Measurements of carbon dioxide (CO2)-evolution from soils are important in evaluating biomass and activity of soil microorganisms, as well as decomposition of soil organic matter. The Respicond VI is a fully computerized system allowing continuous measurement of CO2 evolution in short-and long-term soil incubation experiments in up to 96 incubation vessels. The measurement of CO2 evolution is based on the absorption of CO2 by an electrolyte (KOH solution) producing a change in the cell conductance measured using two electrodes. In this study, the Respicond VI was recalibrated yielding 174.5 mg CO2 as constant A expressing the theoretical maximum amount of CO2 absorbed in 10 ml 0.5 M KOH. This value of A corresponds to 34.9 mg CO2 ml −1 1 M KOH. The constant A does neither depend on the investigated incubation temperatures (5˚C-25˚C) nor on the concentrations of the KOH solutions (0.5, 0.1, 0.05 M KOH). To eliminate any influence of changing incubation temperatures, either induced by uncertainties in temperature control or as a part of the experimental setup, on the conductance of KOH solution, a correction procedure was developed using a factor calculated from changing conductance of KOH solutions in incubation vessels without soil.

Mercury (Hg) pollution of soils is a critical environmental problem. To rehabilitate Hg contaminated soils, arbuscular mycorrhizal (AM) fungi-based phytoremediation may be supportive, yet the functional potential of AM fungi in response to Hg exposure is unclear. In a greenhouse experiment, we assessed the response of Medicago truncatula (Hg tolerance index (TI), Hg partitioning) to different Hg concentrations [0 (Hg0), 25 (Hg25), 50 (Hg50) µg g−1] in treatments with (AM) and without (NM) inoculation of Rhizophagus irregularis. Additionally, zinc (Zn) uptake and the expression of two Zn transporter genes (ZIP2, ZIP6) were examined because Zn is an essential element for plants and shares the same outer electronic configuration as Hg, implying potential competition for the same transporters. The results showed that AM plants had a higher TI than NM plants. Plant roots were identified as dominant Hg reservoirs. AM inoculation reduced the root Hg concentration under Hg50 compared to the NM treatment. There was an interaction between Hg treatment and AM inoculation on Hg stem concentration, i.e., at Hg25, AM inoculation decreased the Hg translocation from roots to stems, while Hg translocation was increased at Hg50 compared to the NM treatment. Zn acquisition was improved by R. irregularis. The negative relationship between Hg and Zn concentrations in the roots of AM and NM plants implied potential competition for the same transporters, although the expression of Zn transporters was upregulated by AM inoculation at all Hg levels. In conclusion, this baseline study demonstrated that R. irregularis may play an important role in Hg tolerance of M. truncatula, suggesting its potential for Hg-contaminated phytoremediation.

To improve the efficiency of Striga hermonthica biological control in the context of an integrated biocontrol system, the role/impact of coinoculating the mycoherbicide Fusarium oxysporum f. sp. strigae (Fos) with a plant growth promoting rhizobacterium, Bacillus subtilis isolate GB03, on S. hermonthica attachment and sorghum biomass was investigated. Fos isolates (Foxy-2, FK3) and GB03, including Trichoderma viride (IMB12098 strain) as reference, were applied as single and combined treatments to S. hermonthica-infested rhizoboxes with sorghum as the host crop. In vivo localization/interaction of Fos and GB03 in S. hermonthica was monitored by fluorescent gene expression of transformed Fos and transformed GB03. Combined treatments of FK3 + GB03 and Foxy-2 + GB03 increased sorghum aboveground dry biomass (P < 0.05), but not IMB12098 + GB03. None of the combined treatments suppressed S. hermonthica attachment. Single treatments of FK3 and GB03 increased sorghum aboveground dry biomass (P < 0.05), but Foxy-2 and IMB12098 did not. Only FK3, of all single treatments, suppressed S. hermonthica attachment (P < 0.05). GB03 promoted sorghum yield when applied alone or combined with Fos (P < 0.05). Fos penetration of S. hermonthica through trichome entry was revealed. Although Fos, either as sporal (conidia) or vegetative form (mycelium), co-occupied common ecological niches with GB03 (they colocalize) in diseased S. hermonthica shoots, mainly in flavonoid-rich regions. Nevertheless, GB03 thwarted Fos suppressive activity against S. hermonthica attachment. In the given setup, coinoculation of Fos with GB03 presented no added advantage in suppressing the incidence of the sampled S. hermonthica. [Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

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Chemical weed control is an effective method, but has proved hazardous for humans, environment, and soil biodiversity. Use of allelopathic bacteria (AB), may be more efficient and sustainable weed control measure. The bacterial inoculants never studied in context of their interaction with weed root exudates and precursor-dependent production of the natural phytotoxins (cyanide, cytolytic enzymes and auxin) by these stains to understand their weed suppression and wheat growth promotion abilities. Therefore, root exudates of Avena fatua, Phalaris minor, Rumex dentatus and wheat were quantified and their role in microbial root colonization and secondary metabolites production i.e. cyanide, cytolitic enzymes, phenolics and elevated auxin concentration was studied. The results depicted L-tryptophan and glycine as major contributor of elevated cyanide and elevated levels in weed rhizosphere by the studied Pseudomonas strains, through their higher root colonization ability in weeds as compared to wheat. Furthermore, the higher root colonization also enhanced p-coumaric acid (photosynthesis inhibitor by impairing cytochrome c oxidase activity in plants), and cytolytic enzymes (root cell wall degradation) concentration in weed rhizosphere. In conclusion, the differential root colonization of wheat and weeds by these strains is responsible for enhancing weed suppression (enhancing phytotoxic effect) and wheat growth promotion (lowering phytotoxic effect).