Effects of cadmium on soil nitrification in the rhizosphere of Robinia pseudoacacia L. seedlings under elevated atmospheric CO2 scenarios

Diversity of nifH Gene in Culturable Rhizobia from Black Locust (Robinia pseudoacacia L.) Grown in Cadmium-Contaminated Soils

(1) Background: Rhizobia can promote plant growth by providing essential nutrients such as NH4+ and PO43-; thus, rhizobia that can tolerate the stress of heavy metals will be conducive to the phytoremediation of heavy-metal-contaminated soils. Therefore, understanding the dominant heavy-metal-tolerant rhizobia that can be cultured is important for the establishment of an indigenous legume-rhizobia symbiotic remediation system; (2) Methods: Here, we investigated nifH gene diversity in culturable rhizobia from black locust (Robinia pseudoacacia L.) grown in cadmium (Cd)-contaminated soils using high-throughput sequencing.; (3) Results: A total of 16 genera and 26 species were identified from the cultures of root nodules of black locust exposed to five Cd levels. Cadmium did not show a significant effect on the abundance, diversity, and evenness of the culturable rhizobia community. However, Cd significantly affected the community structure of culturable rhizobia containing nifH. Mesorhizobium, Sinorhizobium, and Rhizobium were the absolute dominant genera present in the cultures under five Cd treatments. Additionally, Cd significantly affected the relative abundance of Azohydromonas, Xanthobacter, Skermanella, Bradyrhizobium, Paenibacillus, and Pseudacidovorax in the cultures. Soil pH, total Cd, DTPA-Cd, and C/H ratio were the significant factors on culturable rhizobia community.; (4) Conclusions: Cd showed a negative effect on nifH gene community of culturable rhizobia from black locust, which will provide insight into the selection of excellent strains that can promote phytoremediation of heavy-metal-contaminated soils.

Two strains of cadmium (Cd)-resistant bacteria isolated from soils and their ability to promote oilseed rape (Brassica juncea L.) to grow and absorb Cd in soils

As a highly toxic, mobile, and persistent heavy metal, cadmium (Cd) in soils is becoming a crucial environmental problem. Most of classical physical and chemical remediation measures for Cd-contaminated soils possibly cause some dangers to soil structure and characteristics and potential secondary pollution, however, Cd-resistant microbial which can sequestrate Cd by releasing extracellular polymeric substances (EPS) capable of ion exchange, coordination, and adsorption and improve plant growth should be favorable for remediation of Cd-contaminated soils due to being environmentally friendly and cost-effective. Therefore, the plant-microbe combination is becoming a priority option in the remediation of Cd-contaminated soils. Here, we isolated two strains of Cd-resistant bacteria from soils and investigated the ability of the two strains to promote growth of oilseed rape (Brassica juncea L.) and Cd uptake by the plants. Citrobacter farmeri and Cupriavidus gilardii were isolated from soils via culture media containing 30 and 50 mg/L Cd, respectively, which could release EPS including proteins, polysaccharide, and DNA. The EPS from C. gilardii was significantly higher than that from C. farmeri, and the proportion of protein in EPS was the highest for two strains. Additionally, two strains secreted indole-3-acetic acid (IAA) and could solubilize phosphorus, and the ability of C. gilardii to secret IAA was significantly higher than that of C. farmeri. The pot experiment indicated that C. farmeri and C. gilardii significantly enhanced oilseed rape biomass (by 81.99% and 76.57%, respectively), C and N contents, Cd accumulation in plants by 229.03% and 264.63%, respectively, and remediation efficiency at 40 days after emergence (flowering stage). However, the difference in promoting plant growth and Cd uptake and phytoremediation efficiency of Cd-contaminated soils between the two strains was not significant. Overall, C. farmeri and C. gilardii isolated from soils might be promising strains in enhancing phytoremediation of Cd-contaminated soils.

Edaphic factors mediate the response of nitrogen cycling and related enzymatic activities and functional genes to heavy metals: A review

Soil nitrogen (N) transformations control N availability and plant production and pose environmental concerns when N is lost, raising issues such as soil acidification, water contamination, and climate change. Former studies suggested that soil N cycling is chiefly regulated by microbial activity; however, emerging evidence indicates that this regulation is disrupted by heavy metal (HM) contamination, which alters microbial communities and enzyme functions critical to N transformations. Environmental factors like soil organic carbon, soil texture, water content, temperature, soil pH, N fertilization, and redox status play significant roles in modulating the response of soil N cycling to HM contamination. This review examines how different HMs affect soil N processes, including N fixation, mineralization, nitrification, denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and immobilization, as well as microbial activities and functional genes related to soil N transformations. The review additionally outlines the impact of HMs on environmental degradation, including the risk of soil N losses (e.g., leaching, runoff, and gaseous emissions) and depletion of soil fertility, thus threatening the sustainability of the ecosystem. The effect of edaphic factors and fertilization on soil N cycling response to HM contamination was also examined. The effect of phytoremediation, a sustainable approach to remediate HM polluted soils, on N cycling was also reviewed. Thus, this review underscores the importance of increasing research and innovative strategies to combat HM pollution's effects to enhance soil health, boost crop yields, and protect soil stability and productivity.

Calcium oxide nanoparticles ameliorate cadmium toxicity in alfalfa seedlings by depriving its bioaccumulation, enhancing photosystem II functionality and antioxidant gene expression

Cadmium (Cd) is a highly toxic and carcinogenic pollutant that poses significant risks to living organisms and the environment, as it is absorbed by the plant roots and accumulates in different parts of crop during its production. A promising sustainable strategy to counteract these threats to use calcium oxide nanoparticles (CaO-NPs) as soil supplements in fodder crops. This approach has shown notable morpho-physiological and biochemical improvements under metal toxicity conditions. However, the specific mechanisms driving Cd tolerance, particularly at physio-biochemical level and antioxidant related genes expression in fodder crops including alfalfa remain unexplored. CaO-NPs supplementation can trigger various signaling pathways that lead to enhance the photosynthetic pigments formation, stomatal conductance, CO2 assimilation rate and quantum yield of photosystem II. In this study, we evaluated various doses of CaO-NPs (0, 25, 50, and 100 mg kg-1) for their efficacy in reducing Cd bioavailability and toxicity in alfalfa plants. Our results demonstrated that Ca2+ and Cd2+, which share the same ionic radius, compete for ion transport through channels. The small size and high availability of CaO-NPs facilitate their rapid translocation within plant tissues, reducing metal uptake by 61 % in shoots and 30 % in roots. Notably, application of CaO-NPs at 100 mg kg-1 significantly increased shoot length (44 %) and root length (35 %) as compared to Cd-treated control plants. The highest dose of CaO-NPs also improved photosynthetic efficiency and gas exchange attributes including gs, Tr, Pn and Ci by 66 %, 27 %, 33 % and Ci 21 %, respectively, compared with the Cd treated control. Moreover, CaO-NPs (at 100 mg kg-1) alleviated metal-induced oxidative stress by boosting antioxidant enzyme activities like superoxide dismutase (25 %) peroxidase (42 %), catalase (72 %) and ascorbate peroxidase (87 %) and diminishing reactive oxygen species (ROS) production when compared with sole Cd treatment. Scanning and transmission electron microscopy revealed that CaO-NPs positively impacted stomatal conductance and mitigated Cd toxicity in leaf ultrastructure. Additionally, the highest dose of CaO-NPs markedly upregulated the expression of antioxidant-related genes, MsCu/Zn SOD, MtPOD, MtCAT, and MtAPX in roots and shoots by 0.67 and 1.03 fold-change (FC), 0.61 and 0.53 FC, 0.54 and 0.88 FC, and 0.46 and 0.66 FC, respectively. In conclusion, CaO-NPs demonstrate significant potential for environmentally friendly mitigation of Cd stress in alfalfa by reducing its uptake, thereby supporting sustainable agriculture.

pH-Related Changes in Soil Bacterial Communities in the Sanjiang Plain, Northeast China

Soil bacteria are crucial components of terrestrial ecosystems, playing an important role in soil biogeochemical cycles. Although bacterial community diversity and composition are regulated by many abiotic and biotic factors, how soil physiochemical properties impact the soil bacteria community diversity and composition in wetland ecosystems remains largely unknown. In this study, we used high-throughput sequencing technology to investigate the diversity and composition of a soil bacterial community, as well as used the structural equation modeling (SEM) method to investigate the relationships of the soil's physicochemical properties (i.e., soil pH, soil organic carbon (SOC), total nitrogen (TN), ammonium nitrogen (NH4+N), electrical conductivity (EC) and nitrate nitrogen (NO3-N)), and soil bacterial community structures in three typical wetland sites in the Sanjiang Plain wetland. Our results showed that the soil physicochemical properties significantly changed the α and β-diversity of the soil bacteria communities, e.g., soil TN, NH4+N, NO3-N, and SOC were the main soil factors affecting the soil bacterial α-diversity. The soil TN and pH were the key soil factors affecting the soil bacterial community. Our results suggest that changes in soil pH indirectly affect soil bacterial communities by altering the soil nitrogenous nutrient content.

Adaptive response of flavonoids in Robinia pseudoacacia L. affected by the contamination of cadmium and elevated CO2 to arbuscular mycorrhizal symbiosis

As a key component in non-enzyme resistance system, flavonoids play a crucial role in the plant growth and defenses, which are significantly affected by biotic and abiotic factors such as fungi, bacteria, viruses, heavy metals, and atmospheric CO2. Arbuscular mycorrhizal fungi (AMF) play an important role in enhancing plant tolerance to adverse environments, which can significantly affect the synthesis of flavonoids by forming mycorrhizal symbionts with plant roots. However, few studies explored the combined effects of AMF, elevated CO2, and heavy metals on flavonoids in plants. Here, we investigated the adaptive response of flavonoids accumulation in Robinia pseudoacacia L. seedlings affected by the contamination of cadmium (Cd) and elevated CO2 to arbuscular mycorrhizal symbiosis. The results showed that G. mosseae decreased (p < 0.05) Cd content in leaves by 62.2% under elevated CO2. Moreover, G. mosseae colonization led to significant decreases in robinin, quercetin, kaempferol and acacetin by 17.4%, 11.1%, 15.5% and 23.1% under elevated CO2 + Cd, respectively. Additionally, G. mosseae down-regulated (p < 0.05) expression levels of phenylalanine ammonia-lyase (PAL) and chalcone synthase (CHS) genes under elevated CO2 + Cd, and CHS and uridine diphosphate flavonoid glucosyltransferase (UFGT) activities decreased (p < 0.05). Quercetin, kaempferol and acacetin showed positive (p < 0.05) correlation with PAL and CHS genes expression and PAL, CHS, and UFGT activities. Cadmium, C/N ratio, carotenoids, leaf biomass, total chlorophyll, P, and starch in leaves and G. mosseae colonization rate in roots influenced (p < 0.05) flavonoids content. Overall, G. mosseae reduced flavonoids synthesis by down-regulating gene expression levels and activities of key enzymes under elevated CO2 + Cd. The results improved our understanding of the regulation of AMF on non-enzymatic resistance of plants grown in heavy metal-contaminated soils under increasing atmospheric CO2 scenarios.

Correlation of heavy metals' exposure with the prevalence of coronary heart disease among US adults: findings of the US NHANES from 2003 to 2018

We sought to explore the association between heavy metal exposure and coronary heart disease (CHD) based on data from the US National Health and Nutrition Examination Survey (NHANES, 2003-2018). In the analyses, participants were all aged > 20 and had participated in heavy metal sub-tests with valid CHD status. The Mann-Kendall test was employed to assess the trends in heavy metals' exposure and the trends in CHD prevalence over 16 years. Spearman's rank correlation coefficient and a logistics regression (LR) model were used to estimate the association between heavy metals and CHD prevalence. 42,749 participants were included in our analyses, 1802 of whom had a CHD diagnosis. Total arsenic, dimethylarsonic acid, monomethylarsonic acid, barium, cadmium, lead, and antimony in urine, and cadmium, lead, and total mercury in blood all showed a substantial decreasing exposure level tendency over the 16 years (all Pfor trend < 0.05). CHD prevalence varied from 3.53 to 5.23% between 2003 and 2018. The correlation between 15 heavy metals and CHD ranges from - 0.238 to 0.910. There was also a significant positive correlation between total arsenic, monomethylarsonic acid, and thallium in urine and CHD by data release cycles (all P < 0.05). The cesium in urine showed a negative correlation with CHD (P < 0.05). We found that exposure trends of total arsenic, dimethylarsonic acid, monomethylarsonic acid, barium, cadmium, lead, and antimony in urine and blood decreased. CHD prevalence fluctuated, however. Moreover, total arsenic, monomethylarsonic acid, and thallium in urine all showed positive relationships with CHD, while cesium in urine showed a negative relationship with CHD.

Detrimental effects of Cd and temperature on rice and functions of microbial community in paddy soils

Heavy metal (HM) contamination and high environmental temperature (HT) are caused by anthropogenic activities that negatively impact soil microbial communities and agricultural productivity. Although HM contaminations have deleterious effects on microbes and plants; there are hardly any reports on the combined effects of HM and HT. Here, we reported that HT coupled with cadmium (Cd) accumulation in soil and irrigated water could seriously affect crop growth and productivity, alternatively influencing the microbial community and nutrient cycles of paddy soils in rice fields. We analyzed different mechanisms of plants and microflora in the rhizospheric region, such as plant rhizospheric nitrification, endophytes colonization, nutrient uptake, and physiology of temperature-sensitive (IR64) and temperature-resistant Huanghuazhan (HZ) rice cultivars against different Cd levels (2, 5 and 10 mg kg^-1) with rice plants grown under 25 °C and 40 °C temperatures. Consequently, an increment in Cd accumulation was observed with rising temperature leading to enhanced expression of OsNTRs. In contrast, a greater decline in the microbial community was detected in IR64 cultivar than HZ. Similarly, ammonium oxidation, root-IAA, shoot-ABA production, and 16S rRNA gene abundance in the rhizosphere and endosphere were significantly influenced by HT and Cd levels, resulting in a significant decrease in the colonization of endophytes and the surface area of roots, leading to a decreased N uptake from the soil. Overall, the outcomes of this study unveiled the novel effects of Cd, temperature, and their combined effect on rice growth and functions of the microbial community. These results provide effective strategies to overcome Cd-phytotoxicity on the health of endophytes and rhizospheric bacteria in Cd-contaminated soil by using temperature-tolerant rice cultivars.

Nitrification, denitrification, and related functional genes under elevated CO2 : A meta-analysis in terrestrial ecosystems

Global change may have profound effects on soil nitrogen (N) cycling that can induce positive feedback to climate change through increased nitrous oxide (N₂ O) emissions mediated by nitrification and denitrification. We conducted a meta-analysis of the effects of elevated CO₂ on nitrification and denitrification based on 879 observations from 58 publications and 46 independent elevated CO₂ experiments in terrestrial ecosystems. We investigated the effects of elevated CO₂ alone or combined with elevated temperature, increased precipitation, drought, and N addition. We assessed the response to elevated CO₂ of gross and potential nitrification, potential denitrification, and abundances of related functional genes (archaeal amoA, bacterial amoA, nirK, nirS, and nosZ). Elevated CO₂ increased potential nitrification (+28%) and the abundance of bacterial amoA functional gene (+62%) in cropland ecosystems. Elevated CO₂ increased potential denitrification when combined with N addition and higher precipitation (+116%). Elevated CO₂ also increased the abundance of nirK (+25%) and nirS (+27%) functional genes in terrestrial ecosystems and of nosZ (+32%) functional gene in cropland ecosystems. The increase in the abundance of nosZ under elevated CO₂ was larger at elevated temperature and high N (+62%). Four out of 14 two-way interactions tested between elevated CO₂ and elevated temperature, elevated CO₂ and increased precipitation, and elevated CO₂ and N addition were marginally significant and mostly synergistic. The effects of elevated CO₂ on potential nitrification and abundances of bacterial amoA and nirS functional genes increased with mean annual temperature and mean annual precipitation. Our meta-analysis thus suggests that warming and increased precipitation in large areas of the world could reinforce positive responses of nitrification and denitrification to elevated CO₂ and urges the need for more investigations in the tropical zone and on interactive effects among multiple global change factors, as we may largely underestimate the effects of global change on soil N₂ O emissions.

Effect of gradual increase of atmospheric CO2 concentration on nitrification potential and communities of ammonia oxidizers in paddy fields

Currently, the influence of elevated atmospheric CO₂ concentration (eCO₂) on ammonia oxidation to nitrite, the rate-limiting step of nitrification in paddy soil, is poorly known. Previous studies that simulate the effect of eCO₂ on nitrification are primarily based on an abrupt increase of atmospheric CO₂ concentration. However, paddy ecosystems are experiencing a gradual increase of CO₂ concentration. To better understand how the nitrification potential, abundance and communities of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) respond to eCO₂ in paddy ecosystems, a field experiment was conducted using the following two treatments: a gradual increase of CO₂ (EC, increase of 40 ppm per year until 200 ppm above ambient) and ambient CO₂ (CK). The results demonstrated that the EC treatment significantly (P < 0.05) stimulated the soil potential nitrification rate (PNR) at the jointing and milky stages, which increased by 127.83% and 27.35%, respectively, compared with CK. Furthermore, the EC treatment significantly (P < 0.05) stimulated the AOA and AOB abundance by 56.60% and 133.84%, respectively, at the jointing stage. Correlation analysis showed that the PNR correlated well with the abundance of AOB (R² = 0.7389, P < 0.001). In addition, the EC treatment significantly (P < 0.05) altered the community structure of AOB, while it had little effect on that of AOA. A significant difference in the proportion of Nitrosospira was observed between CO₂ treatments. In conclusion, the gradual increase of CO₂ positively influenced the PNR and abundance of ammonia oxidizers, and AOB could be more important than AOA in nitrification under eCO₂.

Effect of arbuscular mycorrhizal fungi (Glomus mosseae) and elevated air temperature on Cd migration in the rhizosphere soil of alfalfa

Cadmium (Cd) migration in the rhizosphere soil is easily affected by plants and microorganisms. Global warming significantly affects plant growth, and arbuscular mycorrhizal fungi (AMF) can chelate heavy metals by mycelium, cell wall components, and mycelial secretion. Here, we investigated the regulation of Glomus mosseae on Cd migration in the rhizosphere soil of alfalfa under elevated temperature (ET, + 3 °C). Elevated temperature significantly decreased G. mosseae colonization rate in the roots by 49.5% under Cd exposure. Under ET + G. mosseae + Cd relative to ET + Cd, the contents of free amino acids, total and easily extractable glomalin-related soil protein (GRSP), and root Cd increased significantly; however, the changes in DTPA-Cd in the rhizosphere soil and Cd in the shoots were insignificant. In addition, G. mosseae colonization enhanced the bioconcentration factor of Cd in the roots and the total removal rate of Cd in the rhizosphere soil by 63.4% and 16.3%, respectively, under ET + Cd. However, the changes in the expression of iron-regulated transport 1 (IRT1) and natural resistance-associated macrophage protein 1 genes were insignificant under ET + G. mosseae + Cd relative to ET + Cd. In summary, temperature and G. mosseae significantly affected Cd fate in the rhizosphere soil, and IRT1 gene and rhizosphere soil pH, N, and C/N ratio were significant factors influencing Cd migration. Additionally, G. mosseae improved the remediation efficiency of Cd-contaminated soils by alfalfa under ET. The results will help us understand the regulation of AMF on the phytoremediation of heavy metal-contaminated soils under global warming scenarios.

Ecological restoration performance enhanced by nano zero valent iron treatment in constructed wetlands under perfluorooctanoic acid stress

Perfluorooctanoic acid (PFOA) of widespread use can enter constructed wetlands (CWs) via migration, and inevitably causes negative impacts on removal efficiencies of conventional pollutants due to its ecotoxicity. However, little attention has been paid to strengthen performance of CWs under PFOA stress. In this study, influences of nano zero valent iron (nZVI), which has been demonstrated to improve nutrients removal, were explored after exemplifying threats of PFOA to operation performance in CWs. The results revealed that 1 mg/L PFOA suppressed the nitrification capacity and phosphorus removal, and nZVI distinctly improved the removal efficiency of ammonia and total phosphorus in CWs compared to PFOA exposure group without nZVI, with the maximum increases of 3.65 % and 16.76 %. Furthermore, nZVI significantly stimulated dehydrogenase (390.64 % and 884.54 %) and urease (118.15 % and 246.92 %) activities during 0-30 d and 30-60 d in comparison to PFOA group. On the other hand, nitrifying enzymes were also promoted, in which ammonia monooxygenase increased by 30.90 % during 0-30 d, and nitrite oxidoreductase was raised by 117.91 % and 232.10 % in two stages. Besides, the content of extracellular polymeric substances (EPS) under nZVI treatment was 72.98 % higher than PFOA group. Analyses of Illumina Miseq sequencing further certified that nZVI effectively improved the community richness and caused the enrichment of microorganisms related to nitrogen and phosphorus removal and EPS secreting. These results could provide valuable information for ecological restoration and decontamination performance enhancement of CWs exposed to PFOA.