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Characterization of soil dissolved organic matter (DOM) and understanding of the interactions between soil microbial communities and DOM molecules along elevation gradients in alpine ecosystems are still limited. To unravel these interactions and how they change along alpine elevation gradients, we sampled topsoil in the Sygera Mountains (Tibet, China) at elevations between 3800 and 4600 m. The molecular characteristics of soil DOM were determined using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and soil microbial composition was identified by high-throughput sequencing. Among the seven components of DOM, the lignins/CRAM (carboxyl-rich alicyclic molecules)-like structure dominated at all elevations, followed by tannins, while the relative abundance of unstable substances, including lipids, aliphatic/protein, and carbohydrates, was lower. As elevation increased, the molecular diversity, degree of oxidation, aromaticity, and unsaturation of soil DOM decreased. The abundance and diversity of soil bacteria and fungi also generally decreased with elevation. Both bacteria and fungi play an important role in the degradation of DOM molecules, but bacteria appear to have greater degradation ability. Among them, Proteobacteria and Bacteroidetes mainly promote the degradation of lignins/CRAM-like structure molecules, while Basidiomycota mainly degrade more unstable substrates. Co-occurrence network analysis revealed complex correlations between specific microbial groups and DOM molecules. Our results suggest that more active cycling of soil DOM could occur in alpine ecosystems due to climate warming, as the result of increased vegetation productivity and litter input in response to rising temperature promoting the relative abundance of microbial groups capable of degrading lignins/CRAM-like structures in soil DOM.

Background Under natural conditions, soil nutrients are heterogeneously distributed, and plants have developed adaptation strategies to efficiently forage patchily distributed nutrient. Most previous studies examined either patch strength or patch size separately and focused mainly on root morphological plasticity (increased root proliferation in nutrient-rich patch), thus the effects of both patch strength and size on morphological and physiological plasticity are not well understood. In this study, we examined the foraging strategy of Neyraudia reynaudiana (Kunth) Keng ex Hithc, a pioneer grass colonizing degraded sites, with respect to patch strength and size in heterogeneously distributed phosphorus (P), and how foraging patchily distributed P affects total plant biomass production. Plants were grown in sand-culture pots divided into ½, ¼, 1/6 compartments and full size and supplied with 0 + 0/30, 0 + 7.5/30 and 7.5 + 0/30 mg P/kg dry soil as KH2PO4 or 0 + 15/15, 0 + 18.5/ 18.5, 7.5 + 15/15 mg kg − 1 in the homogenous treatment. The first amount was the P concentration in the central region, and that the second amount was the P concentration in the outer parts of the pot. Results After 3 months of growth under experimental conditions, significantly (p Conclusion The result demonstrates that root morphological and physiological plasticity are important adaptive strategies for foraging patchily distributed P and the former is largely determined by patch strength and size. The results also establish that foraging patchily distributed P resulted in increased total plant biomass production compared to homogeneous P distribution.

Cadmium (Cd) is one of the most toxic environmental pollutants affecting the growth and reproduction of various plants. Analysis of the biological adaptation and tolerance mechanisms of the hyperaccumulator Erigeron annuus to Cd stress may help identify new plant species for phytoremediation and in optimizing the process. This study is to the first to analyze the molecular composition and diversity of dissolved organic matter (DOM) secreted by roots using FT-ICR MS, and multiple physiological and biochemical indexes of E. annuus seedlings grown in solutions containing 0–200 Cd μmol L−1. The results showed that E. annuus had strong photosynthetic adaptation and protection ability under Cd stress. Cd was immobilized or compartmentalized by cell walls and vacuoles in the plant, thus alleviating Cd stress. Activation of anti-oxidation defense mechanisms also played an important role in alleviating or eliminating Cd toxicity in E. annuus. High Cd stress promoted production of a higher proportion of new molecules in DOM secreted by E. annuus roots compared to low Cd stress. DOM secreted by roots contributed to plant resistance to Cd-induced stress via producing more carbohydrates, aromatic structures and tannins. Results indicate the mechanisms underpinning the potential use of E. annuus as a phytoremediator in environments with moderate Cd pollution.

To investigate the effects of biochar addition (1 or 3%) to the soil of a China fir plantation with or without litter, we conducted a 90-day incubation experiment. We also studied the C and N dynamics and the microbial community structure of the soil. In soil without litter, the application of biochar at a rate of 3% significantly decreased CO2 emissions, while addition of 1% biochar had no effect. Biochar application did not affect the net N mineralization rate but significantly reduced the NH4+ concentration after 90 days. In litter-enriched soil, biochar application had no significant effect on total CO2 emissions; however, application of 3% biochar significantly reduced the net N mineralization rate. Biochar application to soil with or without litter immediately reduced the dissolved organic carbon (DOC) concentration independent of the application rate, which was primarily due to sorption of DOC by the biochar. Phospholipid fatty acid analysis demonstrated that both concentrations of added biochar to soil (with or without litter) altered the soil microbial community structure at the end of incubation, although the effect of biochar was not as strong as the effect of time or litter application. The effect of biochar addition alone on microbial community structure was inconsistent over time. Litter added to soil significantly increased fungi and reduced Gram-positive bacteria. In the presence of litter, biochar applied at both 1% and 3% significantly increased (p

Pogonatherum crinitum (Thunb.) Kunth is a lead (Pb) tolerant, perennial grass and a hyperaccumulator of Pb based on field surveys and greenhouse experiments. However, only few attempts have been made to understand the response mechanisms of P. crinitum to high Pb concentrations. In this study, we measured leaf chlorophyll fluorescence, antioxidative enzyme activities, organic acid contents, and Pb concentrations of P. crinitum grown in soil containing four concentrations of Pb (0, 500, 1000, and 2500 mg kg−1, respectively) for a period of three months. Our results indicated that chlorophyll a and total chlorophyll concentrations were significantly lower in Pb treated plants compared to the control, while chlorophyll b concentrations were not significantly different from those in the control. Furthermore, Pb stress increased the photosystem II (PSII) maximum photochemical efficiency. The activities of antioxidant enzymes such as superoxide dismutase (SOD) and peroxidase (POD), and organic acid contents such as oxalic and malic acids in roots were significantly increased by Pb treatments. Furthermore, the malondialdehyde content in leaves was significantly reduced by higher Pb concentration treatments. The Pb concentration was significantly increased by Pb treatments, and transfer coefficient exceeded 1 (Pb2500 treatment). These results suggest that increase in light use efficiency, scavenging reactive oxygen by strengthening anti-oxidative enzyme activity, and complexation of Pb with organic acids may play important roles in alleviating the toxicity of Pb in P. crinitum.

Dissolved Organic Matter (DOM) affects the fundamental biogeochemical processes in soil including circulation and distribution of carbon and nutrients. Likewise, soils are hotspots of microbial diversity, supporting their ability to provide key soil functions and ecosystem services. Detailed understanding of the interaction between DOM chemical diversity and soil microbial community is essential to predict the direction of C in soil after vegetation restoration. In this study we have characterized the chemical diversity of soil DOM, evaluated the associated changes of the soil microbial community composition and determined the specific relationship between DOM and microbial community composition. The results indicated that the unsaturated degree, obstinacy, oxidation degree, and average molecular mass of soil DOM increased with the duration of vegetation restoration. DOM with low double-bond equivalents (DBE) was less stable and was more likely to undergo transformation reactions to produce persistent organic materials with high carbon and high double-bond equivalents. The relative abundance of soil bacteria and fungi showed different trends in response to vegetation restoration efforts which may be related to the change of soil chemical composition as suggested by redundancy analysis. In addition, network analysis identified complex connectivity and strong interaction between the soil microbial community and DOM. Overall, this study has important guiding and prediction significance for the relationship between DOM and soil microbial community during vegetation restoration. Moreover, it provides a theoretical basis for the quantitative evaluation of the ecological restoration effect to alleviate soil and water loss on the Loess Plateau.

Additional file 1. Supplementary material.

Plants used for phytoremediation of contaminated soil are often enriched in certain metals present in the soil. However, the role of the inherent metal content of the plants on their recycling by hydrothermal liquefaction (HTL) has not been considered in previous studies. The present study showed that Rhus chinensis plants grown in highly Pb-polluted soil can release inherent metals (such as Pb, K, Ca, Na, and Mg) into the HTL solution, further enhancing the production of formic and acetic acids and decreasing the yield of levulinic acid. Theoretical calculations using HTL reactions of model compounds showed that a low Pb content could enhance production of levulinic and formic acids via catalysis of the rehydration reaction for 5-(hydroxymethyl)furfural, while a high Pb content promoted the decomposition of levulinic acid to acetic acid. Fourier transform ion cyclotron resonance mass spectrometry analysis confirmed that Pb2+ preferentially promoted the depolymerization of macromolecular compounds with th...

Vegetation restoration greatly alters soil phosphorus (P) fractions and microbial community diversity, but the response of soil P dynamics to microbial activity, particularly to that of phosphorus-solubilizing bacteria (PSB), remains unclear. This study aimed to evaluate the effect of vegetation restoration on the content of various P fractions and by extension on the diversity of PSB. We also attempted to discover the role of PSB in promoting the availability of P in vegetation restoration chronosequence. According to the vegetation restoration sequence, soil samples were collected from bare land and forests stands of 6, 10, 34, and 80 years old. The PSB was detected by phoD gene function and the soil P fractions were determined by the Hedley sequential fractionation method as modified by Tiessen and Moir. We found that vegetation restoration effectively improved soil available P content, but there was no significant difference in soil available P content in 80-year and 34-year stands. Residual-P accounted for the largest proportion in all stages of vegetation restoration, up to 55%. With stand development, the diversity of soil PSB increased and reached its maximum in R34. The rare PSB groups greatly promoted the increase of PSB abundance. Proteobacteria was the main soil PSB expressing a large amount of acid phosphatase in the whole vegetation restoration sequence, which played an important role in the mineralization of HCl-Po. Although the relative abundance of Ramlibacter and Lysobacter was relatively low, they also played important roles in the mineralization of Residual-P and NaHCO3-Po, respectively.

In this study, we subjected the root systems of eight Eucalyptus hybrids currently cultivated in southern China to heterogeneous phosphorus stress to provide a scientific basis for the selection of a highly phosphorus-efficient Eucalyptus variety. When the ability of these hybrids to locate phosphorus under different experimental conditions (phosphorus supply in a homogeneous or heterogeneous manner vs. no phosphorus supply) was compared, the main growth characteristics of Eucalyptus, such as plant height, diameter, dry mass, and phosphorus content, significantly improved when the phosphorus supply was increased from no phosphorus or heterogeneous phosphorus (half of the phosphorus amount) to homogeneous phosphorus. Across these three conditions, the growth traits of different Eucalyptus hybrids differed significantly, indicating different adaptabilities of the hybrids to various phosphorus conditions. The growth traits of the aboveground tissues of Eucalyptus under different phosphorus conditions were largely influenced by the morphology of the underground root system. In addition, the root morphology of Eucalyptus under heterogeneous phosphorus treatment suggested that there were two mechanisms for locating nutrients. Eucalyptus hybrids such as Urophylla 3229, Grandis 9, Guanglin 3, 201-2, and Dunn produced more roots proximal to the phosphorus supply; the other hybrids, Urophylla 3216, Grandis 5, and Guanglin 9, relied mainly on the growth of roots opposite the phosphorus supply to obtain adequate nutrients for growth. With these two strategies, a wide range of nutrients was obtained, root distribution was greater, more soil volume was covered, the contact area of the roots with soil phosphorus was increased, and the uptake of phosphorus by the root system was increased. These results demonstrate that Eucalyptus relies on changes to morphological characteristics of the root system to increase accessibility to phosphorus resources.

Depletion of soil nutrients is a major cause of decline in productivity of forest plantations in successive rotations. Biochar amendment in agricultural systems has been shown to yield various beneficial effects, including increasing soil phosphorus (P) availability. However, the direct and indirect effects of biochar addition on forest soil P dynamics have largely been unexplored. The objective of this study was to examine how biochar produced from harvest residue (leaves and woodchips) affect the P dynamics in second rotation Cunninghamia lanceolata (Chinese fir) plantation soil. An incubation experiment which involved mixing of forest soil with 1% or 3% w/w leaf or woodchip biochar, pyrolyzed at 300 °C or 600 °C, was conducted for 80 days at 20 °C. After 7, 40 and 80 days of incubation, soil samples were analyzed for total and available P, inorganic and organic P pools, and soil phosphatase activity. At the end of the incubation period, bacterial community composition and diversity were analyzed by 16S rDNA sequencing. The leaf biochar produced at both pyrolysis temperatures was more alkaline and had significantly higher soluble P, nitrogen and calcium contents than the woodchip biochar. Soil total and available P increased significantly in all leaf biochar treatments after 80 days incubation compared to the untreated control soil, but the woodchip biochar treatments had no significant effects. At the end of the experiment, Al-P content was significantly lower and Ca10-P content higher in soil amended with both biochar types compared to the control soil, and Fe-P content was significantly higher in the leaf biochar treatments. Contrary to expectations, acid and alkaline phosphatase enzyme activities were significantly lower in some of the biochar treatments after 80 days incubation compared to the control soil. Nevertheless, the diversity of the bacterial community was higher in leaf biochar-amended forest soil than the woodchip biochar-amended and untreated soil at the end of the experiment. In particular, the abundance increased in the leaf biochar soil treatments of P-solubilizing bacteria, such as Burkholderia-Paraburkholderia, Planctomyces, Sphingomonas and Singulisphaera, which can indirectly improve P availability in soil. Thus, conversion of tree harvest residues, particularly leaves, into biochar and recycling back into the soil could be a viable option to boost P availability and help to conserve nutrients or reduce nutrient losses for the next rotation. Before recommending plantation management with biochar, long-term studies are required assessing the life cycle of biochar under field conditions and its promoting effect on growth of C. lanceolata.

Experiments were conducted to examine the effects of different concentrations of Cd on the performance of the Cd accumulator Conyza canadensis. Cd accumulation in roots and leaves (roots>leaves) increased with increasing Cd concentration in soil. High Cd concentration inhibited plant growth, increased the membrane permeability of leaves, and caused a significant decline in plant height and chlorophyll [chlorophyll (Chl) a, Chl b, and total Chl] content. Leaf ultrastructural analysis of spongy mesophyllic cells revealed that excessive Cd concentrations cause adverse effects on the chloroplast and mitochondrion ultrastructures of C. canadensis. However, the activities of antioxidant enzymes, such as superoxide dismutase, catalase, peroxidase, total non-protein SH compounds, glutathione, and phytochelatin (PC) concentrations, showed an overall increase. Specifically, the increase in enzyme activities demonstrated that the antioxidant system may play an important role in eliminating or alleviating the toxicity of Cd in C. canadensis. Furthermore, results demonstrate that PC synthesis in plant cells is related to Cd concentration and that PC production levels in plants are related to the toxic effects caused by soil Cd level. These findings demonstrate the roles played by these compounds in supporting Cd tolerance in C. canadensis.

A greenhouse experiment was conducted to assay the bioaccumulation and tolerance characteristics of Rhus chinensis Mill. to lead (Pb). The effects of exposing R. chinensis Mill seedlings to increasing Pb concentrations (0, 250, 500, 100mgkg-1) in the soil were assessed by measuring Pb accumulation, subcellular distribution, ultrastructure, photosynthetic characteristics, antioxidative enzyme activity, malondialdehyde content, and phytochelatin content. The majority of Pb taken up by R. chinensis Mill was associated with the cell wall fraction in the roots, where the absorption of Ca increased to maintain cell wall stability, and Pb deposits were found in the intercellular space or in the cell wall structures. In leaves, Pb was primarily stored in the cell wall, while it was compartmentalized into the vacuolar structures in the stem. Pb concentrations adversely affected the morphology of Rhus chinensis Mill cellular substructures. Furthermore, increased Peroxidase (POD) and catalase (CAT) activity was observed in plants grown in Pb-amended soil, and this may have led to reduced ROS to maintain the function of the membrane. Changes in phytochelatin levels (PCs) that were observed in Pb treated plants suggest that PCs formed complexes with Pb in the cytoplasm to reduce Pb2+ toxicity in the metabolically active cellular compartment. This mechanism may allow for the plant to accumulate higher concentrations of toxic Pb and survive for a longer period of time. Our study provides a better understanding of how Rhus chinensis Mill detoxifies Pb.

In the present study, the effects of low-molecular-weight organic acids (OAs) on the toxicity of copper (Cu) to the earthworm Eisenia fetida (E. fetida) were investigated in a simulated soil solution. We exposed E. fetida to soil solution containing Cu and a variety of OAs (acetic acid, oxalic acid, citric acid, and EDTA). We found that the addition of OAs reduced the toxicity of Cu to E. fetida, where the reduction was strongest in EDTA and weakest in acetic acid. These compounds decreased the mortality rate of E. fetida that were exposed to Cu and reduced levels of antioxidant enzymes and malondialdehyde to unexposed control levels. E. fetida were exposed to Cu with OAs had reduced Cu2+, which were likely caused by Cu forming complexes with the OAs, reducing the availability of Cu. The presence of OAs also reduced Cu-induced damage on earthworm cellular ultrastructures and changed the subcellular distribution of Cu. These results demonstrated that OAs could reduce the toxicity, as well as the bioavailability, of heavy metals in soil solutions where both OAs and heavy metals often coexist.

The effects of increasing concentrations of lead (Pb) on Pb accumulation, subcellular distribution, ultrastructure, photosynthetic characteristics, antioxidative enzyme activity, malondialdehyde content, and phytochelatin contents were investigated in Neyraudia reynaudiana seedlings after a 21-day exposure. A Pb analysis at the subcellular level showed that the majority of Pb in the roots was associated with the cell wall fraction, followed by the soluble fraction. In contrast, the majority of the Pb in the leaves was located in the soluble fraction based on transmission electron microscopy and energy dispersive X-ray analyses. Furthermore, high Pb concentrations adversely affected N. reynaudiana cellular structure. The changes in enzyme activity suggested that the antioxidant system plays an important role in eliminating or alleviating Pb toxicity, both in the roots and leaves of N. reynaudiana. Additionally, the phytochelatin contents in the roots and leaves differed significantly between Pb-spiked treatments and control plants. Our results provide strong evidence that cell walls restrict Pb uptake into the protoplasm and establish an important protective barrier. Subsequent vacuolar compartmentalization in leaves could isolate Pb from other substances in the cell and minimize Pb toxicity in other organelles over time. These results also demonstrated that the levels of antioxidant enzymes and phytochelatin in leaves and roots are correlated with Pb toxicity. These detoxification mechanisms promote Pb tolerance in N. reynaudiana.