Metagenomic binning analyses of swine manure composting reveal mechanism of nitrogen cycle amendment using kaolin

The efficient control of nitrogen loss in composting and the enhancement of product quality have become prominent concerns in current research. The positive role of varying concentrations kaolin in reducing nitrogen loss during composting was revealed using metagenomic binning combined with reverse transcription quantitative polymerase chain reaction. The results indicated that the addition of 0.5 % kaolin significantly (P < 0.05) up-regulated the expression of nosZ and nifH on day 35, while concurrently reducing norB abundance, resulting in a reduction of NH3 and N2O emissions by 61.4 % and 17.5 %, respectively. Notably, this study represents the first investigation into the co-occurrence of nitrogen functional genes and heavy metal resistance genes within metagenomic assembly genomes during composting. Emerging evidence indicates that kaolin effectively impedes the binding of Cu/Zn to nirK and nosZ gene reductases through passivation. This study offers a novel approach to enhance compost quality and waste material utilization.

A slow-release fertilizer containing cyhalofop-butyl reduces N2O emissions by slowly releasing nitrogen and down-regulating the relative abundance of nirK

To simplify the process of the application of fertilizers and herbicides for farmers, a slow-release fertilizer containing cyhalofop-butyl (SFC) was developed to prolong the combined effect of the herbicide-fertilizer and achieve a synergistic effect on weeding and reducing N2O emissions. A greenhouse pot experiment was conducted using five treatments: CK (no fertilizer), CF (compound fertilizers), FC (fertilizers combined with cyhalofop-butyl), FF (film-coated compound fertilizers), and SFC (a slow-release fertilizer containing cyhalofop-butyl). The findings indicated that SFC exhibited the lowest N2O emissions, the highest paddy yield, and the highest nitrogen utilization rate among all the treatments. When compared to CF, the nitrogen release was notably delayed, leading to a significant reduction in cumulative N2O emissions under FF and SFC. When compared to CF, N2O emissions under FC were significantly decreased, suggesting that cyhalofop-butyl exerted a reduction role in N2O emissions. The SFC-treated nirK abundance was significantly lower than FF and FC, suggesting that the cyhalofop-butyl of SFC interacted with film of SFC inhibited the denitrification process in the paddy soil. Thus, the SFC reduced N2O emissions by slowing nitrogen release and down-regulating the relative abundance of nirK.

Efficient nitrogen removal pathways and corresponding microbial evidence in tidal flow constructed wetlands for saline water treatment

The artificial tidal wetlands ecosystem was believed to be a useful device in treating saline water, and it played a significant part in global nitrogen cycles. However, limited information is available on nitrogen-cycling pathways and related contributions to nitrogen loss in tidal flow constructed wetlands (TF-CWs) for saline water treatment. This study operated seven experimental tidal flow constructed wetlands to remove nitrogen from saline water at salinities of 0-30‰. Stable and high NH4+-N removal efficiency (∼90.3%) was achieved, compared to 4.8-93.4% and 23.5-88.4% for nitrate and total nitrogen (TN), respectively. Microbial analyses revealed the simultaneous occurrence of anaerobic ammonium oxidation (anammox), dissimilatory nitrate reduction to ammonium (DNRA), nitrification and denitrification, contributing to nitrogen (N) loss from the mesocosms. The absolute abundances were 5.54 × 103-8.35 × 107 (nitrogen functional genes) and 5.21 × 107-7.99 × 109 copies/g (16S rRNA), while the related genera abundances ranged from 1.81% to 10.47% (nitrate reduction) and from 0.29% to 0.97% (nitrification), respectively. Quantitative response relationships showed ammonium transformation were controlled by nxrA, hzsB and amoA, and nitrate removal by nxrA, nosZ and narG. Collectively, TN transformation were determined by narG, nosZ, qnorB, nirS and hzsB through denitrification and anammox pathways. The proportion of nitrogen assimilation by plants was 6.9-23.4%. In summary, these findings would advance our understanding of quantitative molecular mechanisms in TF-CW mesocosms for treating nitrogen pollution that caused algal blooms in estuarine/coastal ecosystems worldwide.

Effect of antibiotic and/or heavy metal on nitrogen cycle of sediment-water interface in aquaculture system: Implications from sea cucumber culture

Antibiotics and heavy metals can have a negative impact on the nitrogen (N) cycle and microbial metabolism in coastal aquaculture environment. An indoor simulated culture experiment was conducted to explore how sulfadiazine and lead influence the N cycling in aquatic environment. Specifically, the experiment involved adding sulfadiazine (SDZ), lead (Pb), a combination of SDZ and Pb (SP), and a control group (CK). The fluxes and contents of ammonia nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N) and nitrite nitrogen (NO₂^--N) in sediment-water interface and sediments, the abundance of N cycle function genes (amoA_AOB, hzsA, nar, nirK, nirS, norB and nosZ) and microbiota in sediments were analyzed. The results showed that the presence of SDZ and Pb inhibited the nitrification function gene and nitrifiers abundance in surface sediment, and thus leading to more accumulation of NH₄⁺ and NO₂^- in overlying water. Pb exposure increased the abundances of denitrifying bacteria stimulated the first three steps of denitrification in the sediment, resulting in more removal of NO₃^- from the sediment, but possibly had the risk of releasing more greenhouse gas N₂O. Conversely, the presence of SDZ ultimately inhibited denitrification and anammox bacterial activities in the sediment. This study revealed how heavy metal and antibiotic impair the microbial communities and N cycling function gene expression, leading to the deterioration of typical coastal aquaculture environments.

Nitrogen loss, nitrogen functional genes, and humification as affected by hydrochar addition during chicken manure composting

This study investigated the effect of hydrochar addition on nitrogen (N) transformation, N functional genes, and humification during chicken manure composting. The addition of 10 % hydrochar reduced cumulative ammonia (NH₃) and nitrous oxide emissions by 55.24 % and 45.30 %, respectively, and N losses by 32.07 %. Further, it increased the relative abundance of amoA while decreasing that of nirK, nirS, and nosZ in compost. Hydrochar reduces NH₃ emissions during composting owing to its acid-carbon properties that lower the pH of the composting pile and promote ammonia oxidation. Moreover, hydrochar addition enhances the humification of the composting pile and significantly increases the content of humic substances. Moreover, after hydrochar addition, the germination index of the compost product reached >80 % 10 days earlier. The results demonstrate that hydrochar is a suitable composting additive for reducing N loss and shortening the composting time.

Enhanced soil potential N2O emissions by land-use change are linked to AOB-amoA and nirK gene abundances and denitrifying enzyme activity in subtropics

Conversion of forestland to intensively managed agricultural land occurs worldwide and can increase soil nitrous oxide (N₂O) emissions by altering the transformation processes of nitrogen (N) cycling related microbes and environmental conditions. However, little research has been conducted to assess the relationships between nitrifying and denitrifying functional genes and enzyme activities, the altered soil environment and N₂O emissions under forest conversion in subtropical China. Here, we investigated the long-term (two decades) effect of converting natural forests to intensively managed tea (Camellia sinensis L.) plantations on soil potential N₂O emissions, inorganic N concentrations, functional gene abundances of nitrifying and denitrifying bacteria, as well as nitrifying and denitrifying enzyme activities in subtropical China. The conversion significantly increased soil potential N₂O emissions, which were regulated directly by increased denitrifying enzyme activity (52 %) and nirS + nirK gene abundance (38 %) as shown by structural equation modeling, and indirectly by AOB-amoA gene abundance and inorganic N concentration. Our results indicate that converting natural forests to tea plantations directly increases soil inorganic N concentration, resulting in increases in the abundance of soil nitrifying and denitrifying microorganisms and the associated N₂O emissions. These findings are crucial for disentangling the factors that directly and indirectly affect soil potential N₂O emissions respond to the conversion of forest to tea plantation.

Long-term metal pollution shifts microbial functional profiles of nitrification and denitrification in agricultural soils

The increasing contamination of heavy metals in agricultural soils and its impact on the nitrogen (N) cycle and N use efficiency have attracted considerable attention in recent years. In this study, agricultural soils neighboring the Dabaoshan copper mining area (DBS) and Qingyuan electronic-waste recycling area (QY), in Guangdong, China, were sampled to study the interaction between heavy metals and nitrification/denitrification processes, especially the related microbial functional profiles. Results showed that the contamination of heavy metals affected nitrifiers and denitrifiers differently. The potential nitrification activity was about four times lower in metal-polluted soils compared with the unpolluted ones, with a significant decrease in the abundance of amoA and nxrB (p < 0.05) in the polluted samples. On the other hand, the potential denitrification activity was more metal-resistant, which attributed to its complex species composition as shown by a slightly higher α-diversity index, and was slightly higher (p > 0.05) in the polluted samples. Among the five denitrifying genes tested, nosZ gene had the highest increase and the nirK gene the most decrease in numbers and in the polluted soils. The metal-polluted soils had fewer correlations among N functional genes based on the co-occurrence network analysis. In addition, the core taxa of the whole bacterial community changed from copiotrophic to oligotrophic bacteria in the presence of heavy metals. Mantel test indicated that heavy metals were the dominant factors determining N-related genes while the bacterial community composition was due to a combination of heavy metal presence and soil properties such as TOC, NO₂^-, and pH. It is concluded that long-term heavy metals pollution potentially affected nitrifiers and denitrifiers differently as indicated by the shift in N functional genes and the change in nitrification/denitrification processes.

Effect of thermophilic microbial agents on nitrogen transformation, nitrogen functional genes, and bacterial communities during bean dregs composting

This study explored how a thermophilic microbial agent altered nitrogen transformation, nitrogen functional genes, and bacterial communities during bean dregs composting with (T) and without (CK) a thermophilic microbial agent for 15 days. The results showed that the maximum temperature in T reached 73 °C and remained above 70 °C for 8 days, while that in CK was only 65 °C. The pH in T had essentially stabilized on day 7, while that in CK was still increasing. On day 15, the seed germination index (GI) of T (95%) reached maturity (defined by GI ≥ 85%), while the GI of CK was only 36%. The concentrations of total nitrogen, water-soluble nitrogen, ammonia nitrogen, and nitrate nitrogen in T (2.5%, 18.9 g/kg, 8.75 g/kg, and 1.69 g/kg) were all lower than those in CK (3.6%, 28.9 g/kg, 12.75 g/kg, and 6.82 g/kg). During composting, Bacillus played a major role in nitrogen reduction, nitrogen mineralization, denitrification, and the conversion between nitrite and nitrate. Weissella played a major role in nitrogen assimilation. Komagataeibacter and Bacillus played a major role in nitrogen fixation in CK and T, respectively. Nitrification was not observed during composting. The nosZ gene, which converts nitrous oxide to nitrogen, was found only in T. Network analysis suggested that the average number of neighbours in T was 3.30% higher than that in CK and the characteristic path length in T was 14.15% higher than that in CK. Therefore, the thermophilic microbial agents could cause nitrogen loss but promote the maturity of bean dregs, which have great potential application.

Enhancing denitrification in constructed wetland with algae addition

Constructed wetlands (CWs) can be used for tertiary treatment of wastewater; however, carbon source shortages limit denitrification. We studied the effect of algae addition as an external carbon source in CWs and found that the nitrogen removal efficiency of CWs is highly dependent on the algae dosage. Optimal nitrogen removal percentage (80.5%) can be achieved by adding 81.1 mg·L^-1 dry weight algae to the influent when the chemical oxygen demand/nitrogen (COD/N) ratio reaches 5.3. Longitudinal changes in the nitrogen concentrations, organic matter concentrations, and nitrogen functional genes were also analyzed. The algae addition strengthened the anoxic environment, boosted the volatile fatty acid concentrations, and improved the ratio of nitrite reductase gene (nirS) and copper-containing nitrite reductase (nirK)/16S rRNA, as well as the ratio of nitrate reductase gene (narG)/16S rRNA, thereby expanding the active space for denitrification. The addition of algae could potentially provide enough carbon to enhance denitrification during treatment of wastewater with a low COD/N ratio.

Response of microbial nitrogen transformation processes to antibiotic stress in a drinking water reservoir

Effects of antibiotics on microbial nitrogen transformation processes in natural aquatic ecosystems are largely unknown. In this study, we utilized the ¹⁵N stable isotope tracers and metagenomic sequencing to identify how antibiotics drive nitrogen transformation processes in Danjiangkou Reservoir, which is the largest artificial drinking water reservoir in China. We retrieved 51 nitrogen functional genes, and found that the highest abundances of nitrate reduction and denitrification-related genes occurred in dissimilatory nitrogen transformation pathways. ¹⁵N-labelling analysis substantiated that denitrification was the main pathway for nitrogen removal, accounting for 57.1% of nitrogen loss. Nitrogen functional genes and antibiotic resistance genes co-occurred in Danjiangkou Reservoir, and they were mainly carried by the denitrifying bacteria such as Rhodoferax, Polaromonas, Limnohabitans, Pararheinheimera, Desulfobulbus, and Pseudopelobacter. Genome annotation revealed that antibiotic deactivation, Resistance-Nodulation-Division and facilitator superfamily efflux pumps were responsible for the multiple-resistance to antibiotics in these bacteria. Moreover, antibiotics showed non-significant effects on nitrogen transformation processes. It is speculated that denitrifying bacteria harboring ARGs played crucial roles in protecting nitrogen transformation from low-level antibiotics stress in the reservoir. Our results highlight that denitrifying bacteria are important hosts of ARGs, which provides a novel perspective for evaluating the effects of antibiotics on nitrogen cycle in natural aquatic ecosystems.

Impacts of Ag and Ag2S nanoparticles on the nitrogen removal within vertical flow constructed wetlands treating secondary effluent

In this study, the effects of silver (Ag NPs) and sliver sulfide nanoparticles (Ag₂S NPs) on nitrogen removal and nitrogen functional microbes in constructed wetlands were investigated. The obtained results demonstrated that inhibition extent on nitrogen removal relied on NPs types and high concentrations NPs showed higher negative effects. 0.5 mg/L Ag NPs had no influence on NH₄⁺-N removal, amoA and nxrA gene copies, whereas Ag₂S NPs and Ag⁺ decreased NH₄⁺-N removal by reducing abundances of nitrifying genes. The concentrations of NO₃^--N and TN in all 0.5 mg/L obviously increased compared with control, resulting from decreasing functional genes and denitrifying bacteria. And 0.5 mg/L Ag NPs exhibited largest inhibitory effects, with the highest NO₃^--N effluent concentrations. 2 mg/L Ag NPs decreased NH₄⁺-N removal, but adverse effects gradually vanished with extension of time, whereas both Ag₂S NPs and Ag⁺ at 2 mg/L influenced NH₄⁺-N transformation and decreased the abundance of amoA and nxrA genes and the AOB Nitrosomonas in CWs. Moreover, 2 mg/L of Ag NPs reduced NO₃^--N removal by decreasing abundance of nirS and key denitrifying bacteria. To sum up, the inhibition mechanisms concluded from current results were possibly in that Ag NPs exhibited nanotoxicity rather than ionic toxicity.

Impacts of human disturbance on the biogeochemical nitrogen cycle in a subtropical river system revealed by nitrifier and denitrifier genes

Human activities have largely modified nitrogen (N) sources supply, cycling and export from land to ocean. Nitrification and denitrification are vital processes alleviating N pollution in aquatic ecosystems but the diverse responses and niche of microbial N retention to human disturbance are still understudied. Here we investigated the changes in N species and functional genes in the urban, agriculture and reservoir river sections of the Jiulong River (southeast China). Our results show that ammonia-oxidizing bacteria (AOB) (Nitrosomonas) were dominant in the urban river section receiving ammonium-rich sewage that enhanced nitrification and subsequent denitrification, while ammonia-oxidizing archaea (AOA) was more abundant than AOB in the river section flowing through areas of pomelo (Citrus maxima) agriculture with low pH, low ammonium and very high nitrate input. Warm temperatures and large total suspended matter (TSM) in the wet season promoted growth of nitrifiers and denitrifiers, which were mostly particle-attached. The potential river N retention through gaseous N removal (PR_N2O and PR_N2) in the agriculture section with huge N loading was among the lowest. Strong nitrification and denitrification were suspected to occur in the agricultural acid soil system rather than in the river network. In addition, the decreased TSM and N concentration promoted free-living microbes in the reservoir. The highest PR_N2 and N₂ production observed in the reservoir in the dry season implied that denitrification and anammox occurring in sediments was likely to increase N retention. This study suggests the diverse factors involved in processing of N pollution among diverse landscapes.

Investigation of nitrogen pollutants transformation and its pathways along the long-distance prechlorinated raw water distribution system

Understanding the transformation pattern of nitrogen (N) pollutants and its pathways in the prechlorinated raw water distribution system (PRWDS) is vital for controlling the stablitiy and safety of raw water qulity. This study investigated the N transformation, N functional genes and their correlations to find the N transformation pathways along the PRWDS. Results suggested that simultaneous nitrification, anaerobic ammonium oxidation and denitrification (SNAD) contribute to the N transformationin the PRWDS. Along the pipeline, anammox 16S rRNA (9.18 × 10⁷-8.41 × 10⁸ copies/g), limited by prechlorination, was the most abundant N functional genes and anammox process was the main pathway of ammonia nitrogen (NH₄⁺-N). The decreasing NH₄⁺-N was connected with Planctomycetes, Nitrospira and abundance of nxrA attributing to the joint effort of anammox and declined nitrification. The concentration of nitrate (NO₃^--N) increasing at first and then decreasing, was correlated positively with Sphingomonas. because of the declined nitritication and increased denitrification. Besides, the NO₃^--N→NO₂^--N process was considered to be primary NO₃^--N transformation pathways. Increases in the concentration of dissolved organic nitrogen (DON) and nitrite (NO₂^--N) observed in the PRWDS had positive correlation with relative abundance of Pseudomonas. We believe that prechlorination shaped the particular bacterialcharacteristics in biofilms and influenced the N transformation pathways indirectly, resulting in the varying N transformation rules in PRWDSs. Moreover, systematic and extended research is particularly vital for determining the effects of changes in source water quality and environmental conditions on bacterial community structure and N conversion along PRWDSs.

The responses of soil nitrogen transformation to nitrogen addition are mainly related to the changes in functional gene relative abundance in artificial Pinus tabulaeformis forests

The increase of soil nitrogen (N) availability may alter soil microbial community composition and the natural N cycle in forest ecosystems. However, the responses of soil microbial nitrogen functional genes (NFGs) to N addition and their consequent effect on the N-cycle processes are poorly understood. In this study, soil samples were collected from an artificial Pinus tabulaeformis forest located in Loess Plateau (China) to which N at four different concentrations was added (0 [N0], 3 [N3], 6 [N6], and 9 [N9] g N m^-2 y^-1) for 4 years. We quantified the relative abundance of NFGs using functional gene microarray GeoChip 5.0 and determined net N transformation and N₂O emission rates in a 14-day incubation experiment. The results showed that N3 and N6 treatments did not significantly affect the total relative abundance and diversity of NFGs assemblage but significantly increased the relative abundance of specific genes for the NH₃ cycle (ureC, nirA, and nrfA), and nitrification (amoA) and denitrification (norB). These positive effects were related to the increase in soil organic C, NO₃^--N, and microbial biomass C (MBC). N9 treatment significantly decreased the relative abundance of all NFGs, and this negative impact was correlated with reduced dissolved organic C and MBC. Moreover, N addition significantly changed net N nitrification, mineralization, and N₂O emission rates, and NFGs explained the higher variances in the N transformation processes than soil properties. Specifically, ammonia-oxidizing archaea (amoA-AOB) and MBC were the key factors related to net N nitrification; ureC, nirK, and MBC were the key factors related to net N mineralization; and narG and nirS were the key factors related to N₂O emission. Our results show that global N deposition may mainly influence N transformation processes by regulating the corresponding NFG relative abundance, thereby affecting the N cycle in forest soils.

Nitrogen removal in recirculating aquaculture water with high dissolved oxygen conditions using the simultaneous partial nitrification, anammox and denitrification system

The efficient removal of nitrogen pollutants in the aquaculture systems is still a challenge due to the low concentration of organic carbon and high concentration of dissolved oxygen (DO) in the wastewater. The simultaneous partial nitrification, anammox and denitrification (SNAD) bioreactor was firstly used for the treatment of aquaculture wastewater in recirculating aquaculture system. The bioreactor operated for 180 days without adding extra organic carbon. After 60-day operation, the bioreactor reached the stable stage with the average concentration of ammonia/nitrate/nitrite/COD in the effluent with 0.26/0.75/0.47/0.27 mg/L. The Pseudoxanthomonas was the dominant genus in the biofilm samples. The typical nitrogen functional bacteria and genes for nitrification, anammox and denitrification were detected with different abundance in different procedures along the bioreactor. Network analysis revealed the significant correlations between nitrogen functional bacteria and genes. The SNAD bioreactor achieved the effective removal for nitrogen and COD under high DO conditions in recirculating aquaculture system.

Composting swine carcasses with nitrogen transformation microbial strains: Succession of microbial community and nitrogen functional genes

In this study, nitrogen transformation strains, including three ammonium transformation strains, one nitrite strain and one nitrogen fixer, were inoculated at different swine carcass composting stages to regulate the nitrogen transformation and control the nitrogen loss. The final total nitrogen content was significantly increased (p < 0.01). The bacterial communities were assessed by amplicon sequencing and association analysis. Proteobacteria, Actinobacteria, Bacteroidetes and Firmicutes were the four most dominant phyla.,Brevibacterium, Streptomyces and Ochrobactrum had a significant (p < 0.05) and positive correlation with total nitrogen and ammonium nitrogen content in both groups. The quantitative results of nitrogen transformation genes showed that ammonification, nitrification, denitrification and nitrogen fixation were simultaneously present in the composting process of swine carcasses, with the latter two accounting for a higher proportion. The ammonium transformation strains significantly (p < 0.05) strengthened nitrogen fixation and remarkably (p < 0.01) weakened nitrification and denitrification, which, however, were notably (p < 0.05) enhanced by the nitrite strain and nitrogen fixer. In this research, the inoculated strains changed the bacterial structure by regulating the abundance and activity of the highly connected taxa, which facilitated the growth of nitrogen transformation bacteria and regulated the balance/symbiosis of nitrogen transformation processes to accelerate the accumulation of nitrogen.

Changes in nitrogen functional genes in soil profiles of grassland under long-term grazing prohibition in a semiarid area

Grazing prohibition has been used to restore degraded grassland ecosystems in semiarid areas; however, the impact of this measure on soil nitrogen (N) cycling is poorly understood. Furthermore, recent studies have tended to focus on the topsoil and ignored a steep gradient of nutrient accumulation with soil depth. Here, we investigated changes in N functional genes (NFGs) involved in organic N decomposition (chiA), archaeal and bacterial ammonia oxidation (amoA-AOA and amoA-AOB), respectively, denitrification (nirK and nirS), and N fixation (nifH) in soil profiles from a chronosequence of grazing prohibition (0, 10, 15, 25, and 35 years) in the semiarid grasslands of the Loess Plateau, China. The abundance of all the investigated NFGs in grassland soils after 35 years' grazing prohibition was higher than in grazed grassland. This result suggests that microbial N turnover potential is facilitated by grazing prohibition, probably through enhanced biomass production via increases in nutrient input into the soil. The higher ratio of (chiA + nifH)/(amoA-AOA + amoA-AOB) and values of (nirK + nirS) in grazing-prohibited grasslands than in grazed grassland suggest that prohibition of grazing not only improved microbial N storage potential but also increased N gas emission potential. The abundances of NFGs varied along the soil profiles and responded differently to environmental factors. The chiA and nifH abundances decreased with soil depth and were associated with variation in aboveground biomass, NH₄⁺-N, and organic carbon, while amoA-AOA, nirK, and nirS genes increased with depth and were more affected by soil organic carbon, moisture, and bulk density. Multivariate regression tree analysis demonstrated that aboveground biomass was the best explanatory variable for the changes in NFGs in grazed grassland, while soil organic carbon was the best in the grazing-prohibited grasslands. Our results provide new insight into the soil N cycling potential of degraded and restored semiarid grassland ecosystems.

Population characteristics and influential factors of nitrogen cycling functional genes in heavy metal contaminated soil remediated by biochar and compost

Sixteen treatments of soil contaminated by Cu, Pb, and Zn by the addition of a different percentage of biochar and compost were incubated for 120 days. The abundance of denitrifying genes such as narG, nirK, nirS and nosZ and the ammonia-oxidizing amoA genes of ammonia-oxidizing archaea/bacteria (AOA/AOB), soil nitrite reductase activity (S-NiR) and their shaping factors were also determined. The relationships between functional genes, S-NiR, and physico-chemical parameters were analyzed using the Pearson correlation method. The study found that the changes in physico-chemical parameters, including water-soluble organic carbon (WSC), nitrate (NO₃^-) and ammonium (NH₄⁺), were predominant in different treatments. The abundance of nirK and narG genes is most sensitive to the changes in the properties of the soil sample. Bacterial 16S rDNA gene abundance was significantly affected by NO₃^- and S-NiR (P < 0.05). Nitrifying genes were mainly correlated to WSC and S-NiR, while denitrifying genes were associated with pH, electrical conductivity, NO₃^- and S-NiR. The systematic study for the relationship between the genes and the environmental parameters will help us to deep understand the biological mechanisms of nitrogen cycle in heavy metal contaminated soils remediated by biochar and compost.

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.