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PurposeNitrogen (N) fertilizer could promote rice root growth and alter methane (CH4) emissions in paddy fields. This study investigated the mechanism of different N application rates and timing affecting CH4 emissions, focusing on the interactions between rice root growth, soil properties, and CH4-related microbes in the rhizosphere.MethodsN fertilizer was applied only as basal and tillering fertilizer (BF) in 2016–2017 and applied as panicle fertilizer (PF) as top-dressing in 2018–2019. A validation experiment was conducted in 2020 to compare the soil chemical properties and CH4 production and oxidation in rhizosphere soil with those in soil with root effects eliminated by removing rice aboveground parts.ResultsBased on the local high-yielding BF rate, applying PF had a more significant effect on CH4 mitigation than increasing BF application rate. PF notably increased the rice root biomass, carbon substrates in root exudate (including organic acids) during rice reproductive growth phase (RGP), and the dissolved organic carbon and dissolved oxygen concentration in rhizosphere soil, simultaneously promoting the abundance and potential activities of methanogens and methanotrophs. A mantel test and the validation experiment confirmed that the increase of CH4 oxidation activity and a high ratio of methanotrophs to methanogens in rhizosphere soil driven by rice root was the main reason for CH4 mitigation after N application.ConclusionAn appropriate PF rate can stimulate rice root growth, root exudation, and oxygen secretion, increasing the CH4 oxidation in rhizosphere soil that contributes significantly to the CH4 mitigation in paddy fields.

Background Recently, it has been indicated that anaerobic co-digestion of waste activated sludge with other waste streams at wastewater treatment plants is a promising strategy for enhancing methane production and materials recovery. The enhanced methane production can be used as a renewable source of energy in wastewater treatment plants. It can also reduce the amount of greenhouse gas emission in landfilling of the waste streams. Results According to the results obtained in this study, anaerobic co-digestion of waste activated sludge with mixed fruit waste and cheese whey improves methane production and the quality of digested sludge in comparison to the anaerobic digestion of waste activated sludge individually. It was indicated that carbon/nitrogen ratio (C/N) in the mixture of waste activated sludge, fruit waste and cheese whey improved considerably, leading to better anaerobic organisms’ activity during digestion. With assessing the activity of protease and cellulase, as the main enzymes hydrolyzing organic matter in anaerobic digestion, it was indicated that co-digestion of waste activated sludge with mixed fruit waste and cheese whey enhances the activity of these enzymes by 22 and 9% respectively. At the end of digestion, the amount of cumulative methane production significantly increased by 31% in the reactor with 85% waste activated sludge and 15% mixed fruit waste and cheese whey, compared to the reactor with 100% waste activated sludge. In addition, chemical oxygen demand (COD) and volatile solid (VS) in digested sludge was improved respectively by 9 and 7% when mixed fruit waste and cheese whey was used. Conclusions This study revealed that mixed fruit waste and cheese whey is potentially applicable to anaerobic digestion of waste activated sludge, as fruit waste and cheese whey have high C/N ratio that enhance low C/N in waste activated sludge and provide a better diet for anaerobic organisms. This is of significant importance because not only could higher amount of renewable energy be generated from the enhanced methane production in wastewater treatment plants, but also capital costs of the companies whose waste streams are being transported to wastewater treatments plants could be reduced considerably.

The population growth is causing an increase in the generation of effluents (mainly organic fraction of municipal solid waste (OFMSW) and agro-industrial waste), which is an old problem in agro-industrial countries such as Brazil. Contrastingly, it is possible to add value to these residual biomasses (residues) through the application of new technologies for the production of bioenergy. Anaerobic digestion (AD) of sewage sludge is being applied in many effluent treatment plants for the sustainable and economically viable production of biogas. However, the biogas produced from AD (sludge) or co-digestion (sludge with other residues) presents a concentration of methane between 60 and 70% on average, which is relatively low. This review is aimed at analyzing studies involving (i) production of lipases by solid-state fermentation (SSF) by different microorganisms for the application in enzymatic pretreatments prior to the anaerobic treatment of effluents; (ii) pretreatment followed by AD of various residues, with an emphasis on OFMSW and sewage sludge; and (iii) more recent studies on anaerobic co-digestion (AcoD) and hybrid technologies (pretreatment + AD or AcoD). There are many studies in the literature that demonstrate the enzymatic pretreatment or AcoD applied to the optimization of methane production. Nevertheless, few studies report the combination of these two technologies, which can improve the process and reduce or eliminate the costs of biogas purification, which are major challenges for the viability of this route of bioenergy production.Key points•Municipal and agro-industrial wastes have potential as medium for lipase production.•Enzymatic pretreatment and anaerobic co-digestion are low cost for high-methane production.

This study aimed to determine the silage characteristics of oat lines developed through a rigorous 10-year selection process. During this period, 100 plants with high biomass yield were meticulously chosen from oat lines obtained from the Louisiana State University gene bank. Researchers designed experiments using an incomplete randomized block format across two growing seasons: 2014–2015 and 2015–2016. Oat plants were carefully harvested at the milk stage, chopped into pieces using a specialized plant chopper, and ensiled in plastic vacuum bags for subsequent analysis. The samples were then stored at room temperature for a period of 60 days. The comprehensive analysis conducted over this two-year research period revealed significant variations in the silage yields of the genotypes, ranging from 5.71 to 24.42 t ha−1. Moreover, notable variations were observed in the content of crude ash (ranging from 3.952 to 9.380%), ADF (ranging from 27.899 to 40.782%), NDF (ranging from 40.688 to 63.999%), crude protein (ranging from 6.673 to 11.470%), dry matter (ranging from 14.954 to 41.817%), pH levels (ranging from 3.812 to 5.668), in vitro gas production (ranging from 49.793 to 67.847 ml), methane production (ranging from 15.036 to 18.442 ml), as well as concentrations of acetic acid (ranging from 0.000 to 0.144%), lactic acid (ranging from 1.974 to 2.599%), butyric acid (ranging from 0.000 to 0.064%), and propionic acid (ranging from 0.000 to 0.306%). The study clearly indicated that many oat lines showed promising silage properties. Consequently, it is inferred that the utilization of high-yield lines in silage production holds considerable potential in providing a valuable feed source for livestock.

The agricultural industry produces a substantial quantity of organic waste, and finding a suitable method for disposing of this highly biodegradable solid waste is a difficult task. The utilisation of anaerobic digestion for agricultural waste is a viable technological solution for both renewable energy production (biogas) and waste treatment. The primary objective of the study was to assess the composition of biogas, namely the percentages of methane, carbon dioxide, ammonia, and hydrogen sulphide. Additionally, the study aimed to quantify the amount of biogas produced and determine the methane yield (measured in NmL/g VS) from different agricultural substrates. The biochemical methane potential (BMP) measurements were conducted in triplicate using the BPC Instruments AMPTS II instrument. The substrates utilised in the investigation were chosen based on their accessibility. The substrates used in this study comprise cattle manure, chicken manure, pig manure, tomato plants, tomatoes, cabbage, mixed fruits, mixed vegetables, dog food, and a co-digestion of mixed vegetables, fruits, and dog food (MVMFDF). Prior to the cleaning process, the makeup of the biogas was assessed using the BIOGAS 5000, a Geotech Analyser. The AMPTS II flow cell automatically monitored and recorded the volume of bio-methane produced after the cleaning stage. The data were examined using the Minitab-17 software. The co-digestion of mixed vegetables, mixed fruits, and dog food (MVMFDF) resulted in the highest methane level of 77.4%, followed by mixed fruits at 76.6%, pig manure at 72.57%, and mixed vegetables at 70.1%. The chicken manure exhibited the greatest levels of ammonia (98.0 ppm) and hydrogen sulphide (589 ppm). Chicken manure had the highest hydrogen sulphide level, followed by pig manure (540 ppm), tomato plants (485 ppm), mixed fruits (250 ppm), and MVMFDF (208 ppm). Ultimately, the makeup of biogas is greatly affected by the unique qualities of each substrate. Substrates containing elevated quantities of hydrogen sulphide, such as chicken manure, require the process of biogas scrubbing. This is because they contain substantial amounts of ammonia and hydrogen sulphide, which can cause corrosion to the equipment in biogas plants. This emphasises the crucial need to meticulously choose substrates, with a specific focus on their organic composition and their capacity to generate elevated methane levels while minimising contaminants. Substrates with a high organic content, such as agricultural waste, are optimal for maximising the production of methane. Furthermore, the implementation of biogas scrubbing procedures is essential for efficiently decreasing carbon dioxide and hydrogen sulphide levels in biogas. By considering and tackling these problems, the effectiveness of biogas generation can be enhanced and its ecological consequences alleviated. This strategy facilitates the advancement of biogas as a sustainable energy source, hence contributing to the attainment of sustainable development goals (SDGs).

The contribution of CH 4 emissions from paddy soils to greenhouse gas emissions is key in the evaluation of future climate change scenarios. Most studies in this field have investigated the effects of elevated CO 2 concentrations (e[CO 2 ]s) on CH 4 fluxes and methanogenic communities in paddy soils under constant CO 2 concentrations ([CO 2 ]s). However, atmospheric [CO 2 ] is gradually increasing and the relationship between future climate change and CH 4 emissions from paddy fields is poorly understood. This study explored the responses of CH 4 fluxes and methanogenic communities in paddy soils to different e[CO 2 ]s using open-top chambers. The rice biomass, CH 4 fluxes, methane production potential, and methanogenic characteristics were analyzed under CK (ambient [CO 2 ]), C 1 (e[CO 2 ] by 120 µmol mol –1 ), and C 2 (e[CO 2 ] by 200 µmol mol –1 ) treatments. The results indicated that the C 1 and C 2 treatments insignificantly increased the CH 4 flux in paddy fields. However, the C 1 treatment significantly increased the CH 4 flux/biomass at the elongation stage, while the C 2 treatment significantly increased the CH 4 flux/biomass at all of the growth stages. The C 1 and C 2 treatments had a positive effect on both methane production potential and methanogenic abundance at all of the growth stages, but this effect was not always significant. In addition, the C 1 and C 2 treatments significantly altered the methanogenic community structure at the elongation stage. Notably, there was a significant linear relationship between the CH 4 flux/biomass and [CO 2 ] at all of the growth stages; between the methane production potential and [CO 2 ] at the tillering, elongation, and milk-ripening stages; and between the mcrA g ene abundance and [CO 2 ] at the milk-ripening stage. A linear model based on rice biomass, methane production potential, and soil DOC concentration explained 72.7% of the variation in the CH 4 fluxes. Overall, the linear relationship between CH 4 fluxes and atmospheric [CO 2 ] levels was controlled by the rice biomass, soil carbon substrate, and methanogenic communities.

Methane production characteristics of anaerobic co-digestion of pig manure (PM) and fermented liquid feed (FLF) were investigated in a continuous digester under mesophilic conditions. The experiment followed three phases. PM alone was digested in phase I. In phases II and III, PM and FLF were mixed in a ratio of 95:5 and 90:10 (% v/v), respectively. The specific methane yields (SMYs) during phases I, II, and III were 238, 278, and 326.8 mLCH4·gVS−1-added, respectively. It was due to the effect of balancing the feedstock carbon-to-nitrogen ratio by adding FLF. This improvement can also be attributed to the readily biodegradable compounds in the FLF. The higher SMY obtained in this study showed a positive synergistic effect in the anaerobic co-digestion of PM and FLF. The results also indicate that adding the FLF positively affected and maintained a constant pH level, avoiding volatile fatty acid accumulation and ammonia inhibition in the anaerobic digestion (AD). Thus, this study provides valuable information regarding the usage of unused or wasted FLF as a co-substrate for the practical AD of PM. The production of fermented liquid additives such as FLF to improve the methane production from the AD of PM is a potential novel alternative to food waste recycling in Japan, besides compost and animal feeding.

Anaerobic conversion rate of phenol to methane was low due to its biological toxicity. In this study, the coupling of granular activated carbon (GAC) and exogenous hydrogen (EH) could enhance greatly methane production of phenol anaerobic digestion, and the metagenomic was firstly used to analyze its potential mechanism. The results indicated that a mass of syntrophic acetate-oxidizing bacteria and hydrogen-utilizing methanogens were enriched on the GAC surface, and SAO-HM pathway has become the dominant pathway. The energy transfer analysis implied that the abundance of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide (NADH) oxidase increased. Furthermore, direct interspecies electron transfer (DIET) was formed by promoting type IV e-pili between Methanobacterium and Syntrophus , thereby improving the interspecies electron transfer efficiency. The dominant SAO-HM pathway was induced and DIET was formed, which was the internal mechanism of the coupling of GAC and EH to enhance anaerobic biotransformation of phenol.

Soil methane generation mainly driven by soil prokaryotic microbes can be coupled with the degradation of petroleum hydrocarbons (PHCs); however, the relationship between prokaryotic community structure and methane production activity in soil with the potential risk of PHC contamination is seldom reported. In this study, 3 soil samples (CS-1 to CS-3) in the area nearby an exploratory gas well and 5 soil samples (DC-1 to DC-5) in a drill cutting dump area were obtained from the Fuling shale gas field (Chongqing City, China). Then, the prokaryotic community structure was examined by Illumina Miseq sequencing, and the linkage between soil methane production rate (MPR) and prokaryotic community composition was analyzed. The results indicated that 2 samples (DC-4 and DC-5) collected from the drill cutting dump area had significantly higher MPR than the other samples, and a significant and positive relationship ( r  = 0.44, P  < 0.05) was found between soil MPR and soil organic matter (OM) content. The prokaryotic community composition in the sample (DC-5) with the highest MPR was different from those in the other samples, and soil OM and MPR were the major factors significantly correlated with the prokaryotic community structure in this soil. The samples (DC-4 and DC-5) with higher MPR had a higher relative abundance of Archaea and different archaeal community structures from the other samples, and the MPR was the sole factor significantly correlated with the archaeal genus composition in this soil. Therefore, both the prokaryotic and archaeal community structures are essential in the determination of soil MPR, and the bacterial genus of Saccharibacteria and the archaeal genus of Methanolobus might be the key contributors for methane generation in this soil from the shale gas field.

Effects of hosting trees on feed quality traits of mistletoe samples were investigated in this study. Mistletoe samples were collected from pine, willow, hawthorn, locust, apricot, wild pear and almond trees. Effects of hosting trees on nutritional traits of mistletoe leaves were found to be significant. The greatest crude protein content was observed in samples collected from locust tree and the greatest crude oil from pine tree, the greatest crude ash from apricot tree, the lowest ADF and NDF ratios from hawthorn tree. Condensed tannin contents of mistletoe samples varied between 0.68 and 0.97% and the differences in condensed tannin contents of mistletoe samples were not found to be significant. Apricot (53.85 ml) and wild pear (54.48 ml) had the greatest gas production and willow tree (10.61 ml) had the greatest methane production. Metabolizable energy (ME) values varied between 8.49 and 10.82 MJ (kg/DM) and organic matter digestibility (OMD) values varied between 58.26 and 73.97%. Although all samples were rich in minerals, pine tree was prominent for micro elements. The major saturated acids found in the leaf oils were palmitic acid and the major unsaturated fatty acids were oleic, linoleic and linolenic acids. The total saturated fatty acid levels of mistletoe collected from different trees were between 13.42 and 25.68%, while the total unsaturated fatty acids were between 74.32 and 86.58%. Mistletoe samples have amino acid content similar to forage crops. In addition, they have high value of proline. It was concluded based on present findings that mistletoe samples might constitute a quality feed source for animals. Use of parasitic mistletoe species in animal feeding may offer a well method in reduction of damages on forest and culture trees and supply a quality feed source for ruminants. Graphical Abstract