Long-term maintenance of high yield and soil fertility with integrated soil-crop system management on the Loess Plateau.

Ridge-furrow with full film mulching has been widely applied to increase crop yield and water productivity on the Loess Plateau, but it may stimulate carbon (C) mineralization. How to integrate other technological benefits based on this technology for long-term maintenance of high yield and soil fertility is a pressing issue. With the local farmers' practice (FP) as a control, three integrated soil-crop system management (ISSM) practices integrating fertilizer rates, fertilizer types and planting densities (ISSM-N1, ISSM-N2 and ISSM-MN) were established to improve maize yield and soil quality. Compared with the FP, the maize yield increased by 13.34%, 21.83% and 30.24%, and the soil quality index (SQI) increased by 9.66%, 14.91% and 38.38% for ISSM-N1, ISSM-N2 and ISSM-MN, respectively. However, ISSM-N1 did not significantly increase yield, and ISSM-N2 increased residual soil nitrate and decreased nitrogen (N) partial factor productivity significantly. Compared to the FP, ISSM practices increased soil organic carbon (SOC), labile organic C fractions (LOCFs) and potassium permanganate organic C fractions in the topsoil to varying degrees, but only ISSM-MN reached significant levels for most C fractions. The sensitivity index indicated very easily oxidizable C (24.6%), easily oxidizable C (24.7%), hot-water extractable C (30.8%), labile organic C (24.7%) and particulate organic C (57.3%) were more sensitive than SOC (22.7%). ISSM-MN sequestered significantly higher C than the other treatments. The results of the relative importance analysis and the structural equation model indicated that LOCFs were the direct contributors to yield, while recalcitrant C (CO) was the indirect contributor, revealing the underlying mechanism that CO decomposed to replenish LOCFs and the total N pool with the water soluble C pool as the transit station. Overall, ISSM-MN is the most promising strategy to improve crop yield and soil fertility in the long term on the Loess Plateau.

Nitrogen transformation promotes the anaerobic degradation of PAHs in water level fluctuation zone of the Three Gorges Reservoir in Yangtze River, China: Evidences derived from in-situ experiment.

Polycyclic aromatic hydrocarbons (PAHs) pose a great threat to human health and ecological system safety. The interception of nitrogen is common found in the riparian zone. However, there is no evidence on how nitrogen addition affects the anaerobic degradation of PAHs in soil of the water-level-fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) in Yangtze River, China. Here, we investigated the PAHs degradation rate, the variation of key functional genes and microbial communities after nitrogen addition in soil that experienced a flooding period of water-level-fluctuation. The results revealed that the ∑16PAHs were decreased 16.19 %-36.65 % and more 3-5-rings PAHs were biodegraded with nitrogen addition in WLFZ. The most genes involved in PAHs-anaerobic degradation and denitrification were up-regulated by nitrate addition, and phyla Firmicutes, Actinobacteria and Proteobacteria were more advantages in nitrogen addition groups. The Tax4Fun based genome function analysis revealed that the microbial activity of PAHs-degradation increased with nitrate addition. The co-occurrence network analysis indicated that nitrogen addition accelerated the metabolism of nitrogen and PAHs. It is the first time to provide the direct experimental evidences that nitrogen transformation in the WLFZ soil promotes anaerobic PAHs degradation. This work is of importance to understand the effect of nitrogen intercepted in the WLFZ soil of TGR in Yangtze River, China.

Temporal changes of microbial community structure and nitrogen cycling processes during the aerobic degradation of phenanthrene.

Phenanthrene (PHE) is frequently detected in worldwide soils. But it is still not clear that how the microbial community succession happens and the nitrogen-cycling processes alter during PHE degradation. In this study, the temporal changes of soil microbial community composition and nitrogen-cycling processes during the biodegradation of PHE (12 µg g-1) were explored. The results showed that the biodegradation of PHE followed the second-order kinetics with a half-life of 7 days. QPCR results demonstrated that the bacteria numbers increased by 67.1%-194.7% with PHE degradation, whereas, no significant change was observed in fungi numbers. Thus, high-throughput sequencing based on 16 S rRNA was conducted and showed that the abundances of Methylotenera, Comamonadaceae, and Nocardioides involved in PHE degradation and denitrification were significantly increased, while those of nitrogen-metabolism-related genera such as Nitrososphaeraceae, Nitrospira, Gemmatimonadacea were decreased in PHE-treated soil. Co-occurrence network analysis suggested that more complex interrelations were constructed, and Proteobacteria instead of Acidobacteriota formed intimate associations with other microbes in responding to PHE exposure. Additionally, the abundances of nifH and narG were significantly up-regulated in PHE-treated soil, while that of amoA especially AOAamoA was down-regulated. Finally, correlation analysis found several potential microbes (Methylotenera, Comamonadaceae, and Agromyces) that could couple PHE degradation and nitrogen transformation. This study confirmed that PHE could alter microbial community structure, change the native bacterial network, and disturb nitrogen-cycling processes.

Nitric oxide-dependent biodegradation of phenanthrene and fluoranthene: The co-occurrence of anaerobic and intra-aerobic pathways.

Polycyclic aromatic hydrocarbons (PAHs) pollution as well as the emissions of nitric oxide (NO) and greenhouse gas nitrous oxide (N2O) in denitrification processes are currently two environmental issues of great concern. Although bioremediation of PAHs under denitrification is considered a promising approach, denitrification was an important contributor to N2O and NO emissions. This long-term study confirmed for the first time that microorganisms could utilize NO to efficiently degrade phenanthrene and fluoranthene. When the two systems of NO-dependent phenanthrene and fluoranthene degradation were stable, the first-order rate constants of phenanthrene and fluoranthene in the two systems (0.1940 and 0.0825 day-1, respectively) were close to those values (0.2290 and 0.1085 day-1, respectively) observed at nitrate-reducing conditions. Further analysis of functional genes revealed that phenanthrene and fluoranthene might be degraded under the combined action of the anaerobic pathway mediated by NO reduction and intra-aerobic pathway mediated by NO dismutation. The genomic analysis showed that Nod genes had high diversity and most of them were similar to aquifer cluster group in the two systems. Microbial community structure analysis indicated that Pseudomonas and Ochrobactrum might be key participants in NO-dependent phenanthrene degradation system, and Azoarcus, Alicycliphilus and Moheibacter might play vital roles in NO-dependent fluoranthene degradation system. This study provides new perspective for anaerobic remediation of PAH pollution and simultaneously reducing NO and N2O emissions during bioprocesses, which has important ecological significance for amending sediment and soil PAHs contamination and potential application for the removal of PAHs in flue gas.

Nitrate-assisted biodegradation of polycyclic aromatic hydrocarbons (PAHs) in the water-level-fluctuation zone of the three Gorges Reservoir, China: Insights from in situ microbial interaction analyses and a microcosmic experiment.

An increase in polycyclic aromatic hydrocarbon (PAH) pollution poses significant challenges to human and ecosystem health in the Three Gorges Reservoir (TGR) of the Yangtze River. Based on the combination of PAH analysis with qPCR and high-throughput sequencing of bacteria, 32 topsoil samples collected from 16 sites along the TGR were used to investigate the distribution and biodegradation pathways of PAHs in the water-level-fluctuation zone (WLFZ). The results indicated that the concentrations of PAHs were 43.8-228.2 and 30.8-206.3 ng/g soil (dry weight) under the high- and low-water-level (HWL and LWL) conditions, respectively. The PAH concentration in urban areas was higher than that in rural areas. Under both the HWL and LWL conditions, the abundance of the bamA gene, a biomarker of anaerobic PAH biodegradation, was significantly higher than that of the ring-hydroxylating-dioxygenase (RHD) gene, a biomarker of aerobic PAH biodegradation. The abundance of the bamA gene was significantly positively correlated with PAHs (R2 = 0.8), and the biodegradation percentage of PAHs incubated anaerobically was greater than that in the aerobically incubated microcosm experiments. These data implicated a key role of the anaerobic pathway in PAH biodegradation. Co-occurrence network analysis suggested that anaerobic Anaerolineaceae, Dechloromonas, Bacteroidetes_vadin HA17 and Geobacter were key participants in the biodegradation of PAHs. The diversity analysis of functional bacteria based on the bamA gene and microcosm experiments further demonstrated that nitrate was the primary electron acceptor for PAH biodegradation. These findings provide a new perspective on the mechanism of PAH biodegradation in the TGR and knowledge that can be used to develop strategies for environmental management.

The Co-occurrence of DNRA and Anammox during the anaerobic degradation of benzene under denitrification.

It was revealed that Anammox process promotes the anaerobic degradation of benzene under denitrification. This study investigates the effect of dissimilatory nitrate reduction to ammonium (DNRA) and exogenous ammonium on anaerobic ammonium oxidation bacteria (AnAOB) during the anaerobic degradation of benzene under denitrification. The results indicate that anammox occurs synergistically with organisms using the DNRA pathway, such as Draconibacterium and Ignavibacterium. Phylogenetic analysis showed 64% (16/25) and 36% (5/25) hzsB gene sequences, a specific biomarker of AnAOB, belonged to Candidatus 'Brocadia fuldiga' and Candidatus 'Kuenenia', respectively. Exogenous ammonium addition enhanced the anammox process and accelerated benzene degradation at a 1.89-fold higher average rate compared to that in the absence of exogenous ammonium and AnAOB belonged to Ca. 'Kuenenia' (84%) and Ca. 'Brocadia fuldiga' (16%). These results indicate that Ca. 'Brocadia fuldiga' could also play a role in DNRA. However, the diversity of abcA and bamA, the key anaerobic benzene metabolism biomarkers, remained unchanged. These findings suggest that anammox occurrence may be coupled with DNRA or exogenous ammonium and that anammox promotes anaerobic benzene degradation under denitrifying conditions. The results of this study contribute to understanding the co-occurrence of DNRA and Anammox and help explore their involvement in degradation of benzene, which will be crucial for directing remediation strategies of benzene-contaminated anoxic environment.

Occurrence, distribution, and source track of antibiotics and antibiotic resistance genes in the main rivers of Chongqing city, Southwest China.

In this study, the occurrence of 14 antibiotics, four corresponding antibiotic resistance genes (ARGs) and two microbial source tracker (MST) indicators were analyzed in two rivers of Chongqing city, southwest China. The results showed that 13 antibiotics were detected in all 12 sites and their detection frequencies were much higher in September, but concentrations were lower than that in March. Of them, erythromycin (ETM) and ofloxacin (OFL) were the predominant antibiotics in both seasons. The remarkably higher concentration of antibiotics in sediments of these rivers than those in other rivers were found. Environmental risk assessment found that four antibiotics posed high risk toward some sensitive algae. For ARGs, their relative abundances were higher in waters than those in sediments, higher in March than in September. Correlation analysis showed that antibiotics were not the exclusive selective pressure of ARGs; many environmental factors like dry matter contents on a mass basis, organic matter, total organic carbon, dissolved organic carbon, temperature, oxidation reduction potential and nitrite could affect the occurrence of ARGs. MST indicators analysis demonstrated that this river basin was largely polluted by human and pig feces, and human feces might be one main source of the four ARGs and five antibiotics.

Inhibition of benzene, toluene, phenol and benzoate in single and combination on Anammox activity: implication to the denitrification-Anammox synergy.

A previous study demonstrated that denitrification synergized with Anammox could accelerate the anaerobic degradation of benzene. The inhibitory effects of benzene, toluene, phenol and benzoate in single and combination on Anammox activity were investigated by short-term batch tests. The results indicated that the inhibition of single compounds on Anammox could be well fitted with the extended non-competitive and Luong inhibition kinetic models. The inhibitions of the individual compound were in order as follows: benzene > toluene > phenol > benzoate. The joint inhibitions of bi-component mixtures of benzene with toluene, benzene with phenol and benzene with benzoate on Anammox activity were additive; the joint inhibition of a tri-component mixture (benzene, toluene and phenol) was partly additive; and the joint inhibition of a multicomponent mixture (benzene, toluene, phenol and benzoate) was synergistic. The effect of benzoate on the denitrification-Anammox synergy for benzene degradation was evaluated using a long-term test. Although the average rate of benzene degradation decreased by 13% with the addition of 10 mg L-1 benzoate, the average rate of NO3- and NH4+ increased by approximately 1- and 0.56-fold, respectively, suggesting that benzoate favors the stability of the denitrification-Anammox synergy. The carboxylation of benzene would be a more favorable pathway for the anaerobic degradation of benzene under denitrification synergized with Anammox.

Long-term nitrate removal through methane-dependent denitrification microorganisms in sequencing batch reactors fed with only nitrate and methane.

Denitrifying anaerobic methane oxidation (damo) bioprocesses can remove nitrate using methane as the electron donor, which gains great concern due to the current stringent discharge standard of nitrogen in wastewater treatment plants. To obtain an engineering acceptable nitrogen removal rate (NRR) and demonstrate the long-term stable ability of damo system under conditions of nitrate and methane, two sequencing batch reactors (SBRs) fed with only nitrate and methane were operated for more than 600 days at 30 °C. The NRR of 21.91 ± 0.73 mg NO3--N L-1 day-1 was obtained which is, to the best of our knowledge, the highest rate observed in the literatures under such conditions. The temperature was found to significantly affect the system performance. Furthermore, the microbial community was analyzed by using real-time PCR technique. The results showed that the microbial consortium contained damo archaea and bacteria. These two microbes cooperated to maintain the long-term stability. And the number of damo archaea was higher than that of damo bacteria with the ratio of 1.77. By using methane as the electron donor, damo archaea reduced nitrate to nitrite coupled to methane oxidation and damo bacteria reduce the generated nitrite to nitrogen gas. The first step of nitrate to nitrite taken by damo archaea might be the limiting step of this cooperation system. SBR could be a suitable reactor configuration to enrich slow-growing microbes like damo culture. These results demonstrated the potential application of damo processes for nitrogen removal of wastewater containing low C/N ratios.

Effect of trace hydrazine addition on the functional bacterial community of a sequencing batch reactor performing completely autotrophic nitrogen removal over nitrite.

A sequencing batch reactor (SBR) was conducted to perform completely autotrophic nitrogen removal over nitrite (CANON). The effect of long-term trace N2H4 addition on ammonium oxidizing bacteria (AOB) and anaerobic AOB (AnAOB) in the CANON system was investigated. AOB and AnAOB primarily related to Nitrosococcus, Nitrosomonas and Candidatus scalindua, respectively. Before and after trace N2H4 addition, the estimates of AOB population decreased from 1.03×10(7) to 6.25×10(4)copies/g (dry sludge), but that of AnAOB increased from 3.14×10(9) to 5.86×10(10)copies/g (dry sludge). Despite there was a partially negative impact on AOB growth, the trace N2H4 addition exerted a stronger inhibition on nitrite oxidizing bacteria (NOB) and promoted AnAOB growth, which improved the nitrogen removal of the CANON system. Sludge granules enriched under long-term trace N2H4 addition were spherical and ellipsoidal, and the aerobic AOB were mainly located on the outer layers while AnAOB occupied most of the interior parts.

Distribution of antibiotic-resistant bacteria in chicken manure and manure-fertilized vegetables.

Veterinary manure is an important pollution reservoir of antibiotics and antibiotic-resistant bacteria (ARB). However, little is known of the distribution of ARB in plant endophytic bacteria and the number/types of ARB in chicken manure. In this study, 454-pyrosequencing was used to investigate the distribution and composition of ARBs in chicken manure and fertilized vegetables. The prevalence of ARB in the samples of the chicken manure compost recovered from farms on which amoxicillin, kanamycin, gentamicin, and cephalexin were used was 20.91-65.9% for ARBs and 8.24-20.63% simultaneously resistant to two or more antibiotics (multiple antibiotic resistant bacteria (MARB)). Antibiotic-resistant endophytic bacteria were widely detected in celery, pakchoi, and cucumber with the highest rate of resistance to cephalexin. The pyrosequencing indicated that the chicken manure dominantly harbored Firmicutes, Bacteroidetes, Synergistetes, and Proteobacteria and that Bacteroidetes was significantly enhanced in farms utilizing antibiotics. In the total cultivable colonies, 62.58-89.43% ARBs and 95.29% MARB were clustered in Bacteroidetes with the dominant species (Myroides ordoratimimus and Spningobacterium spp., respectively) related to human clinical opportunistic pathogens.