Assessment of the effects of phenanthrene and its nitrogen heterocyclic analogues on microbial activity in soil

Can biochar and oxalic acid alleviate the toxicity stress caused by polycyclic aromatic hydrocarbons in soil microbial communities?

It remains unclear whether biochar amendment can mediate changes in soil microbial communities caused by organic contaminants in the rhizosphere. In this study, phenanthrene-contaminated soil was amended with biochar and oxalic acid (OA) alone or in combination and incubated for 21 days. Phospholipid fatty acids (PLFAs) and high-throughput sequencing were used to evaluate shifts in bacterial and fungal community structure. Phenanthrene stress led to significant shifts in both soil bacterial and fungal community structure, in particularly, 82% of microbial phyla decreased in abundance. Biochar and/or OA improved the phenanthrene-polluted soil by positively mediating shifts in soil microbial communities stressed by phenanthrene. Specifically, biochar and/or OA led to the survival of certain microbial taxa that were inhibited by phenanthrene stress. In addition, many functional microbial individuals and genes participating in polycyclic aromatic hydrocarbon (PAH) degradation were positively stimulated by high phenanthrene stress and further stimulated by the simultaneous application of biochar and OA. Based on these findings, tandem biochar and rhizoremediation may be a feasible strategy for relieving PAH toxicity to soil microbial communities.

Influence of biochar aged in acidic soil on ecosystem engineers and two tropical agricultural plants

Biochar amendment to soil is predicted globally as a means to enhance soil health. Alongside the beneficial result on soil nutrient availability and retention, biochar is presumed to increase soil macro / microbiota composition and improve plant growth. However, evidence for such an effect remains elusive in many tropical agricultural soils. The influence of biochar aged in soil was assessed on soil microbiota, macrobiota (Eudrilus eugeniae), seedling emergence and early plant growth of Oryza sativa and Solanum lycopersicum in tropical agricultural soil, over a 90 d biochar-soil contact time. Results showed negative impacts of increased loading of biochar on the survival and growth of E. eugeniae. LC₅₀ and EC₅₀ values ranged from 34.8% to 86.8% and 0.9-23.7% dry biochar kg^-1 soil, over time. The growth of the exposed earthworms was strongly reduced (R² = -0.866, p < 0.05). Biochar significantly increased microbiota abundance relative to the control soil (p < 0.001). However, fungal population was reduced by biochar addition. Biochar application threshold of 10% and 5% was observed for (O. sativa) and (S. lycopersicum), respectively. Furthermore, the addition of biochar to soil resulted in increased aboveground (shoot) biomass (p < 0.01). However, the data revealed that biochar did not increase the belowground (root) biomass of the plant species during the 90 d biochar-soil contact time. The shoot-to-root-biomass increase indicates a direct toxic influence of biochar on plant roots. This reveals that nutrient availability is not the only mechanism involved in biota-biochar interactions. Detailed studies on specific biota-plant-responses to biochars between tropical, temperate and boreal environments are needed to resolve the large variations and mechanisms behind these effects.

Impact of nitrogen-polycyclic aromatic hydrocarbons on phenanthrene and benzo[a]pyrene mineralisation in soil

When aromatic hydrocarbons are present in contaminated soils, they often occur in mixtures. The impact of four different (3-ring) nitrogen-containing polycyclic aromatic hydrocarbons (N-PAHs) on ^12/14C-phenanthrene and ^12/14C-benzo[a]pyrene (B[a]P) mineralisation in soil was investigated over a 90 d incubation period. The results revealed that both ^12/14C-phenanthrene and ^12/14C-benzo[a]pyrene showed no significant mineralisation in soils amended with 10mgkg ^-1 and 100mgkg ^-1 N-PAHs (p>0.05). However, increases in lag-phases and decreases in the rates and extents of mineralisation were observed, over time. Among the N-PAHs, benzo[h]quinoline impacted ¹⁴C-phenanthrene mineralisation with extended and diauxic lag phases. Furthermore,^12/14C-B[a]P and ¹⁴C-benzo[a]pyrene-nitrogen-containing polycyclic aromatic hydrocarbons (¹⁴C-B[a]P-N-PAHs) amended soils showed extensive lag phases (> 21 d); with some ¹⁴C-B[a]P-N-PAH mineralisation recording <1% in both concentrations (10mgkg ^-1 and 100mgkg ^-1), over time. This study suggests that the presence of N-PAHs in contaminated soil may impact the microbial degradation of polycyclic aromatic hydrocarbons (PAHs) and the impact was most likely the result of limited success in achieving absolute biodegradation of some PAHs in soil.