We investigated the beneficial role of different concentrations of exogenous oxalic acid (OA) or citric acid (CA) for improving Pb tolerance and mitigating Pb-induced physiological toxicity in Changbai larch (Larix olgensis A. Henry) seedlings in northeast China. The seedlings were exposed to 100 mg·kg⁻¹ Pb in soil alone or in combination with OA or CA irrigation for 10, 20, or 30 days. Pb-induced damage in L. olgensis was evident from elevated lipid peroxidation that significantly inhibited plant growth. Malondialdehyde (MDA) contents also increased in the presence of elevated Pb; however, superoxide dismutase (SOD) and peroxidase (POD) activities, as well as proline and pigment contents, all decreased. The damage increased in controls over the application periods. Pb contents in fine roots and leaves generally decreased with low-concentration organic acids (<1.0 mmol·L⁻¹), but often increased at 5.0 and 10.0 mmol·L⁻¹. Alternatively, when Pb-stressed plants were exposed to an organic acid (especially 5.0 or 10.0 mmol·L⁻¹ for 10 days), the damage, as indicated by the physiological parameters, was reversed, and plant growth was promoted; CA was more effective in inducing these changes than OA. Therefore, exogenous organic acids have the potential to alleviate Pb-induced oxidative injuries, and can improve the tolerance of L. olgensis seedlings to Pb stress. Under lower OA and CA concentrations, the detoxification mechanism appears to be an external resistance mechanism; however, under higher concentrations (5.0–10.0 mmol·L⁻¹) internal resistance mechanisms appear dominant. It is also possible that the two mechanisms work in tandem.

Graphene is the thinnest and toughest two-dimensional nanomaterial yet discovered. However, it inevitably enters the biosphere, where it may pose potential risks to ecosystems. We investigated the impact of applied graphene concentrations on bacterial community diversity, physicochemical properties, and enzyme activities of Haplic Cambisols, the zonal soil of Northeastern China. Soils receiving 0, 10, 100, or 1000 mg kg⁻¹ of graphene were incubated for 7, 15, 30, 60, or 90 days. Adding graphene significantly increased the community richness and diversity index of the bacterial community in Haplic Cambisols, as well as their abundances, but this impact varied with graphene concentration and incubation time. Compared with 0 mg kg⁻¹ of graphene applied, soil bacteria abundance and diversity increased significantly during early stages of incubation (i.e., 7 and 15 days) under different concentrations of graphene, and was inhibited or remained unchanged by a longer incubation time, reaching a minima at 60 days but then following an upward trend. Graphene treatments influenced the bacterial community structure and metabolic function in Haplic Cambisols, and the bacterial community’s metabolic regulation mechanism varied with both incubation time and graphene concentration. The rank order of bacterial similarity in soils treated with graphene was 15 > 7 > 30 > 60 > 90 days. Throughout the incubation periods, except for a few unidentified bacteria, the relative abundances of Proteobacteria and Acidobacteria in the soil samples were the highest, with the number of Pseudomonas of Proteobacteria being particularly large. The rank order of bacterial abundance at the phylum level in Haplic Cambisols was 15 > 7 > 30 > 90 > 60 days. Graphene also influenced bacterial community diversity by affecting several key soil environmental factors, such as organic matter and hydrolytic nitrogen contents, as well as urease and catalase activities.

We investigated the impacts of graphene application at different concentrations on the growth and physiological characteristics of Changbai larch (Larix olgensis A. Henry) seedlings and the chemical properties and enzyme activities of Haplic Cambisols under these seedlings. The aim is to evaluate the environmental effects of graphene on the afforestation species and the zonal forest soils of Northeast China. Seedlings receiving 0 (CK), 25, 50, 100, 250, or 500 mg L⁻¹ graphene were incubated for 30, 40, or 50 days. Low concentrations (25–50 mg L⁻¹) of graphene increased the dry masses of root, stem, and leaf; however, high concentrations (100–500 mg L⁻¹) inhibited them. Compared with those under 0 mg L⁻¹ graphene, the root length, surface area, volume, and average diameter all increased during the early stages of incubation (i.e., 30 and 40 days) under low concentration of graphene (100 mg L⁻¹); at 50 days, they were significantly inhibited. At 30 days, graphene decreased superoxide dismutase (SOD) and peroxidase (POD) activities, as well as pigment, soluble protein, and proline contents, and the decline increased with increasing graphene concentration; at 40 and 50 days, the above parameters increased initially and then decreased, reaching a maximum at 50 mg L⁻¹. The changes in relative conductivity and malondialdehyde (MDA), superoxide anion and hydrogen peroxide contents were the opposite of those in the physiological indexes mentioned above. Therefore, graphene caused different degrees of oxidative stress in L. olgensis seedlings. At 30 days, graphene generally increased the organic matter, hydrolytic nitrogen, and available phosphorus and potassium contents of Haplic Cambisols, but these parameters decreased at 40 and 50 days. Graphene generally decreased acid phosphatase, urease, dehydrogenase, and catalase activities. Therefore, when graphene reaches a certain content level in this soil, it may also affect nitrogen and phosphorus cycling.

Trace metal contamination of soil is an increasing problem. Organic acid application can restore trace metal elements such as cadmium (Cd) in contaminated soil. Changbai larch (Larix olgensis A. Henry) is an economically important forestry species in northeast China; however, growth is inhibited by severe Cd contamination. We investigated the effects of different concentrations of exogenous succinic acid (SA) on Cd tolerance and physiological and morphological toxicity in L. olgensis seedlings. Seedlings were planted in pots containing Cd-contaminated or uncontaminated Haplic Cambisol. Seedlings in Cd-contaminated soil were treated daily with SA solution at 0, 0.04, 0.2, 1.0, and 2.0 mmol kg⁻¹ of soil for 10, 20 or 30 days. Cd treatment induced seedling damage and significantly increased the relative conductivity and malondialdehyde content of the leaves, inhibiting soluble protein and proline contents, superoxide dismutase and peroxidase activity, chlorophyl fluorescence and pigment content. Decreases in the length, surface area, volume of roots and leaves, and specific root length were also observed. Effects increased in control plants with time. SA treatment also reduced the Cd content of the fine roots and leaves and Mg, K, and Ca contents. Moreover, plant growth was significantly promoted and damage was reversed, especially at 5.0 and 10.0 mmol L⁻¹ SA for 30 days. SA therefore alleviated Cd-induced injury, improving tolerance to Cd stress. SA application combined with afforestation could therefore help restore Cd-contaminated soil in northeast China. Further studies aimed at determining the detoxification mechanism of L. olgensis seedlings are now required.