial oxygen (O2) consumption and consequently faster and more intensive Fe(III) to Fe(II) reduction. This conclusion is supported by a faster Eh decrease in the oxisol than the ultisol corresponding to early P mobilization after input of labile C, and it suggests that Eh-driven Fe transformations and P mobilization are strongly modulated by labile OC mineralization.

Effects of peat decomposition on δ13C and δ15N depth profiles of Alpine bogs

Decomposition of organic substances is one of the main processes responsible for the signatures of stable carbon and nitrogen isotopes (δ13C and δ15N) in soils and peats. However, the applicability of δ13C and δ15N signatures at the natural abundance level as indicators of the degree of peat decomposition is still debatable. We evaluated δ13C and δ15N depth patterns of peat cores sampled at nine sites in two nearby Alpine peat bogs with varying degree of organic matter degradation. Based on water table depths and past drainage intensities, the peat cores were divided into three degradation classes. We found similar overall depths patterns of δ13C and δ15N across the nine depth profiles and distinct differences between aerobic and anaerobic peat layers. Considerable differences in stable C and N isotope signatures of same depths were detected between profiles of the three classes, whereas depth profiles of peat cores with similar degree in peatland degradation were nearly identical. In the aerobic peat layers, δ13C and δ15N increased with depths at all study sites from 2.6‰ to 4.9‰ for δ13C and 3.2‰ to 7.0‰ for δ15N compared to the initial signatures of the plant biomass. Standardised δ13C of aerobic layers differ distinctly between slightly degraded peats at the open peat bog area, intermediately degraded peats at the tree-covered edge areas and strongly degraded peats at the former peat-cutting site. δ13C signatures of aerobic layers of strongly degraded peats were markedly more negative compared to the slightly degraded peats because of the selective 12C losses by microbial respiration. δ15N were more positive at strongly degraded than at slightly degraded sites in both, aerobic and anaerobic peat layers. The uniform stable isotope ratios in the anaerobic layers deeper than the local maxima of the isotopic signatures support the assumption that minor 13C fractionation occurs under anaerobic conditions. δ13C slightly declining with depth in the waterlogged layers of strongly degraded peat reflects the preferential utilisation and loss of labile organic compounds enriched in 13C. δ15N of strongly degraded peats was higher compared to well-conserved peat. The close relationship between the measured δ15N to δ15N modelled based on C:N ratios and bulk densities supports the assumption that the δ15N signature is the result of isotopic fractionation by peat decomposition. We conclude that peat decomposition strongly affects the δ13C and δ15N depth profiles of peat bogs and most likely overrides other factors, such as differences between plant species, litter components, atmospheric δ13C shift during peat formation, temperature effects, or type of mycorrhizal symbiosis.

Interactive priming of soil N transformations from combining biochar and urea inputs: A 15N isotope tracer study

Biochar has been found to interact with N transformations in soil but the mechanisms remain largely unknown. In this study we investigated the priming effect of combined biochar and urea inputs on soil inorganic N pools through an isotope tracer approach. Biochar was applied in combination with urea in two complementary laboratory experiments: (i) in the first one, three 15N-labeled organic amendments (wheat straw (WS)), its biochars produced at 350 °C (B350) and at 550 °C (B550) were added to soil in combination with unlabeled urea; (ii) in the second experiment the three same, but unlabeled, amendments were added to soil in combination with 15N labeled urea. This system allowed partitioning between three N sources: native soil N, biochar-derived N and urea-derived N. In addition, CO2 fluxes were measured to follow total C mineralization in soil and N2O emissions were monitored. The proportion of N that mineralized from biochar was always below 0.5% of the added N. The co-addition of urea increased the concentration of NH4+ derived from B350, but not from B550, demonstrating the lower mineralization of N in biochars produced at 550 °C. Whereas the addition of WS led to a rapid immobilization of N, we found that despite their high C:N, none of the biochars, applied at a rate of 1.5%, immobilized inorganic N in soil. On the contrary, significantly higher NH4+concentrations derived from native soil organic N (SON) and urea were found throughout the incubation when B550 was added. This effect can be attributed to an apparent priming effect since a net decrease in CO2 fluxes was recorded when biochar was added to the soil. The addition of glucose (a low molecular weight carbon source) stimulated an increase in CO2 fluxes in all treatments along with a net N immobilization in soil. However, both biochars significantly reduced C readily available to microbes, as proved by lower soil CO2 fluxes, and limited the immobilization of NH4+ induced by glucose addition. Our results suggest that biochar may partially offset the mineralization of easily available organic C, buffering the immobilization of inorganic N in soil when labile organic compounds (e.g. root exudates, fresh manure, etc.) are incorporated.

Nonthermal air plasma dehydration of hydrochar improves its carbon sequestration potential and dissolved organic matter molecular characteristics

Labile organic compounds are associated with high environmental risk and are common on hydrochar surfaces. However, a comprehensive re-evaluation of hydrochar properties after the removal of labile compounds has long been overlooked. This study confirms that air-based nonthermal plasma can successfully modify hydrochar properties and change hydrochar's environmental benefits. NMR and FTIR results indicate that, aliphatic and alkyl structures are more reactive, while aromatic structures are highly resistant to the hydrochar modification process, leading to increased carbon sequestration potential and decreased dissolved organic matter (DOM). Van Krevelen diagram results indicate that dehydration controls the hydrochar modification process and leads to a decrease in oxygen content and O/C atomic ratio in the hydrochar; this weakens the ability of the hydrochar to immobilize hydrophobic organic pollutants (such as triclosan) due to the decrease in O‑alkyl C species within the hydrochar. Most importantly, air-based nonthermal plasma changes the structures of hydrochar associated DOM, and high molecular weight (>351 Da), and high degree of unsaturation and oxidation in the modified-hydrochar DOM compounds is observed. This study is therefore considered to have important implications for the carbon cycle and sustainable application of hydrochar.

Quantifying biochar content in a field soil with varying organic matter content using a two-temperature loss on ignition method

While the use of biochar as a soil amendment for agronomic and environmental management is gaining popularity, quantification of biochar in soil is still challenging. The objective of this work was to develop a fast, simple and inexpensive method to quantify biochar content in field soil with varying organic matter content – the two-temperature loss on ignition (LOI) method. In this approach, biochar mass fraction in a biochar-amended soil is computed by measuring the dry mass of biochar/soil mixture after heating sequentially at two temperatures: low temperature (LT), and high temperature (HT). This method requires the LOI profile for pure soil and pure biochar that are representative of soil and biochar in the field. Although the soil LOI profile may vary due to spatial variation in soil organic matter (SOM) content, the method only requires that the relative soil LOI at LT with respect to LOI at HT is uniform because of similarity in SOM chemical composition. In this method, LT and HT are selected such that the maximum difference in LOI exists at these temperatures between pure soil and biochar. The method was tested by quantifying the biochar content in roadway filter strips with and without a wood biochar pyrolyzed at high temperature (550 °C). The estimates of biochar content from the method matched independent measurements for soils with low (−0.23 ± 0.09 CI%, CI = 95% confidence interval, versus actual 0%) and high (3.9 ± 0.3 CI% versus actual 4.0 ± 1.1 CI%) biochar mass fraction. The method is applicable when SOM content is low to moderate (e.g. <15%) and mostly composed of labile organic compounds, and when biochars are pyrolyzed at moderate to high temperatures (i.e. >400 °C) and composed of relatively low ash content (e.g. <30%).

Characterization of organic compounds from hydrogen peroxide-treated subbituminous coal and their composition changes during microbial methanogenesis

Natural gas burns cleaner than coal by reducing ∼50% of the carbon footprint and emission of toxic substances and particulates. Coalbed natural gas can be produced from coal by indigenous microorganisms. Hydrogen peroxide treatment to coal has been shown to enhance the production of biogenic coalbed natural gas. In this study, we investigated methane generation from a subbituminous coal pretreated by hydrogen peroxide and the changes of the organic composition during gas production. We demonstrate that there is a great potential to produce natural gas from hydrogen peroxide-treated coal by microorganisms. The organic composition of the coal-derived compounds and the bulk organic profiling were changed because of the microbial degradation. The liquid samples before and after gas production were characterized by fluorescence spectrometer, GC/MS, HPLC, GPC, and TOCs. The results indicated that the chemical treatment has produced both labile organic components and compounds that are recalcitrant to microbial degradation. Labile compounds including short-chain carboxylic acids (C1 to C6) were found to contribute to the gas production. HPSEC analysis has shown shifts of molecular weight distributions, confirming organic composition changes. In addition, the analyses suggested that the labile organic compounds were conducive to shorten the lag phase of the gas production, whereas the presumed-recalcitrant constituents were surprisingly slightly biodegraded. The analysis also showed that the chemical treatment and biotransformation could produce compounds that are toxic to the environment. Environmental impacts should be fully evaluated before field applications.

Opposite photocatalytic oxidation behaviors of BiOCl and TiO2: Direct hole transfer vs. indirect [rad]OH oxidation

Although all the semiconductor-based photocatalysis is initiated by photoinduced conduction band electron and valence band hole, the reaction pathway and activity of the electron and hole would be largely dependent on the type of the photocatalyst. It is essential to distinguish and understand the photocatalyst-dependence of a specific reaction. In the present study, we compared the photocatalytic behaviors of BiOCl and TiO2 for the degradation of different pollutants including perfluorooctanoic acid (PFOA), chloroacetic acids and benzoic acid. We found that the decompositions of perhalocarboxylate acids (PHCAs) such as PFOA and trichloroacetic acid (TCA) were much rapider over BiOCl than on the TiO2 (commercial P25). The surface-area- normalized rate constants for the oxidation of TCA have five orders of magnitude of difference between these two systems. By contrast, the degradation rates of OH-sensitive organic pollutants such as dichloroacetic acid, monochloroacetic acid and benzoic acid were much higher on the TiO2. Moreover, much more meta-substituted hydroxylated intermediate was observed during the photocatalytic oxidation of benzoic acid on BiOCl. In addition, PFOA and TCA were degraded efficiently in the BiOCl system even in the presence of other labile organic compounds (such as acetic acid). All the experimental results definitely indicate that BiOCl photocatalyst prefers to directly oxidize the PHCAs and benzoic acid by the hole transfer, while TiO2 tends to oxidize the solvent water molecule to OH radical. The mechanism underlying on the different activity of BiOCl and TiO2 are further discussed.