Factoring distinct materials and nitrogen-related microbes into assessments of nitrogen pollution risks during composting

Phosphate additives promote humic acid carbon and nitrogen skeleton formation by regulating precursors and composting bacterial communities

This study aimed to compare the effect of different phosphate additives including superphosphate (CP) and MP [Mg(OH)2 + H3PO4] on nitrogen conversion, humus fractions formation and bacterial community in food waste compost. The results showed the ratio of humic acid nitrogen in total nitrogen (HA-N/TN) in CP increased by 49 %. Ammonium nitrogen accumulation was increased by 75 % (CP) and 44 % (MP). Spectroscopic techniques proved that phosphate addition facilitated the formation of complex structures in HA. CP enhanced the dominance of Saccharomonospora, while Thermobifida and Bacillus were improved in MP. Structural equation modeling and network analysis demonstrated that ammonium nitrogen can be converted to HA-N and has positive effects on bacterial composition, reducing sugars and amino acids, especially in CP with more clustered network and synergic bacterial interactions. Therefore, the addition of phosphate provides a new idea to regulate the retained nitrogen toward humification in composting.

Green waste and sewage sludge feeding ratio alters co-composting performance: Emphasis on the role of bacterial community during humification

Composting with five levels of green waste and sewage sludge was compared to examine how feeding ratios affected composting performance with special focus on humification, and the underlying mechanisms. The results showed that the raw material ratio persistently affected compost nutrients and stability. Humification and mineralization were promoted by higher proportion of sewage sludge. Bacterial community composition and within-community relationships were also significantly affected by the raw material feeding ratio. Network analysis indicated that clusters 1 and 4 which dominated by Bacteroidetes, Proteobacteria, and Acidobacteria shown significantly positive correlation with humic acid concentration. Notably, the structural equational model and variance partitioning analysis demonstrated that bacterial community structure (explained 47.82% of the variation) mediated the effect of raw material feeding ratio on humification, and exceeded the effect of environmental factors (explained 19.30% of the variation) on humic acid formation. Accordingly, optimizing the composting raw material improves the composting performance.

The key role of denitrification and dissimilatory nitrate reduction in nitrogen pollution along vertical landfill profiles from metagenomic perspective

Landfill are persistent sources of nitrogen (N) pollution even in the decades after closure. However, the biological pathways of N-pollution, particularly N₂O and NH₄⁺, at different landfill depths have received little attention. In this study, metagenomic analysis was conducted on landfill refuse from vertical reservoir profiles in two closed landfills named XT and MT. NH₄⁺ concentrations were found to be higher in deeper layers of MT, while greater potential for N₂O emissions occurred in XT and the shallow layers of MT. Furthermore, the community structure and function of N-metabolizing microbes were more strongly defined by landfill depth than landfill type. Denitrification, involving abundant nirK and norB genes, was identified as the major pathway for N₂O production in both XT and MT-shallow, while dissimilatory nitrate reduction with abundant nirBD genes was identified as the major pathway for NH₄⁺ accumulation. Microbes of norB-type and nirBD-type were positively affected by NO₃^- in XT, whereas negatively affected by contents of organic material and moisture in MT-shallow. The mechanism by which nitrogen fixation, with abundant nifH genes, contributes to NH₄⁺ accumulation in MT-deep should be further elucidated. These findings can provide a theoretical basis for governing scientific N-pollution control strategies throughout the entire landfill process.

Competition and interaction between DNRA and denitrification in composting ecosystems: insights from metagenomic analysis

This study investigated denitrification and dissimilatory nitrate reduction to ammonium (DNRA) competition for nitrite in composting of sugarcane pith and cow manure. Metagenomic analysis showed that Actinobacteria was the main DNRA microorganism. During heating phase and thermophilic phase, the abundances of denitrification functional genes (nirK and nirS decreased by 40.22% and 98.60%, respectively) and DNRA functional genes (nirB, nirD increased by 195.24% and 176.61%, and nrfA decreased by 45%, respectively) showed different trends. Interestingly, the abundance of nrfA increased by 250% during cooling and maturity phases. Mantel test revealed that competition between denitrification and DNRA microorganisms for NO₂^--N limited the succession of their respective communities (P < 0.01). Network analysis showed that unclassified Solirubrobacterales, Altererythrobacter and Microbacterium were the key microorganisms in DNRA microbial communities. The results provided new insights into the key microorganisms and their driving factors affecting DNRA and nitrogen management in the composting ecosystems.

Effect of superphosphate addition on heavy metals speciation and microbial communities during composting

Superphosphate fertilizer (SSP) as an additive can reduce the nitrogen loss and increase available phosphorus in composting but few studies investigated the effect of SSP addition on heavy metal and microbial communities. In this study, different ratios (10%, 18%, 26%) of SSP were added into pig manure composting to assess the changes of heavy metal (Cu, Mn, As, Zn, and Fe) fractions, bacterial and fungal communities as well as their interactions. SSP addition at 18% had lower ecological risk but still increased the bioavailability of Cu, Mn, and Fe in composts compared to control. Adding 18% SSP into compost decreased bacterial number and increased the fungal diversity compared to CK. Redundancy analysis indicated heavy metal fractions correlated significantly with bacterial and fungal community compositions in composting with 18% SSP. Network analysis showed adding 18% SSP increased microbial interaction and positive cooperation especially enhanced the proportion of Proteobacteria and Ascomycota.

Red mud conserved compost nitrogen by enhancing nitrogen fixation and inhibiting denitrification revealed via metagenomic analysis

The objective of this study was to investigate the effects of adding red mud (RM) on denitrification and nitrogen fixation in composting. The results revealed that the retentions of NH₄⁺-N and NO₃^--N in experimental group (T) with RM were 16.20% and 7.27% higher than that in control group (CK) at the mature stage, respectively. The composition and structure of RM can effectively inhibit denitrification and enhance nitrogen fixation. Moreover, metagenomic analysis revealed that Actinobacteria and Proteobacteria were the main microorganisms in denitrification process, while Firmicutes were the main microorganisms in nitrogen fixation process. In T, denitrifying genes nirK and nosZ were 11% and 18% lower than those in CK, respectively, while nitrogen-fixing genes nifK and nifD were 18% and 34% higher than those in control group, respectively. Therefore, adding RM could reduce nitrogen loss and improve the quality of compost via enhancing nitrogen fixation and inhibiting denitrification process.

Effects of heavy metals stress on chicken manures composting via the perspective of microbial community feedback

Heavy metal pollution was the main risk during livestock manures composting, in which microorganisms played a vital role. However, response strategies of microbial community to heavy metals stress (HMS) remained largely unclear. Therefore, the objective of this study was to reveal the ecological adaptation and counter-effect of bacterial community under HMS during chicken manures composting, and evaluating environmental implications of HMS on composting. The degradation of organic matters (more than 6.4%) and carbohydrate (more than 19.8%) were enhanced under intense HMS, suggesting that microorganisms could quickly adapt to the HMS to ensure smooth composting. Meanwhile, HMS increased keystone nodes and strengthened significant positive correlation relationships between genera (p < 0.05), indicating that bacteria resisted HMS through cooperating during composting. In addition, different bacterial groups performed various functions to cope with HMS. Specific bacterial groups responded to HMS, and certain groups regulated bacterial networks. Therefore, bacterial community had the extraordinary potential to deal with HMS and guarantee chicken manures composting even in the presence of high concentrations of heavy metals.

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.

Insight into the effects of regulating denitrification on composting: Strategies to simultaneously reduce environmental pollution risk and promote aromatic humic substance formation

Denitrification during composting is a hidden danger that causes environmental pollution risk and aromatic humic substance damage, which needs to be better regulate urgently. In this study, two denitrification regulation methods, moisture and biochar amendment, were conducted during chicken manure composting. Denitrification performance data showed two regulation methods obviously reduced NO₃^--N, NO₂^--N and N₂ contents. Humic substance increased by 25.3 % and 29.1 % under two regulations. Microbiological analysis indicated that two regulation methods could decreasing denitrifying functional microbes with aroma degradation capability. Subsequently, denitrification gene narG, nirS, nosZ were significantly inhibited (p < 0.05) and the aromatic degradation metabolism pathways were down-regulated. Correlation analysis further revealed the important influence of interspecific interactions and non-biological characteristics on functional microbes. These results provided important scientific basis to denitrification regulation in the practice of composting, which achieved the purpose of simultaneously controlling environmental pollution risk and conducing end-product formation.

Nitrate shifted microenvironment: Driven aromatic-ring cleavage microbes and aromatic compounds precursor biodegradation during sludge composting

The aim of this study was to clarify the aromatic cleavage pathways and microbes involved in the adverse effect of nitrate on aromatic compounds humic substances during sludge composting. Results showed that the functional microbes involved in aromatic compounds humic substances precursors (catechol, tyrosine, tryptophan and phenylalanine) cleavage pathways significantly enriched after nitrate addition. Linear regression analysis showed that aromatic-ring cleavage functional microbes exhibited significant negative correlation with aromatic humic substances (p < 0.05). Furthermore, network analysis indicated that most of microbial communities prefer cooperative with aromatic-ring cleavage functional microbes. Structural equation model further revealed that composting microenvironment drove aromatic-ring cleavage functional microbes activities, resulting in the biodegradation of complex aromatic compounds. This study parsed the effect of a negative factor on aromatic compounds humic substances from an opposing perspective. Properly controlling nitrate concentration and aromatic-ring cleavage functional microbes involved in precursors cleavage was suggested to the practice of composting.