Cr isotopic insights into ca. 1.9 Ga oxidative weathering of the continents using the Beaverlodge Lake paleosol,Northwest Territories,Canada

The ca. 1.9 Ga Beaverlodge Lake paleosol was studied using redox‐sensitive Cr isotopes in order to determine the isotopic response to paleoweathering of a rhyodacite parent rock 500 million years after the Great Oxidation Event. Redox reactions occurring in modern weathering environments produce Cr(VI) that is enriched in heavy Cr isotopes compared to the igneous inventory. Cr(VI) species are soluble and easily leached from soils into streams and rivers, thus, leaving particle‐reactive and isotopically light Cr(III) species to build up in soils. The Beaverlodge Lake paleosol and two other published weathering profiles of similar age, the Flin Flon and Schreiber Beach paleosols, are not as isotopically light as modern soils, indicating that rivers were not as isotopically heavy at that time. Considering that the global average δ⁵³Cr value for the oxidative weathering flux of Cr to the oceans today is just 0.27 ± 0.30‰ (1σ) based on a steady‐state analysis of the modern ocean Cr cycle, the oxidative weathering flux of Cr to the oceans at ca. 1.9 Ga would have likely been shifted to lower δ⁵³Cr values, and possibly lower than the igneous inventory (–0.12 ± 0.10‰, 2σ). Mn oxides are the main oxidant of Cr(III) in modern soils, but there is no evidence that they formed in the studied paleosols. Cr(VI) may have formed by direct oxidation of Cr(III) using molecular oxygen or H₂O₂, but neither pathway is as efficient as Mn oxides for producing Cr(VI). The picture that emerges from this and other studies of Cr isotope variation in ca. 1.9 Ga paleosols is of atmospheric oxygen concentrations that are high enough to oxidize iron, but too low to oxidize Mn, resulting in low Cr(VI) inventories in Earth surface environments.     

Microbial Communities Associated with Tree Bark Foliose Lichens: A Perspective on their Microecology

Tree‐bark, foliose lichens occur widely on a global scale. In some locales, such as forests, they contribute a substantial amount of biomass. However, there are few research reports on microbial communities including eukaryotic microbes associated with foliose lichens. Lichens collected from tree bark at 11 locations (Florida, New York State, Germany, Australia, and the Arctic) were examined to determine the density and C‐biomass of bacteria and some eukaryotic microbes, i.e. heterotrophic nanoflagellates (HNF) and amoeboid protists. A rich microbial diversity was found, including large plasmodial slime molds, in some cases exceeding 100 μm in size. The densities of HNF and amoeboid protists were each positively correlated with densities of bacteria, r = 0.84 and 0.80, respectively (p < 0.01, N = 11 for each analysis) indicating a likely bacterial‐based food web. Microbial densities (number/g lichen dry weight) varied markedly across the geographic sampling sites: bacteria (0.7–13.1 × 10⁸), HNF (0.2–6.8 × 10⁶) and amoeboid protists (0.4–4.6 × 10³). The ranges in C‐biomass (μg/g lichen dry weight) across the 11 sites were: bacteria (8.8–158.5), HNF (0.03–0.85), and amoeboid protists (0.08–540), the latter broad range was due particularly to absence or presence of large slime mold plasmodia.     

Multiplying the heterologous production of spinosad through tandem amplification of its biosynthetic gene cluster in Streptomyces coelicolor

Heterologous expression of the biosynthetic gene cluster (BGC) is important for studying the microbial natural products (NPs), especially for those kept in silent or poorly expressed in their original strains. Here, we cloned the spinosad BGC through the Cas9-Assisted Targeting of Chromosome segments and amplified it to five copies through a ZouA-dependent DNA amplification system in Streptomyces coelicolor M1146. The resulting strain produced 1253.9 ± 78.2 μg l⁻¹ of spinosad, which was about 224-fold compared with that of the parent strain carrying only one copy of the spinosad BGC. Moreover, we further increased spinosad to 1958.9 ± 73.5 μg l⁻¹ by the dynamic regulation of intracellular triacylglycerol degradation. Our study indicates that tandem amplification of the targeted gene cluster is particularly suitable to enhance the heterologous production of valuable NPs with efficiency and simplicity.     

Isolation,identification and characterization of soil microbes which degrade phenolic allelochemicals

Aims: To isolate and characterize microbes in the soils containing high contents of phenolics and to dissolve the allelopathic inhibition of plants through microbial degradation. Methods and Results: Four microbes were isolated from plant soils using a screening medium containing p‐coumaric acid as sole carbon source. The isolates were identified by biochemical analysis and sequences of their 16S or 18S rDNA, and designated as Pseudomonas putida 4CD1 from rice (Oryza sativa) soil, Ps. putida 4CD3 from pine (Pinus massoniana) soil, Pseudomonas nitroreducens 4CD2 and Rhodotorula glutinis 4CD4 from bamboo (Bambusa chungii) soil. All isolates degraded 1 g l^?1 of p‐coumaric acid by 70–93% in inorganic and by 99% in Luria‐Bertani solutions within 48 h. They also effectively degraded ferulic acid, p‐hydroxybenzoic acid and p‐hydroxybenzaldehyde. The microbes can degrade p‐coumaric acid and reverse its inhibition on seed germination and seedling growth in culture solutions and soils. Low pHs inhibited the growth and phenolic degradation of the three bacteria. High temperature inhibited the R. glutinis. Co²⁺ completely inhibited the three bacteria, but not the R. glutinis. Cu²⁺, Al³⁺, Zn²⁺, Fe³⁺, Mn²⁺, Mg²⁺ and Ca²⁺ had varying degrees of inhibition for each of the bacteria. Conclusions: Phenolics in plant culture solutions and soils can be decomposed through application of soil microbes in laboratory or controlled conditions. However, modification of growth conditions is more important for acidic and ions‐contaminated media. Significance and Impact of the Study: The four microbes were first isolated and characterized from the soils of bamboo, rice or pine. This study provides some evidence and methods for microbial control of phenolic allelochemicals.     

Denitrification and nitrous oxide emissions from riparian forests soils exposed to prolonged nitrogen runoff

Compared to upland forests, riparian forest soils have greater potential to remove nitrate (NO₃) from agricultural runoff through denitrification. It is unclear, however, whether prolonged exposure of riparian soils to nitrogen (N) loading will affect the rate of denitrification and its end products. This research assesses the rate of denitrification and nitrous oxide (N₂O) emissions from riparian forest soils exposed to prolonged nutrient runoff from plant nurseries and compares these to similar forest soils not exposed to nutrient runoff. Nursery runoff also contains high levels of phosphate (PO₄). Since there are conflicting reports on the impact of PO₄ on the activity of denitrifying microbes, the impact of PO₄ on such activity was also investigated. Bulk and intact soil cores were collected from N-exposed and non-exposed forests to determine denitrification and N₂O emission rates, whereas denitrification potential was determined using soil slurries. Compared to the non-amended treatment, denitrification rate increased 2.7- and 3.4-fold when soil cores collected from both N-exposed and non-exposed sites were amended with 30 and 60 μg NO₃-N g⁻¹ soil, respectively. Net N₂O emissions were 1.5 and 1.7 times higher from the N-exposed sites compared to the non-exposed sites at 30 and 60 μg NO₃-N g⁻¹ soil amendment rates, respectively. Similarly, denitrification potential increased 17 times in response to addition of 15 μg NO₃-N g⁻¹ in soil slurries. The addition of PO₄ (5 μg PO₄-P g⁻¹) to soil slurries and intact cores did not affect denitrification rates. These observations suggest that prolonged N loading did not affect the denitrification potential of the riparian forest soils; however, it did result in higher N₂O emissions compared to emission rates from non-exposed forest soils.     

Shifts in priming partly explain impacts of long‐term nitrogen input in different chemical forms on soil organic carbon storage

Input of labile organic carbon can enhance decomposition of extant soil organic carbon (SOC) through priming. We hypothesized that long‐term nitrogen (N) input in different chemical forms alters SOC pools by altering priming effects associated with N‐mediated changes in plants and soil microbes. The hypothesis was tested by integrating field experimental data of plants, soil microbes and two incubation experiments with soils that had experienced 10 years of N enrichment with three chemical forms (ammonium, nitrate and both ammonium and nitrate) in an alpine meadow on the Tibetan Plateau. Incubations with glucose–¹³C addition at three rates were used to quantify effects of exogenous organic carbon input on the priming of SOC. Incubations with microbial inocula extracted from soils that had experienced different long‐term N treatments were conducted to detect effects of N‐mediated changes in soil microbes on priming effects. We found strong evidence and a mechanistic explanation for alteration of SOC pools following 10 years of N enrichment with different chemical forms. We detected significant negative priming effects both in soils collected from ammonium‐addition plots and in sterilized soils inoculated with soil microbes extracted from ammonium‐addition plots. In contrast, significant positive priming effects were found both in soils collected from nitrate‐addition plots and in sterilized soils inoculated with soil microbes extracted from nitrate‐addition plots. Meanwhile, the abundance and richness of graminoids were higher and the abundance of soil microbes was lower in ammonium‐addition than in nitrate‐addition plots. Our findings provide evidence that shifts toward higher graminoid abundance and changes in soil microbial abundance mediated by N chemical forms are key drivers for priming effects and SOC pool changes, thereby linking human interference with the N cycle to climate change.     

Leaching of Zinc,Cadmium, and Lead in a Sandy Soil Due to Application of Poultry Litter

Dissolved organic matter in poultry litter could contribute organic ligands to form complexes with heavy metals in soil. The soluble complexes with heavy metals can be transported downward and possibly deteriorate groundwater quality. To better understand metal mobilization by soluble organic ligands in poultry litter, soil columns were employed to investigate the movement of zinc (Zn), cadmium (Cd), and lead (Pb). Uncontaminated soil was amended with Zn, Cd, and Pb at rates of 400, 8, and 200 mg kg ^{? 1} soil, respectively. Glass tubes, 4.9-cm-diameter and 40-cm-long, were packed with either natural or metal-amended soil. The resulting 20-cm-long column of soils had bulk density of about 1.58 g cm ^{? 3} . Columns repacked with natural or amended soil were leached with distilled water, 0.01 M EDTA, 0.01 M CaCl ₂ , or poultry litter extract (PLE) solutions. Low amounts of Zn, Cd, and Pb were leached from natural soil with the solutions. Leaching of Zn, Cd, or Pb was negligible with distilled water. In the metal-amended soil, EDTA solubilized more Zn, Cd, and Pb than CaCl ₂ and PLE. The breakthrough curves of Zn and Pb in the PLE and CaCl ₂ were similar, indicating they have similar ability to displace Zn and Pb from soils. Compared with Zn and Cd the PLE had a small ability to solubilize Pb from metal-amended soil. Thus, the application of poultry litter on metal-contaminated soils might enhance the mobility of Zn and Cd.     

Soil microbial community shifts explain habitat heterogeneity in two Haloxylon species from a nutrient perspective

Haloxylon ammodendron and Haloxylon persicum (as sister taxa) are dominant shrubs in the Gurbantunggut Desert. The former grows in inter-dune lowlands while the latter in sand dunes. However, little information is available regarding the possible role of soil microorganisms in the habitat heterogeneity in the two Haloxylon species from a nutrient perspective. Rhizosphere is the interface of plant–microbe–soil interactions and fertile islands usually occur around the roots of desert shrubs. Given this, we applied quantitative real-time PCR combined with MiSeq amplicon sequencing to compare their rhizosphere effects on microbial abundance and community structures at three soil depths (0–20, 20–40, and 40–60 cm). The rhizosphere effects on microbial activity (respiration) and soil properties had also been estimated. The rhizospheres of both shrubs exerted significant positive effects on microbial activity and abundance (e.g., eukarya, bacteria, and nitrogen-fixing microbes). The rhizosphere effect of H. ammodendron on microbial activity and abundance of bacteria and nitrogen-fixing microbes was greater than that of H. persicum. However, the fertile island effect of H. ammodendron was weaker than that of H. persicum. Moreover, there existed distinct differences in microbial community structure between the two rhizosphere soils. Soil available nitrogen, especially nitrate nitrogen, was shown to be a driver of microbial community differentiation among rhizosphere and non-rhizosphere soils in the desert. In general, the rhizosphere of H. ammodendron recruited more copiotrophs (e.g., Firmicutes, Bacteroidetes, and Proteobacteria), nitrogen-fixing microbes and ammonia-oxidizing bacteria, and with stronger microbial activities. This helps it maintain a competitive advantage in relatively nutrient-rich lowlands. Haloxylon persicum relied more on fungi, actinomycetes, archaea (including ammonia-oxidizing archaea), and eukarya, with higher nutrient use efficiency, which help it adapt to the harsher dune crests. This study provides insights into the microbial mechanisms of habitat heterogeneity in two Haloxylon species in the poor desert soil.     

Fe(III), Cr(VI), and Fe(III) mediated Cr(VI) reduction in alkaline media using a <Emphasis Type="Italic">Halomonas</Emphasis> isolate from Soap Lake,Washington

Hexavalent chromium is one of the most widely distributed environmental contaminants. Given the carcinogenic and mutagenic consequences of Cr(VI) exposure, the release of Cr(VI) into the environment has long been a major concern. While many reports of microbial Cr(VI) reduction are in circulation, very few have demonstrated Cr(VI) reduction under alkaline conditions. Since Cr(VI) exhibits higher mobility in alkaline soils relative to pH neutral soils, and since Cr contamination of alkaline soils is associated with a number of industrial activities, microbial Cr(VI) reduction under alkaline conditions requires attention. Soda lakes are the most stable alkaline environments on earth, and contain a wide diversity of alkaliphilic organisms. In this study, a bacterial isolate belonging to the Halomonas genus was obtained from Soap Lake, a chemically stratified alkaline lake located in central Washington State. The ability of this isolate to reduce Cr(VI) and Fe(III) was assessed under alkaline (pH = 9), anoxic, non-growth conditions with acetate as an electron donor. Metal reduction rates were quantified using Monod kinetics. In addition, Cr(VI) reduction experiments were carried out in the presence of Fe(III) to evaluate the possible enhancement of Cr(VI) reduction rates through electron shuttling mechanisms. While Fe(III) reduction rates were slow compared to previously reported rates, Cr(VI) reduction rates fell within range of previously reported rates.     

Chromium(VI) resistance and removal by <Emphasis Type="Italic">Acinetobacter haemolyticus</Emphasis>

The present work highlighted the studies on Cr(VI) reduction by cells of Acinetobacter haemolyticus (A. haemolyticus). The strain tolerated 90 mg Cr(VI) l⁻¹ in LB broth compared to only 30 mg Cr(VI) l⁻¹ in LB agar. From the FTIR analysis, the Cr(III) species formed was also most likely to form complexes with carboxyl, hydroxyl, and amide groups from the bacteria. A TEM study showed the absence of precipitates on the cell wall region of the bacteria. Instead, microprecipitates were observed in the cytoplasmic region of the cells, suggesting the transportation of Cr(VI) into the cells. Intracellular reduction of Cr(VI) was supported by a reductase test using soluble crude cell-free extracts. The specific reductase activity obtained was 0.52 μg Cr(VI) reduced per mg of protein an hour at pH 7.2 and 37°C. Our results indicated that A. haemolyticus can be used as a promising microorganism for Cr(VI) reduction from industrial wastewaters.     

Light emission from the Fe2+–EDTA–ascorbic acid–H2O2 system strongly enhanced by plant phenolic acids

Oxidative reactions can result in the formation of electronically excited species that undergo radiative decay depending on electronic transition from the excited state to the ground state with subsequent ultra‐weak photon emission (UPE). We investigated the UPE from the Fe²⁺–EDTA (ethylenediaminetetraacetic acid)–AA (ascorbic acid)–H₂O₂ (hydrogen peroxide) system with a multitube luminometer (Peltier‐cooled photon counter, spectral range 380–630 nm). The UPE, of 92.6 μmol/L Fe²⁺, 185.2 μmol/L EDTA, 472 μmol/L AA, 2.6 mmol/L H₂O₂, reached 1217 ± 118 relative light units during 2 min measurement and was about two times higher (P < 0.001) than the UPE of incomplete systems (Fe²⁺–AA–H₂O₂, Fe²⁺–EDTA–H₂O₂, AA–H₂O₂) and medium alone. Substitution of Fe²⁺ with Cr²⁺, Co²⁺, Mn²⁺ or Cu²⁺ as well as of EDTA with EGTA (ethylene glycol‐bis(β‐aminoethyl ether)‐N,N,N′,N′‐tetraacetic acid) or citrate powerfully inhibited UPE. Experiments with scavengers of reactive oxygen species (dimethyl sulfoxide, mannitol, sodium azide, superoxide dismutase) revealed the dependence of UPE only on hydroxyl radicals. Dimethyl sulfoxide at the concentration of 0.74 mmol/L inhibited UPE by 79 ± 4%. Plant phenolics (ferulic, chlorogenic and caffec acids) at the concentration of 870 μmol/L strongly enhanced UPE by 5‐, 13.9‐ and 46.8‐times (P < 0.001), respectively. It is suggested that augmentation of UPE from Fe²⁺–EDTA–AA–H₂O₂ system can be applied for detection of these phytochemicals.     

Ocean acidification reduces induction of coral settlement by crustose coralline algae

Crustose coralline algae (CCA) are a critical component of coral reefs as they accrete carbonate for reef structure and act as settlement substrata for many invertebrates including corals. CCA host a diversity of microorganisms that can also play a role in coral settlement and metamorphosis processes. Although the sensitivity of CCA to ocean acidification (OA) is well established, the response of their associated microbial communities to reduced pH and increased CO₂ was previously not known. Here we investigate the sensitivity of CCA‐associated microbial biofilms to OA and determine whether or not OA adversely affects the ability of CCA to induce coral larval metamorphosis. We experimentally exposed the CCA Hydrolithon onkodes to four pH/pCO₂ conditions consistent with current IPCC predictions for the next few centuries (pH: 8.1, 7.9, 7.7, 7.5, pCO₂: 464, 822, 1187, 1638 μatm). Settlement and metamorphosis of coral larvae was reduced on CCA pre‐exposed to pH 7.7 (pCO₂ = 1187 μatm) and below over a 6‐week period. Additional experiments demonstrated that low pH treatments did not directly affect the ability of larvae to settle, but instead most likely altered the biochemistry of the CCA or its microbial associates. Detailed microbial community analysis of the CCA revealed diverse bacterial assemblages that altered significantly between pH 8.1 (pCO₂ = 464 μatm) and pH 7.9 (pCO₂ = 822 μatm) with this trend continuing at lower pH/higher pCO₂ treatments. The shift in microbial community composition primarily comprised changes in the abundance of the dominant microbes between the different pH treatments and the appearance of new (but rare) microbes at pH 7.5. Microbial shifts and the concomitant reduced ability of CCA to induce coral settlement under OA conditions projected to occur by 2100 is a significant concern for the development, maintenance and recovery of reefs globally.     

EDTA and industrial waste water improving the bioavailability of different Cu forms in contaminated soil

It is common to find that low bioavailability can prevent the phytoremediation process of heavy metal-contaminated soils. Heavy metals in soil are associated with various forms having different bioavailability. In this study, the bioavailability of various Cu forms in contaminated soils was investigated using ion-exchange resins, a sequential extraction procedure, and combined with methods including partial dissolution procedure, simulated Cu forms, seedling culture, pot experiment when treated with EDTA, or waste water from monosodium glutamate and citric acid production. Results showed that the bioavailability, in decreasing order of different Cu forms to tall fescue (Festuca arundinacea Schreb.) is: exchangeable Cu (EX-Cu) and organic matter bound Cu (OM-Cu)> Cu bound to carbonate (CAB-Cu)> Fe/Mn oxide bound Cu (OX-Cu)> residual Cu (RES-Cu). Effect of EDTA on the activation of Cu contaminated soil or simulated Cu and the uptake and translocation of tall fescue was better than that of monosodium glutamate waste water (MGW) and citric acid waste water (CAW). EDTA, CAW and MGW all improved the plant availability of different Cu forms in contaminated soil, which could be used in chelate-assisted phytoremediation of heavy metal polluted soil.     

The trade-off between growth rate and yield in microbial communities and the consequences for under-snow soil respiration in a high elevation coniferous forest

Soil microbial respiration is a critical component of the global carbon cycle, but it is uncertain how properties of microbes affect this process. Previous studies have noted a thermodynamic trade-off between the rate and efficiency of growth in heterotrophic organisms. Growth rate and yield determine the biomass-specific respiration rate of growing microbial populations, but these traits have not previously been used to scale from microbial communities to ecosystems. Here we report seasonal variation in microbial growth kinetics and temperature responses (Q₁₀) in a coniferous forest soil, relate these properties to cultured and uncultured soil microbes, and model the effects of shifting growth kinetics on soil heterotrophic respiration (R_h). Soil microbial communities from under-snow had higher growth rates and lower growth yields than the summer and fall communities from exposed soils, causing higher biomass-specific respiration rates. Growth rate and yield were strongly negatively correlated. Based on experiments using specific growth inhibitors, bacteria had higher growth rates and lower yields than fungi, overall, suggesting a more important role for bacteria in determining R_h. The dominant bacteria from laboratory-incubated soil differed seasonally: faster-growing, cold-adapted Janthinobacterium species dominated in winter and slower-growing, mesophilic Burkholderia and Variovorax species dominated in summer. Modeled R_h was sensitive to microbial kinetics and Q₁₀: a sixfold lower annual R_h resulted from using kinetic parameters from summer versus winter communities. Under the most realistic scenario using seasonally changing communities, the model estimated R_h at 22.67 mol m⁻² year⁻¹, or 47.0% of annual total ecosystem respiration (R_e) for this forest.     

A decade of land use contributes to changes in the chemistry,biochemistry and bacterial community structures of soils in the Cerrado

The bacterial community structures (BCSs) of Cerrado soils cultivated under conventional tillage (CT), no-tillage (NT) and under native Cerrado (NC) vegetation were evaluated using PCR/DGGE of bacterial 16S rRNA (rrs) and rpoB genes and of Pseudomonas group genes. Soil chemical analysis, microbial biomass and the enzyme activities were also evaluated and correlated with the BCS measurements. The multivariate ordinations of DGGE profiles showed differences between the BCS of the NC area and those from cultivated areas. The BCSs of the CT and NT areas also differed in all DGGE fingerprints, including changes in the profile of Pseudomonas populations, indicating that agricultural systems can also be responsible for changes within specific microbial niches, although the clearest differences were found in the rpoB profiles. The MRPP analysis demonstrated significant differences between the BCSs from different soil layers of NT areas based on all gene fingerprints and those of NC areas based on bacterial 16S rRNA and rpoB genes fingerprints. No differences were observed in the microbial fingerprints of CT samples from different depths, indicating that ploughing affected the original BCS stratification. The BCS from NC areas, based on all gene fingerprints, could be related to higher levels of soil acidity and higher amounts of MBC and of phosphatase activity. In contrast, the BCSs from cultivated areas were related to higher levels of Ca + Mg, P and K, likely as a result of a history of chemical fertilisation in these areas. The relationships between rpoB and Pseudomonas BCSs and all chemical and biochemical properties of soils were significant, according to a Mantel test (P < 0.05), indicating that the different changes in soil properties induced by soil use or management may drive the formation of the soil BCS.     

Isolation and Molecular Characterization of Heavy Metal-Resistant Alcaligenes faecalis from Sewage Wastewater and Synthesis of Silver Nanoparticles

Environmental pollution with toxic heavy metals is increasing throughout the world alongside industrial development. Microorganisms and microbial products can be highly efficient bioaccumulators of soluble and particulate forms of metals, especially dilute external solutions. Microbe related technologies (Biotechnology) may provide an alternative or additive conventional method for metal removal or metal recovery. This study dealt with isolation, identification and characterization of heavy metal-resistant (Pb²⁺, Cd²⁺, Al³⁺, Cu²⁺, Ag²⁺ and Sn²⁺) bacteria from sewage wastewater at Taif Province, Saudi Arabia. Nine bacterial isolates were selected by using an enrichment isolation procedure based on high level of heavy metal resistance. All the isolates showed high resistance to heavy metals with Minimum Inhibitor Concentration (MIC) ranging from 800 μg/ml to 1400 μg/ml. All nine resistant isolates showed multiple tolerances to heavy metals. On the basis of morphological, biochemical and 16S rRNA characterization, the most potent isolates (Cd1-1, Ag1-1, Ag1-3 and Sn1-1) were identified as Alcaligenes faecalis. Scanning electron microscope analysis showed that the morphology of Alcaligenes faecalis Ag1-1 was unchanged after growth in medium without and with addition of Ag²⁺ indicative Ag²⁺ is not toxic to the isolate under the conditions tested. The ability of Alcaligenes faecalis Ag1-1 to synthesize sliver nanoparticles was examined. The heavy metal-resistant bacteria obtained could be useful for the bioremediation of heavy metal-contaminated environment.     

Comparison of different microbial biomass and activity measurement methods in metal-contaminated soils

The aims of this study were: (1) to compare different microbial methods of detecting the effects of heavy metals on the functioning of the soil ecosystem; and (2) to evaluate the effect of incubation on microbial biomass and microbial activity in soils that were not pre-incubated after sampling in order to determine their suitability for measuring the effects of heavy metals on the soil microbial ecosystem. The microbial biomass methods (included: biomass C, N and ninhydrin-N by fumigation-extraction (FE); substrate-induced respiration (SIR); soil ATP content and microbial activity as evolved CO2-C and arginine ammonification. All were tested in soils from the Woburn Market Garden Experiment. Due to past sludge application the soils contained, Zn, Cu or Ni at around current European Union upper limits and Cd at up to three times the limit. The amount of microbial biomass in metal-contaminated soils was about half of that found in soils from the experiment that received uncontaminated organic manure or inorganic fertilizer. The amount of biomass measured by FE and soil ATP content in incubated soils showed little change over 20 days incubation. However, SIR measurements were statistically affected over the first few days of incubation. The rates of arginine ammonification were higher in this order: farmyard manure (FYM)>inorganic fertilizer>sewage-sludge throughout the incubation. However, the evolved CO2-C rates were not significantly different among the treatments. Discriminant analysis confirmed smaller amounts of biomass in the metal-contaminated soils than in the other treatments. Linked properties, such as relationships between biomass and soil organic matter, or biomass-specific respiration rates, may provide "internal control" which may help overcome problems of establishing suitable control, or comparative measurements, when moving from experimental to natural environments.     

Methane and nitrous oxide emissions under no‐till farming in China: a meta‐analysis

No‐till (NT) practices are among promising options toward adaptation and mitigation of climate change. However, the mitigation effectiveness of NT depends not only on its carbon sequestration potential but also on soil‐derived CH₄ and N₂O emissions. A meta‐analysis was conducted, using a dataset involving 136 comparisons from 39 studies in China, to identify site‐specific factors which influence CH₄ emission, CH₄ uptake, and N₂O emission under NT. Comparative treatments involved NT without residue retention (NT0), NT with residue retention (NTR), compared to plow tillage (PT) with residue removed (PT0). Overall, NT0 significantly decreased CH₄ emission by ~30% (P < 0.05) compared to PT0 with an average emission 218.8 kg ha⁻¹ for rice paddies. However, the increase in N₂O emission could partly offset the benefits of the decrease in CH₄ emission under NT compared to PT0. NTR significantly enhanced N₂O emission by 82.1%, 25.5%, and 20.8% (P < 0.05) compared to PT0 for rice paddies, acid soils, and the first 5 years of the experiments, respectively. The results from categorical meta‐analysis indicated that the higher N₂O emission could be mitigated by adopting NT within alkaline soils, for long‐term duration, and with less N fertilization input when compared to PT0. In addition, the natural log (lnR) of response ratio of CH₄ and N₂O emissions under NT correlated positively (enhancing emission) with climate factors (temperature and precipitation) and negatively (reducing emission) with experimental duration, suggesting that avoiding excess soil wetness and using NT for a long term could enhance the benefits of NT. Therefore, a thorough understanding of the conditions favoring greenhouse gas(es) reductions is essential to achieving climate change mitigation and advancing food security in China.