The influence of flue gas desulphurization gypsum additive on characteristics and evolution of humic substance during co-composting of dairy manure and sugarcane pressmud

Bacterial necromass as the main source of organic matter in saline soils

Soil salinity poses a major threat to crop growth, microbial activity, and organic matter accumulation in agroecosystems in arid and semiarid regions. The limitations of carbon (C) accrual due to salinity can be partly mitigated by the application of organic fertilizers. Although microorganisms are crucial for soil organic carbon (SOC) stabilization, the relationships between living and dead microbial C pools and the community features of SOC accrual in saline soils are not known. A two-year field experiment was conducted to examine the effects of organic fertilizers on the microbial regulatory mechanisms of C sequestration in saline soil (chloride-sulfate salinity). Compared to manure addition alone, manure plus commercial humic acid increased SOC stock by 11% and decreased CO2 emissions by 10%, consequently facilitated soil C sequestration. We explain these results by greater bacterial necromass formation due to the dominance of r-strategists with faster turnover rate (growth and death), as well as larger necromass stability as supported by the increased aggregate stability under the addition of humic acids with manure. Humic acids increased the abundance of bacterial phylum Proteobacteria (copiotrophs) and decreased Acidobacteria (oligotrophs) compared with straw, indicating that r-strategists outcompeted K-strategists, leading to bacterial necromass accumulation. With larger C/N ratio (88), straw increased leucine aminopeptidase to mine N-rich substrates (i.e., from necromass and soil organic matter) and consequently reduced SOC stock by 8%. The decreased salinity and increased organic C availability under straw with manure addition also led to a 13% higher CO2 flux compared with manure application alone. Thus, humic acids added with manure benefited to SOC accumulation by raising bacterial necromass C and reducing CO2 emissions.

Artificial humic acid produced from wet distillers grains in a microwave-assisted hydrothermal process: Physicochemical characteristics and stimulation to plant growth

Wet distillers grains, as a waste biomass with a large annual output, pose a threat to the environment and food industry. Herein, artificial humic acid (AHA) was first produced from wet distillers grains in a dual-stage microwave-assisted hydrothermal process. The influence of temperature on AHA's characteristics was investigated and compared with natural humic acid (NHA) and standard humic acid (SHA). A high yield of AHA at 20.6% was obtained at 200 °C with a total reaction time of 1 h, which is 1.8-3.1 times that obtained in traditional single-stage hydrothermal process. Increasing the reaction temperature induced the formation of phenolic hydroxyl in AHA. AHA was rich in aromaticity and carboxylic acid structure, showing similar spectral characteristics to NHA. The distribution of molecular weight of AHA was mostly 5797 Da, which decreased by 15% compared to SHA. The optimal concentration of AHA to promote seedling growth was 0.2 g/L, and the root length was 2.0 times that of the control. The microwave hydrothermal process is a facile and efficient approach to preparing AHA from waste biomass with high moisture content.

Malonic acid shapes bacterial community dynamics in compost to promote carbon sequestration and humic substance synthesis

The incorporation of malonic acid (MA) into compost as a regulator of the tricarboxylic acid (TCA) cycle has the potential to increase carbon sequestration. However, the influence of MA on the transformation of the microbial community during the composting process remains unclear. In this investigation, MA was introduced at different stages of chicken manure (CM) composting to characterize the bacterial community within the compost using high-throughput sequencing. We assess the extent of increased carbon sequestration by comparing the concentration of total organic carbon (TOC). At the same time, this study examines whether increased carbon sequestration contributes to humus formation, which was elucidated by evaluating the content and composition of humus. Our results show that the addition of MA significantly improved carbon sequestration within the compost, reducing the carbon loss rate (C loss (%)) from 64.70% to 52.94%, while increasing HS content and stability. High throughput sequencing and Random Forest (RF) analysis show that the introduction of MA leads to a reduction in the diversity of the bacterial communities, but enhanced the ability of bacterial communities to synthesize humus. Furthermore, the addition of MA favors the proliferation of Firmicutes. Also, the hub of operational taxonomic units (OTUs) within the community co-occurrence network shifts from Proteobacteria to Firmicutes. Remarkably, our study finds a significant decrease in negative correlations between bacteria, potentially mitigating substrate consumption due to negative interactions such as competition. This phenomenon contributes to the improved retention of TOC in the compost. This research provides new insights into the mechanisms by which MA regulates bacterial communities in compost, and provides a valuable theoretical basis for the adoption of this innovative composting strategy.

Understanding the compositional changes of organic matter in torrefied olive mill pomace compost using infrared spectroscopy and chemometrics

Composting olive mill pomace (OMP), the major by-product of the olive oil industry, is an attractive waste management practice in the context of sustainable food production. Thermal treatment of compost at mild temperatures (torrefaction) can aid to improve its characteristics as a soil amendment. This study aims to understand the chemical changes occurring during torrefaction of olive mill pomace-based (OMP) compost, as well as to evaluate the treatment effects on compost at different stages of maturation. Here, treatments at different temperatures (175, 225, and 275 °C) and duration (from 1 to 5 h) have been employed to obtain a sort of torrefied samples. In general, the H/C and O/C atomic ratios of compost samples decreased with torrefaction temperatures, which suggests an incipient coalification of the organic matter. Furthermore, the results showed that a combination of FT-NIR and FT-MIR spectroscopy using a low-level data fusion strategy is very sensitive to the molecular changes occurring both in the composting process and during heating. Principal Component Analysis (PCA) of the merged spectra revealed that the changes at 175 °C are mainly the loss of water (O-H contributions at 3300 and 5169 cm^-1) together with the degradation of proteins (observed in the decrease of amide I and II characteristic bands). Furthermore, the samples heated at this temperature can still be differentiated by their initial maturation stage. On the other hand, thermochemical changes occurring at higher temperatures are more intense and make the samples more alike, independently of the composting time. When heating above 225 °C, the loss of O-H happens together with the decrease of aliphatic moieties, reflected in the bands 2920 and 2850 cm^-1 (FT-MIR) and 4258, 4323, 5665, and 5781 cm^-1 (FT-MIR). This can be attributed to the thermal degradation of cellulosic materials and, additionally, to the degradation of the residual oil in the case of poorly composted samples. Heated samples are characterized by the presence of carbonyl groups (1709 cm^-1) and humic-like complex and polymerized aromatic structures (1579 cm^-1). Since the characteristics of the torrefied compost at 275 °C are very similar regardless of the initial maturation stage, torrefaction may be a very interesting way to reduce the composting time of olive mill pomace to obtain a high-quality organic amendment for soil application.

Effects of persulfate-assisted hydrothermal treatment of municipal sludge on aqueous phase characteristics and phytotoxicity

Hydrothermal technology (HT) has received much attention in recent years as a process to convert wet organic waste into hydrochar. The aqueous phase (HTAP) produced by this process is still a burden and has become a bottleneck issue for HT process development. In this study, we provide the first investigation of the HTAP characteristics, phytotoxicity, and their correlation with persulfate (PS) (PS, 2.0 mmol/g TS)-assisted municipal sludge HT. The results showed that PS accelerated the hydrolysis of protein substances and increased the concentration of NH₄⁺ by 13.4% to 190.5% and that of PO₄^3- by 24.2% to 1103.7% in HTAP at hydrothermal temperatures of 120 to 240 °C. PS can reduce the phytotoxicity of HTAP by reducing aldehydes, ketones, N heterocyclic compounds, and particle size and by increasing its humification index. The maximum values of the root length and biomass of pakchoi (Brassica chinensis L.) seedlings occurred when electrical conductivity was 0.2 mS/cm of HTAP. This work provided a new strategy for the selection and design of HTAP management strategies.

Improving the humification by additives during composting: A review

Humic substances (HSs) are key indicators of compost maturity and are important for the composting process. The application of additives is generally considered to be an efficient and easy-to-master strategy to promote the humification of composting and quickly caught the interest of researchers. This review summarizes the recent literature on humification promotion by additives in the composting process. Firstly, the organic, inorganic, biological, and compound additives are introduced emphatically, and the effects and mechanisms of various additives on composting humification are systematically discussed. Inorganic, organic, biological, and compound additives can promote 5.58-82.19%, 30.61-50.92%, 2.3-40%, and 28.09-104.51% of humification during composting, respectively. Subsequently, the advantages and disadvantages of various additives in promoting composting humification are discussed and indicated that compound additives are the most promising method in promoting composting humification. Finally, future research on humification promotion is also proposed such as long-term stability, environmental impact, and economic feasibility of additive in the large-scale application of composting. It is aiming to provide a reference for future research and the application of additives in composting.

Humic acid and phosphorus fractions transformation regulated by carbon-based materials in composting steered its potential for phosphorus mobilization in soil

This study investigated the effects of different carbon-based additives including biochar, woody peat, and glucose on humic acid, fulvic acid, and phosphorus fractions in chicken manure composting and its potential for phosphorus mobilization in soil. The results showed that the addition of glucose effectively increased the total humic substance content (90.2 mg/g) of composts, and the fulvic acid content was significantly higher than other groups (P < 0.05). The addition of biochar could effectively improve the content of available phosphorus by 59.9% in composting. The addition of carbon-based materials to the composting was beneficial for the production of more stable inorganic phosphorus in the phosphorus fraction. The highest proportion of soluble inorganic phosphorus components of sodium hydroxide was found in group with woody peat addition (8.7%) and the highest proportion of soluble inorganic phosphorus components of hydrochloric acid was found in group with glucose addition (35.2%). The compost products with the addition of biochar (humic acid decreased by 17.9%) and woody peat (fulvic acid decreased by 72.6%) significantly increased soil humic acid mineralization. The compost products with the addition of biochar was suitable as active phosphate fertilizer, while the compost products with the addition of glucose was suitable as slow-release phosphate fertilizer.

Co-Thermal Oxidation of Lignite and Rice Straw for Synthetization of Composite Humic Substances: Parametric Optimization via Response Surface Methodology

In this study, Baoqing lignite (BL) and rice straw (RS), which were the representatives of low-rank coal and biomass, were co-thermally oxidized to produce composite humic substances (HS), including humic acid (HA) and fulvic acid (FA). Taking HS content as the output response, the co-thermally oxidizing conditions were optimized through single factor experiment and response surface methodology (RSM). The structures of HA and FA prepared under optimized conditions were analyzed by SEM, UV, and FTIR. Results showed that HS content was clearly influenced by the material ratio, oxidation time, and oxidation temperature, as well as their interactions. The optimized co-thermal oxidization condition was as follows: BL and RS pretreated with a material ratio of 0.53, oxidation time of 59.50 min, and oxidation temperature of 75.63 °C. Through verification, the experimental value (62.37%) had a small relative error compared to the predicted value (62.27%), which indicated that the developed models were fit and accurate. The obtained HA had a tightly packed block structure; FA had a loosely spherical shape. The molecular weight of FA was 2487 Da and HA was 20,904 Da; both had a smaller molecular weight than that reported in other literature. FA showed strong bands at 1720 cm^-1, thus confirming the presence of more oxygen-containing functional groups. The appearance of double peaks at 2900~2980 cm^-1 indicated that HA contains more aliphatic chains. The co-thermal oxidation of BL and RS gives a new method for the synthesis of HS, and the optimization of co-thermal oxidation conditions will provide fundamental information for the industrialization of composite HS.

Biochemical Fulvic Acid Modification for Phosphate Crystal Inhibition in Water and Fertilizer Integration

Biochemical fulvic acid (BFA), produced by organic wastes composting, is the complex organic matter with various functional groups. A novel modified biochemical fulvic acid (MBFA) which possessed stronger chelating ability had been synthesized by the grafting copolymerization of BFA and acrylic acid (AA). Results showed that MBFA effectively inhibited the crystallization of calcium phosphate and increased the concentration of phosphate in water solution. The optimum reaction conditions optimized by Box–Behnken design and response surface methodology were reaction temperature 69.24 °C, the mass of monomer to fulvic acid ratio 0.713, the initiator dosage 19.78%, and phosphate crystal-inhibition extent was 96.89%. IR spectra demonstrated AA was grafted onto BFA. XRD data and SEM images appeared the formation and growth of calcium phosphate crystals was effectively inhibited by MBFA.

Effects of additives on physical, chemical, and microbiological properties during green waste composting

Composting is an environmentally friendly and sustainable way to transform Green waste (GW) into a useful product. GW, however, contains substantial quantities of lignocelluloses that extend the composting period unless substances that accelerate composting are added. The objective of this research was to assess the influence of the following additives on GW composting (w/w dry matter contents of the additives were indicated): sugarcane bagasse at 15%; bean dregs at 35%; silage at 45%; flue gas desulfurization gypsum at 5%; maifanite at 4%; and furfural residue at 20%. Based on the composting temperature, compost density, porosity, particle-size distribution, water retention, pH, cation exchange capacity, available nutrient contents, humification coefficient, organic matter loss, microbial populations, and phytotoxicity, the best additives were 45% silage and 5% flue gas desulfurization gypsum. The latter two additives produced a high-quality product in only 35 and 37 days, respectively.

Sustainability of the Dairy Industry: Emissions and Mitigation Opportunities

Dairy cattle provide a major benefit to the world through upcycling human inedible feedstuffs into milk and associated dairy products. However, as beneficial as this process has become, it is not without potential negatives. Dairy cattle are a source of greenhouse gases through enteric and waste fermentation as well as excreting nitrogen emissions through their feces and urine. However, these negative impacts vary widely due to how and what these animals are fed. In addition, there are many promising opportunities for further reducing emissions through feed and waste additives. The present review aims to further expand on where the industry is today and the potential avenues for improvement. This area of research is still not complete and additional information is required to further improve our dairy systems impact on sustainable animal products.

Protein-derived structures determines the redox capacity of humic acids formed during hyperthermophilic composting

Humic acid (HA) in compost has received widespread attention for its high redox activity, which can mediate the degradation of organic pollution and the passivation of heavy metals in the environment. Hyperthermophilic composting (HTC) can accelerate HA formation. However, few studies have examined whether and how the structures of different organics affect the formation of the HA and HA redox structure at the molecular level in HTC. Detailed molecular information and the redox capacity (electron transfer capacity, ETC) of HA in HTC and thermophilic composting (TC) were characterized using pyrolysis gas chromatography/mass spectrometry and the electrochemical method, respectively. HTC promoted the formation of redox structure, leading to the improvement of the ETC of HA. Aromatics and N-containing compounds were mainly derived from protein components, and the rate at which they were transferred into HA was accelerated in HTC, while the relative abundance of lipids decreased. Partial least squares regression and correlation analysis demonstrated that protein-derived compounds were the key factor determining the HA redox capacity. Finally, partial least squares path modeling suggested that the influence mechanism of protein-derived structures on HA redox capacity might differ in HTC and TC. HTC may promote the relative abundance of N-containing components into the C-skeleton and accelerate the accumulation of the aromatic products, thereby improve the HA redox capacity. These findings provided new insight into how the redox capacity of the HA in compost could be improved and how compost products could be prepared for use in environmental remediation.

An analysis of the versatility and effectiveness of composts for sequestering heavy metal ions, dyes and xenobiotics from soils and aqueous milieus

The persistence and bioaccumulation of environmental pollutants in water bodies, soils and living tissues remain alarmingly related to environmental protection and ecosystem restoration. Adsorption-based techniques appear highly competent in sequestering several environmental pollutants. In this review, the recent research findings reported on the assessments of composts and compost-amended soils as adsorbents of heavy metal ions, dye molecules and xenobiotics have been appraised. This review demonstrates clearly the high adsorption capacities of composts for umpteen environmental pollutants at the lab-scale. The main inferences from this review are that utilization of composts for the removal of heavy metal ions, dye molecules and xenobiotics from aqueous environments and soils is particularly worthwhile and efficient at the laboratory scale, and the adsorption behaviors and effectiveness of compost-type adsorbents for agrochemicals (e.g. herbicides and insecticides) vary considerably because of variabilities in structure, topology, bond connectivity, distribution of functional groups and interactions of xenobiotics with the active humic substances in composts. Compost-based field-scale remediation of environmental pollutants is still sparse and arguably much challenging to implement if, furthermore, real-world soil and water contamination issues are to be addressed effectively. Hence, significant research and process development efforts should be promptly geared and intensified in this direction by extrapolating the lab-scale findings in a cost-effective manner.

Effect of bean dregs amendment on the organic matter degradation, humification, maturity and stability of pig manure composting

The purpose of this study was to effectively dispose of bean dregs (BD) using composting technology, which could provide a theoretical basis for the disposal of BD. Therefore, the influence of different quantities of bean dregs (BD) (0%, 5%, 10% and 15%, w/w with a dry base of pig manure (PM)) on the decomposition and humification of organic matter during PM-composting was investigated, and a 0% BD amendment was used as the control (CK). Wheat straw was used as a bulking agent. Compared to the CK, the BD amendment promoted the degradation of organic matter. The degree of organic matter degradation increased by 16.46-25.04% upon BD amendment. Furthermore, the BD amendment improved humification and increased indices of the humification ratio (HR), percentage of humic acids (PHA), degree of polymerization (DP) and the humification index (HI). Furthermore, Fourier transform infrared (FTIR) spectroscopy indicated that the aromatic structure was enhanced with the BD amendment, and excitation-emission matrix (EEM) fluorescence spectra showed increased humic-like substance production. Additionally, the dissolved organic carbon (DOC), germination index (GI), electrical conductivity (Ec) and carbon/nitrogen (C/N) influenced the maturity and stability of composting. Comparatively, a 10% BD addition showed the optimal performance among all PM-composting treatments.

Exploring the characteristics of dissolved organic matter and succession of bacterial community during composting

The objective of this study was to explore the relationships among physico-chemical parameters, dissolved organic matters (DOM), and bacterial community during composting to better understand composting performances. The results showed total Kjeldahl nitrogen (TKN) (57%), temperature (39%), and pH (3%) were main factors driving the succession of bacterial communities. Firmicutes was a crucial phylum degrading organic matters for DOM formation, whereas the aromaticity and humification of DOM were closely related to Luteimonas (R² = 0.971, p < 0.05) and Sphingobacteriaceae (R² = 0.931, p < 0.05). Additionally, total phosphorus (TP), total potassium (TK), and TKN increased by 34.84%, 43.66%, and 65.91%, respectively, while organic matter decreased by 61.79%. The final compost had a C/N of 6.91 (<15) and a germination index of 97.81% (>80%), indicating that compost reached maturity and could be safely applied for soil amendment.

Humic substances developed during organic waste composting: Formation mechanisms, structural properties, and agronomic functions

Aerobic composting is a typical biochemical process of stabilization and harmlessness of organic wastes during which organic matter degrades, and then aggregates, to produce humic substances (HSs). HSs are a core product of-and a crucial indicator of-the maturation of compost that can be used in soil amendments. The formation of HSs is affected by the characteristics of the raw materials involved, the presence of compost additives, microbial activity, temperature, pH, the C/N ratio, moisture content, oxygen content and particle size, all of which can interact with each other. The formation of HSs is therefore complex. Moreover, it is difficult to identify definitive structures of humic acids (HAs) and fulvic acids (FAs), which are the two major components of HSs. However, HSs represent the same functional groups and structural arrangements, which helps to predict their structures. Functional groups represented by phenol and carboxylic acid groups of HAs and FAs can provide various agronomic functions, such as plant growth enhancement, water and nutrient retention, and disease suppression capacity. Overall, HSs can act as a soil amendment, fertilizer, and plant growth regulator. These functions of HSs enhance the reuse potential of organic waste compost products; however, this requires scientific control of various composting parameters and appropriate application of final products.

Effect of the addition of exogenous precursors on humic substance formation during composting

The aim of this work was to explore the effect of the addition of exogenous precursors on humic substance (HS) formation during composting. HS formation is a complex biochemical process that occurs during composting. In addition, HS precursors and bacterial communities were recognized as the key factors that affect HS formation. The addition of exogenous precursors can promote the humification process during composting, but few studies have explored the potential relationships between the proportion of additional exogenous precursors, the bacterial community and HS formation. Jointly adding benzoic acid (BA) and soybean residue after extracted oil (SR) treatment can promote HS formation, especially humic acid formation. In addition, the increase in the proportion of exogenous precursors added could strengthen the relationship among different precursors, thereby changing the bacterial community composition and further promoting the humification process during composting. In addition, a structural equation model (SEM) showed that precursors were the key factors to regulate HS formation and certain bacteria as the direct drivers to affect HS formation. This model provides more possibilities to regulate HS formation during composting and enhances its potential applicability under real conditions.

The Effect of Co-Additives (Biochar and FGD Gypsum) on Ammonia Volatilization during the Composting of Livestock Waste

The effectiveness of co-additives for improving livestock waste composting (reduction of air pollution and conservation of nutrients) was investigated. Biochar and Flue gas desulphurization gypsum (FGD gypsum) were used to supplement the composting of a mixture of slaughter waste, swine slurry, and sawdust. Different compositions of additives (0% or 5% each, 10% biochar or FGD gypsum) were tested in triplicate on the laboratory scale. In addition, the effects of two different aeration schemes (continuous and intermittent) were also investigated. Ammonia volatilization, physicochemical characteristics, and compost maturity indices were investigated. The results indicated that the use of the co-additive (Biochar and FGD gypsum) during composting of livestock waste led to a reduction of ammonia volatilization by 26–59% and to a 6.7–7.9-fold increase of nitrate accumulation. The total ammonia volatilization of intermittent aeration treatment was lower than that of continuous aeration using co-additives treatment. It was concluded that co-additives (biochar and FGD gypsum) might be utilized in livestock waste composting to reduce ammonia volatilization and improve nutrient conservation.

The properties and dynamic changes of DOM subfractions during food waste and sugarcane leaves co-composting

The purpose of this research was to evaluate the properties and dynamic changes of humic acids (HA), fulvic acids (FA), hydrophobic neutrals (HoN), and hydrophilic (Hi) fractions of dissolved organic matter (DOM) during food waste and sugarcane leaves co-composting process. The pools of HA, FA, HoN, and Hi were separated from DOM by fractionation method, and characterized using spectroscopic (UV-vis, FTIR) and pyrolysis-GC/MS analyses. The least peaks in the HA pool were found in FTIR spectra with the simple structure in HA. The highest value of SUV₂₅₄ was observed in HA, indicating that the HA pool played a dominant role in aromaticity of DOM. Hydrophobic compounds (HA, FA, HoN) had higher percentages of alkanes and cyclo-alkanes at the end of composting, while lower contents in the Hi pool. Both DOM and its subfractions increased the Chinese flowering cabbage (Brassica campestris L.) seed germination rate (SGR), whereas HA had a significant effect on promoting the root growth.

Improvement of pig manure compost lignocellulose degradation, organic matter humification and compost quality with medical stone

The present study aimed to investigate the effect of different concentrations (0%, 2.5%, 5.0%, 7.5% and 10.0%) of medical stone (MS) on the lignocellulose degradation and organic matter humification during pig manure (PM) composting. The results indicated that the addition of MS drastically promoted the organic carbon and lignin degradation. Compared to the control, the decomposition rate of hemicellulose and cellulose was increased by 9.64-27.08% and 2.11-12.07% in MS added treatments. Meanwhile, MS amendment significantly improved the humification of composting process, and the humic acid contents in MS added treatments were 5.58-9.75% higher than control. The FTIR and synchronous fluorescence spectra indicated that the aromatization of final compost was promoted with increasing the MS amount. In addition, the application of MS blended composts could significantly improve the biomass and chlorophyll content of pachoi (Brassica chinensis L.). Due to the effective performance of MS, the 10.0% MS was suggested for PM composting.

Properties and evolution of dissolved organic matter during co-composting of dairy manure and Chinese herbal residues

Composting is an effective method in treating solid organic wastes, in which dissolved organic matter (DOM) plays an important role in transformation of organic matter and microbial activity. Therefore, an understanding of the properties and evolution of DOM during composting is crucial. In this study, DOM was studied using elemental analysis, spectroscopic analysis (UV-vis, FTIR, and pyrolysis-GC/MS), and colloidal analysis during a 120-day composting. Results showed that the content of N and O in DOM increased while C and H content declined progressively over the composting time. Aliphatic C-H stretching, aromatic C=C or C=O stretching of amide groups, and C-O stretch (carbohydrates) showed an obvious decrease, while COO- and C-N groups had a significant increase. The evolution of DOM indicated a gradual decrease of the lipid and polysaccharide fractions, whereas an increase of aromatic and nitrogenous compounds was observed. The DOM also showed a more stable status, and an accumulation of small molecular compounds occurred with composting proceeded. Taken together, these results shed a good insight into the properties and evolution of DOM during a composting process.