Effects of Corn Straw and Citric Acid on Removal of PAHs in Contaminated Soil Related to Changing of Bacterial Community and Functional Gene Expression

Modifying soil bacterial communities in saline mudflats with organic acids and substrates

Aims

The high salinity of soil, nutrient scarcity, and poor aggregate structure limit the exploitation and utilization of coastal mudflat resources and the sustainable development of saline soil agriculture. In this paper, the effects of applying exogenous organic acids combined with biological substrate on the composition and diversity of soil bacterial community were studied in moderately saline mudflats in Jiangsu Province.

Methods

A combination of three exogenous organic acids (humic acid, fulvic acid, and citric acid) and four biological substrates (cottonseed hull, cow manure, grass charcoal, and pine needle) was set up set up on a coastal saline mudflat planted with a salt-tolerant forage grass, sweet sorghum. A total of 120 kg ha-1 of organic acids and 5,000 kg ha-1 of substrates were used, plus two treatments, CK without application of organic acids and substrates and CK0 in bare ground, for a total of 14 treatments.

Results

No significant difference was found in the alpha diversity of soil bacterial community among all treatments (p ≥ 0.05), with the fulvic acid composite pine needle (FPN) treatment showing the largest increase in each index. The beta diversity differed significantly (p < 0.05) among all treatments, and the difference between citric acid-grass charcoal (CGC) and CK treatments was greater than that of other treatments. All treatments were effective in increasing the number of bacterial ASVs and affecting the structural composition of the community. Citric acid-cow manure (CCM), FPN, and CGC treatments were found to be beneficial for increasing the relative abundance of Proteobacteria, Chloroflexi, and Actinobacteria, respectively. By contrast, all treatments triggered a decrease in the relative abundance of Acidobacteria.

Conclusion

Among the 12 different combinations of exogenous organic acid composite biomass substrates applied to the coastal beach, the CGC treatment was more conducive to increasing the relative abundance of the salt-tolerant bacteria Proteobacteria, Chloroflexi and Actinobacteria, and improving the community structure of soil bacteria. The FPN treatment was more conducive to increase the species diversity of the soil bacterial community and adjust the species composition of the bacterial community.

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.

Bioremediation of Diesel-Contaminated Soil by Fungal Solid-State Fermentation

To address the poor removal of diesel in soil by indigenous microorganisms, we proposed a fungal solid-state fermentation (SSF) method for bioremediation. We screened Pycnoporus sanguineus 5.815, Trametes versicolor 5.996, and Trametes gibbosa 5.952 for their diesel-degrading abilities, with Trametes versicolor 5.996 showing the most promise. The fungal inoculum was obtained through SSF using wood chips and bran. Trametes versicolor 5.996 was applied to two treatments: natural attenuation (NA, diesel-contaminated soil) and bioremediation (BR, 10% SSF added to diesel-contaminated soil). Over 20 days, NA removed 12.9% of the diesel, while BR achieved a significantly higher 38.3% degradation rate. BR also increased CO2 and CH4 emissions but reduced N2O emissions. High-throughput sequencing indicated SSF significantly enriched known diesel-degrading microorganisms like Ascomycota (83.82%), Proteobacteria (46.10%), Actinobacteria (27.88%), Firmicutes (10.35%), and Bacteroidota (4.66%). This study provides theoretical support for the application of fungal remediation technology for diesel and improves understanding of microbiologically mediated diesel degradation and soil greenhouse gas emissions.

Effects of different carbon substrates on PAHs fractions and microbial community changes in PAHs-contaminated soils

Different types of carbon substrates were widely used in soil remediation. However, differences of their impacts and related mechanisms on degradation of polycyclic aromatic hydrocarbons (PAHs) and microbial community structures in contaminated soil still remain unclear. Here, we investigated the effects of corn straw (S), glucose (G), straw combined with glucose (SG), and sodium azide (N, as an abiotic control) on PAHs fractions and bacterial communities in soil. After 70 days' microcosm experiments, total PAHs concentrations were significantly reduced by 30.9%, 19.5% and 44.6% under S, G and SG treatments. Water soluble, acid soluble and residual PAHs under all treatments were significantly decreased after 70 days of incubation, while organically bound PAHs were increased by 11.4%, 22.7% and 36.1% under G, S and SG treatments. Additionally, straw and glucose application increased relative abundance related PAHs-degrading bacteria and the copy numbers of gram-negative (PAHs-RHDα GN) and gram-positive genes (PAHs-RHDα GP) in the contaminated soil. Redundancy analysis (RDA) and Random Forest (RF) indicated that PAHs fractions are crucial factors for biodegradation of PAHs in PAHs-contaminated soils amended with carbon substrates. These suggested that carbon substrates contributed to PAHs conversion from residual PAHs (nonlabile fractions) to organically bound PAHs and thus increased the potential for PAHs conversion to water-soluble and organic acid-soluble PAHs, which were more easy to be utilized by soil microorganisms. This study revealed the new insights of different carbon substrates on degradation and dynamic changes of PAHs fractions and the better potential of combined application of straw and glucose in enhancing degradation of PAHs in PAHs-contaminated soils.