Cadmium influences the litter decomposition of Solidago canadensis L. and soil N-fixing bacterial communities

Decomposition of exotic versus native aquatic plant litter in a lake littoral zone: Stoichiometry and life form analyses

The decomposition rates and stoichiometric characteristics of many aquatic plants remain unclear, and our understanding of material flow and nutrient cycles within freshwater ecosystems is limited. In this study, an in-situ experiment involving 23 aquatic plants (16 native and 7 exotic species) was carried out via the litter bag method for 63 days, during which time the mass loss and nutrient content (carbon (C), nitrogen (N), and phosphorus (P)) of plants were measured. Floating-leaved plants exhibited the highest decomposition rate (0.038 ± 0.002 day-1), followed by submerged plants and free-floating plants (0.029 ± 0.002 day-1), and emergent plants had the lowest decomposition rate (0.019 ± 0.001 day-1). Mass loss by aquatic plants correlated with stoichiometric characteristics; the decomposition rate increased with an increasing P content and with a decreasing C content, C:N ratio, and C:P ratio. Notably, the decomposition rate of submerged exotic plants (0.044 ± 0.002 day-1) significantly exceeded that of native plants (0.026 ± 0.004 day-1), while the decomposition rate of emergent exotic plants was 55 ± 4 % higher than that of native plants. The decomposition rates of floating-leaved and free-floating plants did not significantly differ between the native and exotic species. During decomposition, emergent plants displayed an increase in C content and a decrease in N content, contrary to patterns observed in other life forms. The P content decreased for submerged (128 ± 7 %), emergent (90 ± 5 %), floating-leaved (104 ± 6 %), and free-floating plants (32 ± 6 %). Exotic plants released more C and P but accumulated more N than did native plants. In conclusion, the decomposition of aquatic plants is closely linked to litter quality and influences nutrient cycling in freshwater ecosystems. Given these findings, the invasion of the littoral zone by submerged and emergent exotic plants deserves further attention.

Soil Cd increased the leaf litter Cd remains of Solanum nigrum and Solanum lycopersicum

Plant litter decomposition is a natural pathway of heavy metal cycling in soil ecosystems, but the dynamics of heavy metal release during litter decomposition are relatively poorly understood. The purpose of this study was to investigate the effects of species, soil fauna and soil Cd addition on litter decomposition and Cd release dynamics. Therefore, we selected two plants, Solanum nigrum and S. lycopersicum with large differences in Cd accumulation capacity. First, they were enriched with Cd during the growing period and leaf litter was harvested after 6 months of pretreatment. Then, the decomposition of leaf litter was conducted with or without soil Cd and Eisenia fetida through lab pot tests. Our results showed that leaf litter Cd led to a significant decrease in litter decomposition rate (K value), with a maximum decrease of 32.1% in S. nigrum and 30.1% in S. lycopersicum. We observed that the presence of E. fetida significantly increased K value, but the effect was similar in the +leaf Cd treatment and the -leaf Cd treatment, both for S. nigrum and S. lycopersicum. Interestingly, the litter Cd concentration did not decrease during decomposition, but showed an increasing trend, especially for S. nigrum in the +soil Cd treatment. Moreover, the litter Cd remains was higher in the +soil Cd treatment compared to the -soil Cd treatment for both S. nigrum and S. lycopersicum, no matter whether with or without E. fetida. This result suggests that the Cd may be transferred from soil to litter, thus increasing the litter Cd remains. Overall, our study shows that leaf litter Cd slowed down the carbon cycling in ecosystems. In addition, the release of litter Cd has a lag, and the litter has a certain adsorption capacity for soil Cd, which intensifies the harm to the ecology during litter transfer.

Root microbiota of tea plants regulate nitrogen homeostasis and theanine synthesis to influence tea quality

The flavor profile of tea is influenced not only by different tea varieties but also by the surrounding soil environment. Recent studies have indicated the regulatory role of soil microbes residing in plant roots in nutrient uptake and metabolism. However, the impact of this regulatory mechanism on tea quality remains unclear. In this study, we showed that a consortium of microbes isolated from tea roots enhanced ammonia uptake and facilitated the synthesis of theanine, a key determinant of tea taste. Variations were observed in the composition of microbial populations colonizing tea roots and the rhizosphere across different seasons and tea varieties. By comparing the root microorganisms of the high-theanine tea variety Rougui with the low-theanine variety Maoxie, we identified a specific group of microbes that potentially modulate nitrogen metabolism, subsequently influencing the theanine levels in tea. Furthermore, we constructed a synthetic microbial community (SynCom) mirroring the microbe population composition found in Rougui roots. Remarkably, applying SynCom resulted in a significant increase in the theanine content of tea plants and imparted greater tolerance to nitrogen deficiency in Arabidopsis. Our study provides compelling evidence supporting the use of root microorganisms as functional microbial fertilizers to enhance tea quality.

Heavy Metal Contamination Alters the Co-Decomposition of Leaves of the Invasive Tree Rhus typhina L. and the Native Tree Koelreuteria paniculata Laxm

Invasive and native plants can coexist in the same habitat; however, the decomposition process may be altered by the mixing of invasive and native leaves. Heavy metal contamination may further alter the co-decomposition of both leaf types. This study evaluated the effects of two concentrations (35 mg·L^-1 and 70 mg·L^-1) and three types (Pb, Cu, and combined Pb + Cu) of heavy metal contamination on the co-decomposition of leaves of the invasive tree Rhus typhina L. and the native tree Koelreuteria paniculata Laxm, as well as the mixed effect intensity of the co-decomposition of the mixed leaves. A polyethylene litterbag experiment was performed over six months. The decomposition coefficient of the two trees, mixed effect intensity of the co-decomposition, soil pH and enzymatic activities, soil bacterial alpha diversity, and soil bacterial community structure were determined. A high concentration of Pb and combined Pb + Cu significantly reduced the decomposition rate of R. typhina leaves. A high concentration of Pb or Cu significantly reduced the decomposition rate of the mixed leaves. In general, R. typhina leaves decomposed faster than K. paniculata leaves did. There were synergistic effects observed for the co-decomposition of the mixed leaves treated with combined Pb + Cu, regardless of concentration, but there were antagonistic effects observed for the co-decomposition of the mixed leaves treated with either Pb or Cu, regardless of concentration. A high concentration of Pb or Cu may increase antagonistic effects regarding the co-decomposition of mixed-leaf groups. Thus, heavy metal contamination can significantly affect the intensity of the mixed effect on the co-decomposition of heterogeneous groups of leaves.

Cyperus rotundus L. drives arable soil infertile by changing the structure of soil bacteria in the rhizosphere, using a maize field as an example

Rhizosphere microorganisms can greatly affect plant growth, especially the plant growth-promoting rhizobacteria (PGPR), which can improve plant root development and growth because they contain various biological functions including nitrogen fixation, phosphate solubilization, and phytosiderophore production. This study demonstrates that Cyperus rotundus L. is capable of developing and forming complex underground reproductive systems at arbitrary burial depths and cutting modes due to its extremely strong multiplication and regeneration ability. With the densities of C. rotundus increasing, the abundance of PGPR, soil enzymes invertase and urease, the nutrient contents of the field soil, and maize quality were impacted. Notably, more abundance of PGPR-most notably, the nitrogen-fixing microorganisms (NFMs) such as Azospirillum, Burkholderia, Mycobacterium, and Rhizobium-enriches in the rhizosphere of C. rotundus than in that of maize. In addition, the activities of soil enzymes invertase (S_SC) and urease (S_SU) were significantly higher in its rhizosphere than in maize, further proving that more NFMs enrich the C. rotundus rhizosphere. The nutrient contents of the field soil of TN, SOM, and SOC were reduced, indicating that the presence of C. rotundus made the soil infertile. Hence, these pieces of evidence indicate that C. rotundus may drive the field soil infertile as reflected by reduced soil nutrients via altering rhizosphere bacteria community structure.

Effect of poplar ecological retreat project on soil bacterial community structure in Dongting Lake wetland

As an important environmental protection measure, the Poplar Ecological Retreat (PER) project aims to restore the ecology of the Dongting Lake (DL, China’s second largest freshwater lake) wetland. And its ecological impact is yet to be revealed. This study selected soil bacterial community structure (BCS) as an indicator of ecological restoration to explore the ecological impact of PER project on DL wetland. Soil samples were collected from reed area (RA, where poplar had never been planted, as the end point of ecological restoration for comparison in this study), poplar planting area (PA), poplar retreat for 1-year area (PR1A) and poplar retreat for 2 years area (PR2A), then their soil properties and BCS were measured. The results showed that the PER project caused significant changes in soil properties, such as the soil organic matter (SOM) and moisture, and an increase in the diversity and richness index of soil BCS. The Shannon-wiener index of RA, PA, PR1A and PR2A were 3.3, 2.63, 2.75 and 2.87, respectively. The number of operational taxonomic units (OTUs) changed similarly to the Shannon-wiener index. The Pearson correlation analysis and redundancy analysis (RDA) showed that the poplar retreat time, SOM and moisture content were the main factors leading to the increase of BCS diversity. All of these indicated that after the implementation of the PER project, the ecology of the lake area showed a trend of gradual recovery.

Acid deposition at higher acidity weakens the antagonistic responses during the co-decomposition of two Asteraceae invasive plants

Co-invasion by two invasive plant species (IPS) can occur in the same habitat. Diversified acid deposition may change the co-invasion process by altering litter decomposition and plant-soil feedback signalling. This study examined the co-decomposition of two Asteraceae IPS (Solidago canadensis L. and Bidens pilosa L.) on litter decomposition rate, soil enzyme activities, and soil N-fixing bacterial communities under diversified acid deposition (mixed acid deposition at pH 5.6 and at pH 4.5, sulfuric acid at pH 4.5, and nitric acid at pH 4.5). B. pilosa litter degraded faster than S. canadensis litter. Acid deposition at higher acidity accelerated the decomposition rate of both pure S. canadensis litter and the equally mixed litters from the two Asteraceae IPS. Antagonistic responses may occur during the co-decomposition of the two Asteraceae IPS with mixed acid deposition, regardless of the pH, as well as with nitric acid deposition at pH 4.5; in contrast, there may be neutral responses for the co-decomposition process with sulfuric acid at pH 4.5. The type of acid deposited may be one of the key factors affecting the intensity of the mixing effect affecting the co-decomposition. Acid deposition at higher acidity weakened the antagonistic responses for the co-decomposition of the two Asteraceae IPS compared with the response to weak acids. Together, these results indicate that acid deposition at higher acidity could facilitate the co-invasion of the two Asteraceae IPS mainly through accelerated litter decomposition as well as weakened antagonistic responses for co-decomposition.

Cadmium hyperaccumulation as an inexpensive metal armor against disease in Crofton weed

Invasive plants readily invade metal-contaminated areas. The hyperaccumulation of toxic heavy metals is not an uncommon feature among plant species. Although several hypotheses were proposed to explain this phenomenon, it is currently unclear how hyperaccumulation may benefit plants. The invasive Crofton weed (Ageratina adenophora) is a known hyperaccumulator of chromium and lead. We previously found that the species can also hyperaccumulate cadmium. The role of phytoaccumulation in defense to pathogen attack is unclear. We inoculated A. adenophora plants with a common generalist pathogen (Rhizoctonia solani) to test its resistance under cadmium treatment. We found evidence that cadmium hyperaccumulation reduced pathogen infection in A. adenophora. Our findings indicate elemental defense is highly cost efficient for hyperaccumulators inhabiting metal-contaminated sites, where plants were only modestly affected by cadmium. The reduction in pathogen damage conferred by cadmium was relatively high, particularly under lower cadmium levels. However, the benefits at higher levels may be capped. Elemental defense may be a key mechanism for plant invasion into polluted sites, especially in regions with widespread industrial activity. Our study highlights the importance of testing different metal concentrations when testing plant resistance and the importance of considering enemy attack when selecting plants for phytoremediation.

Environmental toxicology and ecotoxicology: How clean is clean? Rethinking dose-response analysis

Global agendas for sustaining clean environments target remediation of multimedia contaminants, but how clean is clean? Environmental Toxicology and Ecotoxicology focus on issues concerning "clean". However, the models used to assess the effects of environmental multimedia on individual living organisms and communities or populations in Environmental Toxicology and Ecotoxicology may fail to provide reliable estimates for risk assessment and optimize health. Recent developments in low-dose effects research provide a novel means in Environmental Toxicology and Ecotoxicology to improve the quality of hazard and risk assessment.

Indigenous plant species and invasive alien species tend to diverge functionally under heavy metal pollution and drought stress

The functional similarity between indigenous plant species (IPS) and invasive alien species (IAS) governs the invasion process of successful IAS because IPS and coexisting IAS suffer alike or even same ecological selection pressures. The aggravated condition created by heavy metal pollution (HMP) and drought stress may generate a noticeable impact on the invasive competitiveness and invasion process of IAS possibly via the variations in the functional similarity between IPS and IAS. Consequently, it is necessary to illumine the functional similarity between IPS and IAS under HMP and drought stress to clarify the mechanisms underlying the successful invasion of IAS. This study aims to estimate the functional similarity between IPS Amaranthus tricolor L. and IAS A. retroflexus L. under the condition with the alone and combined effects of HMP with different kinds (e.g., Cu and Pb) and drought stress [simulated by polyethylene glycol-6000 (PEG) solution]. HMP notably declines A. tricolor growth but has no remarkable effect on A. retroflexus growth. A. retroflexus displays a strong competitive intensity than A. tricolor under HMP. Further, HMP makes a greater stress intensity on A. tricolor growth than A. retroflexus growth. Therefore, HMP can accelerate A. retroflexus invasion. A. retroflexus displays a poor competitive intensity under drought stress. Thus, drought stress can hinder A. retroflexus invasion. However, drought stress causes a greater stress intensity on A. tricolor growth than A. retroflexus growth. Thus, the continued drought stress may converse the adverse effects of drought stress on A. retroflexus invasion potentially. The two Amaranthus species tend to diverge functionally under the combined HMP and drought stress. Further, A. retroflexus shows a strong competitive intensity than A. tricolor under the combined HMP and drought stress. Moreover, the combined HMP and drought stress induces a greater stress intensity on A. tricolor growth than A. retroflexus growth. Thus, the combined HMP and drought stress can facilitate A. retroflexus invasion. Meanwhile, the competitiveness for sunlight acquisition and leaf photosynthetic capacity may play a key role in the successful invasion of A. retroflexus under the combined HMP and drought stress.

Heavy metal pollution improves allelopathic effects of Canada goldenrod on lettuce germination

Large amounts of heavy metals have been released into the environment. Thus, the allelopathic effects of invasive alien species on the germination performance of co-occurring indigenous species may be altered or even heightened with the rapid growth in heavy metal pollution. This study evaluated the impacts of Canada goldenrod (Solidago canadensis L.) leaf extracts at concentrations of 0, 10 or 20 gl 1 on the germination of lettuce under different forms of heavy metal pollution (Cu2+ , Pb2+ or a combination of Cu2+ and Pb2+ ; 35 mgl 1) during incubation in Petri dishes for 10 days. Goldenrod leaf extracts (high concentration) reduced growth of aboveground and belowground parts of lettuce as well as competition for light and soil nutrients. However, low concentrations of goldenrod leaf extracts dramatically improved growth of lettuce roots, competition for light, soil nutrient availability, leaf photosynthetic area and growth competitiveness. The combination of goldenrod leaf extracts and heavy metal pollution was synergistic on most lettuce germination parameters, probably because high concentrations of goldenrod leaf extracts together with heavy metal pollution had a synergistic negative impact on lettuce germination. Consequently, increased levels of heavy metal pollution may favour invasion of invasive alien species while largely suppressing germination of indigenous species.

Litter decomposition process dramatically declines the allelopathy of Solidago canadensis L. on the seed germination and seedling growth of Lactuca sativa L

A variety of invasive alien species (IAS) can trigger distinct allelopathy on the seed germination and seedling growth (SGeSGr) of native plant species (NPS) mainly through the released allelochemicals. However, the decomposition process of IAS litters may affect their allelopathy on SGeSGr of NPS because part of the allelochemicals will be released during the litter decomposition process, especially under heavy metal pollution. This study focuses on the impacts of the litter decomposition process of the notorious IAS Solidago canadensis L. on its allelopathy on SGeSGr of NPS Lactuca sativa L. under cadmium (Cd) pollution. The decomposition process signally declines the allelopathy of S. canadensis litters on SGeSGr of L. sativa likely because partial allelochemicals in S. canadensis litters discharged during the decomposition process. Cd addition noticeably rises the allelopathy of S. canadensis litters on SGeSGr of L. sativa probably because Cd can reduce plant growth largely via the improved lipid membrane permeability and the induced reactive oxygen molecules which is unfavorable to plant cell metabolism. This phenomenon may also be attributed to the weak acid properties of one of the most abundant allelochemicals in S. canadensis litters, i.e., phenolics (particularly polyphenols), can improve the solubility and the toxicity of Cd.

Erigeron annuus (L.) Pers. and Solidago canadensis L. antagonistically affect community stability and community invasibility under the co-invasion condition

The successful invasion of one invasive alien plant (IAP) can generate a favorable habitat in the invaded communities that beneficial to the successful invasion of the subsequent IAP. Advanced variations in the species number of IAP have the potential to alter the functional similarity and dissimilarity between IAP and co-existing native plant species (NPS), plant taxonomic diversity, plant functional diversity, community stability, and community invasibility. This study aims to evaluate the effects of the co-invasion of two notorious IAP, Erigeron annuus (L.) Pers. and Solidago canadensis L., on the functional similarity and dissimilarity between IAP and co-existing NPS, plant taxonomic diversity, plant functional diversity, community stability, and community invasibility in East China by using a comparative study. Results presented that: (I) IAP and co-existing NPS tend to converge functionally under E. annuus invasion and the functional similarity between IAP and co-existing NPS under E. annuus invasion supports the habitat filtering; (II) IAP and co-existing NPS tend to diverge functionally under S. canadensis invasion and the co-invasion condition and the functional dissimilarity between IAP and co-existing NPS under S. canadensis invasion and the co-invasion condition supports the niche differentiation; (III) plant taxonomic diversity was dramatically reduced under invasion condition, especially under S. canadensis invasion; (IV) Mason's α functional diversity was remarkably elevated under S. canadensis invasion and the co-invasion condition; (V) E. annuus and S. canadensis antagonistically affect community stability and community invasibility under the co-invasion condition compared with their independent invasion.