Unplanted Soil Research Articles

Short-term plant legacy alters the resistance and resilience of soil microbial communities exposed to heat disturbance in a Mediterranean calcareous soil

Abstract Plant legacy is a concept representing the effects exerted by plants on soil once they are no longer growing. We hypothesized that plant species and mixture (intercropping) would induce different short-term legacy effects impacting carbon and nitrogen-related soil microbial activities and resistance and resilience after a heat disturbance. A microcosm experiment was conducted using a calcareous Mediterranean soil conditioned by a complete vegetative cycle in a greenhouse with four planting modalities (W = monoculture of Wheat (Triticum aestivum L.); L = monoculture of white Lupin (Lupinus albus L.); WL = both species intercropped; U = unplanted soil). Half of microcosms were incubated at 28 °C (C = control conditions) whereas the remaining half were exposed at 48 °C for 2 days (S = stress conditions), with an immediately return to control conditions. Microcosms were destructively sampled at 2, 7, 16 and 28 days (T2, T7, T16, T28) after the end of the heat disturbance and the following soil measurements were performed: Basal Respiration (BR), Substrate-Induced Respiration (SIR), Nitrification Enzyme Activity (NEA) and N mineral concentrations. Our results demonstrated that monocultures and intercropping promoted different legacy effects under control conditions especially for SIR. WL soils presented lower values of SIR than L and higher than W soils. For SIR, W and WL soils conferred greater resistance to the heat stress, whereas L and WL soils conferred higher resilience at T28. For NEA, no differences between soils were observed for resistance to heat stress, but at T16, soils having WL legacy were more resilient than L soils, but comparable to those having W legacy. Our results highlight that a short-term legacy effect is measurable but greatly differs between C- and N-related microbial activities. We estimated that intercropping had modified ability of soil microorganisms to face heat stress, suggesting that plant legacy effect has to be considered to mitigate extreme climatic events in Mediterranean soils.

Plant economic strategies of grassland species control soil carbon dynamics through rhizodeposition

Abstract The plant economics spectrum is increasingly recognized as a major determinant of plant species effects on terrestrial ecosystem functioning related to carbon cycling. However, the role of plant economic strategies in the effects of living root activity on soil organic carbon (SOC) dynamics through rhizodeposition remains unexplored, despite SOC being the largest terrestrial carbon pool. Using a continuous 13C‐labelling method allowing partitioning of plant and soil sources to carbon fluxes and pools, we studied here the linkages between plant economic strategies and SOC cycling processes in a ‘common garden’ greenhouse experiment. It includes a panel of 12 grassland species selected along a gradient of economic traits and belonging to three functionnal groups (C3 grasses, forbs and legumes). All species induced an acceleration of native SOC mineralization but this rhizosphere priming effect (RPE) substantially differed across species and varied eleven‐fold by the end of the experiment (from +26% to +295% relative to unplanted soil). Interspecific variation in RPE was primarily linked to plant photosynthetic activity associated to species economic strategies of light and CO2 resource acquisition and processing. Fast‐growing acquisitive species, such as legumes, featured large RPE, in relation with their high canopy photosynthesis coupled to high leaf photosynthetic capacity and large net primary productivity allocated above‐ground. This large RPE was further associated with high root metabolic activity, rhizodeposition and soil microbial activity. In contrast, fine‐root growth and economic traits related to soil resource foraging ability were poor predictors of RPE. The formation of new root‐derived SOC varied nine‐fold across species and was similarly positively related to the net primary productivity allocated above‐ground. Fast‐growing acquisitive species with a high photosynthetic activity induced a disproportionately large RPE relative to SOC formation. Synthesis. Overall, our study demonstrates that rhizodeposition is a major mechanism through which plant economic strategies of grassland species control soil carbon dynamics. Acquisitive versus conservative species were associated with high versus low rates of photosynthesis and rhizodeposition, in turn leading to fast versus slow SOC turnover. This emphasizes the importance of considering rhizosphere processes for understanding plant species effects on soil biogeochemistry.

Treatability of hexabromocyclododecane using Pd/Fe nanoparticles in the soil-plant system: Effects of humic acids

Hexabromocyclododecane (HBCD) is a persistent organic pollutant that accumulates in soil and sediments, however, it has been difficult to degrade HBCD with developed remediation technologies so far. In this study, degradation of HBCD by bimetallic iron-based nanoparticles (NPs) under both aqueous and soil conditions considering the effects of humic acids (HAs) and tobacco plant was investigated. In the aqueous solution, 99% of the total HBCD (15 mM) was transformed by Pd/nFe (1 g L−1) within 9 h of treatment and the HBCD debromination by Pd/nFe increased with the addition of HAs. In the soil system, 13%, 15%, 41% and 27% of the total HBCD were removed by treatments consisting of plant only, plant with HAs, plant with NPs and plant + NPs + HAs, respectively, compared to the HBCD removal in an unplanted soil. The 221–986 ng/g of HBCD were detected inside the plant after the treatments, and HAs showed considerable influence on the selective bioaccumulation of HBCD stereoisomers in the plant. Overall, this approach represents a meaningful attempt to develop an efficient and eco-friendly technology for HBCD removal, and it provides advantages for the sustainable remediation of recalcitrant emerging contaminants in soils.

Complexity of bacterial communities within the rhizospheres of legumes drives phenanthrene degradation

Determining the mechanisms underlying complex plant-microbe interactions and feedbacks is crucial for effective rhizoremediation of contaminated soils. Here, rhizosphere bacterial communities of 10 common legumes planted in arable and aged oil-contaminated soils for 90 days were used to degrade phenanthrene (PHE). Bacterial community structures were analyzed to reveal the mechanisms underlying the varying PHE degradation capacities. We found that bacterial β-diversity within the rhizospheres played a major role in predicting PHE degradation rates, and more complex bacterial communities exhibited higher PHE degradation capacities. The highest PHE degradation rates were obtained with bacterial communities of Coronilla varia and Vigna unguiculata planted in oil-contaminated soils (almost 80%). There were significant differences in bacterial community composition between arable and contaminated soils, and among different legumes rhizospheres and the unplanted soils, although minimal differences were observed in bacterial α-diversity. The potential degradation-related taxa were more abundant in bacterial communities from contaminated soils, indicating more effective PHE degradation than in arable soils. Network analysis revealed the vital ecological roles of the potential degradation-related taxa in the maintenance of complex associations among bacterial taxa. Oil contamination reduced the differences in bacterial community composition between rhizosphere soils with high and low PHE degradation rates. These results suggest that the formulation of effective rhizoremediation strategies ought to focus on more diverse and complex interactions between rhizosphere-inhabiting organisms from aged oil-contaminated soils through the selection of optimal plant-microbiota association.

Combination of rhamnolipid and biochar in assisting phytoremediation of petroleum hydrocarbon contaminated soil using Spartina anglica

Biochar (BC) and rhamnolipid (RL) is used in bioremediation of petroleum hydrocarbons, however, the combined effect of BC and RL in phytoremediation has not been studied until now. In this paper, the phytoremediation of petroleum hydrocarbon-contaminated soil using novel plant Spartina anglica was enhanced by the combination of biochar (BC) and rhamnolipid (RL). Samples of petroleum-contaminated soil (10, 30 and 50 g/kg) were amended by BC, BC+ RL and rhamnolipid modified biochar (RMB), respectively. After 60 day's cultivation, the removal rate of total petroleum hydrocarbons (TPHs) for unplanted soil (UP), planted soil (P), planted soil with BC addition (P-BC), planted soil with BC and RL addition (P-BC + RL) and planted soil with addition of RMB (P-RMB) were 8.6%, 19.1%, 27.7%, 32.4% and 35.1% in soil with TPHs concentration of 30 g/kg, respectively. Compared with UP, the plantation of Spartina anglica significantly decreased the concentration of C8–14 and tricyclic PAHs. Furthermore, the application of BC and RMB alleviated the toxicity of petroleum hydrocarbons to Spartina anglica via improving plant growth with increasing plant height, root vitality and total chlorophyll content. High-throughput sequencing result indicated that rhizosphere microbial community of Spartina anglica was regulated by the application of BC and RMB, with increase of bacteria and plant mycorrhizal symbiotic fungus in biochar and RMB amended soil.

Cover crop species have contrasting influence upon soil structural genesis and microbial community phenotype

Cover crops (plants grown in an agricultural rotation between cash crops) can significantly improve soil quality via sequestering carbon, retaining nutrients, decreasing soil erosion, and maintaining belowground biodiversity. However, little is known about the effects of such plants upon soil structure. The aim of the study was to assess the impact of four species typically used as cover crops and which have contrasting root architecture (viz. clover, black oat, phacelia, tillage radish) on soil structural genesis and the associated modification of microbial community structure in a clay soil. The four plant species were grown in a replicated pot experiment with sieved soil (<2 mm), with unplanted soil as control for 8 weeks. X-ray Computed Tomography was used to quantify the formation of pore networks in 3D and phospholipid fatty acid analysis was performed to characterise the microbial community phenotype. Black oats developed a greater soil-pore connectivity than the other species throughout the growth period, whereas phacelia decreased both the porosity and pore-connectivity. The microbial community phenotype under phacelia was notably different from the other species, with a greater proportion of fungal markers. Thus, different plant species have differential effects upon soil structural genesis and microbial community phenotype, which provides evidence that certain species may be more suitable as cover crops in terms of soil structural conditioning depending upon specific contexts.

Combined Effects of Plant Cultivation and Sorbing Carbon Amendments on Freely Dissolved PAHs in Contaminated Soil.

We report freely dissolved concentrations ( Cfree) of PAHs in soils amended with 2.5% biochar and activated carbon (AC) during a long-term (18-months) field experiment. The study evaluates also the impact of different plants (clover, grass, willow) on Cfree PAHs. The cumulative effect of treatments on nitrogen and available forms of phosphorus, potassium, and magnesium is also assessed. The direct addition of biochar to soil did not cause any immediate reduction of the sum of 16 Cfree PAHs, while AC resulted in a slight reduction of 5- and 6 ring compounds. The efficiency of binding of Cfree PAHs by biochar and AC increased with time. For biochar, the maximum reduction of 4-6-ring PAHs (18-67%) was achieved within 6 months. For 2- and 3-ring PAHs, a gradual decrease of Cfree was observed which reached 60-66% at 18 months. AC proved to be better in reducing Cfree PAHs than biochar, though for 2- and 3-ring PAHs, the differences in AC and biochar performances were smaller than those for 4-6-ring PAHs. After 18 months, a significantly lower content of Cfree PAHs was observed in the soil with plants compared to the unplanted soil. Except for potassium, AC or biochar did not negatively impact nutrient availability.

Effect of root density of wheat and okra on hydraulic properties of an unsaturated compacted loam

Root‐induced changes of soil hydraulic properties, that is soil water retention curve (SWRC) and unsaturated hydraulic conductivity (K(ψ)), depend on plant species and root density. This study aimed to investigate the effects of wheat roots and okra roots on both SWRC andK(ψ) of soil within an extended range of matric potential (−300 ~ 0 kPa) and to investigate the effects of root density of wheat (i.e. grass species) and okra (i.e. shrub species) on SWRC andK(ψ) of a loamy soil. The SWRC andK(ψ) of soil planted with wheat and okra were measured by the simplified evaporation method. Soil matric potential was measured directly by high‐capacity tensiometers. Different root densities were obtained by establishing different seeding densities in the soil. Meanwhile, root characteristics in planted soil were obtained using image analysis. The results showed that wheat‐ and okra‐planted soil had significantly larger volumetric water content (VWC) within the range of matric potential from −90 to 0 kPa than that of unplanted soil. Within the range of matric potential from around −300 to −90 kPa, the effects of wheat and okra roots on SWRC were not significant. The effects of wheat and okra roots onK(ψ) were not significant within the range of matric potential from around −300 to −10 kPa. Wheat‐planted soil with a root length density (RLD) of 10.04 cm cm−3had larger values of VWC (ψ = 0, −33, −50 kPa) than those of wheat‐planted soil with RLD of 2.69 cm cm−3. The difference in hydraulic properties between okra‐planted soil and the two RLDs, 0.89 and 2.96 cm cm−3, was not significant. Okra roots were more effective in increasing saturated water content (θs) and field capacity (θfc) than wheat roots. This might be because okra roots are relatively coarser than wheat roots.HighlightsEffect of root density of wheat and okra on SWRC andK(ψ) was investigated.TheK(ψ) of soil planted with wheat and okra was measured at matric potentials less than −100 kPa.Okra roots were more effective in increasingθsandθfcthan wheat roots.Effect of wheat and okra roots onK(ψ) was not significant at matric potentials from −300 to −10 kPa.

Arbuscular mycorrhizal colonization of selected herbaceous plants under arid protected area in Tunisia

ABSTRACT Arbuscular mycorrhizal fungi (AMF) play an important role in plants growth and soils dynamic in all most ecosystems. The main objective of the present study was to evaluate the plant-AMF interactions on soil functions under arid protected area ‘Zarat-Gabès’ in Tunisia. AMF colonization was evaluated by visual observation of AMF in fine roots of eight herbaceous plants. The level of mycorrhizal colonization varied between plants. Astragalus corrugatus and Hippocrepis areolata showed the highest mycorrhizal performance. The relative spore number was significantly different across rhizosphere soils. Statistical analysis showed a clearly positive correlation between the number of spores and plant-mycorrhizal intensity. For microbiological parameters, our results showed that mycorrhizal plants improved significantly the various microbiological parameters. Rhizosphere soils of Astragalus corrugatus and Hippocrepis areolata presented the necessary microbial densities and microorganisms more stable compared to unplanted soil. This study allowed obtaining a new result that challenges us about the need for efficient management of natural resources in the objective of nature conservation.

Successive phytoextraction alters ammonia oxidation and associated microbial communities in heavy metal contaminated agricultural soils

Phytoextraction is an attractive strategy for remediation of soils contaminated by heavy metal (HM), yet the effects of this practice on biochemical processes involved in soil nutrient cycling remain unknown. Here we investigated the impact of successive phytoextraction with a Cd/Zn co-hyperaccumulator Sedum alfredii (Crassulaceae) on potential nitrification rates (PNRs), abundance and composition of nitrifying communities and functional genes associated with nitrification using archaeal and bacterial 16S rRNA gene profiling and quantitative real-time PCR. The PNRs in rhizosphere were significantly (P < 0.05) lower than in the unplanted soils, and decreased markedly with planting time. The decrease of PNR was more paralleled by changes in numbers of copy and transcript of archaeal amoA gene than the bacterial counterpart. Phylogenetic analysis revealed that phytoextraction induced shifts in community structure of soil group 1.1b lineage-dominated ammonia-oxidizing archaea (AOA), Nitrosospira cluster 3-like ammonia-oxidizing bacteria (AOB) and Nitrospira-like nitrite-oxidizing bacteria (NOB). A strong positive correlation was observed between amoA gene transcript numbers and PNRs, whereas root exudates showed negative effect on PNR. This effect was further corroborated by incubation test with the concentrated root exudates of S. alfredii. Partial least squares path model demonstrated that PNR was predominantly controlled by number of AOA amoA gene transcripts which were strongly influenced by root exudation and HM level in soil. Our result reveals that successive phytoextraction of agricultural soil contaminated by HMs using S. alfredii could inhibit ammonia oxidation and thereby reduce nitrogen loss.

The Effects of Different Lead Pollution Levels on Soil Microbial Quantities and Metabolic Function with/without Salix integra Thunb. Planting

Background and Objectives: Salix integra Thunb., a fast-growing woody species, has been used in phytoremediation in recent years. It has the potential to accumulate high amounts of lead (Pb) in its growth, however, its effects on soil microbial community structure and function during its phytoextraction processes are not well understood, especially at different pollution levels. Materials and Methods: In our study, we set unplanted and planted Salix integra in areas with four levels of Pb treatments (0, 500, 1000, and 1500 mg/kg). After six months of planting, the rhizospheric soil, bulk soil, and unplanted soil were collected. Soil properties and microbes participating in nitrogen and phosphorus cycling were measured, following standard methods. Microbial metabolic functions were assessed using a Biolog-ECO microplate. Results: The bacteria (nitrogen-fixing bacteria, ammonifying bacteria, inorganic phosphorus-solubilizing bacteria, and nitrosobacteria) all increased in the 500 mg/kg treatment and decreased in the 1500 mg/kg treatment compared with the 0 mg/kg treatment, especially in rhizospheric soil. The microbial metabolisms decreased along with the increase of Pb levels, with the exception of the rhizospheric soil with a 500 mg/kg treatment. The metabolic patterns were relative to the pollution levels. The utilization of carbohydrates was decreased, and of amino acids or fatty acids was increased, in the 500 mg/kg treatment, while the opposite occurred in the 1500 mg/kg treatment. The values of soil properties, microbial quantities, and metabolic activities were higher in rhizospheric than bulk soil, while the differences between bulk and unplanted soil were different among the different Pb treatments. The soil properties had little effect on the microbial quantities and metabolic activities. Conclusions: S. integra planting and Pb levels had an interactive effect on the microbial community. In general, S. integra planting promoted microbial quantities and metabolic activity in rhizospheric soil. Lower Pb pollution increased microbial quantities and promoted the utilization of amino acids or fatty acids, while higher Pb concentrations decreased microbial quantities and metabolic activities, and promoted the utilization of carbohydrates.

Effects of plant density on the bioremediation of soils contaminated with polyaromatic hydrocarbons

This study aims to evaluate the effects of plant density on the degradation of polyaromatic hydrocarbon (PAH) compounds in soils contaminated with crude oil. In this work, Axonopus compressus was applied as a bioremediator given its extensive and successful use in phytoremediation. A. compressus was planted at the density of three (pot-3), five (pot-5) or seven (pot-7) plants in pots containing soil contaminated with crude oil at the concentration of 20 g kg−1. Two pots were designated as controls: one containing unplanted soil contaminated with crude oil (pot-C) and one containing uncontaminated soil as control (pot-0). All planted and unplanted pots were irrigated with nutrient solution as needed. This study was performed for 2 months. The degradation of PAH compounds in soil was evaluated through ultraviolet fluorescence technique. Results showed that PAH degradation was enhanced as plant density increased. Specifically, after 2 months, PAH contents decreased by 69%, 75.8% and 82.3% in pot-3, pot-5 and pot-7, respectively, relative to those in pot-C. High plant densities mitigated the adverse effects of crude oil contamination on plant growth. Thus, this study demonstrated that high plant densities promote the phytoremediation of soil contaminated with hydrocarbon compounds.

Growth and Phytoremediation Efficiency of Winged Bean in Fluorene- and Pyrene-Contaminated Soil.

Winged bean is a tropical legume that has been reported to enhance polycyclic aromatic hydrocarbon (PAH) biodegradation in soil. However, there is insufficient information about the susceptibility of winged bean to PAH toxicity in long term study. In this study, winged bean was planted in soil contaminated with either fluorene (124.5mg/kg) or pyrene (98.4mg/kg) for 90days. Plant growth parameters and PAH disappearances from soil were measured every 30days. Neither fluorene nor pyrene led to decreased shoot and root length of winged bean and all the winged bean plants flowered on day 90. However, the chlorophyll b content in the leaves decreased since day 60 and further decreased significantly by day 90 when winged bean was grown in the presence of fluorene or pyrene. The presence of fluorene and pyrene led to reduced root nodule formation at 30 and 60days. Despite the reduced chlorophyll b content and decreased number of root nodules, winged bean could enhance pyrene removal significantly on day 30 compared to unplanted soil. Subsequently, pyrene degradation in the unplanted soil caught up and there was no statistically significant difference between the two treatments at 60 or 90 days. Negligible amounts of PAHs were accumulated in the shoot and root tissues of winged bean. These results showed that winged bean can speed up the removal of high MW PAHs from contaminated soil and we conclude that this plant is suitable for PAH phytoremediation.

Removal of soil biota alters soil feedback effects on plant growth and defense chemistry.

Summary We examined how the removal of soil biota affects plant–soil feedback (PSF) and defense chemistry of Jacobaea vulgaris, an outbreak plant species in Europe containing the defense compounds pyrrolizidine alkaloids (PAs).Macrofauna and mesofauna, as well as fungi and bacteria, were removed size selectively from unplanted soil or soil planted with J. vulgaris exposed or not to above‐ or belowground insect herbivores. Wet‐sieved fractions, using 1000‐, 20‐, 5‐ and 0.2‐μm mesh sizes, were added to sterilized soil and new plants were grown. Sieving treatments were verified by molecular analysis of the inocula.In the feedback phase, plant biomass was lowest in soils with 1000‐ and 20‐μm inocula, and soils conditioned with plants gave more negative feedback than without plants. Remarkably, part of this negative PSF effect remained present in the 0.2‐μm inoculum where no bacteria were present. PA concentration and composition of plants with 1000‐ or 20‐μm inocula differed from those with 5‐ or 0.2‐μm inocula, but only if soils had been conditioned by undamaged plants or plants damaged by aboveground herbivores. These effects correlated with leaf hyperspectral reflectance.We conclude that size‐selective removal of soil biota altered PSFs, but that these PSFs were also influenced by herbivory during the conditioning phase.

Improvement of petroleum hydrocarbon remediation using the oat plant in the soil treated by poultry manure

This greenhouse experiment aimed to investigate the effects of poultry manure (PM) on the growth of oat plant (Avena sativa) and removal of total petroleum hydrocarbons (TPHs) from soil. The treatments consisted of three TPH levels (4%, 6%, and 8% w/w) and two PM levels (zero and 1%) at three replications. According to the findings, shoot height, number of leaves, and the fresh and dry weight of the shoots and roots of oat decreased significantly with 6% and 8% TPHs compared to 4% contamination level. On the other hand, the removal of TPHs and microbial respiration rate significantly increased by 16.8% and 78.18%, respectively in the PM treatment compared to the non-PM treatment. The rate was estimated at 28.85% and 27.03% in the soil cultivated by oat compared to the unplanted soil, respectively. The highest and lowest removal rate of TPHs was observed in the soil cultivated with oats in the PM treatments containing 4% of the TPHs (45.25%) and unplanted treatments without manure containing 8% of TPHs (11.38%). Therefore, it could be concluded that the addition of PM to the soil cultivated with oat in the soils with TPH contamination could increase the microbial respiration rate of the soil, as well as the removal of TPHs.

High PAH degradation and activity of degrading bacteria during alfalfa growth where a contrasted active community developed in comparison to unplanted soil.

PAH biodegradation in plant rhizosphere has been investigated in many studies, but the timescale of degradation and degrading bacteria activity was rarely considered. We explored the impact of plants on the temporal variability of PAH degradation, microbial abundance, activity, and bacterial community structure in a rhizotron experiment. A historically contaminated soil was spiked with PAHs, planted or not with alfalfa, over 22days with sampling once a week. In both conditions, most of the spiked PAHs were dissipated during the first week, conducting to polar polycyclic aromatic compound production and to decreased richness and diversity of bacterial communities. We showed a rapid impact of the rhizosphere on PAH degradation via the increased activity of PAH-degrading bacteria. After 12days, PAH degradation was significantly higher in the planted (100% degradation) than in unplanted (70%) soil. Gram-negative (Proteobacteria) PAH-dioxygenase genes and transcripts were higher in planted than unplanted soil and were correlated to the spiked PAH degradation. Conversely, Gram-positive (Actinobacteria) PAH-dioxygenase gene transcription was constant over time in both conditions. At 12days, plant growth favored the activity of many Gammaproteobacteria (Pseudomonadaceae, Stenotrophomonas, and Acinetobacter) while in unplanted soil Alphaproteobacteria (Sphingomonadaceae, Sphingobium, and Magnetospirillum) and Actinobacteria (Iamia, Geodermatophilaceae, and Solirubrobacterales) were more active.

Enhanced polycyclic aromatic hydrocarbons degradation in rhizosphere soil planted with tall fescue: Bacterial community and functional gene expression mechanisms

To investigate the bacterial mechanisms of polycyclic aromatic hydrocarbons (PAHs) degradation in an aged-contaminated agricultural soil planted with tall fescue (Festuca arundinacea), a rhizo-box experiment was carried out for 60 d. Shifts in bacterial community structure in the soils during the experiment were performed using denaturing gradient gel electrophoresis. The abundance and activity of total bacteria and PAH-degraders were measured by quantification of 16S rDNA, PAH-ring hydroxylating dioxygenase (PAH-RHDα) genes and their transcripts, respectively. The residual PAH concentrations were monitored using high-performance liquid chromatography analysis. Results showed that the removal percentage of total PAHs in rhizosphere soil was 11% higher than that in unplanted soil. Soil bacteria were dominated by Alphaproteobacteria (48.4%) and Gammaproteobacteria (25.8%). Tall fescue positively affected the abundance and activity of total bacteria in the soil, and stimulated RHDα gram-negative (GN) gene expression while inhibiting RHDα gram-positive gene expression. PAH dissipation in rhizosphere soil could be ascribed to modifications in the bacterial community structure, increase in the abundance of PAH-degraders, and enhancement of the RHDα GN gene expression during the incubation.

Revegetation of the riparian zone of the Three Gorges Dam Reservoir leads to increased soil bacterial diversity.

As one of the most active components in soil, bacteria can affect soil physicochemical properties, its biological characteristics, and even its quality and health. We characterized dynamics of the soil bacterial diversity in planted (with Taxodium distichum) and unplanted soil in the riparian zone of the Three Gorges Dam Reservoir (TGDR), in southwestern China, in order to accurately quantify the changes in long-term soil bacterial community structure after revegetation. Measurements were taken annually in situ in the TGDR over the course of 5years, from 2012 to 2016. Soil chemical properties and bacterial diversity were analyzed in both the planted and unplanted soil. After revegetation, the soil chemical properties in planted soil were significantly different than in unplanted soil. The effects of treatment, time, and the interaction of both time and treatment had significant impacts on most diversity indices. Specifically, the bacterial community diversity indices in planted soil were significantly higher and more stable than that in unplanted soil. The correlation analyses indicated that the diversity indices correlated with the pH value, organic matter, and soil available nutrients. After revegetation in the riparian zone of the TGDR, the soil quality and health is closely related to the observed bacterial diversity, and a higher bacterial diversity avails the maintenance of soil functionality. Thus, more reforestation should be carried out in the riparian zone of the TGDR, so as to effectively mitigate the negative ecological impacts of the dam. Vegetating the reservoir banks with Taxodium distichum proved successful, but planting mixed stands of native tree species could promote even higher riparian soil biodiversity and improved levels of ecosystem functioning within the TGDR.

Changes in Soil Enzyme Activities and Microbial Biomass after Revegetation in the Three Gorges Reservoir, China

Soil enzymes and microbes are central to the decomposition of plant and microbial detritus, and play important roles in carbon, nitrogen, and phosphorus biogeochemistry cycling at the ecosystem level. In the present study, we characterized the soil enzyme activity and microbial biomass in revegetated (with Taxodium distichum (L.) Rich. and Cynodon dactylon (L.) Pers.) versus unplanted soil in the riparian zone of the Three Gorges Dam Reservoir (TGDR), in order to quantify the effect of revegetation on the edaphic microenvironment after water flooding in situ. After revegetation, the soil physical and chemical properties in revegetated soil showed significant differences to those in unplanted soil. The microbial biomass carbon and phosphorus in soils of T. distichum were significantly higher than those in C. dactylon and unplanted soils, respectively. The microbial biomass nitrogen in revegetated T. distichum and C. dactylon soils was significantly increased by 273% and 203%, respectively. The enzyme activities of T. distichum and C. dactylon soils displayed no significant difference between each other, but exhibited a great increase compared to those of the unplanted soil. Elements ratio (except C/N (S)) did not vary significantly between T. distichum and C. dactylon soils; meanwhile, a strong community-level elemental homeostasis in the revegetated soils was found. The correlation analyses demonstrated that only microbial biomass carbon and phosphorus had a significantly positive relationship with soil enzyme activities. After revegetation, both soil enzyme activities and microbial biomasses were relatively stable in the T. distichum and C. dactylon soils, with the wooded soil being more superior. The higher enzyme activities and microbial biomasses demonstrate the C, N, and P cycling and the maintenance of soil quality in the riparian zone of the TGDR.

A Study of Some Physical Properties of Two Different Soil Textures Planted with Conocarpus Trees (Conocarpuslancifolius. Engl)

&#x0D; &#x0D; &#x0D; &#x0D; This research has been conducted to study the effect of Conocarpus trees and their roots on some of soilphysical properties. The soil physical properties are moisture content , bulk density , total porosity , mean weight diameter (dry sieveing) and soil penetration resistance . Some soil samples are collected from two locations :the first location is Agric. College research, Garmat Ali, stations , Basra university , and the second location isZuwber province farm . The soil texture of the first location is silty clay which is classified as fine clay mixed Calcarioushy perthermictypictorrifluvent, while the soil texture of the second location is sandy loam. This soil is classified with in species Entisol and under species psamments and high group , underhigh group and family (Typictorripsamments, Calcarious Mixed Hyperthermic). The soil samples are collected from two soil depths( 0 – 30 and 30 – 60) from both locations are planted with Conocarpus trees, their ages ranged between 4 to 5 years . The trees height is 2.5 – 3.0 m . Another soil samples are also collected from unplanted soil with Conocarpus trees. The results reveated that the silty clay soil is significantly surpassed the sandy loam soil in moisture content and mean weight diameter by a percentage of 68.76% and 32.91% respectively . Whereas, the bulk density and soil penetration resistance decreased , while the total porosity of the silty clay soil as compared with sandy loam soil .For unplanted soil, moisture content, the bulk density and the soil penetration resistance are increased as compared with planted soil . The soil depth (30 – 60)cm is surpassed soil depth of ( 0- 30)cm in giving higher values of moisture content andbulk density whereas it does not significantly affect the mean weight diameter and soil penetration resistance .&#x0D; &#x0D; &#x0D; &#x0D;