Large-scale importance of microbial carbon use efficiency and necromass to soil organic carbon.

Optimal methods for incorporating soil microbial mechanisms of carbon (C) cycling into Earth system models (ESMs) are still under debate. Specifically, whether soil microbial physiology parameters and residual materials are important to soil organic C (SOC) content is still unclear. Here, we explored the effects of biotic and abiotic factors on SOC content based on a survey of soils from 16 locations along a ~4000 km forest transect in eastern China, spanning a wide range of climate, soil conditions, and microbial communities. We found that SOC was highly correlated with soil microbial biomass C (MBC) and amino sugar (AS) concentration, an index of microbial necromass. Microbial C use efficiency (CUE) was significantly related to the variations in SOC along this national-scale transect. Furthermore, the effect of climatic and edaphic factors on SOC was mainly via their regulation on microbial physiological properties (CUE and MBC). We also found that regression models on explanation of SOC variations with microbial physiological parameters and AS performed better than the models without them. Our results provide the empirical linkages among climate, microbial characteristics, and SOC content at large scale and confirm the necessity of incorporating microbial biomass and necromass pools in ESMs under global change scenarios.

Transport of sulfanilamide in saturated porous media under different solution chemistry conditions: role of physicochemical characteristics of soils.

Identification of the transport of sulfonamide antibiotics in soils facilitates a better understanding of the environmental fate and behaviors of these ubiquitous contaminants. In this study, the mobility properties of sulfanilamide (SNM, a typical sulfonamide antibiotic) through saturated soils with different physicochemical characteristics were investigated. The results showed that the physicochemical characteristics controlled SNM mobility. Generally, the mobility of SNM was positively correlated with CEC values and soil organic matter content, which was mainly related to the interactions between the organic matter in soils and SNM molecules via π-π stacking, H-bonding, ligand exchange, and hydrophobic interaction. Furthermore, higher clay mineral content and lower sand content were beneficial for restraining SNM transport in the soils. Unlike Na+, Cu2+ ions could act as bridging agents between the soil grains and SNM molecules, contributing to the relatively weak transport of SNM. Furthermore, the trend of SNM mobility in different soil columns was unaffected by solution pH (5.0-9.0). Meanwhile, for a given soil, the SNM mobility was promoted as the solution pH values increased, which was caused by the enhanced electrostatic repulsion between SNM- species and soil particles as well as the declined hydrophobic interaction between SNM and soil organic matter. The obtained results provide helpful information for the contribution of soil physicochemical characteristics to the transport behaviors of sulfonamide antibiotics in soil-water systems.

Biochars assisted phytoremediation of polycyclic aromatic hydrocarbons contaminated agricultural soil: Dynamic responses of functional genes and microbial community.

A biochar-intensified phytoremediation experiment was designed to investigate the dynamic effects of different biochars on polycyclic aromatic hydrocarbon (PAH) removal in ryegrass rhizosphere contaminated soil. Maize and wheat straw biochar pyrolyzed at 300 °C and 500 °C were amended into PAH-contaminated soil, and then ryegrass (Lolium multiflorum L.) was planted for 90 days. Spearman's correlations among PAH removal, enzyme activity, abundance of PAH-ring hydroxylating dioxygenase (PAH-RHDα), and fungal and bacterial community structure were analyzed to elucidate the microbial degradation mechanisms during the combined remediation process. The results showed that 500 °C wheat straw biochar had higher surface area and more nutrients, and significantly accelerated the phytoremediation of PAHs (62.5 %), especially for high molecular weight PAH in contaminated soil. The activities of urease and dehydrogenase and the abundance of total and PAH-degrading bacteria, which improved with time by biochar and ryegrass, had a positive correlation with the removal rate of PAHs. Biochar enhanced the abundance of gram-negative (GN) PAH-RHDα genes. The GN PAH-degraders, Sphingomonas, bacteriap25, Haliangium, and Dongia may play vital roles in PAH degradation in biochar-amended rhizosphere soils. Principal coordinate analysis indicated that biochar led to significant differences in fungal community structures before 30 days, while the diversity of the bacterial community composition depended on planting ryegrass after 60 days. These findings imply that the structural reshaping of microbial communities results from incubation time and the selection of biochar and ryegrass in PAH-contaminated soils. Applying 500 °C wheat straw biochar could enhance the rhizoremediation of PAH-contaminated soil and benefit the soil microbial ecology.

Rice husk and its derived biochar assist phytoremediation of heavy metals and PAHs co-contaminated soils but differently affect bacterial community.

In order to evaluate the feasibility of rice husk and rice husk biochar on assisting phytoremediation of polycyclic aromatic hydrocarbons (PAHs) and heavy metals (HMs) co-contaminated soils, a 150-day pot experiment planted with alfalfa was designed. Rice husk and its derived biochar were applied to remediate a PAHs, Zn, and Cr co-contaminated soil. The effects of rice husk and biochar on the removal and bioavailability of PAHs and HMs, PAH-ring hydroxylating dioxygenase gene abundance and bacterial community structure in rhizosphere soils were investigated. Results suggested that rice husk biochar had better performance on the removal of PAHs and immobilization of HMs than those of rice husk in co-contaminated rhizosphere soil. The abundance of PAH-degraders, which increased with the culture time, was positively correlated with PAHs removal. Rice husk biochar decreased the richness and diversity of bacterial community, enhanced the growth of Steroidobacter, Bacillus, and Sphingomonas in rhizosphere soils. However, Steroidobacter, Dongia and Acidibacter were stimulated in rice husk amended soils. According to the correlation analysis, Steroidobacter and Mycobacterium may play an important role in PAHs removal and HMs absorption. The combination of rice husk biochar and alfalfa would be a promising method to remediate PAHs and HMs co-contaminated soil.

Transport of dissolved organic matters derived from biomass-pyrogenic smoke (SDOMs) and their effects on mobility of heavy metal ions in saturated porous media.

Biomass-pyrogenic smoke-derived dissolved organic matter (SDOMs) percolating into the underground environment profoundly impacts the transport and fate of environmental pollutants in groundwater systems. Herein, SDOMs were produced by pyrolyzing wheat straw at 300-900 °C to explore their transport properties and effects on Cu2+ mobility in quartz sand porous media. The results indicated that SDOMs exhibited high mobility in saturated sand. Meanwhile, the mobility of SDOMs was enhanced at a higher pyrolysis temperature due to the decrease in their molecular sizes and the declined H-bonding interactions between SDOM molecules and sand grains. Furthermore, the transport of SDOMs was elevated as pH values were raised from 5.0 to 9.0, which resulted from the strengthened electrostatic repulsion between SDOMs and quartz sand particles. More importantly, SDOMs could facilitate Cu2+ transport in the quartz sand, which stemmed from forming soluble Cu-SDOM complexes. Intriguingly, the promotional function of SDOMs for the mobility of Cu2+ was strongly dependent on the pyrolysis temperature. Generally, SDOMs generated at higher temperatures exhibited superior effects. The phenomenon was mainly due to the differences in the Cu-binding capacities of various SDOMs (e.g., cation-π attractive interactions). Our findings highlight that the high-mobility SDOM can considerably affect heavy metal ions' environmental fate and transport.

Biochar enhanced polycyclic aromatic hydrocarbons degradation in soil planted with ryegrass: Bacterial community and degradation gene expression mechanisms.

Biochar and ryegrass have been used in the remediation of polycyclic aromatic hydrocarbons (PAHs)-contaminated soils; however, the effects of different biochar application levels on the dissipation of PAHs, bacterial communities, and PAH-ring hydroxylating dioxygenase (PAH-RHDα) genes in rhizosphere soil remain unclear. In this study, enzyme activity tests, real-time quantitative polymerase chain reaction (PCR), and high-throughput sequencing were performed to investigate the effects of different proportions of rape straw biochar (1%, 2%, and 4% (w/w)) on the degradation of PAHs, as well as the associated changes in the soil bacterial community and PAH-RHDα gene expression. The results revealed that biochar enhanced the rhizoremediation of PAH-contaminated soil and that 2% biochar-treated rhizosphere soil was the most effective in removing PAHs. Furthermore, urease activity, abundance and activity of total bacteria, and PAH-degrading bacteria were enhanced in soil that was amended with biochar and ryegrass. Additionally, the activity of 16S rDNA and PAH-RHDα gram-negative (GN) genes increased with increasing biochar dosage and had a positive correlation with the removal of PAHs. Biochar changed the rhizosphere soil bacterial composition and α-diversity, and promoted the growth of Pseudomonas and Zeaxanthinibacter. In addition, the relative abundance of Pseudomonas was positively correlated with PAH removal. These findings imply that rape straw biochar can enhance the rhizoremediation of PAH-contaminated soil by changing soil bacterial communities and stimulating the expression of PAH-RHDα GN genes. The 2% of rape straw biochar combined with ryegrass would be an effective method to remediate the PAH-contaminated soil.

Effects of Fire Phoenix (a genotype mixture of Fesctuca arundinecea L.) and Mycobacterium sp. on the degradation of PAHs and bacterial community in soil.

Phytomicrobial remediation technology of PAH-contaminated soils has drawn great attention due to its low-cost, eco-friendly, and effective characteristics, but the mechanism underlying the removal of PAHs by rhizosphere in wastewater-irrigated soil is so far not clear. To evaluate the dissipation of PAHs and the shifts of bacterial community structure under plant-microorganism symbiotic system in an agricultural soil, a rhizo-box experiment with Fire Phoenix (a genotype mixture of Fesctuca arundinecea L.) or/and inoculated Mycobacterium sp. was conducted for 60 days. The changes of bacterial community structure and the contents of PAHs were analyzed by denaturing gradient gel electrophoresis (DGGE) and high-performance liquid chromatography (HPLC), respectively. The results showed that the removal rate of PAHs in phytomicrobial combined treatment was 53.7% after 60 days. The PAH-degraders were dominated by Microbacterium sp., Sphingomonas sp., Mycobacterium sp., and Flavobacterium sp. The plant of Fire Phoenix induced the appearance of Pseudomonas sp. and TM7 phylum sp. oral clone. The highest of bacterial diversity index was observed in unrhizosphere soils (MR-), rather than that in rhizosphere soils (MR+). In combination, phytomicrobial combined treatment of Fire Phoenix and Mycobacterium strain enhanced the removal rate of PAHs and changed the structure of bacterial community and bacterial diversity. Bacterial community has great effect on PAH degradation in PAH-contaminated soil from the wastewater-irrigated site. Our study can provide support information for PAH degradation enhancement by the synergetic effect of Fire Phoenix and Mycobacterium sp.

Response of soil bacterial communities to polycyclic aromatic hydrocarbons during the phyto-microbial remediation of a contaminated soil.

Rhizo-box experiments were conducted to analyze the phyto-microbial remediation potential of a grass (Lolium multiflorum L.) and a crop (Glycine max L.) combined with exogenous strain (Pseudomonas sp.) for polycyclic aromatic hydrocarbons (PAHs) contaminated soils. The dynamics of bacterial community composition, abundances of 16 S rDNA and ring hydroxylating dioxygenases (RHDα) genes, and removal of PAHs were evaluated and compared on four culture stages (days 0, 10, 20, and 30). The results showed that 8.65%-47.42% of Σ12 PAHs were removed after 30 days of cultivation. Quantitative polymerase chain reaction (qPCR) analysis indicated that treatments with soybean and ryegrass rhizosphere markedly increased the abundances of total bacteria and PAH-degraders, especially facilitated the growth of gram-negative degrading bacteria. Flavobacterium sp. and Pseudomonas sp. were the main and active strains in the control soil. However, the presence of plants and/or exogenous Pseudomonas sp. changed the soil bacterial community structure and modified the bacterial diversity of PAH-degraders. On the whole, this study showed that the high molecular weight PAHs removal efficiency of phyto-microbial remediation with ryegrass was better than those of remediation with soybean. Furthermore, the removals of PAHs strongly coincided with the abundance of PAH-degraders and bacterial community structure.

Enhanced polycyclic aromatic hydrocarbons degradation in rhizosphere soil planted with tall fescue: Bacterial community and functional gene expression mechanisms.

To investigate the bacterial mechanisms of polycyclic aromatic hydrocarbons (PAHs) degradation in an aged-contaminated agricultural soil planted with tall fescue (Festuca arundinacea), a rhizo-box experiment was carried out for 60 d. Shifts in bacterial community structure in the soils during the experiment were performed using denaturing gradient gel electrophoresis. The abundance and activity of total bacteria and PAH-degraders were measured by quantification of 16S rDNA, PAH-ring hydroxylating dioxygenase (PAH-RHDα) genes and their transcripts, respectively. The residual PAH concentrations were monitored using high-performance liquid chromatography analysis. Results showed that the removal percentage of total PAHs in rhizosphere soil was 11% higher than that in unplanted soil. Soil bacteria were dominated by Alphaproteobacteria (48.4%) and Gammaproteobacteria (25.8%). Tall fescue positively affected the abundance and activity of total bacteria in the soil, and stimulated RHDα gram-negative (GN) gene expression while inhibiting RHDα gram-positive gene expression. PAH dissipation in rhizosphere soil could be ascribed to modifications in the bacterial community structure, increase in the abundance of PAH-degraders, and enhancement of the RHDα GN gene expression during the incubation.

Fire rather than nitrogen addition affects understory plant communities in the short term in a coniferous-broadleaf mixed forest.

Increasing fire risk and atmospheric nitrogen (N) deposition have the potential to alter plant community structure and composition, with consequent impacts on biodiversity and ecosystem functioning. This study was conducted to examine short-term responses of understory plant community to burning and N addition in a coniferous-broadleaved mixed forest of the subtropical-temperate transition zone in Central China. The experiment used a pair-nested design, with four treatments (control, burning, N addition, and burning plus N addition) and five replicates. Species richness, cover, and density of woody and herbaceous plants were monitored for 3 years after a low-severity fire in the spring of 2014. Burning, but not N addition, significantly stimulated the cover (+15.2%, absolute change) and density (+62.8%) of woody species as well as herb richness (+1.2 species/m2, absolute change), cover (+25.5%, absolute change), and density (+602.4%) across the seven sampling dates from June 2014 to October 2016. Light availability, soil temperature, and prefire community composition could be primarily responsible for the understory community recovery after the low-severity fire. The observations suggest that light availability and soil temperature are more important than nutrients in structuring understory plant community in the mixed forest of the subtropical-temperate transition zone in Central China. Legacy woody and herb species dominated the understory vegetation over the 3 years after fire, indicating strong resistance and resilience of forest understory plant community and biodiversity to abrupt environmental perturbation.

Effects of nitrogen addition and mowing on rodent damage in an Inner Mongolian steppe.

Rodent damage is a serious threat to sustainable management of grassland. The effects of nitrogen (N) deposition and grassland management on rodent damage have been scarcely studied. Here, we reported the effects of 2 years of N addition and mowing on burrow density and damage area of Citellus dauricus in a semiarid steppe in Inner Mongolia. N addition significantly aggravated, while mowing alleviated rodent damage in the grassland under study. Burrow density and damage area increased 2.8-fold and 4.7-fold, in N addition plots compared to the ambient N addition treatment, respectively. Conversely, mowing decreased burrow density and damage area by 75.9% and 14.5%, respectively, compared to no mowing plots. Observed changes in rodent damage were mainly due to variations in plant community cover, height, and aboveground net primary productivity. Our findings demonstrate that N addition and mowing can affect the rodent density and activity in grassland, suggesting that the effects of a changing atmospheric composition and land use on rodent damage must be considered in order to achieve better grassland management.

Variations in the bioavailability of polycyclic aromatic hydrocarbons in industrial and agricultural soils after bioremediation.

The aim of this study was to demonstrate the variations in bioavailability remaining in industrial and agricultural soils contaminated by polycyclic aromatic hydrocarbons (PAHs) after bioremediation. After inoculation of Mycobacterium sp. and Mucor sp., PAH biodegradation was tested on a manufactured gas plant (MGP) soil and an agricultural soil. PAH bioavailability was assessed before and after biodegradation using solid-phase extraction (Tenax-TA extraction) and solid-phase micro-extraction (SPME) to represent bioaccessibility and chemical activity of PAHs, respectively. Only 3- and 4-ring PAHs were noticeably biodegradable in the MGP soil. PAH biodegradation in the agricultural soil was different from that in the MGP soil. The rapidly desorbing fractions (F(rap)) extracted by Tenax-TA and the freely dissolved concentrations of 3- and 4-ring PAHs determined by SPME from the MGP soil decreased after 30 days biodegradation; those values of the 5- and 6-ring PAHs changed to a lesser degree. For the agricultural soil, the F(rap) values of the 3- and 4-ring PAHs also decreased after the biodegradation experiment. The Tenax-TA extraction and the SPME have the potential to assess variations in the bioavailability of PAHs and the degree of biodegradation in contaminated MGP soils. In addition, Tenax-TA extraction is more sensitive than SPME when used in the agricultural soil.