Response of denitrifying genes coding for nitrite (nirK or nirS) and nitrous oxide (nosZ) reductases to different physico-chemical parameters during agricultural waste composting

Biodegradation of volatile solids and water mass balance of bio-drying sewage sludge after electro-dewatering pretreatment

Using pressurized electro-osmotic dewatering (PEOD) as the pretreatment process for sewage sludge (SS) bio-drying can improve the dewatering performance, but the kinetics of volatile solids biodegradation and the water mass balance are still unknown. These processes were first investigated in this study. Experiments were conducted with three different initial materials, which were composed of SS, bio-dried product and SS dewatered by PEOD (EDSS) as different mass ratios. Six kinetic models and a nonlinear regression method were used to estimate the kinetic parameters, and the models were analyzed using four statistical indicators. Satisfactory fitting of the proposed kinetic model to the experimental data was achieved. Through the water mass balance, the results showed that EDSS had the best dewatering performance for bio-drying. EDSS provided the most appropriate conditions for the bio-drying process; the highest correlation coefficient was 0.9291 and the total water removal rate was 51.13% in the bio-drying of all EDSS.

Evaluating the net effect of sulfadimidine on nitrogen removal in an aquatic microcosm environment

Antibiotics enter into aquatic pond sediments by wastewater and could make detrimental effects on microbial communities. In this study, we examined the effects of sulfadimidine on nitrogen removal when added to experimental pond sediments. We found that sulfadimidine increased the number of sulfadimidine resistant bacteria and significantly increased the abundance of sul2 at the end of the incubation time (ANOVA test at Tukey HSD, P < 0.05). In addition, sulfadimidine decreased the N₂O reduction rate as well as the amount of nitrate reduction. Pearson correlation analysis revealed that the N₂O reduction rate was significantly and negatively correlated with narG (r = -0.679, P < 0.05). In contrast, we found a significant positive correlation between the amount of nitrate reduction and the abundance of narG (r = 0.609, P < 0.05) and nirK (r = 0.611, P < 0.05). High-throughput sequencing demonstrated that Actinobacteria, Euryarchaeota, Gemmatimonadetes, Nitrospirae, Burkholderiaceae (a family of Proteobacteria), and Thermoanaerobaculaceae (a family of Firmicutes) decreased with sulfadimidine exposure. In sediments, Actinobacteria, Bacteroidetes, Cyanobacteria, Epsilonbacteraeota, Euryarchaeota, Firmicutes, Gemmatimonadetes, and Spirochaetesat may play key roles in nitrogen transformation. Overall, the study exhibited a net effect of antibiotic exposure regarding nitrogen removal in an aquatic microcosm environment through a combination of biochemical pathways and molecular pathways, and draws attention to controlling antibiotic pollution in aquatic ecosystems.

Effects of Physico-Chemical Parameters on Actinomycetes Communities during Composting of Agricultural Waste

The objective of this study was to investigate the influence of physico-chemical parameters on Actinomycetes communities and to prioritize those parameters that contributed to Actinomycetes community composition during the composting of agricultural waste. Denaturing gradient gel electrophoresis of polymerase chain reaction (PCR-DGGE) and redundancy analysis (RDA) were used to determine the relationships between those parameters and Actinomycetes community composition. Quantitative PCR (qPCR) and regression analysis were used to monitor the 16S rDNA copy numbers of Actinomycetes and to analyse the correlations between physico-chemical parameters and Actinomyces 16S rDNA gene abundance, respectively. The RDA results showed that moisture content, water soluble carbon (WSC) and pH (p < 0.05) made the main contributions to the temporal variations of Actinomycetes community composition. The output of the regression analysis indicated that moisture content (R2 = 0.407, p < 0.01) showed a negative linear relationship with the Actinomyces 16S rDNA gene abundance.

Moderate Grazing Promotes Grassland Nitrous Oxide Emission by Increasing Ammonia-Oxidizing Archaea Abundance on the Tibetan Plateau

Grasslands are suffering from long-term overgrazing because of the population inflation. Furthermore, nitrous oxide (N₂O) is a major greenhouse gas that also depletes stratospheric ozone. However, the emission rate of grassland N₂O and underlying mechanisms remained unclear under different grazing intensities. We conducted a field manipulation under four grazing intensities to compare its N₂O fluxes and main affected factors. It was indicated that alpine meadow N₂O emission rates increased from 39.7 ± 3.1 to 47.8 ± 2.3 μg m^-2 h^-1 (p < 0.05), then decreased to 43.4 ± 4.1 and 32.9 ± 1.4 μg m^-2 h^-1 with grazing intensity increasing from 4 to 8, 12 and 16 sheep ha-1, respectively. Multiple-stepwise regression analysis indicated that the predominant affected soil factors were separately TN and BD, pH and BD, also pH and BD, SOC and BD. Simple linear regression models revealed that ammonia-oxidizing archaea (AOA) contributed much to N₂O emission (R² = 0.77). Additionally, the R² coefficient of linear regression was 0.87 between nosZ genes and N₂O emission rates in alpine meadow. Much attention should be paid to protecting alpine meadow from degradation to mitigate N₂O emission source on the Tibetan Plateau.

Effects of amoxicillin on nitrogen transformation and bacterial community succession during aerobic composting

This study systematically analyzed the effects of amoxicillin (AMX) on the nitrogen transformation and its corresponding functional bacterial communities by conducting two aerobic composting experiments, and AMX impact on bacterial community succession was also evaluated. It provides theoretical and methodological support for harmless composting treatment of large quantities of manure containing AMX in China and for the high-quality compost products. The results showed that AMX exerted several effects on basic physicochemical and biological compost parameters. Notably, temperature changes typically accompanying compost maturation were delayed in AMX compost, reflecting altered compost maturation kinetics and bacterial community structure. Moreover, relative to control, AMX inhibited growth and reproduction of dominant bacterial phyla Firmicutes and Bacteroidetes, with respective reductions of 17.8-26.1% and 0-7.76% in relative abundance (RA) and significantly increased Proteobacteria RA by 1.9-24.8%. Thus, AMX altered both compost bacterial community structure and succession. From the perspective of various nitrogen content changes, AMX has a significant effect on nitrogen conversion and release. Simultaneously, AMX may inhibit ammoniated and ammonia-oxidizing bacterial activity, while significantly increasing the RA of denitrifying bacteria. Indeed, during early composting with AMX, the RA of denitrifying bacteria was 1361.9-1435.0% of control, highlighting differences in nitrogen transformation and release between groups.

Microbial diversity and nitrogen-metabolizing gene abundance in backyard food waste composting systems

AIMS

The microbial diversity of backyard compost piles is poorly understood compared to large-scale, highly regulated composting systems. The purpose of this study was the identification of the microbial community composition and associated change over time among three different backyard composting styles.

METHODS AND RESULTS

Food waste was composted in a household backyard compost bin, a small-scale aerated windrow or a semi-aerated static pile. Samples were obtained from each sequential phase of the composting process for 16s rRNA sequencing and relationships between temperature, moisture and microbial communities were examined. The Bacilli dominated in the early phases of composting then transitioned to Proteobacteria in the later stages. Different bacterial species increased and decreased dramatically in different composting systems and at different phases of the composting process. We performed qPCR to quantify gene abundance of nirS to profile the nitrogen-metabolizing bacteria present in each composting system. Gene abundance of nirS varied with temperature, but peaked during the cooling phase in the aerated windrow.

CONCLUSIONS

Although the phases of decomposition were not as distinct as large-scale regulated piles, the microbial diversity mirrored the appropriate phases. Interestingly, different backyard composting styles were marked by the predominance of certain bacterial species. In particular, nitrogen-metabolizing bacterial communities peaked in the later stages of decomposition.

SIGNIFICANCE AND IMPACT OF THE STUDY

A profile of the compost microbiome yields important clues about how differences in backyard food waste composting systems influence bacterial species that may facilitate or hinder nitrogen metabolism.

Multivariate relationships between microbial communities and environmental variables during co-composting of sewage sludge and agricultural waste in the presence of PVP-AgNPs

This study evaluated the contributions of environmental variables to the variations in bacterial 16S rDNA, nitrifying and denitrifying genes abundances during composting in the presence of polyvinylpyrrolidone coated silver nanoparticles (PVP-AgNPs). Manual forward selection in redundancy analysis (RDA) indicated that the variation in 16S rDNA was significantly explained by NO₃^--N, while nitrifying genes were significantly related with pH, and denitrifying genes were driven by NO₃^--N and TN. Partial RDA further revealed that NO₃^--N solely explained 28.8% of the variation in 16S rDNA abundance, and pH accounted for 61.8% of the variation in nitrifying genes. NO₃^--N and TN accounted for 34.2% and 9.2% of denitrifying genes variation, respectively. The RDA triplots showed that different genes shared different relationships with environmental parameters. Based on these findings, a composting with high efficiency and quality may be conducted in the future work by adjusting the significant environmental variables.

Aerobic deterioration of corn stalk silage and its effect on methane production and microbial community dynamics in anaerobic digestion

Ensilage is a commonly used method of preserving energy crops for biogas production. However, aerobic deterioration of silage is an inevitable problem. This study investigated the effect of aerobic deterioration on methane production and microbial community dynamics through anaerobic digestion (AD) of maize stalk silage, following 9days air exposure of silage. After air exposure, hydrolytic activity and methanogenic archaea amount in AD were reduced, decreasing the specific methane yield (SMY); whereas lignocellulose decomposition during exposure improved the degradability of silage in AD and enhanced SMY, partially compensating the dry matter (DM) loss. 29.3% of the DM and 40.7% of methane yield were lost following 0-9days exposure. Metagenomic analysis showed a shift from Clostridia to Bacteroidia and Anaerolineae in AD after silage deterioration; Methanosaetaceae was the dominant methanogenic archaea.

Impacts of delayed addition of N-rich and acidic substrates on nitrogen loss and compost quality during pig manure composting

Delayed addition of Nitrogen (N)-rich and acidic substrates was investigated to evaluate its effects on N loss and compost quality during the composting process. Three different delayed adding methods of N-rich (pig manure) and acidic substrates (phosphate fertilizer and rotten apples) were tested during the pig manure and wheat straw is composting. The results showed that delayed addition of pig manure and acidic materials led two temperature peaks, and the durations of two separate thermophilic phase were closely related to the amount of pig manure. Delayed addition reduced total N loss by up to 14% when using superphosphate as acidic substrates, and by up to 12% when using rotten apples as acidic substrates, which is mainly due to the decreased NH₃ emissions. At the end of composting, delayed the addition of pig manure caused a significant increase in the HS (humus substance) content, and the highest HS content was observed when 70% of the pig manure was applied at day 0 and the remaining 30% was applied on day 27. In the final compost, the GI in all treatments almost reached the maturity requirement by exceeding 80%. The results suggest that delayed addition of animal manure and acidic substrates could prevent the N loss during composting and improve the compost quality.

Role of psychrotrophic bacteria in organic domestic waste composting in cold regions of China

To study the influence of psychrotrophic bacteria on organic domestic waste (ODW) composting in cold regions, twelve new efficient psychrotrophic composting strains were isolated. Together with the published representative composting strains, a phylogenetic tree was constructed showing that although the strains belong to the same phylum, the genera were markedly different. The twelve strains were inoculated into the ODW in the composting reactor at 13°C. After treatment, the indices of temperature, moisture content, pH, electrical conductivity, C/N, ammonium nitrogen, and nitrate nitrogen indicated that the compost had reached maturity. The thermophilic phase was reached at 17d, and composting was completed at 42d, a markedly shorter composting time than that in previous studies. High-throughput sequencing indicated that the inoculative strains became the dominant community during the mesophilic phase and that they enhanced the stability of the microbial community structure. Thus, psychrotrophic bacteria played a key role in low-temperature composting.

Effect of phosphate additive on the nitrogen transformation during pig manure composting

Previous studies revealed that phosphate, as an additive to composting, could significantly reduce NH₃ emission and nitrogen loss through change of pH and nitrogen fixation to form ammonium phosphate. However, few studies have explored the influence of pH change and phosphate additive on NO _x^--N, NH₄⁺-N, NH₃, and N₂O, which are dominate forms of nitrogen in composting. In this study, the equimolar H₃PO₄, H₂SO₄, and K₂HPO₄ were added into pig manure composting to evaluate the effect of H⁺ and PO₄^3- on nitrogen transformation. As a result, we reached the conclusion that pH displays significant influence on adsorption from PO₄^3- to NH₄⁺. The NH₄⁺-N concentration in H₃PO₄ treatment kept over 3 g kg^-1DM (dry matter) which is obviously higher than that in H₂SO₄ treatment, and NH₄⁺-N concentration in K₂HPO₄ treatment (pH>8.5) is lower than 0.5 g kg^-1DM because adsorption capacity of PO₄^3- is greatly weakened and NH₄⁺-N rapidly transformed to NH₃-N influenced by high pH value. The N₂O emission of composting is significantly correlated with incomplete denitrification of NO _x^--N, and PO₄^3- addition could raise NO _x^--N contents to restrict denitrification and further to promote N₂O emission. The study reveals the influence mechanism of phosphate additive to nitrogen transformation during composting, presents theoretical basis for additive selection in nitrogen fixation, and lays foundation for study about nitrogen circulation mechanism during composting.

The impact of silver nanoparticles on the co-composting of sewage sludge and agricultural waste: Evolutions of organic matter and nitrogen

This study evaluated the influence of silver nanoparticles (AgNPs) on evolutions of organic matter and nitrogen during co-composting of sewage sludge and agricultural waste. Two co-composting piles were conducted, one was treated without AgNPs (pile 1) and the other with AgNPs (pile 2). Results showed that the AgNPs affected the quality of final composts. Less organic matter (OM) losses were determined in pile 2 (57.96%) than pile 1 (61.66%). 27.22% and 30.1% of the initial total organic matter (TOC) was decomposed in pile 1 and pile 2, respectively. The final water soluble carbon (WSC) concentration in pile 2 was 23559.27mg/kg DW compost which was significantly lower than pile 1 (25642.75mg/kg DW compost). Changes of different forms of nitrogen in the two piles showed that AgNPs could reduce the losses of TN but increase the losses of mineral N.

Microbial Abundances Predict Methane and Nitrous Oxide Fluxes from a Windrow Composting System

Manure composting is a significant source of atmospheric methane (CH₄) and nitrous oxide (N₂O) that are two potent greenhouse gases. The CH₄ and N₂O fluxes are mediated by methanogens and methanotrophs, nitrifying and denitrifying bacteria in composting manure, respectively, while these specific bacterial functional groups may interplay in CH₄ and N₂O emissions during manure composting. To test the hypothesis that bacterial functional gene abundances regulate greenhouse gas fluxes in windrow composting systems, CH₄ and N₂O fluxes were simultaneously measured using the chamber method, and molecular techniques were used to quantify the abundances of CH₄-related functional genes (mcrA and pmoA genes) and N₂O-related functional genes (amoA, narG, nirK, nirS, norB, and nosZ genes). The results indicate that changes in interacting physicochemical parameters in the pile shaped the dynamics of bacterial functional gene abundances. The CH₄ and N₂O fluxes were correlated with abundances of specific compositional genes in bacterial community. The stepwise regression statistics selected pile temperature, mcrA and NH₄⁺ together as the best predictors for CH₄ fluxes, and the model integrating nirK, nosZ with pmoA gene abundances can almost fully explain the dynamics of N₂O fluxes over windrow composting. The simulated models were tested against measurements in paddy rice cropping systems, indicating that the models can also be applicable to predicting the response of CH₄ and N₂O fluxes to elevated atmospheric CO₂ concentration and rising temperature. Microbial abundances could be included as indicators in the current carbon and nitrogen biogeochemical models.

Effects of Copper Addition on Copper Resistance, Antibiotic Resistance Genes, and intl1 during Swine Manure Composting

Copper is one of the most abundant heavy metals present in swine manure. In this study, a laboratory-scale aerobic composting system was amended with Cu at three levels (0, 200, and 2000 mg kg^-1, i.e., control, Cu200, and Cu2000 treatments, respectively) to determine its effect on the fate of copper resistance genes [copper resistance genes (CRGs): pcoA, cusA, copA, and tcrB], antibiotic resistance genes [antibiotic resistance genes (ARGs): erm(A) and erm(B)], and intl1. The results showed that the absolute abundances of pcoA, tcrB, erm(A), erm(B), and intl1 were reduced, whereas those of copA and cusA increased after swine manure composting. Redundancy analysis showed that temperature significantly affected the variations in CRGs, ARGs, and intl1. The decreases in CRGs, ARGs, and intI1 were positively correlated with the exchangeable Cu levels. The bacterial community could be grouped according to the composting time under different treatments, where the high concentration of copper had a more persistent effect on the bacterial community. Network analysis determined that the co-occurrence of CRGs, ARGs, and intI1, and the bacterial community were the main contributors to the changes in CRGs, ARG, and intl1. Thus, temperature, copper, and changes in the bacterial community composition had important effects on the variations in CRGs, ARGs, and intl1 during manure composting in the presence of added copper.

Linking N2O emission from biochar-amended composting process to the abundance of denitrify (nirK and nosZ) bacteria community

Manure composting has been recognized as an important anthropogenic source of nitrous oxide (N₂O) contributing to global warming. However, biochar effect on N₂O emissions from manure composting is rarely evaluated, especially by linking it to abundance of denitrifying bacteria community. Results of this study indicated that biochar amendment significantly reduced N₂O emissions from manure composting, primarily due to suppression of the nirK gene abundance of denitrifying bacteria. Pearson’s correlation analysis showed a significant positive correlation between nirK abundance and N₂O fluxes, while a negative correlation between nosZ density and N₂O fluxes. Simultaneously, a linear correlation between nirK gene abundance minus nosZ gene abundance with N₂O fluxes was also observed. In addition, a statistical model for estimating N₂O emissions based on the bacterial denitrifying functional genes was developed and verified to adequately fit the observed emissions. Our results highlighted that biochar amendment would be an alternative strategy for mitigating N₂O emissions during manure composting, and the information of related functional bacterial communities could be helpful for understanding the mechanism of N₂O emissions.

Response of Spatial Patterns of Denitrifying Bacteria Communities to Water Properties in the Stream Inlets at Dianchi Lake, China

Streams are an important sink for anthropogenic N owing to their hydrological connections with terrestrial systems, but main factors influencing the community structure and abundance of denitrifiers in stream water remain unclear. To elucidate the potential impact of varying water properties of different streams on denitrifiers, the abundance and community of three denitrifying genes coding for nitrite (nirK, nirS) and nitrous oxide (nosZ) reductase were investigated in 11 streams inlets at the north part of Dianchi Lake. The DGGE results showed the significant pairwise differences in community structure of nirK, nirS, and nosZ genes among different streams. The results of redundancy analysis (RDA) confirmed that nitrogen and phosphorus concentrations, pH, and temperature in waters were the main environmental factors leading to a significant alteration in the community structure of denitrifiers among different streams. The denitrifying community size was assessed by quantitative PCR (qPCR) of the nirS, nirK, and nosZ genes. The abundance of nirK, nirS, and nosZ was positively associated with concentrations of total N (TN) and PO4 (3-) (p < 0.001). The difference in spatial patterns between nirK and nirS community diversity, in combination with the spatial distribution of the nirS/nirK ratio, indicated the occurrence of habitat selection for these two types of denitrifiers in the different streams. The results indicated that the varying of N species and PO4 (3-) together with pH and temperature would be the main factors shaping the community structure of denitrifiers. Meanwhile, the levels of N in water, together with PO4 (3-), tend to affect the abundance of denitrifiers.