Multivariate and Multiscale Approaches for Interpreting the Mechanisms of Nitrous Oxide Emission during Pig Manure-Wheat Straw Aerobic Composting

SiO2 nanoparticles can enhance nitrogen retention and reduce copper resistance genes during aerobic composting of swine manure

SiO2 nanoparticles (SiO2 NPs) are low-cost, environmentally friendly materials with significant potential to remove pollutants from complex environments. In this study, SiO2 NPs were used for the first time as an additive in aerobic composting to enhance nitrogen retention and reduce the expression of copper resistance genes. The addition of 0.5 g kg-1 SiO2 NPs effectively reduced nitrogen loss by 72.33 % by decreasing denitrification genes (nosZ, nirK, and napA) and increasing nitrogen fixation gene (nifH). The dominant factors affecting nitrification and denitrification genes were Firmicutes and C/N ratio. Additionally, SiO2 NPs decreased copper resistance genes by 28.96 % - 37.52 % in compost products. Copper resistance genes decreased most in the treatment with 0.5 g kg-1 SiO2 NPs. In summary, 0.5 g kg-1 SiO2 NPs have the potential to reduce copper resistance genes and enhance nitrogen retention during aerobic composting, which may be used to improve compost quality.

Effect of iron-based nanomaterials on organic carbon dynamics and greenhouse gas emissions during composting process

Iron-based nanomaterials as effective additives can enhance the quality and safety of compost. However, their influence on organic carbon fractions changes and greenhouse gas emissions during composting remains unclear. This study demonstrated that iron-based nanomaterials facilitate the conversion of light organic carbon fraction into heavy organic carbon fraction, with the iron-based nanomaterials group showing a significantly higher heavy organic carbon fraction content (41.88%) compared to the control group (35.71%). This shift led to an increase in humic substance content (77.5 g/kg) and a reduction in greenhouse gas emissions, with CO2, CH4, and N2O emissions decreasing by 20.5%, 39.7%, and 55.4%, respectively. Additionally, CO2-equivalent emissions were reduced by 42.9%. Microbial analysis revealed that iron-based nanomaterials increased the abundance of Bacillus and reduced the abundance of methane-producing archaea such as Methanothermobacter and Methanomassiliicoccus. These results indicated that the role of iron-based nanomaterials in regulating reactive oxygen species production and specific microbial communities involved in humification process. This study provides a practical strategy for improving waste utilization efficiency and mitigating climate change.

Nitrogen retention and emissions during membrane-covered aerobic composting for kitchen waste disposal

The composting performance and nitrogen transformation during membrane-covered aerobic composting of kitchen waste were investigated. The aerobic composting products of the kitchen waste had a high seed germination index of ∼180%. The application of the membrane increased the mean temperature in the early cooling stage of composting by 4.5℃, resulted in a lower moisture content, and reduced the emissions of NH3 and N2O by 48.5% and 44.1%, respectively, thereby retaining 7.9% more nitrogen in the compost. The adsorption of the condensed water layer under inner-membrane was the reason for reducing NH3 emissions, and finite element modeling revealed that the condensed water layer was present throughout the composting process with a maximum thickness of ∼2 mm in the thermophilic stage. The reduction of N2O emissions was related to the micro-positive pressure in the reactor, which promoted the distribution of oxygen, thus weakening denitrification. In addition, the membrane cover decreased the diversity of the bacterial community and increased the diversity of ammonia-oxidizing strains. This study confirmed that membrane-covered composting was suitable for kitchen waste management and could be used as a strategy to mitigate NH3 and N2O emissions.

Insight into mitigation mechanisms of N2O emission by biochar during agricultural waste composting

The effects and mitigation mechanisms of biochar added at different composting stages on N2O emission were investigated. Four treatments were set as follows: CK: control, BB10%: +10 % biochar at beginning of composting, BB5%&T5%: +5% biochar at beginning and + 5 % biochar after thermophilic stage of composting, BT10%: +10 % after thermophilic stage of composting. Results showed that treatment BB10%, BB5%&T5%, and BT10% reduced total N2O emissions by 55 %, 37 %, and 36 %, respectively. N2O emission was closely related to most physicochemical properties, while it was only related to amoA gene and hydroxylamine oxidoreductase. Different addition strategies of biochar changed the contributions of physicochemical properties, functional genes and enzymes to N2O emission. Organic matter and C/N contributed 23.7 % and 27.6 % of variations in functional gene abundances (P < 0.05), respectively. pH and C/N (P < 0.05) contributed 37.3 % and 17.3 % of variations in functional enzyme activities. These findings provided valuable insights into mitigating N2O emissions during composting.

Reduced greenhouse gas emission by reactive oxygen species during composting

Reactive oxygen species (ROS) is produced in the composting, which effectively promote organic matter transformation and humification process, but the effect of ROS on greenhouse gas emissions in this process has not been understood. This study proposed and validated that ROS can effectively reduce greenhouse gas emissions intheprocessofcomposting. Compared with ordinary thermophilic composting (oTC), thermophilic composting (imTC) that was supplemented by iron mineral increased ROS production by 1.38 times, and significantly reduced greenhouse gas emissions by 45.12%. Microbial community analysis showed no significant difference in the abundance of microbes involved in greenhouse gas production between oTC and imTC. Further correlation analysis proved that ROS played a crucial role in influencing greenhouse gas emissions throughout the composting process, especially in the initial phase. These findings provide new strategies for managing livestock and poultry manure to mitigate climate change.

Effects of C/N ratio on N2O emissions and nitrogen functional genes during vegetable waste composting

Nitrous oxide (N2O) generation during composting not only leads to losses of nitrogen (N) but also reduces the agronomic values and environmental benefits of composting. This study aimed to investigate the effect of the C/N ratio on N2O emissions and its underlying mechanisms at the genetic level during the composting of vegetable waste. The experiment was set up with three treatments, including low C/N treatment (LT, C/N = 18), middle C/N treatment (MT, C/N = 30), and high C/N treatment (HT, C/N = 50). The results showed that N2O emission was mainly concentrated in the cooling and maturation periods, and the cumulative N2O emissions decreased as the C/N ratio increased. Specifically, the cumulative N2O emission was 57,401 mg in LT, significantly higher than 2155 mg in MT and 1353 mg in HT. Lowering the C/N ratio led to increasing TN, NH4+-N, and NO3--N contents throughout the composting process. All detected nitrification-related gene abundances in LT continued to increase during composting, significantly surpassing those in MT during the cooling period. By contrast, in HT, there was a slight increase in the abundance of detected nitrification-related genes but a significant decrease in the abundance of narG, napA, and norB genes in the thermophilic and cooling periods. The structural equation model revealed that hao and nosZ genes were vital in N2O emissions. In conclusion, increasing the C/N ratio effectively contributed to N2O reduction during vegetable waste composting.

Black soldier fly larvae mitigate greenhouse gas emissions from domestic biodegradable waste by recycling carbon and nitrogen and reconstructing microbial communities

Black soldier fly larvae have been proven to reduce greenhouse gas emissions in the treatment of organic waste. However, the microbial mechanisms involved have not been fully understood. The current study mainly examined the dynamic changes of carbon and nitrogen, greenhouse gas emissions, the succession of microbial community structure, and changes in functional gene abundance in organic waste under larvae treatment and non-aeration composting. Thirty percent carbon and 55% nitrogen in the organic waste supplied were stored in larvae biomass. Compared to the non-aeration composting, the larvae bioreactor reduced the proportion of carbon and nitrogen converted into greenhouse gases (CO2, CH4, and N2O decreased by 62%, 87%, and 95%, respectively). 16S rRNA sequencing analysis indicated that the larvae bioreactor increased the relative abundance of Methanophaga, Marinobacter, and Campylobacter during the bioprocess, enhancing the consumption of CH4 and N2O. The metagenomic data showed that the intervention of larvae reduced the ratio of (nirK + nirS + nor)/nosZ in the residues, thereby reducing the emission of N2O. Larvae also increased the functional gene abundance of nirA, nirB, nirD, and nrfA in the residues, making nitrite more inclined to be reduced to ammonia instead of N2O. The larvae bioreactor mitigated greenhouse gas emissions by redistributing carbon and nitrogen and remodeling microbiomes during waste bioconversion, giving related enterprises a relative advantage in carbon trading.

Improving prediction of N2O emissions during composting using model-agnostic meta-learning

Nitrous oxide (N2O) represents a significant environmental challenge as a harmful, long-lived greenhouse gas that contributes to the depletion of stratospheric ozone and exacerbates global anthropogenic greenhouse warming. Composting is considered a promising and economically feasible strategy for the treatment of organic waste. However, recent research indicates that composting is a source of N2O, contributing to atmospheric pollution and greenhouse effect. Consequently, there is a need for the development of effective, cost-efficient methodologies to quantify N2O emissions accurately. In this study, we employed the model-agnostic meta-learning (MAML) method to improve the performance of N2O emissions prediction during manure composting. The highest R2 and lowest root mean squared error (RMSE) values achieved were 0.939 and 18.42 mg d-1, respectively. Five machine learning methods including the backpropagation neural network, extreme learning machine, integrated machine learning method based on ELM and random forest, gradient boosting decision tree, and extreme gradient boosting were adopted for comparison to further demonstrate the effectiveness of the MAML prediction model. Feature analysis showed that moisture content of structure material and ammonium concentration during composting process were the two most significant features affecting N2O emissions. This study serves as proof of the application of MAML during N2O emissions prediction, further giving new insights into the effects of manure material properties and composting process data on N2O emissions. This approach helps determining the strategies for mitigating N2O emissions.

Exploring the nitrogen fixing strategy of bacterial communities in nitrogen cycling by adding calcium superphosphate at various periods during composting

Chicken manure, as an organic solid waste with a high nitrogen content, generates large amounts of ammonia during composting, which leads to pollution of the surrounding environment, and causes a reduction in the quality of the compost product. Nitrogen is transformed through the nitrogen cycle and bacterial communities are the main contributors to the transformation of the nitrogen cycle. The microbial composition changes dramatically at different stages during composting. Therefore, calcium superphosphate (SSP) was added to compost as a nitrogen-fixing agent to elucidate the strategy and function of the bacterial community involved in the nitrogen cycle. The results showed that the addition of SSP at the initial, high temperature and cooling stages increased the inorganic nitrogen (NH4+-N, NO3--N) content by 51.99 %, 202.72 % and 173.37 % compared to CK, respectively. In addition, nitrogen cycle functional genes (gdh, nifH, pmoA-amoA, hao, nxrA, nirK, napA, nosZ, narG) abundance were determined by real-time qPCR. The nitrogen cycle genetic results showed that SSP addition at high temperature phase resulted in a 62.43 % down-regulation of ammonification genes, while nitrogen fixation and nitrification genes were enhanced. Random forests revealed a shift in the participation strategy of bacterial communities (e.g., Mycobacterium, Izemoplasmatales, Paracoccus, Ruminococcus) within the nitrogen cycle, leading to altered importance rankings despite involvement in different nitrogen cycle pathways. Moreover, Regression analysis and structural equation modelling revealed that SSP addition at high temperature stage stimulated the bacterial community engaged in nitrogen fixation and nitrification, resulting in increased nitrogen accumulation as NO3--N during composting. This paper offers the potential to yield novel scientific insights into the impact of microbially mediated nitrogen transformation processes and reduce gaseous pollution.

Microbes from mature compost to promote bacterial chemotactic motility via tricarboxylic acid cycle-regulated biochemical metabolisms for enhanced composting performance

This study aims to reveal the underlying mechanisms of mature compost addition for improving organic waste composting. Composting experiments and metagenomic analysis were conducted to elucidate the role of mature compost addition to regulate microbial metabolisms and physiological behaviors for composting amelioration. Mature compost with or without inactivation pretreatment was added to the composting of kitchen and garden wastes at 0%, 5%, 10%, 15%, and 20% (by wet weight) for comparison. Results show that mature compost promoted pyruvate metabolism, tricarboxylic acid (TCA) cycle, and oxidative phosphorylation to produce heat and energy to accelerate temperature increase for composting initiation and biological contaminant removal (>78%) for pasteurization. Energy requirement drives bacterial chemotactic motility towards nutrient-rich regions to sustain organic biodegradation. Nevertheless, when NADH formation exceeded NAD+ regeneration in oxidative phosphorylation, TCA cycle was restrained to limit continuous temperature increase and recover high intracellular NAD+/NADH ratio to secure stable oxidation reactions.

Effect of calcium peroxide assisted microwave irradiation pretreatment on humus formation and microbial community in straw and dairy manure composting

In this study, the effects of four pretreatment methods on the crystallinity of maize straw were compared, and the CaO₂ assisted microwave pretreatment was selected for straw and dairy manure composting. The humification and microbial community were investigated. Results showed that the pretreatment increased the initial water-soluble carbon, which favored the microbial activity, and the CO₂ release increased by 15.71%. Pretreatment promoted the lignocellulose degradation, with total degradation ratio of 37.06%. The final humic acid content was 11.39 g/kg higher than the control. Spearman correlation analysis indicated that polyphenols and amino acids were significantly related to humus formation. In addition, pretreatment rendered the Firmicutes the most dominant phylum, and increased the metabolic intensity of reducing sugar metabolism, aromatic amino acid biosynthesis and carbon fixation pathways. Redundancy analysis revealed that the dominant genus of Firmicutes was significantly positively correlated with humus, while that of Actinobacteriota was correlated with CO₂ release.

Effect of different-sized bulking agents on nitrification process during food waste digestate composting

Food waste digestate (FWD) disposal is a serious bottleneck in anaerobic digestion plants to achieve a circular bioeconomy. FWD could be recycled into nitrogen-rich compost; however, the co-composting process optimisation along with bulking agents is required to reduce nitrogen loss and unwanted gaseous emissions. In the present study, two different-sized bulking agents, namely, wood shaving (WS) and fine sawdust (FS), were used to investigate their impact on FWD composting performance along with the nitrogen dynamics. The mixing of FWD with different bulking agents altered the physiochemical characteristics of composting matrix and the effective composting performance was observed through reduced ammonium nitrogen and increased seed germination index during 28 days of composting. The carbon loss of 19-22% through CO2 emission indicated similar carbon mineralisation with both types of sawdust; however, the nitrogen transformation pathways were different. Only WS treatment demonstrated the nitrification process, whereas the nitrogen loss was higher with FS. A total nitrogen loss of ∼15% was observed in treatments with FS, whereas WS treatments displayed a nitrogen loss of 12%. The outcome of the present study could significantly contribute to the practical aspect of the FWD composting operation with the promotion of the bio-recycling economy.

Large Semi-Membrane Covered Composting System Improves the Spatial Homogeneity and Efficiency of Fermentation

Homogenous spatial distribution of fermentation characteristics, local anaerobic conditions, and large amounts of greenhouse gas (GHGs) emissions are common problems in large-scale aerobic composting systems. The aim of this study was to examine the effects of a semi-membrane covering on the spatial homogeneity and efficiency of fermentation in aerobic composting systems. In the covered group, the pile was covered with a semi-membrane, while in the non-covered group (control group), the pile was uncovered. The covered group entered the high-temperature period earlier and the spatial gradient difference in the group was smaller compared with the non-covered group. The moisture content loss ratio (5.91%) in the covered group was slower than that in the non-covered group (10.78%), and the covered group had a more homogeneous spatial distribution of water. The degradation rate of organic matter in the non-covered group (11.39%) was faster than that in the covered group (10.21%). The final germination index in the covered group (85.82%) was higher than that of the non-covered group (82.79%) and the spatial gradient difference in the covered group was smaller. Compared with the non-covered group, the oxygen consumption rate in the covered group was higher. The GHG emissions (by 30.36%) and power consumption in the covered group were reduced more significantly. The spatial microbial diversity of the non-covered group was greater compared with the covered group. This work shows that aerobic compost covered with a semi-membrane can improve the space homogeneity and efficiency of fermentation.

A novel method to measure air-immobile regions of the composting pile by inverse calculation combined with gas tracer test

Air-immobile regions in composting piles obstruct O₂ mass transport and exacerbate the formation and emission of harmful off-gases. However, effective methods for measuring the parameters of these air-immobile regions are lacking. With quartz sand piles, this study first adjusted the circumstances of a gas tracer test (gas tracer, its injection volume, and chamber type) using the two-region model (TRM). The effects of β (proportional coefficient of gas in the air-mobile region) and ω (mass exchange coefficient) on the breakthrough curves (BTCs) of the gases were then explored. Finally, an inverse calculation method was used to measure the feature parameters of air-immobile regions in two composting piles (temperature-increasing and thermophilic phases) and estimate the O₂ concentrations in different composting piles (50, 100, 200 cm whole height; layers of 50, 100, 200 cm height in a 200-cm high pile). The results showed that the optimal conditions were achieved when 100 mL helium (He) as the gas tracer and a cylinder with a height/diameter ratio of 3 as the chamber were used. With the simulating composting piles, increasing β or ω slowed breakthrough and decreased peak concentration in BTCs of a gas tracer. Tracer-inverse calculation protocol can be used to efficiently estimate the volume ratios of air-immobile regions (φ) and first-order mass transfer coefficient (α), with the values of 39%/46% and 0.001/0.006 min^-1 in the composting piles during temperature-increasing /thermophilic phase. The TRM also predicted the O₂ concentration in the off-gas or air-mobile/immobile regions of the temperature-increasing-phase composting piles.

Effects of functional-membrane covering technique on nitrogen succession during aerobic composting: Metabolic pathways, functional enzymes, and functional genes

This study investigated and assessed the effect of the functional-membrane covering technique (FMCT) on nitrogen succession during aerobic composting. By comparative experiments involving high-throughput sequencing and qPCR, nitrogen metabolism (including the ko00910 pathway and functional enzyme and gene abundances) was analyzed, and the nitrogen succession mechanism was identified. The FMCT created a micro-positive pressure, improved the aerobic conditions, and increased the oxygen utilization rate and temperature. This strongly affected the nitrogen metabolism pathway and down-regulated the nitrifying and denitrifying bacteria abundances. The FMCT up-regulated the relative abundance of glutamate dehydrogenase and down-regulated the absolute abundances of AOB and nxrA. This and the high temperature increased NH₃ emissions by 13.78%-73.37%. The FMCT down-regulated the abundances of denitrifying gene groups (nirS + nirK)/nosZ and nitric oxide reductase associated with N₂O emissions and decreased N₂O emissions by 16.44%-41.15%. The results improve the understanding of the mechanism involved in nitrogen succession using the FMCT.

Free radicals accelerate in situ ageing of microplastics during sludge composting

Composting is the last "barrier" for microplastics (MPs) in the entry of organic solid wastes into the environment. The transformation of MPs is thought to be mainly driven by microorganisms during composting, whereas the contribution of abiotic processes that involve free radicals is often overlooked. Herein, we provide initial evidence for the generation of free radicals during sludge composting, including environmental persistent free radicals and reactive oxygen species, which accelerate the oxidative degradation of MPs. The ·OH yield of composting fluctuated greatly from 23.03 to 277.18 μmol/kg during composting, which was closely related to the dynamic changes in Fe(II) (R² = 0.926). Analyses of the composted MPs physicochemical properties indicated that MPs were aged gradually with molecular weights decrease from 18% to 27% and carbonyl index value increase from 0.23 to 0.52. Further investigation suggested that the microbially-mediated redox transformation of iron oxides could occur on the MPs surface accompanied by the production of abundant free radicals, thereby leading to the damage of MPs during composting. These results reveal the critical role of free radicals in MPs ageing under oxic/anoxic alternation conditions of composting and provide new insights into the bio-chemical mechanism of contaminant removal or transformation during sludge composting.

Greenhouse gas emissions of biosolid and cow manure during composting and vermicomposting and when applied to soil cultivated with wheat (Triticum sp. L.)

Biosolids are a by-product of wastewater treatment, and their nutritional composition makes them ideal for fertilizing crops. However, pre-treatments, such as conditioning and/or (vermi)composting, are often required to stabilize the product and remove pathogens. Biosolids, cow manure, and a 50-50% mixture were conditioned for 21 days, composted or vermicomposted with Eisenia fetida (Savigny 1826) for 28 days, and applied to soil cultivated with wheat (Triticum sp. L.), while emissions of nitrous oxide (N₂O), methane (CH₄), and carbon dioxide (CO₂) were monitored. Emissions of CH₄ were large from the biosolid and N₂O from the cow manure during conditioning. Emissions of CH₄ remained high during (vermi)composting of the biosolids, while the emissions of N₂O from the cow manure dropped. The addition of E. fetida did not affect the emissions of greenhouse gases during (vermi)composting. The emission of N₂O was higher when (vermi)composted biosolid was applied to soil cultivated with wheat than when (vermi)composted cow manure was applied. The global warming potential (GWP) of the sum of the emitted greenhouse gases (GHG) during conditioning, (vermi)composting, and when the final product was applied to soil was 3 times larger from the cow manure than from the biosolid, but mixing biosolid with cow manure eliminated that difference. It was concluded that mixing biosolid with cow manure might be a simple way to reduce the GWP of the emitted GHG during storage, (vermi)composting, and when applied to soil.

In-situ electrolytic oxygen is a feasible replacement for conventional aeration during aerobic composting

Aerobic composting is an effective recycling method for the disposal and resource utilization of organic solid waste. However, the inappropriate aeration mode used during conventional aerobic composting (CAC) often results in low oxygen utilization efficiency and loss of temperature, which further leads to a long maturation period and large odorous gas (NH₃) pollution. Herein, a novel electrolytic oxygen aerobic composting (EOAC) process was invented first using in-situ oxygen generation for aeration by the electrolysis of water in compost. Our results demonstrated that the germination index (GI) significantly increased during EOAC, and the maturation time of compost was shortened by nearly 50% during EOAC compared to CAC, indicating higher oxygen utilization efficiency during EOAC. Meanwhile, NH₃ emissions, N₂O emissions, and nitrogen loss during the EOAC process decreased by 61%, 46%, and 21%, respectively, compared to CAC. The total relative abundance of thermophilic and electroactive bacteria during EOAC increased remarkably. EOAC inhibited ammoniation, nitrification, and denitrification, and weakened N-associated functional genes. A techno-economic analysis indicated that EOAC had greater technical superiority and cost advantages compared to CAC. This study represents proof-of-principle for EOAC and suggests that in-situ electrolytic oxygen is a feasible replacement for conventional aeration during aerobic composting.

Effects of Turning Frequency on Ammonia Emission during the Composting of Chicken Manure and Soybean Straw

Here, we investigated the impact of different turning frequency (TF) on dynamic changes of N fractions, NH3 emission and bacterial/archaeal community during chicken manure composting. Compared to higher TF (i.e., turning every 1 or 3 days in CMS1 or CMS3 treatments, respectively), lower TF (i.e., turning every 5 or 7 days in CMS5 or CMS7 treatments, respectively) decreased NH3 emission by 11.42-18.95%. Compared with CMS1, CMS3 and CMS7 treatments, the total nitrogen loss of CMS5 decreased by 38.03%, 17.06% and 24.76%, respectively. Ammonia oxidizing bacterial/archaeal (AOB/AOA) communities analysis revealed that the relative abundance of Nitrosospira and Nitrososphaera was higher in lower TF treatment during the thermophilic and cooling stages, which could contribute to the reduction of NH3 emission. Thus, different TF had a great influence on NH3 emission and microbial community during composting. It is practically feasible to increase the abundance of AOB/AOA through adjusting TF and reduce NH3 emission the loss of nitrogen during chicken manure composting.

Nitrifier denitrification dominates nitrous oxide production in composting and can be inhibited by a bioelectrochemical nitrification inhibitor

Targeted options to reduce nitrous oxide (N₂O) emission from composting is scarce due to challenges in disentangling the complex N₂O production pathways. Here, combined approaches of nitrogen form analysis, isotopocule mapping, quantitative PCR, and Illumina MiSeq sequencing were used to differentiate N₂O production pathways and decipher the underlying biochemical mechanisms. Results suggested that most N₂O was produced at the latter stage through nitrifier denitrification. The bioelectrochemical assistance through applying an electric potential reduced N₂O emissions by 28.5-75.5%, and the underlying mitigation mechanism was ammonia oxidation repression, as evidenced by the observed reduction in the proportion of the amoA containing family Nitrosomonadaceae from 99% to 83% at the lower voltage and to a negligible level at the higher voltage assessed, which was attributed to their depressed competitiveness for oxygen with heterotrophs. The findings provide evidence that the bioelectrochemical assistance could function as a nitrification inhibitor to minimize compost derived N₂O emissions.

Insight into the influence of biochar on nitrification based on multi-level and multi-aspect analyses of ammonia-oxidizing microorganisms during cattle manure composting

In this study, influence of biochar on nitrification was explored using multi-level (DNA, RNA, protein) and multi-aspect (diversity, structure, key community, co-occurrence pattern and functional modules) analyses (M-LAA) of ammonia-oxidizing microorganisms (AOMs) during cattle manure composting. Biochar addition increased the copy numbers and diversity of AOMs, restricted (36.02%) the amoA gene transcripts of archaea but increased (24.53%) those of bacteria, and reduced (75.86%) ammonooxygenase (AMO) activity. Crenarchaeota and Thaumarcheota mediated NH₄⁺-N, Unclassified_k_norank_d_Archaea and Crenarchaeota regulated AMO activity and potential ammonia oxidation (PAO) rates. Nitrosomonas and Nitrosospira were the predominant microbial taxa influencing NH₄⁺-N variation and PAO rates, respectively. Additionally, both Crenarchaeota and Nitrosospira played crucial roles in mediating NO₃^--N and NO₂^--N. Furthermore, biochar altered the network patterns of AOMs community by changing the keystone species and the interactivity among communities. These findings indicated that influence of biochar on nitrification could be better explained using M-LAA of AOMs.

Gases emission during the continuous thermophilic composting of dairy manure amended with activated oil shale semicoke

NH₃ and greenhouse gases emission are big problems during composting, which can cause great nitrogen nutrient loss and environmental pollution. This study investigated effects of the porous bulking agent of oil shale semicoke and its activated material on the gases emission during the continuous thermophilic composting. Results showed addition of semicoke could significantly reduce the NH₃ emission by 74.65% due to its great adsorption capacity to NH₄⁺-N and NH₃, further the effect could be enhanced to 85.92% when utilizing the activated semicoke with larger pore volume and specific surface area. In addition, the CH₄ emission in the semicoke and activated semicoke group was also greatly mitigated, with a reduction of 67.23% and 87.62% respectively, while the N₂O emission was significantly increased by 93.14% and 100.82%. Quantification analysis of the functional genes found the abundance of mcrA was high at the massive CH₄-producing stage and the archaeal amoA was dominant at the N₂O-producing stage in all the composting groups. Correlation and redundancy analysis suggested there was a positive correlation between the CH₄ emission and mcrA. Addition of semicoke especially activated semicoke could reduce the CH₄ production by inhibiting the methanogens. For the NH₃ and N₂O, it was closely related with the nitrification process conducted by archaeal amoA. Addition of semicoke especially activated semicoke was beneficial for the growth of ammonia-oxidizing archaea, causing the less NH₄⁺-N transformation to NH₃ but more N₂O emission.

Nitrogen transformation and dynamic changes in related functional genes during functional-membrane covered aerobic composting

The effects of functional membrane covering (FMC) on nitrogen transformation and related functional genes during aerobic composting were investigated by performing a comparable experiment. The FMC increased the pile temperature, promoted compost maturity, and decreased nitrogen loss. The FMC reduced NH₃ and N₂O emissions by 7.34% and 26.27%, respectively. The water film and the micro-positive pressure environment under the membrane effectively prevented NH₃ escaping. The FMC up-regulated the amoA gene copy number (promoting NH₃/NH₄⁺ oxidation). The reduction of N₂O emission by the FMC was mainly related to denitrifying genes (nirK, nirS, and nosZ). The FMC down-regulated the nirK and nirS gene copy numbers, but up-regulated the nosZ gene copy number. Pearson correlation analysis indicated that the functional membrane characteristics and differences between the composting pile environments caused by the FMC significantly affected the nitrogen forms and the related functional genes. The FMC strongly decreased nitrogen emissions and therefore conserved nitrogen.

Differential effects of various reclamation treatments on soil characteristics: an experimental study of newly reclaimed tidal mudflats on the east China coast

Reclamation of coastal land is increasingly being used as a means of raising agricultural productivity and improving food security in China. Applications of organic and inorganic supplements on reclaimed soils can significantly adjust a range of soil properties, C, N, P content and stoichiometry, and extracellular enzyme activities. However, the linkages between soil C꞉N꞉P stoichiometry and extracellular enzyme activities following reclamation of coastal saline soil remain largely unclear. In this experimental study, treatments included control (CK), chicken manure (OM), polyacrylamide plus chicken manure (PAM+OM), straw mulching plus chicken manure (SM + OM), buried straw plus chicken manure (BS + OM), and bio-organic manure plus chicken manure (BM + OM) were conducted to explore the linkages between soil physicochemical characteristics in reclaimed soils under different treatments and to evaluate their impact on oat yield. Soils under all reclamation treatments exhibited higher moisture content and, with the exception of SM + OM, lower soil pH compared to the control. The reclamation treatments also significantly decreased soil bulk density (BD) and soil salt content (SSC), and increased soil organic carbon (SOC), total nitrogen (TN) and organic phosphorus (OP). Our study of soil C꞉N꞉P stoichiometry revealed that newly reclaimed soils in the study area are N limited. Additionally, soil invertase (INV), urease (URE) and alkaline phosphatase (ALP) activity under different reclamation treatments were significantly enhanced compared with CK in surface soil, while soil catalase (CAT) activity was observed to be much higher in BM + OM than in other treatments. Mean oat yields for each of the treatments were ranked as follows: BM + OM > SM + OM > PAM + OM > BS + OM > OM > CK treatment. Our results also indicate that TN (12.1% and 12.4%) was the main factor affecting URE and ALP, whereas BD (13.5%) and pH (8.5) were key factors affecting INV and CAT activity, respectively.

Responses of ammonia-oxidizing microorganisms to biochar and compost amendments of heavy metals-polluted soil

Heavy metal pollution affects soil ecological function. Biochar and compost can effectively remediate heavy metals and increase soil nutrients. The effects and mechanisms of biochar and compost amendments on soil nitrogen cycle function in heavy-metal contaminated soils are not fully understood. This study examined how biochar, compost, and their integrated use affected ammonia-oxidizing microorganisms in heavy metal polluted soil. Quantitative PCR was used to determine the abundance of ammonia-oxidizing archaea (AOA) and bacteria (AOB). Ammonia monooxygenase (AMO) activity was evaluated by the enzyme-linked immunosorbent assay. Results showed that compost rather than biochar improved nitrogen conversion in soil. Biochar, compost, or their integrated application significantly reduced the effective Zn and Cd speciation. Adding compost obviously increased As and Cu effective speciation, bacterial 16S rRNA abundance, and AMO activity. AOB, stimulated by compost addition, was significantly more abundant than AOA throughout remediation. Correlation analysis showed that AOB abundance positively correlated with NO₃^--N (r = 0.830, P < 0.01), and that AMO activity had significant correlation with EC (r = -0.908, P < 0.01) and water-soluble carbon (r = -0.868, P < 0.01). Those seem to be the most vital factors affecting AOB community and their function in heavy metal-polluted soil remediated by biochar and compost.

Alternating magnetic field mitigates N2O emission during the aerobic composting of chicken manure

Nitrous oxide (N₂O) emission is an environmental problem related to composting. Recently, the electric field-assisted aerobic composting process has been found to be effective for enhancing compost maturity and mitigating N₂O emission. However, the insertion of electrodes into the compost pile causes electrode erosion and inconvenience in practical operation. In this study, a novel alternating magnetic field-assisted aerobic composting (AMFAC) process was tested by applying an alternating magnetic field (AMF) to a conventional aerobic composting (CAC) process. The total N₂O emission of the AMFAC process was reduced by 39.8% as compared with that of the CAC process. Furthermore, the results demonstrate that the AMF weakened the expressions of the amoA, narG, and nirS functional genes (the maximum reductions were 96%, 83.7%, and 95.5%, respectively), whereas it enhanced the expression of the nosZ functional gene by a maximum factor of 36.5 as compared with that in CAC. A correlation analysis revealed that the nitrification and denitrification processes for N₂O emission were suppressed in AMFAC, the main source of N₂O emission of which was denitrification. The findings imply that AMFAC is an effective strategy for the reduction of N₂O emission during aerobic composting.

Metagenomic and q-PCR analysis reveals the effect of powder bamboo biochar on nitrous oxide and ammonia emissions during aerobic composting

To investigate the emission mechanism of ammonia (NH₃) and nitrous oxide (N₂O) during aerobic composting and the influence of powder bamboo biochar (PBB) on this process, this paper conducted a systematic study on the nitrogen-transforming functional microbial community, including functional genes, microbial structure and metabolism pathways. PBB reduced N₂O and NH₃ emissions by 1.25%-8.72% and 10.4%-11.8%, respectively. The quantitative PCR results indicated that the reduced N₂O emission by PBB were mainly related to denitrifying genes (nirS, nirK, nosZ, and narG). The metagenome results demonstrated that Nitrosococcus was the main genus that could oxidize ammonia to nitrite decreased by PBB. The PBB significantly affected the nitrogen metabolism pathway, reduced the activity of glutamate dehydrogenase to inhibit the formation of NH₄⁺ to reduce NH₃ emission. The higher N₂O emission in the control group was also related to the higher relative contents of hydroxylamine reductase and nitrite reductase.

Simultaneously enhance iron/sulfur metabolism in column bioleaching of chalcocite by pyrite and sulfur oxidizers based on joint utilization of waste resource

In chalcocite (Cu₂S) bioleaching, the lack of iron metabolism is a key restricting factor. As the most common sulfide mineral, pyrite (FeS₂) can release Fe(Ⅱ) and compensate for the iron metabolism deficiency in chalcocite bioleaching. The bioleaching of chalcocite in an imitated industrial system was improved by enhancing the iron-sulfur metabolism simultaneously using pyrite and sulfur oxidizers based on the joint utilization of waste resources, while the bioleaching performance and community structure in the leachate were systematically investigated. Due to the active sulfur/iron metabolism, the pH reached 1.2, and Fe³⁺ was increased by 77.78%, while the biomass of planktonic cells was improved to 2.19 × 10⁷ cells/mL. Fourier transform infrared reflection (FTIR) and X-ray diffraction (XRD) analysis results showed that more iron-sulfur crystals were produced due to more active iron-sulfur metabolism. Scanning electron microscopy (SEM) revealed that many derivative particles and corrosion marks appeared on the surface of the ore, implying that the mineral-microbe interaction was strengthened. Confocal laser scanning microscopy (CLSM) showed the accumulation of cells and extracellular polymeric substances (EPS) on the ore surface, indicating a stronger contact leaching mechanism. Furthermore, the community structure and canonical correspondence analysis (CCA) demonstrated that the introduction of sulfur-oxidizing bacteria and pyrite could maintain the diversity of dominant leaching microorganisms at a high level. Sulfobacillus (27.75%) and Leptospirllillum (20.26%) were the dominant sulfur-oxidizing and iron-oxidizing bacteria during the bioleaching process. With the accumulation of multiple positive effects, the copper ion leaching rate was improved by 44.8%. In general, this new type of multiple intervention strategy can provide an important guide for the bioleaching of low-grade ores.

Elucidating the role of earthworms in N2O emission and production pathway during vermicomposting of sewage sludge and rice straw

Vermicomposting is a sustainable option for the recycling of biodegradable organic waste. However, it also produces nitrous oxide (N₂O), which is a highly potent greenhouse gas. In this study, the N₂O stable isotope and functional genes for nitrogen cycling were determined to investigate the sources of N₂O during vermicomposting. The results showed that vermicomposting promoted the organic degradation and nitrogen nitrification, and the presence of earthworms increased the emission of N₂O during vermicomposting compared to that during the control treatment with no earthworms. The site preference analysis of N₂O stable isotope showed that both nitrification and denitrification were present during the early stages of vermicomposting, while nitrification was the dominant contributor to N₂O production in the later stages. Moreover, earthworms increased the gene copies of amoA, and stimulated the nitrifying bacteria, and hence, increased the N₂O emission via nitrification. In addition, the activity of earthworms reduced the gene number of nosZ during vermicomposting, while the denitrification was the main source of N₂O in the earthworm gut, as the conditions inside the gut inhibited nosZ. Overall, nitrification was the major pathway (55.8-88.7 %) for N₂O production, which was promoted by the introduction of earthworms through nitrification.

N2O emission factors of full-scale animal manure windrow composting in cold and warm seasons

Emission of nitrous oxide (N₂O) during animal manure composting is of great concern, and its emission factor (EF) is important for greenhouse gas emission inventory, while the EF is still uncertain due to limited on-site full-scale observations worldwide. In this study, N₂O emissions were monitored during different seasons in a full-scale swine manure windrow composting with pile volume of about 76.5 m³. The results showed that the maximum N₂O flux during the cold season (CS) was 23 times higher than during the warm season (WS), significant differences in the contribution to direct N₂O emissions were observed in three composting stages, and shaded-side N₂O emission was higher than sunny-side emission. The direct N₂O emission factors of animal manure composting were 0.0046, 0.0002 kg N₂O-N/kgTN (dry weight) in the CS and WS, respectively. Scenario analysis results showed that windrow composting is a suitable manure management that emits less N₂O than solid storage.

Biochar-mediated Fenton-like reaction for the degradation of sulfamethazine: Role of environmentally persistent free radicals

Swine manure biochar (SBC) pyrolyzed at 300 °C, 600 °C and 900 °C were utilized to degrade sulfamethazine (SMT) in heterogeneous Fenton-like systems which achieved excellent degradation efficiency (over 85% in 30 min). Experiments results demonstrated that SBC possessed the poor SMT adsorption capacity but high catalytic performance. Electron Paramagnetic Resonance (EPR) and X-ray photoelectron spectroscopy (XPS) analysis revealed that there were oxygen-centered environmentally persistent free radicals (EPFRs) and carbon-centered EPFRs with an adjacent oxygen atom in SBC. The oxygen-centered EPFRs played a major role in the catalytic process which tended to convert to carbon-centered EPFRs after the reaction. Besides, the electron transfer pathways were the most likely catalytic mechanism of SBC and the contribution of OH was dominant through Electron capture experiments and Linear sweep voltammetry (LSV) measurements. The acidic or alkaline condition can promote the catalytic ability of SBC. The presence of dissolved salts (NaCl) inhibited the catalytic process but the inhibition was slightly weakened at high concentration of NaCl, which showed the high tolerance of Cl^- in Fenton/Fenton-like systems. Moreover, real wastewater application suggested that SBC600/H₂O₂ system possessed excellent catalytic efficiency and good adaptability. This research provides a novel swine manure reuse process with high practicability and presents a more explicit perspective about the reaction mechanisms of EPFRs in biochar.

Insight into the microbiology of nitrogen cycle in the dairy manure composting process revealed by combining high-throughput sequencing and quantitative PCR

Nitrogen cycling during composting process is not yet fully understood. This study explored the key genes involved in nitrogen cycling during dairy manure composting process using high-throughput sequencing and quantitative PCR technologies. Results showed that nitrogen fixation occurred mainly during the thermophilic and cooling phases, and significantly enhanced the nitrogen content of compost. Thermoclostridium stercorarium was the main diazotroph. Ammonia oxidation occurred during the maturation phase and Nitrosomonas sp. was the most abundant ammonia oxidizing bacteria. Denitrification contributed to the greatest nitrogen loss during the composting process. The nirK community was dominated by Luteimonas sp. and Achromobacter sp., while the nirS community was dominated by Alcaligenes faecalis and Pseudomonas stutzeri. The nosZ community varied in a succession of Halomonas ilicicola, Pseudomonas flexibili and Labrenzia alba dominated communities according to different composting phases. Based on these results, nitrogen cycling models for different phases of the dairy manure composting process were established.

Minimizing ammonia emissions from dairy manure composting by biofiltration using a pre-composted material as the packing media

Compost-based biofiltration is a method widely used to mitigate ammonia emissions during composting. To improve the efficiency of a composting-biofiltration system, it is necessary to determine the most effective degree of composting at which to process the packing media used in the biofiltration system. In this study, materials pre-composted for 20 and 30 d (C20 and C30, respectively), and mature compost (CM) that had been treated for 60 d, were applied as biofilter media to remove ammonia from dairy manure composting exhaust gases. A comparison of the results revealed that the C30 biofilter not only completely removed ammonia, but also produced the least nitrogen loss (1.84%). The C20 biofilter exhibited an inferior performance, indicating that enough pre-composted time is necessary for material used as the packing media. Though the CM biofilter displayed good performance with regard to ammonia removal (97.8%), it had a high nitrogen loss (6.46%). A spearman rank correlation matrix revealed that the abundance of nitrogen cycle genes including amoA, nosZ, nirK, and nirS, had a strong correlation with the physicochemical properties such as nitrate content, carbon source, moisture content, and pH of the biofilter media. C30 provided advantageous conditions and contained a relatively high abundance of nitrifiers and the lowest abundance of denitrifiers. As a result, C30 rather than CM was a more appropriate biofilter media for ammonia removal. Moreover, the occurrence of biological nitrification during the dairy manure composting process indicates the effectiveness of a material for use as biofilter media.

Nitrification plays a key role in N2O emission in electric-field assisted aerobic composting

Nitrous oxide (N₂O) emission is a serious environmental problem in composting. Previous studies have indicated that electric field assistance results in lower N₂O emissions in aerobic composting; however, the exact mechanisms involved in electric-field assisted aerobic composting (EAAC) are not clear. In this study, the biological N transformation processes and the N-associated genes were investigated. The results demonstrated that electric field application inhibited nitrification, weakened the nitrifying functional genes (the hao and nxrA genes declined maximally by 86% and 86.8%, respectively), and increased the N₂O consumption-related gene (nosZ) by a maximum factor of 2.76 compared with that in CAC. The correlation analysis demonstrated that nitrification was the main source of N₂O emission in EAAC. The findings imply that EAAC is a promising process for mitigating N₂O emission at the source during aerobic composting.

N2O emission in full-scale wastewater treatment: Proposing a refined monitoring strategy

Nitrous oxide (N₂O) emissions from wastewater treatment contribute significantly to greenhouse gas emissions. They have been shown to exhibit a strong seasonal and daily profile in previously conducted monitoring campaigns. However, only two year-long online monitoring campaigns have been published to date. Based on three monitoring campaigns on three full-scale wastewater treatment plants (WWTPs) with different activated sludge configurations, each of which lasted at least one year, we propose a refined monitoring strategy for long-term emission monitoring with multiple flux chambers on open tanks. Our monitoring campaigns confirm that the N₂O emissions exhibited a strong seasonal profile and were substantial on all three plants (1-2.4% of the total nitrogen load). These results confirm that N₂O is the most important greenhouse gas emission from wastewater treatment. The temporal variation was more distinct than the spatial variation within aeration tanks. Nevertheless, multiple monitoring spots along a single lane are crucial to assess representative emission factors in flow-through systems. Sequencing batch reactor systems were shown to exhibit comparable emissions within one reactor but significant variation between parallel reactors. The results indicate that considerable emission differences between lanes are to be expected in cases of inhomogeneous loading and discontinuous feeding. For example, N₂O emission could be shown to depend on the amount of treated reject water: lanes without emitted <1% of the influent load, while parallel lanes emitted around 3%. In case of inhomogeneous loading, monitoring of multiple lanes is required. Our study enables robust planning of monitoring campaigns on WWTPs with open tanks. Extensive full-scale emission monitoring campaigns are important as a basis for reliable decisions about reducing the climate impact of wastewater treatment. More specifically, such data sets help us to define general emission factors for wastewater treatment plants and to construct and critically evaluate N₂O emission models.

Electric field induces electron flow to simultaneously enhance the maturity of aerobic composting and mitigate greenhouse gas emissions

The long maturation period and greenhouse gas (GHG) emission are two major problems that arise during aerobic composting, mainly due to the low efficiency of O₂ transmission and utilization. In this study, a novel electric-field-assisted aerobic composting (EAC) process was tested by simply applying a direct-current voltage of 2 V to a conventional aerobic composting (CAC) process. Compared with the CAC process, the maturation time and the total GHG for the EAC process were reduced by 33% and 70%, respectively. Furthermore, the analyses of O₂ consumption and microbial communities demonstrated that the electric field had enhanced O₂ utilization by 30 ± 9% and increased the relative abundance of electroactive bacteria by about 3.4-fold compared to CAC. This work has represented a proof of principle for EAC and suggests that the electric field is an effective and environmentally friendly strategy for enhancing compost maturity and mitigating GHG emissions during aerobic composting.

Effects of composting and carbon based materials on carbon and nitrogen loss in the arable land utilization of cow manure and corn stalks

Recycling organic wastes to arable land as fertilizers has been recognized as a sustainable utilization to reduce environmental pollution. Techniques used for the treatment of organic wastes determine their nutrient contents and thus fertilizer efficiency for agricultural applications. The current study investigated the influences of composting and carbon based materials (biochar and woody peat), on carbon and nitrogen loss in the process of agricultural wastes utilization in the soil batch experiments. The results indicated composting process significantly strengthened the organic matter mineralization, increased the carbon loss rates from 33.46-38.96% to 60.54-86.15% and the nitrogen loss rates from 5.01-22.22% to 48.64-58.16%, dominant lost as carbon dioxide (CO₂) and ammonia (NH₃) emissions. Addition of carbon based materials could effectively reduce the carbon and nitrogen loss during both composting and soil incubation process. When the composted organic wastes were used in the soil batch experiments, woody peat was more effective to reduce nitrogen loss, while biochar was more effective to control carbon loss. When organic wastes were directly fertilized to soil, biochar could effectively reduce nitrogen loss. These results suggested that fertilizing raw agricultural wastes to with carbon based materials could reduce carbon and nitrogen losses, and increased the nutrient bioavailability in soil in comparison with their farmland application after composting.

Drum composting of nitrogen-rich Hydrilla Verticillata with carbon-rich agents: Effects on composting physics and kinetics

Composting of the Hydrilla verticillata, an invasive aquatic weed, signifies aquatic waste management as a safe and hygienic method that produces a nutrient-rich end product, i.e., compost. However, its higher moisture content, higher N-losses, and lower degradation rate have shown negative impacts on the composting process. Therefore the primary objective of this study was to assess the composting physics and the degradation kinetics after addition of three different carbon-rich agents with Hydrilla verticillata. To pursue this objective, three carbon-rich agents (viz. dry leaves in Run A, grass clippings in Run B and wood chips in Run C) each were mixed (10% w/w) to the optimized control mixture of Hydrilla verticillata, cow dung and sawdust (8:1:1) as reported in the earlier study. The composting experiments were performed in 550L rotary drum composter for 20 days to evaluate variation in physical, chemical, nutritional properties as well as degradation kinetics. The Run A and Run B were the only two mixtures that attained the temperature (55-70 °C) that indicates standard sterilization capacity in both with maximum moisture reduction (17%) and total Kjeldahl N increment (48%) in the latter. Organic matter losses throughout the process followed a first-order kinetic equation in all the Run (A-C) and control with the higher loss in Run B whereas least in control. Nevertheless, the addition of all carbon-rich agents is found to be beneficial to improve composting physics. Amongst all Runs (A-C), Run B achieved maximum reduction in the initial value of bulk density (64%) and increment in the initial value of free air space (20%). The study also concluded that all the carbon-rich agents have produced compost with the nutritional concentration suitable for agricultural proposes.