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Forest soil microbes play a crucial role in regulating atmospheric-soil carbon fluxes. Environmental heterogeneity across forest types and regions may lead to differences in soil CO 2 and CH 4 emissions. However, the microbial mechanisms underlying these emission variations are currently unclear. In this study, we measured CO 2 and CH 4 emissions of Robinia pseudoacacia forests along a north-south transect in the Loess Plateau. Using metagenomic sequencing, we investigated the structural and functional profiles of soil carbon cycling microbial communities. Results indicated that the forest CO 2 emissions of Robinia pseudoacacia was significantly higher in the north region than in the south region, while the CH 4 emission was oppositely. This is mainly attributed to changes in gene abundance driven by soil pH and moisture in participating carbon degradation and methane oxidation processes across different forest regions. The gene differences in carbon fixation processes between regions primarily stem from the Calvin cycle, where the abundance of rbcL, rbcS, and prkB genes dominates microbial carbon fixation in forest soils. Random forest models revealed key genes involved in predicting forest soil CO 2 emissions, including SGA1 and amyA for starch decomposition, TYR for lignin decomposition, chitinase for chitin decomposition, and pectinesterase for pectin decomposition. Microbial functional characterization revealed that interregional differences in CH 4 emissions during methane metabolism may originate from methane oxidation processes, and the associated gene abundances (glyA, ppc, and pmoB) were key genes for predicting CH 4 emissions from forest soils. Our results provide new insights into the microbial mechanisms of CO 2 and CH 4 emissions from forest soils, which will be crucial for accurate prediction of the forest soil carbon cycle in the future., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

The increase in atmospheric CO 2 concentration is a significant factor in triggering global warming. CO 2 is essential for plant photosynthesis, but excessive CO 2 can negatively impact photosynthesis and its associated physiological and biochemical processes. The tetraploid Robinia pseudoacacia L., a superior and improved variety, exhibits high tolerance to abiotic stress. In this study, we investigated the physiological and proteomic response mechanisms of the tetraploid R. pseudoacacia under high CO 2 treatment. The results of our physiological and biochemical analyses revealed that a 5% high concentration of CO 2 hindered the growth and development of the tetraploid R. pseudoacacia and caused severe damage to the leaves. Additionally, it significantly reduced photosynthetic parameters such as P n, G s, T r, and C i, as well as respiration. The levels of chlorophyll (Chl a and b) and the fluorescent parameters of chlorophyll ( Fm , Fv / Fm , q P, and ETR ) also significantly decreased. Conversely, the levels of ROS (H 2 O 2 and O 2 ·- ) were significantly increased, while the activities of antioxidant enzymes (SOD, CAT, GR, and APX) were significantly decreased. Furthermore, high CO 2 induced stomatal closure by promoting the accumulation of ROS and NO in guard cells. Through a proteomic analysis, we identified a total of 1652 DAPs after high CO 2 treatment. GO functional annotation revealed that these DAPs were mainly associated with redox activity, catalytic activity, and ion binding. KEGG analysis showed an enrichment of DAPs in metabolic pathways, secondary metabolite biosynthesis, amino acid biosynthesis, and photosynthetic pathways. Overall, our study provides valuable insights into the adaptation mechanisms of the tetraploid R. pseudoacacia to high CO 2 .

Flavonoids participate in several plant processes such as growth and physiological protection in adverse environments. In this study, we investigated the combined effects of eCO 2 and cadmium (Cd)-contaminated soils on the total flavonoid and monomer contents in the leaves of Robinia pseudoacacia L. seedlings. Elevated CO 2 , Cd, and eCO 2 + Cd increased the total flavonoids in the leaves relative to the control, and eCO 2 mostly increased (p < 0.05) the total flavonoid content under Cd exposure. Elevated CO 2 increased (p < 0.05) robinin, rutin, and acacetin contents in the leaves of 45-day seedlings and decreased (p < 0.05) the content of robinin and acacetin at 90 and 135 d under Cd exposure except for robinin at day 45 under Cd1 and acacetin on day 135 under Cd1. Quercetin content decreased (p < 0.05) under the combined conditions relative to Cd alone. Kaempferol in the leaves was only detected under eCO 2 on day 135. The responses of total chlorophyll, total soluble sugars, starch, C, N, S, and the C/N ratio in the leaves to eCO 2 significantly affected the synthesis of total flavonoids and monomers under Cd exposure. Overall, rutin was more sensitive to eCO 2 + Cd than the other flavonoids. Cadmium, CO 2, and time had significant interactive effects on the synthesis of flavonoids in the leaves of R. pseudoacacia L. seedlings. Elevated CO 2 may improve the protection and defense system of seedlings grown in Cd-contaminated soils by promoting the synthesis of total flavonoids, although robinin, rutin, quercetin, and acacetin yields may reduce with time. Additionally, increased Cd in the leaves suggested that eCO 2 could improve the phytoremediation of Cd-contaminated soils., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

Few studies have explored the long-term effects of elevated atmospheric CO 2 combined with lead (Pb) contamination on plants. The objective of this study was to examine the effects of 3 years of elevated CO 2 (700 ± 23 μmol mol -1 ) on Pb accumulation and plant defenses in leaves of Robinia pseudoacacia L. seedlings in exposed to Pb (500 mg kg -1 soil). Elevated CO 2 increased Pb accumulation in leaves and Pb removal rate in soils. In plants exposed to Pb stress, total chlorophyll and carotenoid contents in leaves were lower under elevated CO 2 than under ambient CO 2 , but seedling height and width increased under elevated CO 2 relative to ambient CO 2 . Elevated CO 2 significantly (p < .01) stimulated malondialdehyde content in leaves under Pb exposure. Superoxide dismutase and catalase activity increased significantly (p < .01), peroxidase activity decreased significantly (p < .01), and glutathione, cystine, and phytochelatin contents increased under elevated CO 2  + Pb relative to Pb alone. Elevated CO 2 stimulated the production of soluble sugars, proline, flavonoids, saponins, and phenolics in plants exposed to Pb stress. Ove rall, long-term elevation of CO 2 increased Pb-induced oxidative damage in seedlings, but enhanced the phytoextraction of Pb from contaminated soils., (Copyright © 2018 Elsevier B.V. All rights reserved.)

The relationships of CO 2 assimilation under saturated-light conditions (A sat ) with exposure- (AOTX, Accumulated Ozone exposure over a hourly Threshold of X ppb) and flux-based (POD Y , Phytotoxic Ozone Dose over a hourly threshold Y nmol·m -2 ·s -1 ) O 3 metrics was studied on three common urban trees, Fraxinus chinensis (FC), Platanus orientalis (PO) and Robinia pseudoacacia (RP). Parameterizations for a stomatal multiplicative model were proposed for the three species. RP was the species showing lower species-specific maximum stomatal conductance (g max ) and experiencing lower cumulative O 3 uptake along the experiment, but in contrast it was the most sensitive to O 3 . POD Y was slightly better than AOTX metric at estimating relative A sat (R-A sat )for PO and RB but not for FC. The best fittings obtained for the regressions between R-A sat and AOTX for FC, PO and RP were 0.904, 0.868, and 0.876, when the thresholds of X were 60ppb, 55ppb and 30ppb, respectively. However, AOT40 performed also well for all of them, with R 2 always >0.83. For POD Y , the highest R 2 values for FC, PO and RB were 0.863, 0.897 and 0.911 at thresholds Y=7, 5 and 1nmolO 3 m -2 s -1 , respectively. Given the potentially higher O 3 removal capacity of FC and PO by stomatal uptake and their lower sensitivity to this pollutant than RP, the former two species would be appropriate for urban gardens and areas where O 3 levels are high. Parameterization and modeling of stomatal conductance for the main urban tree species may provide reliable estimations of the stomatal uptake of O 3 and other gaseous pollutants by vegetation, which may support decision making on the most suitable species for green urban planning in polluted areas., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Secondary metabolites play important roles in plant interactions with the environment. The co-occurrence of heavy metal contamination of soils and rising atmospheric CO2 has important effects on plant. It is important to explore the ways in which production of plant secondary metabolites is affected by heavy metals under elevated atmospheric CO2. We examined the effects of elevated CO2 on secondary metabolite contents in Robinia pseudoacacia seedlings grown in Cd- and lead (Pb)-contaminated soils. The increase in secondary metabolites was greater under Cd + Pb exposure than under exposure to individual metals regardless of elevated CO2 with the exception of condensed tannins in leaves and total alkaloids in stems. Except for phenolic compounds and condensed tannins, elevated CO2 was associated with increased secondary metabolite contents in leaves and stems of plants exposed to Cd, Pb, and Cd + Pb compared to plants exposed to ambient CO2 + metals. Changes in saponins in leaves and alkaloids in stems were greater than changes in the other secondary metabolites. Significant interactive effects of CO2, Cd, and Pb on secondary metabolites were observed. Saponins in leaves and alkaloids in stems were more sensitive than other secondary metabolites to elevated CO2 + Cd + Pb. Elevated CO2 could modulate plant protection and defense mechanisms in R. pseudoacacia seedlings exposed to heavy metals by altering the production of secondary metabolites. The increased Cd and Pb uptake under elevated CO2 suggested that R. pseudoacacia may be used in the phytoremediation of heavy metal-contaminated soils under global environmental scenarios., (Copyright © 2016 Elsevier Ltd. All rights reserved.)