Dissolved organic carbon, a critical factor to increase the bioavailability of phosphorus during biochar-amended aerobic composting

Insights into the roles and mechanisms of coconut shell biochar and coke in anaerobic digestion of river snail rice noodle wastewater

The effects of coconut shell biochar and coke on anaerobic digestion of river snail rice noodle wastewater treatment were assessed, and the microbial community, and methane metabolic pathways were investigated. When the hydraulic retention time was 24 h, the average chemical oxygen demand (COD) removal rates in the reactors with coconut shell biochar and coke were 94.02% and 88.68%, respectively; when the hydraulic retention time was 12 h, the average COD removal rates were 91.32% and 85.47%, respectively. The addition of coconut shell biochar and coke increased the activity of protease in the sludge from 0.45% to 29.31% and from 1.00% to 21.35%, respectively. The addition of coconut shell biochar and coke to the two anaerobic reactors promoted the growth of Euryarchaeota, Proteobacteria, and Chloroflexi. In glycolysis, the key genes glk, pfk and pk were upregulated by 3.15%, 5.22%, and 0.44% in the coconut shell biochar reactor and 8.97%, 1.93% and 3.73% in the coke reactor, respectively, and the keytricarboxylic acid (TCA) cycle genes kor, frd, and mdh were also up-regulated.

Redistribution of phosphorus fraction driven by organic carbon and microbial community during composting

Available information on the coupling relationship between phosphorus fraction and organic carbon during composting remains limited. Thus, this research investigated the changes of phosphorus fraction, dissolved organic carbon fluorescent components and microbial community in swine manure composting with different carbon sources including the maize straw (MS), garden waste (GW) and distillers' grains (DG), in order to investigate whether the distribution and availability of phosphorus are influenced by different carbon sources used in the composting of swine manure. The result showed that different carbon sources changed phosphorus availability variously mainly by altering the succession of fungal communities and phosphorus functional genes. The dissolved organic material including tyrosine and tryptophan facilitate the mineralization of organic phosphorus (Org-P) into water-soluble phosphorus, thereby improving phosphorus availability. However, humic acid-like carbon components promote the conversion of inorganic-phosphorus to Org-P, which is the direct cause of the reduced phosphorus availability during composting. The results of this study provide support for the development of phosphorus-rich, stable, and clean compost products.

A comprehensive review on agricultural waste utilization through sustainable conversion techniques, with a focus on the additives effect on the fate of phosphorus and toxic elements during composting process

The increasing trend of using agricultural wastes follows the concept of "waste to wealth" and is closely related to the themes of sustainable development goals (SDGs). Carbon-neutral technologies for waste management have not been critically reviewed yet. This paper reviews the technological trend of agricultural waste utilization, including composting, thermal conversion, and anaerobic digestion. Specifically, the effects of exogenous additives on the contents, fractionation, and fate of phosphorus (P) and potentially toxic elements (PTEs) during the composting process have been comprehensively reviewed in this article. The composting process can transform biomass-P and additive-born P into plant available forms. PTEs can be passivated during the composting process. Biochar can accelerate the passivation of PTEs in the composting process through different physiochemical interactions such as surface adsorption, precipitation, and cation exchange reactions. The addition of exogenous calcium, magnesium and phosphate in the compost can reduce the mobility of PTEs such as copper, cadmium, and zinc. Based on critical analysis, this paper recommends an eco-innovative perspective for the improvement and practical application of composting technology for the utilization of agricultural biowastes to meet the circular economy approach and achieve the SDGs.

Modifying humus-phosphorus-arsenic interactions in sludge composting: The strengthening of phosphorus availability and arsenic efflux detoxification mechanisms

Arsenic (As) in sewage sludge poses a significant threat to environmental and human health, which has attracted widespread attention. This study investigated the value of adding sodium percarbonate (SP) on phosphorus (P) availability and As efflux detoxification through HS-P-As interactions. Due to the unique structure of humus (HS) and the similar chemical properties of P and As, the conditions for HS-P-As interaction are provided. This study discussed the content, morphology and microbial communities of HS, P and As by using metagenomic and correlation analysis. The results showed that the humification index in the experiment group (SPC) was 2.34 times higher than that in the control group (CK). The available phosphorus (AP) content of SPC increased from 71.09 mg/kg to 126.14 mg/kg, and SPC was 1.11 times that of CK. The relative abundance of ACR3/ArsB increased. Pst, Actinomyces and Bacillus commonly participated in P and As conversion. The correlation analysis revealed that the humification process was enhanced, the AP was strengthened, and the As was efflux detoxified after SP amendment. All in all, this study elucidated the key mechanism of HS-P-As interaction and put forward a new strategy for sewage sludge resource utilization and detoxification.

Industrial and agricultural waste amendments interact with microorganism activities to enhance P availability in rice-paddy soils

Adding industrial and agricultural wastes to farmland can increase soil available phosphorus (P) pool and boost crop production, but the process affecting soil P transformation and bioavailability is still poorly understood. We studied the effects of straw (ST), biochar (BC) and Si-modified biochar (Si-BC) amendments on the available-P content and its fraction transformation in rice-paddy soils. Our results showed that these three soil amendments significantly increased the concentrations of both microbial biomass carbon (MBC) and microbial biomass-P (MBP) during the first rice season; by contrast, the effects of ST and BC application were relatively poor on acid-phosphatase (ACP) activity, which was increased by 24 % under ST and 14 % under BC. Soil total P concentrations did not differ significantly, although the concentration and percentage of each P-fraction were altered significantly among treatments. Although all three applications increase soil available-P concentration by promoting the transformation of organic-P (Po) components to inorganic-P (Pi), there are differences in the transformation efficiency of the soil P fraction between these amendments. Redundancy analysis results also showed significant clustering of soil P-fraction transformations after ST and BC treatments. Structural equation model analysis further indicated that all amendments regulated microbial processes by changing soil pH and dissolved organic carbon (DOC), thereby promoting soil P transformation and improving P efficiency. Sodium bicarbonate-extractable Po (NaHCO3-Po) contributed most to soil available-P under the different amendments. Compared to ST and Si-BC, BC application improved more soil microbial status and the transformation of soil unavailable-P into available-P, therefore the application of BC in rice fields is the most beneficial method to promote phosphorus use and production sustainability in rice. These findings helped to understand the effects of using industrial and agricultural waste (e.g. straw, biochar and Si-modified biochar) on soil P-fractions and so provided a reference for sustainable resource use and green production in rice-paddy ecosystems.

Effects of soil organic carbon metabolism on electro-bioremediation of petroleum-contaminated soil

Soil organic carbon (SOC) potentially interacts with microbial metabolism and may affect the degradation of petroleum-derived carbon (PDC) in the electro-bioremediation of petroleum-contaminated soil. This study evaluated the interactions among organic carbon, soil properties, and microbial communities to explore the role of SOC during the electro-bioremediation process. The results showed that petroleum degradation exerted superposition and synergistic electrokinetic and bioremediation effects, as exemplified by the EB and EB-PR tests, owing to the maintenance and enhancement of SOC utilization (P/S value), respectively. The highest P/S value (2.0-2.4) was found in the electrochemical oxidation zone due to low SOC consumption. In the biological oxidation zones, electric stimulation enhanced the degradation of PDC and SOC, with higher average P/S values than those of the Bio test. Soil pH, Eh, inorganic ions, and bioavailable petroleum fractions were the main factors reshaping the microbial communities. SOC metabolism effectively buffered the stress of environmental factors and pollutants while maintaining functional bacterial abundance, microbial alpha diversity, and community similarity, thus saving the weakened PDC biodegradation efficiency in the EB and EB-PR tests. The study of the effect of SOC metabolism on petroleum biodegradation contributes to the development of sustainable low-carbon electro-bioremediation technology.

Biochar preparation and evaluation of its effect in composting mechanism: A review

This article provides an overview of biochar application for organic waste co-composting and its biochemical transformation mechanism. As a composting amendment, biochar work in the adsorption of nutrients, the retention of oxygen and water, and the promotion of electron transfer. These functions serve the micro-organisms (physical support of niche) and determine changes in community structure beyond the succession of composing primary microorganisms. Biochar mediates resistance genes, mobile gene elements, and biochemical metabolic activities of organic matter degrading. The participation of biochar enriched the α-diversity of microbial communities at all stages of composting, and ultimately reflects the high γ-diversity. Finally, easy and convincing biochar preparation methods and characteristic need to be explored, in turn, the mechanism of biochar on composting microbes at the microscopic level can be studied in depth.

Carbon-containing additives changes the phosphorus flow by affecting humification and bacterial community during composting

Phosphorus recycling from organic wastes to prepare a fertilizer by composting is promising. The aim of this study was to compare the effect of diverse carbon-containing additives (T1, glucose; T2, biochar; T3, woody peat) on phosphorus (P) fractions transformations, humus formation and bacterial community succession in chicken manure composting. Results showed that orthophosphate monoester was significantly related to the humification process, and glucose or woody peat addition increased the P in humus. Lentibacillus was a key carbon cycle bacteria related to organics stabilization affected by carbon-containing additives. Redundancy analysis and variation partitioning indicated that phosphatase enzyme activity driven by bacterial community and humic substance had 59.7% contribution to P fractions dynamics. The findings highlight an efficient humus-regulation P stabilization way, notably in composting adding glucose to form humus with a better binding ability to labile P forms and phosphatase.

Biochar reduces bioavailability of phosphorus during swine manure composting: Roles of phoD-harboring bacterial community

The bioavailability of phosphorus is a vital index for evaluating the quality of compost products. This study examined the effects of adding wheat straw biochar (WSB) and bamboo charcoal (BC) on the transformation of various phosphorus fractions during composting, as well as analyzing the roles of the phoD-harboring bacterial community in the transformation of phosphorus fractions. Adding WSB and BC reduced the available phosphorus content in the compost products by 35.2 % and 38.5 %, respectively. Redundancy analysis showed that the alkaline phosphatase content and pH were the most important factors that affected the transformation of phosphorus fractions. The addition of biochar resulted in changes in the composition and structures of the phoD-harboring bacteria communities during composting. In addition, the key bacterial genera that secreted alkaline phosphatase and decomposed different forms of phosphorus under WSB and BC were different compared with those under control. Network and correlation analysis demonstrated that the activities of phoD-harboring bacteria could have been enhanced by biochar to accelerate the consumption of available phosphorus, and the activities of key phosphorus-solubilizing bacteria (Lysobacter, Methylobacterium, and Saccharothrix) might be inhibited when the pH increased, thereby increasing the insoluble phosphorus content.

Insights into enzyme activity and phosphorus conversion during kitchen waste composting utilizing phosphorus-solubilizing bacterial inoculation

The main objective of this research was to investigate the effects of Phosphorus-Solubilizing Bacterial (PSB) inoculant on the bacterial structure and phosphorus transformation during kitchen waste composting. High throughput sequencing, topological roles, and multiple analysis methods were conducted to explain the links between phosphorus fractions, enzyme contents, and microbial community structure and function. The findings indicated that bacterial inoculant improved environmental parameters and increased the concentration of total phosphorus, Olsen phosphorus, citric acid phosphorus, OM decomposition, and bacterial diversity. Network analysis concluded that the inoculation treatment was more complex (nodes and edges) and contained more positive links than the control, implying the inoculation effect. The structural equation model also displayed that pH and enzyme activity directly enhanced the phosphorus conversion and bacterial structure. Overall, these results suggest that bacterial inoculation may considerably increase enzyme activity, thus improving biological phosphorus transformation and nutrient content in composting products.

Review on fate and bioavailability of heavy metals during anaerobic digestion and composting of animal manure

Anaerobic digestion and composting are attracting increasing attention due to the increased production of animal manure. It is essential to know about the fate and bioavailability of heavy metals (HMs) for further utilisation of animal manure. This review has systematically summarised the migration of HMs and the transformation of several typical HMs (Cu, Zn, Cd, As, and Pb) during anaerobic digestion and composting. The results showed that organic matter degradation increased the HMs content in biogas residue and compost (with the exception of As in compost). HMs migrated into biogas residue during anaerobic digestion through various mechanisms. Most of HMs in biogas residue and compost exceeded relevant standards. Then, anaerobic digestion increased the bioavailable fractions proportion in Zn and Cd, decreased the F4 proportion, and raised them more than moderate environmental risks. As (III) was the main species in the digester, which extremely increased As toxicity. The increase of F3 proportion in Cu and Pb was due to sulphide formation in biogas residue. Whereas, the high humus content in compost greatly increased the F3 proportion in Cu. The F1 proportion in Zn decreased, but the plant availability of Zn in compost did not reduce significantly. Cd and As mainly converted the bioavailable fractions into stable fractions during composting, but As (V) toxicity needs to be concerned. Moreover, additives are only suitable for animal manure treated with slightly HM contaminated. Therefore, it is necessary to combine more comprehensive methods to improve the manure treatment and make product utilisation safer.

Evaluate the role of biochar during the organic waste composting process: A critical review

Composting is very robust and efficient for the biodegradation of organic waste; however secondary pollutants, namely greenhouse gases (GHGs) and odorous emissions, are environmental concerns during this process. Biochar addition to compost has attracted the interest of scientists with a lot of publication in recent years because it has addressed this matter and enhanced the quality of compost mixture. This review aims to evaluate the role of biochar during organic waste composting and identify the gaps of knowledge in this field. Moreover, the research direction to fill knowledge gaps was proposed and highlighted. Results demonstrated the commonly referenced conditions during composting mixed biochar should be reached such as pH (6.5-7.5), moisture (50-60%), initial C/N ratio (20-25:1), biochar doses (1-20% w/w), improved oxygen content availability, enhanced the performance and humification, accelerating organic matter decomposition through faster microbial growth. Biochar significantly decreased GHGs and odorous emissions by adding a 5-10% dosage range due to its larger surface area and porosity. On the other hand, with high exchange capacity and interaction with organic matters, biochar enhanced the composting performance humification (e.g., formation humic and fulvic acid). Biochar could extend the thermophilic phase of composting, reduce the pH value, NH₃ emission, and prevent nitrogen losses through positive effects to nitrifying bacteria. The surfaces of the biochar particles are partly attributed to the presence of functional groups such as Si-O-Si, OH, COOH, CO, C-O, N for high cation exchange capacity and adsorption. Adding biochars could decrease NH₃ emissions in the highest range up to 98%, the removal efficiency of CH₄ emissions has been reported with a wide range greater than 80%. Biochar could absorb volatile organic compounds (VOCs) more than 50% in the experiment based on distribution mechanisms and surface adsorption and efficient reduction in metal bioaccessibilities for Pb, Ni, Cu, Zn, As, Cr and Cd. By applicating biochar improved the compost maturity by promoting enzymatic activity and germination index (>80%). However, physico-chemical properties of biochar such as particle size, pore size, pore volume should be clarified and its influence on the composting process evaluated in further studies.