Response strategies of slash pine (Pinus elliottii) to cadmium stress and the gain effects of inoculation with Herbaspirillum sp. YTG72 in alleviating phytotoxicity and enhancing accumulation of cadmium.

Phytoremediation using fast-growing woody plants assisted by plant growth-promoting bacteria (PGPB) on cadmium (Cd)-contaminated sites is considered a promising technique; however, its remediation efficiency is still affected by multiple factors. In this study, the mining areas' soil conditions were simulated with different Cd addition levels (0, 3, 6, 9 mg kg-1) in order to investigate the response strategy to Cd stress of fast-growing economic tree species, slash pine (Pinus elliottii), and the effects of inoculation with the PGPB strain Herbaspirillum sp. YTG72 on the physiological activity and Cd accumulation of plants. The main results showed that there were significant (p < 0.05) increases in contents of chlorophyll and nutrient elements (P, K, Ca, and Mg) at low Cd addition level (3 mg kg-1) compared to non-Cd addition treatment. When the additive amount of Cd increased, the growth of plants was severely inhibited and the content of proline was increased, as well as Cd in plants. Besides, the ratios of K:P, Ca:P, and Mg:P in plants were negatively correlated with the contents of Cd in plants and soils. Inoculation of P. elliottii with the PGPB strain Herbaspirillum sp. YTG72 improved the physiological functions of the plants under Cd stress and activated the antioxidant system, reduced the accumulation of proline, and decreased the ratios of K:P, Ca:P, and Mg:P in plant. More importantly, planting P. elliottii in Cd-contaminated soil could significantly (p < 0.05) reduce the Cd content in the rhizosphere soil, and furthermore, inoculation treatment could promote the reduction of soil Cd content and increased the accumulation of Cd by root. The results of the present study emphasized the Cd response mechanism of P. elliottii based on multifaceted regulation, as well as the feasibility of strain Herbaspirillum sp. YTG72 assisted P. elliottii for the remediation on Cd-contaminated sites.

Free-standing N, S co-doped graphene aerogels coupled with Eucalyptus wood tar-based activated carbon and cellulose nanofibers for high-performance supercapacitor and removal of Cr(VI).

N, S-dual doping graphene aerogels with three dimensional interconnected network and large specific surface area have been fabricated by cellulose nanofibers (CNF), Eucalyptus wood tar-based activated carbon (AC), and reduced graphene oxide (rGO) for the energy storage applications as well as the removal of Cr(VI). Benefiting from the particular pore structural characteristics, the optimized activated carbon aerogel electrode (GDAC) exhibited prominent capacitances of 813.8 F/g at 1 A/g, and prominent cycling stability. The Ragone plot for the GDAC supercapacitor depicted that the energy density reached maximum (50 Wh/kg) when the power density was 370 W/kg. As far as the adsorption capacity of GDAC for Cr(VI), GDAC achieved a removal rate of 97 % for Cr(VI) and a maximum adsorption capacity of 939.20 mg/g. The fabrication method and excellent performance of GDAC proposed in this study provided new perspective into the potential application of Eucalyptus wood tar-based materials in the supercapacitor applications. Additionally, the comprehensive analysis of the structure-function relationship also provided important theoretical foundations for the removal of Cr(VI).

Isolation of cadmium-resistance and siderophore-producing endophytic bacteria and their potential use for soil cadmium remediation.

Endophyte-assisted phytoremediation is an emerging technique for soil heavy metals (HMs) remediation and has become a research focus in the world because of the benefits of endophytes on plant growth and uptake of HMs. In this study, multifunctional endophytic bacteria strains were isolated and screened, and the feasibility of these strains for soil cadmium (Cd) remediation was investigated by soil incubation experiments and pot experiments. All endophytic bacteria were isolated from the roots of woody plants grown on Cd-contaminated soil. Seven endophytic bacteria strains had capacities to tolerate Cd toxicity and produce siderophores, and sequence analysis of the 16S rRNA gene classified these strains as belonging to the genera Burkholderia, Pseudomonas, Pantoea, and Herbaspirillum. All strains were able to produce hydroxamate siderophores (32.40%-91.49%) and had three or more plant growth promoting properties such as phosphorus solubilization, nitrogen fixation, indole acetic acid and 1-aminocyclopropane-1-carboxylate deaminase production. They were all strongly resistant to Cd2+ toxicity, with the minimum inhibitory concentration in LB medium ranging from 1.5 mM to 9.0 mM. Except for strain Burkholderia contaminans JLS17, other strains showed decreasing removal rates within continuously elevated Cd2+ concentration of 10-100 mg L-1. Compared with the uninoculated treatment, the inoculation of strains B.contaminans JLS17, Pseudomonas lurida JLS32, and Pantoea endophytica JLS50 effectively increased the concentration of acid-soluble Cd and decreased the concentration of reducible, oxidizable, and residual Cd in the soils of different Cd contamination levels. In pot experiments, inoculation of strains JLS17 and YTG72 significantly (p < 0.05) promoted the growth of above-ground parts and root system of slash pine (Pinus elliottii) under Cd stress. This study provides a valuable biological resource for endophyte-assisted phytoremediation and a theoretical basis for the application of endophytic bacteria for remediation of Cd-contaminated soil.

Adaptive Pathways of Microorganisms to Cope With the Shift From P- to N-Limitation in Subtropical Plantations.

Ecological stoichiometry is increasingly acknowledged as one of the main control factors for microbial activity and diversity. Soil carbon/nitrogen (C/N) and carbon/phosphorus (C/P) ratios are usually much higher than microbial nutrient requirements and vary with planting density and stand age in forestlands. However, how microorganisms cope with such stoichiometric imbalances and how they regulate nutrient cycling remain unclear. Here, 5- and 35-year-old experimental Cunninghamia lanceolata [Lamb.] Hook plantations with five planting densities (1,667, 3,333, 5,000, 6,667, and 10,000 stems ha-1) were used to explore the underlying mechanism of the response of microorganisms to stoichiometric imbalances. We found that (i) enzyme activity and microbial biomass and diversity increased with planting density at age 5 but decreased at age 35; (ii) soil microorganisms were P-limited at age 5, but gradually shifted from P- to N-limitation during the development of plantations from 5 to 35 years; and (iii) significantly negative relationships between microbial biomass stoichiometry and enzymatic stoichiometry were observed. The adaptive pathways of soil microorganisms to cope with stoichiometric imbalances include (i) adjusting the stoichiometry of microorganisms and enzymes; (ii) changing the relative abundance of the dominant microbial phyla; and (iii) increasing the ratio of fungal to bacterial diversity. These results highlight how to use the ecological stoichiometry method to identify soil microbial nutrient limitations with planting density during the development of plantations. By underlining the important role of stoichiometry on microbial growth and activity, these findings furthermore emphasize the dependency of organic matter transformation and nutrient cycling on the microbial community.

Isolation and screening of multifunctional phosphate solubilizing bacteria and its growth-promoting effect on Chinese fir seedlings.

Phosphorus-solubilizing microorganisms is a microbial fertilizer with broad application potential. In this study, 7 endophytic phosphate solubilizing bacteria were screened out from Chinese fir, and were characterized for plant growth-promoting traits. Based on morphological and 16S rRNA sequence analysis, the endophytes were distributed into 5 genera of which belong to Pseudomonas, Burkholderia, Paraburkholderia, Novosphingobium, and Ochrobactrum. HRP2, SSP2 and JRP22 were selected based on their plant growth-promoting traits for evaluation of Chinese fir growth enhancement. The growth parameters of Chinese fir seedlings after inoculation were significantly greater than those of the uninoculated control group. The results showed that PSBs HRP2, SSP2 and JRP22 increased plant height (up to 1.26 times), stem diameter (up to 40.69%) and the biomass of roots, stems and leaves (up to 21.28%, 29.09% and 20.78%) compared to the control. Total N (TN), total P (TP), total K (TK), Mg and Fe contents in leaf were positively affected by PSBs while showed a significant relationship with strain and dilution ratio. The content of TN, TP, TK, available phosphorus (AP) and available potassium (AK) in the soil increased by 0.23-1.12 mg g-1, 0.14-0.26 mg g-1, 0.33-1.92 mg g-1, 5.31-20.56 mg kg-1, 15.37-54.68 mg kg-1, respectively. Treatment with both HRP2, SSP2 and JRP22 increased leaf and root biomass as well as their N, P, K uptake by affecting soil urease and acid phosphatase activities, and the content of available nutrients in soil. In conclusion, PSB could be used as biological agents instead of chemical fertilizers for agroforestry production to reduce environmental pollution and increase the yield of Chinese fir.

Soil organic carbon fractions, C-cycling hydrolytic enzymes, and microbial carbon metabolism in Chinese fir plantations.

The mechanisms by which planting density affects soil organic carbon (SOC) fractions, C-cycling associated hydrolytic enzyme activities, and microbial carbon metabolism remain unclear. We evaluated the influences of five planting densities (D1: 1667 stems·ha-1, D2: 3333 stems·ha-1, D3: 4444 stems·ha-1, D4: 5000 stems·ha-1, and D5: 6667 stems·ha-1) on the concentrations of SOC, microbial biomass carbon (MBC), easily oxidizable carbon (EOC), and dissolved organic carbon (DOC), the activities of invertase, cellulase, and ß-glucosidase, and microbial carbon metabolism activities in 5- and 35-year-old Chinese fir plantations. Generally, no significant differences in the SOC and DOC concentrations among five planting densities in 5-year-old plantations were found, but the SOC and DOC were significantly higher in high-density plantations (D3, D4, and D5) than in low-density plantations (D1 and D2) in 35-year-old plantations. The EOC concentration in low-density plantations was lower than that in high-density plantations in both 5- and 35-year-old plantations. The high planting density was associated with higher MBC, activities of invertase and ß-glucosidase, and microbial carbon metabolism activity in 5-year-old plantations, but the opposite was found in 35-year-old plantations. The high-density plantations exhibited a significant decrease in cellulase activity in 35-year-old plantations. These results highlight that although increased planting density would enrich SOC storage after a long-term rotation of plantations, it also reduces microbial and enzymatic activities. This has important implications in the formulation of planting density management strategies to increase SOC stocks while maintaining soil fertility.

Acidobacteria Community Responses to Nitrogen Dose and Form in Chinese Fir Plantations in Southern China.

Acidobacteria is a new bacterial group, identified by molecular research, which is widely distributed and has specific ecological functions in forest soil. In this study, we investigated Acidobacteria response to N input, and the effects were related to N form and dose. The experimental design included two N forms (NH4+-N and NO3--N) and five levels of N deposition (0, 20, 40, 60, 80 kg N ha-1) for 2 years. Research into the Acidobacteria community was conducted using 16Sr RNA gene-based high-throughput pyrosequencing methods. Acidobacteria OTUs and N had a negative relationship in 0-60 kg ha-1 year-1; however, at N doses beyond a certain size, nitrogen might promote an increase in Acidobacteria OTUs. The Acidobacteria relative abundance under NH4+-N treatment was higher than under NO3--N treatment. Acidobacteria relative abundance decreased with increasing of NH4+-N dose, but increased with increasing NO3--N dose. Overall, 13 different Acidobacteria subgroups were identified, with Gp1, Gp2, and Gp3 being dominant. Significant differences in Acidobacteria distribution were primarily caused by N input and pH value. The environmental factors of N were all negatively related to Acidobacteria distribution in low N dose treatments (0-20 kg ha-1 year-1), but were positively related in response to N dose treatments (40-80 kg ha-1 year-1).