Effects of artificial aeration and iron inputs on the transformation of carbon and phosphorus in a typical wetland soil

Purpose

Artificial aeration changes the redox conditions at the soil surface. The introduction of iron (Fe) into wetlands can influence carbon (C) and phosphorus (P) cycling under the fluctuating redox conditions. However, artificial Fe introduced into wetlands is uncommon, and there are no Fe dose guidelines. We compared aerobic and anaerobic conditions to test the hypothesis that Fe addition can, although redox-dependent, affect P forms and the coupling of organic C.

Materials and methods

Twenty-four intact soil cores were collected randomly from a lacustrine wetland of Lake Xiaoxingkai. And representative and homogeneous seedlings of Glyceria spiculosa were collected. The incubation was designed with two treatment factors: Fe/P ratio (5 or 10) and high and low dissolved oxygen (DO) concentrations (>?6 and <?2 mg L^?1, respectively). Four groups with three replicates were separated randomly and labeled as aerobic + plant treatment, anaerobic + plant treatment, and aerobic or anaerobic treatment (control).

Results and discussion

The DO concentrations were stratified, decreasing with soil depth and increasing with time, especially under aerobic conditions. The Eh values generally increased with fluctuations under aerobic conditions. The artificial aeration substantially changed the redox environment at the water–soil interface. Of the total P, 45% was in the reactive Fe-bound P, indicating that Lake Xiaoxingkai had high internal P loading. No significant differences were observed in total Fe, amorphous Fe, and organic C at the soil surface between the two Fe/P ratios; however, a significant difference in free Fe was observed. And soil amorphous Fe was found to be a significant correlation with soil organic C, indicating that iron oxides were related with the soil chemical properties.

Conclusions

After short-term incubation, Fe addition can affect the cycling of major elements in wetlands, although this effect is redox dependent. Excessive Fe doses may result in regional environmental risks, such as eutrophication and C sinks of wetland ecosystems. Large-scale controlled experiments are needed to fully understand the behaviors of soil elements in wetlands.

     

Straw amendment to paddy soil stimulates denitrification but biochar amendment promotes anaerobic ammonia oxidation

Purpose

Organic matter amendment is usually used to improve soil physicochemical properties and to sequester carbon for counteracting climate change. There is no doubt that such amendment will change microbial activity and soil nitrogen transformation processes. However, the effects of straw and biochar amendment on anammox and denitrification activity and on community structure in paddy soil are unclear.

Materials and methods

We conducted a 30-day pot experiment using rice straw and rice straw biochar to deepen our understanding about the activity, microbial abundance, and community structure associated with soil nitrogen cycling during rice growth.

Results and discussion

Regarding activity, anammox contributed 3.1–8.1% of N₂ production and denitrification contributed 91.9–96.9% of N₂ production; straw amendment resulted in the highest denitrification rate (38.9 nmol N g^?1 h^?1), while biochar amendment resulted in the highest anammox rate (1.60 nmol N g^?1 h^?1). Both straw and biochar amendments significantly increased the hzsB and nosZ gene abundance (p < 0.05). Straw amendment showed the highest nosZ gene abundance, while biochar amendment showed the highest hzsB gene abundance. Phylogenetic analysis of the anammox bacteria 16S rRNA genes indicated that Candidatus Brocadia and Kuenenia were the dominant genera detected in all treatments.

Conclusions

Straw and biochar amendments have different influences on anaerobic ammonia oxidation and denitrification within paddy soil. Our results suggested that the changes in denitrification and anammox rates in the biochar and straw treatments were mainly linked to functional gene abundance rather than microbial community structure and that denitrification played the more major role in N₂ production in paddy soil.

     

An overlooked nitrogen loss linked to anaerobic ammonium oxidation in estuarine sediments in China

Purpose

Despite its importance, anammox (anaerobic ammonium oxidation) in estuarine sediment systems remains poorly understood, particularly at the continental scale. This study aimed to understand the abundance, diversity, and activity of anammox bacteria and to determine the main factors influencing the anammox process in estuarine sediments in China.

Materials and methods

Estuarine sediments were collected from 18 estuaries spanning over 4000 km. Experiments using an ¹⁵ N–tracer, quantitative PCR, and clone library construction were used to determine the activity, abundance, and diversity of anammox bacteria. The impact of environmental factors on anammox processes was also determined.

Results and discussion

The abundance of the anammox-specific hydrazine synthase (hzsB) gene ranged from 1.8 × 10⁵ ± 3.4 × 10⁴ to 3.6 × 10⁸ ± 7.5 × 10⁷ copies g^?1 dw. Candidatus Scalindua, Brocadia, Kuenenia, Jettenia, and two novel unidentified clusters were detected, with Scalindua dominating the anammox population. Additionally, the abundances of Scalindua, Kuenenia, and Brocadia were found to be significantly correlated with latitude. The anammox rates ranged from 0.29 ± 0.15 to 13.68 ± 3.98 nmol N g^?1 dw h^?1 and contributed to 2.39–82.61% of total N₂ production. Pearson correlation analysis revealed that the anammox rate was positively correlated with total nitrogen, total carbon, and temperature, and was negatively correlated with dissolved oxygen (DO). The key factors influencing the hzsB gene abundance were ammonium concentration, salinity, and DO. Ammonium concentration, pH, temperature, and latitude were main variables shaping the anammox-associated bacterial community.

Conclusions

Our results suggested that anammox bacteria are ubiquitous in coastal estuaries in China and underline the importance of anammox resulting in N loss at a continental scale.

     

The transformation and migration of selenium in soil under different Eh conditions

Purpose

Soil selenium (Se) sequestration and transformation, which are strongly controlled by soil redox conditions, are critical for understanding the mobility and bioavailability in the environment. Thus, the effect of redox potential on Se transformation was investigated for exploring the release mechanism of Se in soil.

Materials and methods

Soils were incubated under anoxic condition in four treatments at room temperature over 56 days, and the soil solution pH, Eh, and Fe and Se concentrations were measured at given reaction time. The sequential extraction and X-ray photoelectron spectroscopy (XPS) were used to obtain the species distribution of Se in soil. High-resolution transmission electron microscopy (HR-TEM) was employed to observe morphology characteristic of soil.

Results and discussion

Parts of soil Se can be released into solution, and Se speciation in soil changed during the incubation period. XPS and sequential extraction analyses revealed that the primary speciation of Se in soil was elemental Se, and metallic selenides were formed under aerobic condition. Moreover, XPS and HR-TEM data revealed the crystalline state of iron oxides in soil changed after anoxic incubation, and certain amorphous iron oxides were formed.

Conclusions

Se release is activated by short-term incubation, whereas Se can be transformed into less soluble state after long-term incubation. Organic matter takes extremely an important role in Fe oxide reductive dissolution and Se transformation. This study is useful to understand the environmental behaviors of Se and enhance the application of Se fertilizers effectively and safely in Se deficiency area.

     

Amendment of sulfate with Se into soils further reduces methylmercury accumulation in rice

Purpose

Both selenium (Se) and sulfate could largely affect methylmercury (MeHg) dynamics and phytoavailability in soil-rice systems, while their combined effects are less understood. Here, we aimed at exploring the potential effects of sulfate on MeHg accumulation in rice in the presence of Se.

Materials and methods

Rice was cultivated in inorganic Hg-spiked soils amended with Se only (selenite/selenate, “Se treatments”) or Se and sulfate (“Se?+?Sulfate treatments”). Soil parameters (e.g., pH and redox potential (Eh)), MeHg concentrations in soils, as well as MeHg or Se accumulation in rice plants were quantified during the rice growth period.

Results and discussion

Soil MeHg concentrations were generally comparable between Se?+?Sulfate and Se treatments. However, MeHg uptake by rice plants in Se?+?Sulfate treatments was 9–31 % lower than those in Se treatments, possibly due to the increased soil pH and formation of iron sulfides, which may reduce MeHg phytoavailability under sulfate amendment. Furthermore, sulfate input enhanced Se accumulation in root (especially in the presence of selenate), which could be responsible for the increased MeHg distribution in root and thus lower MeHg distribution in grain. Consequently, the reduced plant uptake of MeHg together with the decreased MeHg distribution in grain resulted in decline of grain MeHg concentrations in Se?+?Sulfate treatments (8–31 % lower compared to Se treatments).

Conclusions

Our results suggest that sulfate input with Se could further reduce MeHg accumulation in rice, which improved mechanistic understanding of MeHg behaviors in soil-rice systems.

     

Arsenic modulates the composition of anode-respiring bacterial community during dry-wet cycles in paddy soils

Purpose

Bacteria able to extracelluar respiration, which could be enriched in the anode of microbial fuel cells (MFCs), play important roles in dissimilatory iron reduction and arsenic (As) desorption in paddy soils. However, the response of the bacteria to As pollution is unknown.

Materials and methods

Using soil MFCs to investigate the effects of As on anode respiring bacteria (ARB) communities in paddy soils exposed to As stress. The soil MFC performances were evaluated by electrochemical methods. The bacterial community compositions on anodes were studied using Illumina sequencing.

Results and discussion

In wet 1 phase, polarization curves of MFCs showed cathode potentials were enhanced at low As exposure but inhibited at high As exposure. In the meantime, anode potentials increased with As levels. The dry-wet alternation reduced As levels in porewater and their impacts on electrodes microorganisms. Arsenic addition significantly influenced the anode microbial communities. After dry-wet cycles, Deltaproteobacteria dominated in the anode with high As.

Conclusions

The dynamic changes of the communities on cathodes and anodes of soil MFCs in paddy soils with different As addition might be explained by their different mechanisms for As detoxification. These results provide new insights into the microbial evolution in As-contaminated paddy soils.

     

Formation and mineralization kinetics of dissolved humic substances from aquatic macrophytes decomposition

Purpose

Aquatic macrophytes are an important source of autochthonous dissolved organic carbon in aquatic ecosystems. Yield and mass loss of aquatic humic substances released from macrophytes decomposition could be affected by the plant species and oxygen availability. Our aim was to describe the kinetics of dissolved fulvic and humic acids formed from decomposition of four aquatic macrophytes under aerobic and anaerobic conditions.

Materials and methods

Samples of Eichhornia azurea (Sw.) Kunth, Egeria najas Planch, Oxycaryum cubense (Poepp. and Kunth), and Salvinia molesta (Mitchell) were incubated under aerobic and anaerobic conditions. On sampling days, the remaining particulate detritus were weighted and were measured for the pH, the electrical conductivity, and the organic carbon in the dissolved fraction. Humic substances were extracted from the dissolved fraction, separated into fulvic and humic acids, and then quantified. The mass loss of particulate and dissolved fractions were fitted to first order kinetic models.

Results and discussion

Aerobic environment favored mineralization of aquatic macrophyte detritus and humification of organic dissolved carbon. Incubations under aerobic conditions formed 3.6 times more humic acid than incubations under anaerobic conditions. However, incubations in an anaerobic environment formed 1.84 times more fulvic acid. The dissolved humic compounds presented low mineralization rates probably due to the presence of the macrophyte detritus in the incubation representing a more attractive source of resource for microorganisms.

Conclusions

In many cases, the mineralization of HS was not noticed, leading to an increase in humic and fulvic acid concentration in the water. O. cubense detritus presented the highest carbon concentration, were related to refractory features, and generated the highest amounts of dissolved HA (mainly under aerobic condition). Egeria najas detritus presented the lowest carbon concentration, were related to labile features, and generated the highest amounts of dissolved FA (mainly under anaerobic condition). Besides that, high humic substance concentrations in the dissolved organic carbon were related to low mineralization of this fraction.

     

Phosphate adsorption on uncoated and humic acid-coated iron oxides

Purpose

The phosphate adsorption on natural adsorbents is of particular importance in regulating the transport and bioavailability of phosphates in environmental system. In soils, oxides are often associated with organic matter and form mineral-organic complexes. The aim of the present paper was to investigate the mechanisms of phosphate adsorption on these complexes.

Materials and methods

Phosphate adsorption on uncoated and humic acid (HA)-coated iron oxide complexes was investigated at different ionic strengths and pH by isotherm experiments and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy.

Results and discussion

Results showed that HA-coated iron oxide complexes caused a decrease in the specific surface area (SSA) and the isoelectric point (IEP) of oxides. Phosphate adsorption on iron oxides was insensitive to changes of ionic strength, while it increased on the complexes with increasing ionic strength. The presence of HA decreased the maximum adsorption and the affinity of phosphate on the complexes. The zeta potential of phosphate-bound iron oxides linearly reduced with the increment of phosphate surface coverage, while the zeta potential of complexes with adsorbed phosphate kept at the same level. ATR-FTIR analysis suggested the formation of phosphate-metal complexation. The presence of HA promotes the formation of the monodentate phosphate complexes at pH 4.5 and significantly influenced phosphate species at pH 8.5.

Conclusions

The amount of phosphate adsorbed was reduced, and the phosphate speciation was also influenced when phosphate was adsorbed on HA-coated iron oxide complexes compared with phosphate adsorption on pure goethite and hematite.

     

Methane cycling in a drained wetland soil profile

Purpose

Peatlands have an important role in methane cycling in the natural environment. Methane emissions as a result of methanogenesis and methanotrophy in soil are affected by several environmental factors such as temperature, oxygen and groundwater level. The objective of this study was to analyse methane cycling as a function of soil depth.

Materials and methods

In this study, methane cycling and soil organic matter mineralization were investigated in a drained fen grassland area of Ljubljana marsh, Slovenia that has been subjected to reclamation strategies for several centuries. Potential mineralization, methane production and methane oxidation rates were measured in slurry incubation experiments with soil samples from 10 sampling depths of a 1-m profile. In addition, the extent of iron reduction in the soil was determined.

Results and discussion

The potential for methane production was low in the investigated soil profile, even in constantly flooded layers below the water table fluctuations. During anaerobic incubations, the highest accumulated concentrations and production rates of methane were observed in the upper 10-cm layer and the lowest in deeper soil layers, indicating that plant exudates are the main source of energy for heterotrophic soil microbes and that methanogenesis in deeper layers is limited by the availability of appropriate organic substrates. Methane oxidation was on the other hand active throughout the soil profile, suggesting that the potentially active methane oxidizing community is present despite low methane production. The highest abundance and activity of methanotrophs was detected in the water table fluctuation layers.

Conclusions

Together, these findings have implications for understanding the biogeochemical function of drained peat soils and emphasize the influence of drainage on quality of soil organic matter and consequently on methane production even in flooded soils.

     

Remediation of heavy metal(loid)s contaminated sediments—in situ iron treatment and subsequent magnetic extraction

Purpose

Surface sediments contaminated with high levels of multiple heavy metal(loid) species are very common environmental problems. Especially, the labile and bioaccessible fractions of heavy metal(loid)s in the sediments are posing serious risks to the biota and the overlaying water quality. This study aimed at developing a potential method to manage the activity of the labile fractions of heavy metal(loid)s in surface sediments.

Materials and methods

This study assessed the feasibility of adding iron powder, a low-cost industrial by-product, to sediments containing high levels of Pb, As, and Cd to adsorb labile fractions of heavy metal(loid)s onto the sorbent surfaces and to retrieve the heavy metal(loid) laden powders by applying external magnetic field. In addition, the redistribution of Pb, Cd, and As in different sediment fractions, the dissolved fraction and the sorbent-adsorbed fraction, was also investigated and characterized.

Results and discussion

The results indicate that the bioactive labile fractions (exchangeable and carbonate-bound fractions) of heavy metal(loid)s are prone to concentrating onto iron powders and can be selectively removed from the sediments by magnetic retrieval. In addition, iron addition induces conversion of labile fractions of heavy metal(loid)s into more stabilized fractions.

Conclusions

Overall, the process can effectively minimize the activity of labile fractions of heavy metal(loid)s in surface sediments.

     

Long-term leaching prediction of constituents in coal bottom ash used as a structural fill material

Purpose

This study is aimed to assess the long-term leaching of inorganic constituents from structural fills composed of reused coal bottom ash in Korea and identify key parameters that affect the amount of the constituents leached.

Materials and methods

A model for the prediction of long-term leaching by percolation of stormwater through a structural fill is adopted and used. The long-term leaching model is applied to five field sites in Korea using site-specific parameters obtained for each site and coal bottom ash specific parameters determined using column studies for two coal bottom ash samples collected from coal-fired power plants.

Results and discussion

The long-term leaching of trace inorganic constituents, As, Cu, Sb, and Zn, is variable among the sites primarily due to the variation in the total amount of leachable constituents and application depth of a structural fill. First-order leaching rate constant is also one of the key parameters when the leaching rate is relatively small. Because of the significant variability in the leaching rate constants and application depths, the time for the leachate constituent concentration to reach half the initial value, t _50%, ranges from less than a year to more than hundreds of years for the studied sites and constituents.

Conclusions

The long-term leaching characteristics of the trace inorganic constituents are predicted to significantly vary by the type of reused bottom ash and the site conditions, suggesting the need to determine the model parameters in a case-specific manner.

     

Geochemical characterisation of surface waters,topsoils and efflorescences in a historic metal-mining area in Spain

Purpose

Mining activities generate large amounts of wastes that may contain potentially toxic elements (PTE), which, if released into the environment, may cause air, water and soil pollution long after mining operations have ceased. This paper describes the environmental relevance of efflorescences, topsoils and surface waters from the abandoned mine of Sierra Minera of Cartagena-La Unión, SE Spain. The exposure of the population to arsenic and the associated risk were also assessed.

Materials and methods

A total of 10 topsoils and surface waters and 11 efflorescences affected at varying degrees by mining activities were studied. The total potentially toxic element content (As, Cd, Cu, Fe, Pb and Zn) was determined in all samples. In addition, the mineralogical composition of solid samples was determined by X-ray diffraction and some efflorescences were also analysed using a scanning electron microscopy-energy dispersive X-ray spectrometer. Finally, an arsenic-intake risk assessment was carried out, both as regards carcinogenic and non-carcinogenic effects and considering the total and the bioaccesible As content.

Results and discussion

The study area is heavily polluted as a result of historical mining and processing activities, during which time great amounts of wastes were produced, characterised by a high PTE content, acidic pH and minerals resulting from supergene alteration. The supergene mineralogical assemblages include soluble metal salts, mainly sulphates, iron hydroxysulphates and iron oxyhydroxides, all of which form ochreous precipitates. Topsoil samples showed risk values and hazard quotients higher than the reference levels, particularly for children. In the efflorescences, these values were lower but still unacceptable.

Conclusions

The efflorescences are of significance for monitoring purposes because they are involved in cycles of retention release of hydrogen ions, sulphate and potentially toxic elements. In addition, in a semi-arid climate, such as the study area, these minerals contribute to our understanding of the response of the system to episodic rainfall events. In general, it was observed that the arsenic in collected samples represent a potential risk for human health through ingestion.

     

Iron–manganese concretions contribute to benthic release of phosphorus and arsenic in anoxic conditions in the Baltic Sea

Purpose

Deposits of iron–manganese (Fe, Mn) concretions forming a large storage of phosphorus (P) and arsenic (As) are frequently under pressure of oscillating oxygen conditions in the eutrophic Gulf of Finland, the Baltic Sea. Yet, there is a poor understanding how anaerobic microbial processes regulate the cycling of elements in the concretions. The objective of this study was to highlight how the microbial processes control the release of elements from the concretions to brackish water during anoxia.

Materials and methods

Spherical concretions were collected from the oxic bottoms of the Gulf of Finland in the summer. Concretions and autoclaved controls were incubated in anoxic artificial brackish seawater with and without labile carbon, plus supplied with ammonium at 5, 10, and 20 °C for 15 weeks. Concentrations of Fe, Mn, P, and As were measured from the intact concretions and the ambient solutions during the experiment. Also, the consumption of the added ammonium and organic carbon and the formation of dissolved inorganic carbon were measured.

Results and discussion

At near-bottom temperature 5 °C, the concretions released at highest 0.12, 0.42, 0.02, and 0.0002 μmol g^?1 day^?1 of Fe, Mn, P, and As, respectively. The rates were significant only in the microcosms with added labile carbon, and only minor proportions (0.1–0.4 %) of their total contents were released during the incubations. The concretions removed completely the supplied ammonium only without carbon addition. We find that concretion deposit may form a local hot spot for the microbial reduction of Fe and Mn and release significant amounts of P and As, and participate in N cycling besides the bottom sediments of the Gulf of Finland during prolonged anoxia. However, the concretions may maintain their binding capacity for P and As longer than the fine-grained organic-rich sediment during anoxia.

Conclusions

During anoxia concretion deposits may form a temporal source of bioavailable P having ecological significance in the Gulf of Finland when concretions have access to labile organic carbon. Concretions from the Baltic Sea, the oceans, lakes, and soils contain high concentrations of Mn and Fe, but their proportions vary considerably. Anaerobic microbial processes may thus affect the stability of concretions from the different environments, but the outcome may depend on the ambient geochemical conditions.

     

Assessment of the adaptive capacity of plant species in copper mine tailings in arid and semiarid environments

Purpose

The objective of this work was to identify hyperaccumulator plants and evaluate their capacity on copper mine tailings in the Antofagasta Region (Chile), considered one of the most arid in the world.

Materials and methods

Two native plant species, Gazania rigens and Pelargonium hortorum, were grown during 11 weeks on mine tailings. The physico-chemical characterization of the mine tailings under study indicated that the substrate required conditioning to support a phytoremediation system. In this respect, organic and inorganic amendments and mycorrizhal fungi were added to the substrate. Three treatments were designed to assess the effects of the amendments through an analysis of variance.

Results and discussion

Indicators of plant growth and development were measured weekly, and concentrations of Cd, Cu, Fe, Mn, Pb, Al, and Zn in roots of tailing-grown plants and substrate were measured at the end of the experiment.

Conclusions

The results were used to determine the bioconcentration factor (BCF), which demonstrated that both species act as excluders of Fe, Mn, Pb, Al, and Zn. In addition, it was found that both species present characteristics of potential accumulators of Cu.

     

Soil pollution by mining activities in Andalusia (South Spain)—the role of Mineralogy and Geochemistry in three case studies

Purpose

Geochemical and mineralogical studies of soils potentially polluted by trace elements are basic to find the source of pollution, to understand the behavior of the contaminants in the environment, and to propose remediation and reclamation actions. This work reviews the role of the Mineralogy and Geochemistry to assess the hazard of soil contamination by trace elements in mining areas, focusing on three different case studies carried out in the Andalusian community (South Spain), with a Mediterranean temperate climate.

Materials and methods

Two significant mining districts were selected for this work: the Linares-La Carolina (Pb-Zn ores) and the Riotinto (Iberian Pyrite Belt, IPB) mining areas. Another case study was the Guadiamar basin, which soil was polluted by the spill produced in 1998 by the breakage of a mine tailing impoundment in Aznalcóllar mines (IPB). Soils, mine waste, and secondary precipitates were studied to approach the source of the pollution and the fate of the contaminants. Chemical composition (major and trace elements), soil parameters, and mineralogy of all materials selected were studied. In addition, the bioavailability of trace elements was determined by different methods.

Results and discussion

Pyrite and secondary phases are the main sources of pollution in the Riotinto area. Their stability is a key factor in the release of trace elements. The availability of trace elements in soils was lower in unpolluted leptosols than in contaminated orchards. In Linares-La Carolina, a severe pollution by Pb and a high availability (10–70% extracted with EDTA) were found. In Guadiamar basin, the residual pyrite sludge continues releasing trace elements to soil. Cd and Zn were mobile at pH <?6 and As at pH >?8, and Pb was quite immobile. Cd, Zn, and Pb can be coprecipitated by carbonates while As is mainly adsorbed by clays and iron oxyhydroxides.

Conclusions

The geochemical studies of soils polluted by mining activities allow to evaluate the availability of trace elements and their retention in soils. Therefore, geochemical and mineralogical studies are necessary for the assessment of soil pollution and remediation actions.

     

Development of microbial community structure in vegetable-growing soils from open-field to plastic-greenhouse cultivation based on the PLFA analysis

Purpose

The objectives of this study were to investigate (i) how the changes in cultivation pattern of vegetable affect soil microbial communities and (ii) the relationships between soil physico-chemical properties and microbial community structure.

Materials and methods

Soil samples were collected from fields growing vegetable crops with various times of plastic-greenhouse cultivation (0, 1, 4, 7 and 15 years, respectively). Phospholipid fatty acid (PLFA) analysis was conducted to reveal the soil microbial community of the test soils.

Results and discussion

The open-field soil had the highest total PLFAs amount. Although the Shannon-Weaver index was also highest in the open-field soil, the difference was not significant. Plastic-greenhouse cultivation decreased PLFAs species diversity and richness. Cluster analysis and principal component analysis (PCA) of the PLFA profiles revealed distinct groupings at different times during plastic-greenhouse cultivation.

Conclusions

Ultimately, PLFA analyses showed that long-term plastic-greenhouse cultivation make the physiological status of soil microbial community worse and increased stress level of microorganisms. And soil microbial community was significantly affected by field water capacity and water-soluble organic carbon. The study highlights the potential risk of long-term plastic-greenhouse cultivation to soil microbial community.

     

The influence of soil properties and geographical distance on the bacterial community compositions of paddy soils enriched on SMFC anodes

Purpose

Exoelectrogens are important microorganisms playing crucial roles in the biogeochemistry of elements in paddy soils. But it remains unclear how the soil properties and geographical distances affect the exoelectrogen communities of Chinese paddy soils. So the objectives of this study were to investigate the diversity and composition of these microbial communities which were enriched on the anodes of soil microbial fuel cells (SMFCs) and to elucidate the links between the microbial community compositions and their driving factors.

Materials and methods

We used Illumina HiSeq sequencing to determine the bacterial community structures which were enriched on the anodes of SMFCs. Variance partitioning analysis (VPA) was used to obtain the contribution of soil properties and geographical distance to the variations of bacterial communities.

Results and discussion

Active bacterial community on anodes of the closed circuit SMFCs differs significantly from the control open circuit SMFCs. Anodes of all the closed circuit SMFCs were characterized by the presence of high numbers of Nitrospira and Anaerolineae. Taxonomic similarities and phylogenetic similarities of bacterial communities from different paddy soil samples across North and South China were found to be significantly correlated with geographical distances. The relationship between the similarities and the geographic distance exhibited a distance-decay relationship. VPA showed that both geographical distances and soil properties affect the structure of bacterial communities detected on anodes.

Conclusions

Our study gives a foundation for understanding the distribution and diversity of exoelectrogens in paddy soils and elucidates the links between the distribution and the diversity of extracellular respiring bacteria and their driving factors. Furthermore, this study also identifies the crucial factors which should be used to evaluate the response of exoelectrogens to environmental perturbations in Chinese paddy soils.

     

An efficient approach towards the bioremediation of copper,cobalt and nickel contaminated field samples

Purpose

The synthetic soil based bioremediation approach as reasonable and sustainable practice at the farming level where desired bioremediation could be established at lower cost.

Materials and methods

Metal-tolerant bacteria from different environmental field samples, (a) a municipal dump site, (b) an agricultural field and (c) sludge of electro-plating industries, were screened and characterized. Bioremediation of metal contaminants through isolated bacteria was compared under two different conditions, synthetic soil and basic minimal media containing copper, cobalt and nickel.

Results and discussion

The pollutants arising from industrial effluents are imparting a huge negative impact on agricultural land. Microbes are predominant in heavy metal-contaminated sites, which signifies as a potential opportunity for the researchers towards bioremediation. Three bacterial species showed high metal tolerance; 16S ribosomal DNA (rDNA) analysis revealed that the organisms were Proteus vulgaris strain, Stenotrophomonas sp. and Bacillus thuringiensis. Percentage removal of metals was also analysed under different concentrations and pH.

Conclusions

The current tested methods are helpful in streamlining the natural compliance of fragile elements and its uptake into the microbial system under in vitro and in situ conditions.

     

Early detection of the effects of compaction in forested soils: evidence from selective extraction techniques

Purpose

Soil compaction resulting from mechanisation of forest operations reduces air permeability and hydraulic conductivity of soil and can result in the development of hydromorphic and/or anoxic conditions. These hydromorphic conditions can affect physico-chemical properties of the soils. However, early detection of these effects on mineralogical portion of soils is methodologically difficult.

Materials and methods

To analyse the effects of soil compaction on iron minerals in loamy Luvisol, three compacted and three non-compacted soil profiles up to the depth of 50 cm were collected from an artificially deforested and compacted soils after 2 years of treatment. Soil was compacted with the help of 25 Mg wheeler’s load to increase the dry bulk density of soil from 1.21?±?0.05 to 1.45?±?0.1 g cm^?3. Soil samples were analysed by X-ray diffraction (XRD) and were treated by citrate bicarbonate (CB) and dithionite citrate bicarbonate (DCB) under controlled conditions. Major and minor elements (Fe, Al, Mg, Si and Mn) were analysed by ICP-AES in the CB and DCB extracts.

Results and discussion

It was found that X-ray diffraction is not an enough sensitive method to detect the quick mineralogical changes due to soil compaction. Results obtained from CB-DCB extractions showed that soil compaction resulted in larger CB and smaller DCB extractable elements as compared to non-compacted soil. Labile Fe was found 30 % of total Fe oxides in compacted soil against 10–14 % in non-compacted soils. Compaction thus resulted in Fe transfer from non-labile to labile oxides (s.l.). Results showed that soil compaction leads to the reduction of Fe³⁺ to Fe²⁺. The effects of hydromorphic conditions due to soil compaction were observed up to the depth of 35 cm in forest soil profile. Furthermore, a close association of Al with Fe oxides was observed in the soil samples, while Mn and Si were mainly released from other sources, Mg showing an intermediate behaviour.

Conclusions

Hydromorphic conditions owing to soil compaction affect the mobility and crystallisation process of iron mineral. CB-DCB selective extraction technique, in contrast to XRD technique, can be effectively used to examine the possible effects of soil compaction on iron minerals.

     

Soil nitrogen availability alters rhizodeposition carbon flux into the soil microbial community

Purpose

Soil microorganisms are important in the cycling of plant nutrients. Soil microbial biomass, community structure, and activity are mainly affected by carbon substrate and nutrient availability. The objective was to test if both the overall soil microbial community structure and the community-utilizing plant-derived carbon entering the soil as rhizodeposition were affected by soil carbon (C) and nitrogen (N) availability.

Materials and methods

A ¹³C-CO₂ steady-state labeling experiment was conducted in a ryegrass system. Four soil treatments were established: control, amendment with carboxymethyl cellulose (CMC), amendment with ammonium nitrate (NF), combined CMC and NF. Soil phospholipid fatty acid (PLFA) and ¹³C labeling PLFA were extracted and detected by isotope ratio mass spectrometer.

Results and discussion

The combined CMC and NF treatment with appropriate C/N ratio (20) significantly enhanced soil microbial biomass C and N, but resulted in lower soil inorganic N concentrations. There was no significant difference in soil PLFA profile pattern between different treatments. In contrast, most of the ¹³C was distributed into PLFAs 18:2ω6,9c, 18:1ω7c, and 18:1ω9c, indicative of fungi and gram-negative bacteria. The inorganic-only treatment was distinct in ¹³C PLFA pattern from the other treatments in the first period of labeling. Factor loadings of individual PLFAs confirmed that gram-positive bacteria had relatively greater plant-derived C contents in the inorganic-only treatment, but fungi were more enriched in the other treatments.

Conclusions

Amendments with CMC can improve N transformation processes, and the ryegrass rhizodeposition carbon flux into the soil microbial community is strongly modified by soil N availability.