Assessment of the soil phosphorus–mobilization potential by microbial reduction using the Fe(III)‐reduction test

The mobility of soil P is greatly influenced by the redox potential (Eh), which depends on the reducing activity of soil microorganisms. Standard extraction methods for the determination of the mobile soil P disregard the P mobilization caused by the influence of microorganisms on Eh, while P test methods that include soil microbial activities are lacking. Thus, the Fe(III)‐reduction test was investigated for its suitability to determine the P fraction that is mobilized in soil under reducing conditions (P_Red). In this test, the soil‐microbial reducing activity is measured from the microbial Fe(III) reduction combining a bioassay with 7 d incubation and a chemical extraction using 1M KCl. After the incubation, Eh in 26 different soil samples ranged from –282 to –123 mV. The concentration of P_Red in the soil samples ranged from concentrations below the limit of detection to 84.9 mg kg^–1 and was on average of all soil samples by a factor of 2.4 to 18 smaller than the P fractions determined by standard soil P–extraction methods. As standard agronomic and environmental P extractants, respectively, water (P_H2O), dithionite citrate bicarbonate (P_Dith), ammonium oxalate (P_Ox), ammonium lactate (P_AL), double lactate (P_DL), and sodium bicarbonate (P_Olsen) were selected. The P_Red fraction was not correlated with P_AL, P_DL, P_olsen, and the degree of P saturation, but with P_H2O (r = 0.43*), P_Dith (r = 0.60***), and P_Ox (r = 0.61***). Furthermore, P_Red depended on the concentration of amorphous Fe oxides (Fe_Ox, r = 0.53**) and was closely correlated with the concentration of microbially reduced Fe (Fe_Red, r = 0.94***). This indicated the influence of the Fe(III)‐reducing activity of soil microorganisms on P mobilization. In subsoils, low in Fe(III)‐reducing activity, no P was released by the Fe(III)‐reduction test, which was in contrast to the results from the other chemical extraction methods. Additional alterations of the microbial activity by inhibiting and activating amendments, respectively, clearly affected the microbial Fe(III)‐reducing activity and the associated release of P_Red. Thus, P_Red, determined by the Fe(III)‐reduction test, might be termed as the fraction that is potentially released from soil by microbial reduction.     

Einfluß verschiedener Bodeneigenschaften auf die mikrobielle Toxizität von Blei und Cadmium

Effects of different soil properties on the microbial toxicity of lead and cadmium Effects of different soil properties on the microbial toxicity of lead and cadmium were investigated in laboratory experiments on ten arable and nine preserve area soils. Microbial activity was measured by means of the dehydrogenase and the arginine-ammonification tests. The latter was not suitable to show the microbial toxicity of both metals. Effects of lead on dehydrogenase activity were mainly influenced by its concentration in soil solution (r = 0,79). Thus, all soil properties which determined the adsorption of lead correlated significantly with the relative dehydrogenase activities of the contaminated soils. The most important abiotic factors influencing the toxicity of lead were the soil pH and the CEC. Cd solubility and Cd toxicity were influenced by soil pH in a contrary way. Low Cd concentrations caused greater inhibitions of dehydrogenase activity at neutral and slightly alkaline soil reaction than under acidic conditions. Therefore, correlations between Cd concentrations of soil solutions or clay content and dehydrogenase activity were only significant if partial correlation analysis (constant pH) was used.     

Einfluß polychlorierter Biphenyle auf die mikrobielle Aktivität in Böden

Effects of polychlorinated biphenyls on soil microbial activity In laboratory experiments the microbial toxicity of the PCB congeners 5 (2, 3-Dichlorobiphenyl), 8 (2, 4′-Dichlorobiphenyl), 29 (2. 4, 5-Trichlorobiphenyl) and 77 (3, 3′, 4, 4′-Tetrachlorobiphenyl) which is supposed to be extremely toxic to wildlife was investigated using Parabrownearth-Ap and Podsol-Ahe horizon material. In addition the technical PCB mixtures Arochlor 1242 and 1260 were tested. Microbial toxicity was measured by means of long-term respiration (CO₂-evolution), short-term respiration (CO₂-evolution 12 h, after addition of glucose), and dehydrogenase activity (TTC reduction) tests. 1 mg/kg of the Dichlorobiphenyls 5 and 8 reduced the long-term and short-term respiration of the Podsol-Ahe during the whole experiment (35 and 28 days, respectively). The Trichlorobiphenyl 29 became effective after addition of 10 mg/kg. No effect except a short stimulation of long-term respiration was observed for PCB 77 (Tetrachloro-PCB). Due to its higher sorption capacity, all PCB congeners reduced the microbial activity of the Parabrownearth-Ap to a lower degree. In general the toxicity of PCBs decreased with increasing degree of chlorination in both soil horizons. The technical mixtures reduced the long-term respiration only after high additions of 50 mg/kg (Podsol-Ahe) and 100 mg/kg (Parabrownearth-Ap), respectively. Arochlor 1242 proved to be more toxic than Arochlor 1260.     

Reductive dissolution of crystalline and amorphous Fe(III) oxides by microorganisms in submerged soil

Summary Reduction of Fe(III) of amorphous and crystalline Fe(III) oxides to Fe(II) in flooded soils was studied using ⁵⁹Fe(OH)₃ and ⁵⁹Fe₂O₃. The results indicated that Fe(III) in the amorphous oxide was readily amenable to microbial reduction in anaerobic soil condition whereas Fe(III) in the crystalline oxide was not. Following soil submergence, the native as well as the applied crystalline Fe(III) oxides were rapidly converted into the amorphous form. The transformation of the crystalline oxides to the amorphous form appears to be a prerequisite for the reduction of Fe(III) of the oxide. This transformation, probably through hydration, is also mediated by microorganisms.     

Untersuchungen über die toxische Wirkung der in Siedlungsabfällen häufigen Schwermetalle auf die Bodenmikroflora

The toxic effects of heavy metals, commonly found in urban wastes on the soil microflora In several series the influence of soluble salts of Cd, Cr, Zn, Cu, Ni and Hg in various concentrations was tested on the development of bacteria and fungi in cultural studies and in soil model systems. In addition the changes in microbial biomass, in the activity of oxidoreductases and hydrolases, and in nitrification were measured in five soils. Comparison of the results of bacteria-plate-counts in the presence of heavy metals in agar-media and in soils demonstrated that all tested elements have a more toxic effect on isolated soil microorganisms under culture conditions than when tested after heavy metal application to soils. Beside Hg, eucaryotic soil fungi proved to be 10 – 50 fold more resistant to heavy metals in vitro as well as in situ. For calculating the effect of heavy metals on soil microbial activity soil enzymatic methods are not reliable and can be used only to some extent, because some cellfree encymes in the soil are activated or inactivated immediately by heavy metals. In long-term experiments, the microbial biomass and nitrification in soil is not significantly influenced by the elements Cd, Cu and Ni in the concentration range of the tolerable limit content. In contrast, Cr, Zn and Hg reduced these microbiological properties more or less distinctly.     

Chromium and arsenic in contaminated soils (Review of publications)

In the last decades, the chromium clarke in the world’s soils has been revised and reduced; at present, it is equal to 70 mg/kg. No maximal permissible concentration is accepted for the total chromium content in the soils of Russia; it appears reasonable to use the Western European and North American standards in Russia and to take the average value of the maximal permissible concentration equal to 200 mg Cr/kg. Chromium toxicity depends on its oxidizing status. The hazardous effect decreases with the reduction of Cr(VI) to Cr(III). There are various chemical reducers of Cr(VI), including sulfides, dissolved organic substance, aqueous Fe(II) and minerals enriched in Fe(II), and Fe(0). As-containing ore tailings represent a powerful source of technogenic arsenic. Significant environment contamination with natural As is registered in a number of Asian countries. The maximal permissible concentration of total arsenic is equal to 2 mg/kg in Russian soils; it is probably underestimated, because it is lower than the As clarke in soil (5 mg/kg). The approximately permissible concentration (APC) values for As look more reasonable. Arsenic toxicity depends on its oxidation degree: As(III) is 2–3 times more toxic than As(V).     

The use of a microbial contact toxicity test for evaluating cadmium bioavailability in soil

Background, Aims and Scope

Bioavailability of toxic compounds in soil can be defined as the fraction able to come into contact with biota and to cause toxic effects. The contact toxicity tests may detect the total toxic response of all bioavailable contaminants present in a sample. The objectives of this study were to evaluate the use of microbial contact toxicity tests for cadmium bioavailability assessment and to evaluate the relationship between sorption, soil characteristics and cadmium bioavailability.

Methods

A test soil bacterium,Bacillus cereus, was put in direct contact with the solid sample. Four unpolluted soils were selected to provide solid samples with a variety of physicochemical characteristics. The toxicity and sorption behaviour of cadmium spiked to the soil samples were determined.

Results, Discussion and Conclusions

A significant correlation between contact toxicity test results and partitioning of cadmium in the soil samples (r²= 0.79, p <0.05; n = 26) was found. The results confirm that the bioavailability of cadmium in soil depends on its sorption behaviour. Cadmium sorbed to the cation exchange sites associated with fulvic acids is non-bioavailable in the toxicity test employed in this study. It is concluded that the microbial contact toxicity test is a suitable tool for detecting cadmium bioavailablity in the soils used in this study.

Outlook

The application of microbial contact toxicity tests for bioavailability assessment can be very useful for the risk identification and remediation of soil-associated contaminants.     

Comparison and improvement of methods for determining soil dehydrogenase activity by using triphenyltetrazolium chloride and iodonitrotetrazolium chloride

The triphenyltetrazolium chloride (TTC) method described by Thalmann (1968) and the iodonitrotetrazolium chloride (INT) method described by Spothelfer-Magaña and Thalmann (1992), used for measuring soil dehydrogenase activity, have been modified to overcome some methodical short-comings. Absorption maxima of 485 nm for triphenylformazan dissolved in acetone, 491 nm for iodonitrotetrazolium formazan (INTF) dissolved in tetrahydrofuran and 455 nm for INTF dissolved in N,N-dimethylformamide are recommended for measuring wavelengths. Extracting triphenylformazan twice with acetone is less toxic and proved to be at least as efficient as extraction with a mixture of 90% acetone and 10% carbon tetrachloride (Thalmann 1968 method). Tetrahydrofuran and dimethylformamide were equally good in extracting INTF from soils, but the former was less toxic. Anaerobic incubation resulted in the formation of higher amounts of triphenylformazan and INTF as well as reduced standard error. Both TTC and INT reduction showed high reproducibility and good differentiation of the microbial activity of six soils. For several reasons (more easily determined substrate dose depending on different soil types, better reduction, shorter incubation time), INT reduction seems to be a more suitable method of measuring soil microbial activity than TTC reduction.     

Using the Caenorhabditis elegans soil toxicity test to identify factors affecting toxicity of four metal ions in intact soil

A previously developed soil toxicity test for rapidly determining the toxicity of chemicals to the soil-dwelling nematode Caenorhabditis elegans (Donkin and Dusenbery, 1993) was used to measure the toxicity of four metals (Zn²⁺, Cd²⁺, Cu²⁺, and Pb²⁺) added to four soils common to the southeastern United States. Nematode survival after a 24-hour exposure in the presence of a bacterial food source was assessed. All soils reduced the toxicity of most metal ions compared to solutions without soil. Pb was the most strongly affected, while Cd toxicity was not much influenced by the soils. Correlations between the LC₅₀S and various soil or metal characteristics were determined. No significant correlation was found between LC₅₀s and many soil characteristics commonly cited as having large effects on soil bioavailability of metals. Although sample size was limited, the indication was that bioavailability of metals to nematodes is determined by a complex array of many interacting soil, as well as metal, properties. Comparison of the relative mobilities of these ions in other soils with the relative toxicity measured here suggests that mobility may be a good predictor of toxicity. The C. elegans soil toxicity test is shown to be as sensitive and more rapid than the commonly used earthworm soil toxicity test.     

The central role of microbial activity for iron acquisition in maize and sunflower

 Maize (Zea mays L.) and sunflower (Helianthus annuus L.) grown on a calcareous soil showed poor growth and/or were chlorotic in spite of abundant Fe in the roots. It has been hypothesized that microbial siderophores chelate Fe (III) in the soil, and that in this form Fe is transported towards the root apoplast. On the calcareous soil, total and apoplastic root Fe concentrations were high, probably because of a high apoplastic pH depressing Fe (III)-reductase activity and thus the Fe²⁺ supply to the cytoplasm. On the acidic soil, total and apoplastic root Fe concentrations were low, probably because of a low apoplastic pH favouring Fe (III) reduction, hence plants showed no Fe-deficiency symptoms. The main objective of the present work was to investigate the role of microbial soil activity in plant Fe acquisition. For this purpose, plants were grown under sterile and non-sterile conditions on a loess loam soil. Plants cultivated under non-sterile conditions grew well, showed no Fe-deficiency symptoms and had fairly high Fe concentrations in the roots in contrast to plants grown in the sterile medium. Low root and leaf Fe concentrations in the axenic treatments indicated that the production of microbial siderophores was totally suppressed. Accordingly, sunflowers were severely chlorotic and this was associated with very poor growth, whereas in maize only growth was drastically reduced. In maize under sterile conditions, root apoplastic and total Fe concentrations were not as low as in sunflowers, which may have indicated that phytosiderophores produced in maize partly sustained Fe acquisition, but due to poor growth were not as efficient in supplying Fe as microbial activity under natural conditions. It may be therefore assumed that in natural habitats soil microbial activity is of pivotal importance for plant Fe acquisition. Received: 11 March 1999     

The influence of soil characteristics on the toxicity of four chemicals to the earthworm Eisenia fetida andrei (Oligochaeta)

Summary The acute toxicity of Cd (chloride), chloroacetamide, 3,4-dichloroaniline and pentachlorophenol to the earthworm Eisenia fetida andrei was determined using the OECD (1984) artificial soil and contact testing procedures. To investigate the influence of two soil characteristics (pH and organic-matter content), the toxicity of the chemicals was also determined in two natural sandy soils. It is concluded that the filter-paper contact test cannot be recommended to predict earthworm toxicity of these chemicals in soil. Toxicity in soil was influenced by both pH and organic-matter content. Differences between LC₅₀ values in the high-organic-matter artificial soil and in an acid, low-organic-matter sandy soil were, however, not greater than a factor of 3–4. The results of this study therefore support the use of a well-defined artificial soil substrate for standardized earthworm toxicity tests.     

Implications of iron solubilization on soil phosphorus release in seasonally flooded forests of the lower Orinoco River, Venezuela

The microbial reduction of Fe oxides is thought to contribute with the release of P in sedimentary environments. However, secondary reactions of the bioproduced Fe(II) with P in solution, can lead to a decrease in the soluble P concentration. In this study, we examined how the reduction of Fe(III) affects the soluble P concentration, when the soils of a seasonally flooded forest gradient are subjected to anaerobic conditions. Soil samples were collected during the dry season from two zones subjected to different flooding intensity: MAX and MIN zones that were inundated 8 and 2 months per year, respectively. When anaerobic conditions were applied to soils from both zones, a clear stimulatory effect on the Fe(III) reduction was observed. However, bioproduced Fe(II) underwent secondary chemical reactions, masking the extend of Fe(III) reduction of these soils. Iron was reduced mainly during the first 15 days of the anaerobic incubation and it was stimulated by a pulse of labile carbon. Iron dissolution did not lead to an increase of the soluble P content. However, in both zones P was high and positively correlated with Fe(II), implying that soil P mobilization was linked to Fe dissolution. In the MIN zone, soluble P concentration decreased, probably as a consequence of the secondary reactions of solubilized P with other non-redox sensitive soils elements. Fe solubilization also had an effect on the activity of acid phosphatase and consequently in the solubilization of P from the organic pool. In conclusion, the P cycle in these soils is strongly coupled to C and Fe cycles.     

有机肥缓解小麦铝毒效果的研究

以交换性Al为1.83［1／3Al3+］cmol／kg的淋溶型水田砂质土（低Al毒）和交换性Al为6.05［1／3Al3+］cmol／kg的黄筋泥（红壤,高Al毒）为供试土壤,研究了有机肥对缓解小麦铝毒的效果。盆栽试验（均有充足N、P、K基肥）结果表明,小麦（扬表5号）在这两种土壤上均发生Al毒。在前一种土壤上生育不良,减产约一半左右,后一种土壤上几近绝收。施用生产中常量猪厩肥（约22.5／hm2）,与施用石灰比较,在低Al毒土壤上,产量接近施石灰处理（90％～100％）,能基本消除Al毒;在高Al毒土壤上,同样表现出一定的解Al毒效果,但还不够明显,产量只为石灰处理的1／4。土壤测定表明,施用猪厩肥使交换性Al下降18.1％～38.7％。植株分析也表明,吸收的Al相应减少。土壤Al毒明显抑制小麦对K、Ca、Mg的吸收;对S则有促进吸收的趋势;对N、P、Fe、Zn的吸收影响不大。在Al毒土壤上施用猪厩肥,不仅降低Al的吸入,并可减缓Al对小麦Ca、Mg、K吸收的抑制作用,有利于体内营养元素间的平衡。     

Sorption und Wirkung von Arsen auf die mikrobielle Aktivität von Waldhumusformen

Arsenate adsorption and effects of arsenate pollution on microbial activity of different types of humus soils Perfusion experiments using Na₂HAsO₄ were carried out in order to investigate adsorption and effects of arsenate on microbial activity of L-, O-, and A-horizons of a mull, a moder, and a mor soil. Arsenic adsorption of mineral A-horizons was positively influenced by their contents of amorphous Al- and Fe-oxides. It is assumed that arsenic was mainly precipitated as Al-, Mn- or Ca-arsenate in the organic layers during the perfusion experiments. The Freundlich equation was valid to describe As-adsorption of all layers under investigation. Microorganisms seemed to be very tolerant to arsenate pollution. Soil respiration and enzyme activities were reduced when liquid concentrations exceeded 8 μg As/ml. Extremely high As-additions (up to 10 mg As/g) did not reduce microbial processes to a greater degree than lower ones of 0,5 mg As/g.     

Einfluss eisenhaltiger Klärschlämme auf Kenngrößen der P‐Verfügbarkeit in Ackerböden

Influence of iron content in sewage sludges on parameters of phosphate availability in arable soils The use of iron salts for the P elimination in sewage plants is widely used. But it is not clear whether the P availability in arable soils is negatively influenced by iron compounds or not. The aim of the investigations was, therefore, to study the influence of two sewage sludges with a high and a low Fe content respectively on P sorption and phosphate concentration (P_i) in the soil solution after application of CaHPO₄ or sewage sludge to 5 loamy and 4 sandy soils (pot experiments and 1 silty loam (field experiment)). Soils were analyzed 1, 6, and 13 months after P application. Sludge Gö contained 12 kg P and 65 kg Fe (t DM)^—1 (P : Fe = 1 : 5.4) and sludge Sh 25 kg P and 39 kg Fe (t DM)^—1 (P : Fe = 1 : 1.5). The basic P application was 60 kg P ha^—1 (= 30 mg P (kg soil)^—1 in the pot experiment, as sludge or as CaHPO₄). P uptake by maize was determined in a separate pot experiment with a loamy soil and the same P application rate. The P sorption capacity remained similar in all soils after application of sludge Sh (P : Fe = 1:1.5) compared with soils without sludge, however, after application of sludge Gö the P sorption increased by 16% (0—59%). After application of sludge Sh the mean P_i concentration increased in loamy soils by 34% and in sandy soils by 15%. On the other hand the P_i concentration decreased after applying sludge Gö by 13% and 36% as compared to the controls of the respective soils. In the field experiment the P_i concentration of plots with a high P level (50 mg lactate soluble P (kg soil)^—1) was also significantly decreased after application of 10 t sludge Gö (126 kg P ha^—1) in comparison with triple phosphate. One month after the application of increasing amounts of sludge Gö (5, 10, 15 t DM ha^—1) both the concentration of oxalate‐soluble Fe in the soil and the P sorption were increased. The elevated relationship between these two parameters was highly significant (r² = 0.6 — 0.97). Plant uptake of P was less after application of sludge Gö than after application of sludge Sh and much less than P uptake from CaHPO₄. Sewage sludges with a P : Fe ratio of 1 : 5 should not be recommended for agricultural use, as the P availability is significantly reduced. Iron salts should not be used for conditioning of sludges.     

Specific features of iron behavior in soddy-podzolic and alluvial gleyed soils of the Middle Cis-Urals region

The regime of observations revealed that the Eh dynamics in soddy-podzolic and alluvial soils in the Middle Cis-Urals region depends not only on the rate of iron (hydr)oxides reduction but also on the rate of opposite reactions in the gleyed horizons. Both processes depend on the temperature. The Eh value decreases on heating in automorphic soils, when the reduction of Fe(III)-(hydr)oxide particles accelerates. On the contrary, in gley soils, the Eh decreases on cooling, probably, because of the reactions opposing the reduction of Fe(III)-(hydr)oxide particles, including Fe(II) fixation on the surface of mineral particles. Fe(III)-(hydr)oxides are, for the most part, preserved in gleyed soils of the Cis-Urals; the content of (Fe₂O₃)_dit reaches 3.3% with iron minerals being usually represented by goethite. The increase in moistening influences the soil parameters (i.e., the redoxpotential rH and the content of conventional red pigment Hem_conv) in an intricate manner. Both direct and reverse branches on the curve of the Hem_conv-rH dependence point to the equilibrium and nonequilibrium conditions in the soil. The reverse branch probably stands for the initial phase of gleying in strongly humified soils, where, despite extra electrons in the solution, the brown pigment in the form of Fe(III)-(hydr)oxides is preserved.     

Influence of balsam poplar tannin fractions on carbon and nitrogen dynamics in Alaskan taiga floodplain soils

The feedbacks between plant and soil processes play an important role in driving forest succession. One poorly understood feedback mechanism is the interaction between plant secondary chemicals and soil microbes. In the Alaskan taiga, changes in nutrient cycling caused by balsam poplar (Populus balsamifera) secondary chemicals may affect the transition from alder (Alnus tenuifolia) to balsam poplar on river floodplains. We examined the effects of four poplar condensed tannin fractions on N cycling in alder and poplar soils. Tannins were added to forest floor samples from both poplar and alder sites. Samples were incubated for 1 month in the laboratory with soil respiration rates measured over the course of the incubation. At the end of the incubation we measured both net and gross nitrogen mineralization and nitrification, microbial biomass C and N, and the activity of various exoenzymes. In all soils, tannin additions reduced N availability, however, the mechanisms differed depending on the molecular weight of the tannin and the native soil microbial community. Low molecular weight tannin fractions served as a labile C source in poplar Oi, poplar Oe, and alder Oe horizons but were toxic to microbes in alder Oi. High molecular weight tannin fractions appeared to act primarily by binding extracellular substrates and thus limiting C and N mineralization, with the strongest effects observed in the alder soils.     

Evaluation of ecological doses of the nitrification inhibitors 3,4-dimethylpyrazole phosphate (DMPP) and 4-chloromethylpyrazole (ClMP) in comparison to dicyandiamide (DCD) in their effects on dehydrogenase and dimethyl sulfoxide reductase activity in soils

Risk assessment of the nitrification inhibitors (NIs) 3,4-dimethylpyrazole phosphate (DMPP), 4-chloromethylpyrazole (ClMP), and dicyandiamide (DCD) on nontarget microbial activity in soils was determined by measuring dehydrogenase and dimethyl sulfoxide reductase activity (DHA, DRA, respectively) in three differently textured soils under laboratory conditions. Dehydrogenase activity was measured with standard procedure recommended to evaluate side effects of environmental chemicals on general microbial activity in soils. The kinetic of inhibition were obtained by dose–response relationships and used to calculate the no observable effect levels (NOEL values) and the effective doses at 10% and 50% inhibition (ED₁₀ and ED₅₀), respectively. Negative effects on DHA and DRA, respectively, were observed only at rates approximately 40–100 times higher than the concentrations recommended in the field. Both DHA and DRA were affected more in the sandy than in the silty or clayey soil. Consequently, NOEL, ED₁₀, and ED₅₀ values were considerably higher in the clayey than in the silty or sandy soil. The heterocyclic N compounds DMPP and ClMP, respectively, were more effective in inhibiting DHA and DRA than DCD. At application rates used in the field as well as at concentration up to 25 to 90 times higher, the NIs concerned failed to affect general soil microbial activity in soils. Among the three NIs tested, the not marketed ClMP exhibited the strongest negative effects on soil microbial activity. At recommended application rates, the NIs tested should be considered as enviromentally safe.     

Mitigating Negative Microbial Effects of <Emphasis Type="Italic">p</Emphasis>-Nitrophenol,Phenol, Copper and Cadmium in a Sandy Loam Soil Using Biochar

Biochars are adsorptive solids potentially of benefit to soil microbes by providing improved nutrient retention, a carbon substrate and contaminant adsorption. A 28-day incubation experiment gauged the interactive effects of biochar application and contaminants on the microbial biomass and respiration of a sandy loam soil. Soil was amended with 250 mg/kg phenol or p-nitrophenol (two toxic but nevertheless biodegradable organic contaminants) or 50 mg/kg cadmium or copper. Biochar application generally caused increased microbial respiration and biomass relative to non-amended controls. Of the heavy metal-amended soils, Cu effected significant reductions in microbial biomass carbon and basal respiration, which were improved with concurrent biochar amendment. The biochar’s functional groups are likely to have mitigated the metals’ negative effects via complexation and sorption, while the soil’s proportion of negative pH-dependent sites was increased by the pH rise induced by biochar application, allowing more cationic retention. Organic contaminant-spiked soils had higher microbial biomass-specific respiration without biochar amendment, indicating that surviving microbes utilised the compounds and necromass as substrates. Paranitrophenol proved to be particularly toxic without biochar application, causing marked reductions in the microbial quotient and biomass carbon. Remarkably, concurrent biochar and pNP application led to hugely increased microbial biomass carbon and nitrogen, significantly higher than those in contaminant-free replicates. It is likely this arose from biochar sorbing the contaminant and allowing its microbial utilisation as a carbon and nitrogen source, stimulating growth. Biochar application is a highly promising strategy for reducing the soil microbial toxicity of heavy metals and aromatic organic contaminants, particularly p-nitrophenol.     

Efficacy of an artificial microbial siderophore-Fe(III) with high redox potential on correcting Fe chlorosis in rice

ABSTRACT

Microbial siderophore-chelated Fe(III) is suggested to be an important source of Fe for plants, although it is hardly reduced by plant roots. Here, we investigated the efficacy of the easily reducible artificial microbial siderophore tris[2-{(N-acetyl-N-hydroxy)glycylamino}ethyl]amine (TAGE)-Fe(III) as an alternative Fe source to correct Fe deficiency in rice plants, and compared it to that of the natural siderophore deferoxamine B (DFOB)-Fe(III). We also evaluated the absorption of Fe from TAGE-Fe(III) by the Strategy I-like system of gramineous plants using nicotianamine aminotransferase 1 (naat1) mutant rice, which does not synthesize phytosiderophores. Fe(III)-siderophores were synthesized in vitro. Nipponbare rice and its naat1 mutant were reared in soil and gel cultures to determine Fe availability. Hydroponically grown naat1 mutant seedlings were used for reducibility assays to determine the ability of rice roots to reduce Fe(III) chelated by TAGE or DFOB. The expression of a Fe-deficiency inducible gene was also determined, as well as chlorophyll and Fe concentrations. Reduci bility assays on naat1 mutant seedlings revealed that the reduction level of TAGE-Fe(III) was approximately three times higher than that of DFOB-Fe(III). Application of TAGE-Fe(III) to both culture medium and alkaline soil improved Fe chlorosis, growth, and Fe concentration in both naat1 and wild type plants, whereas application of DFOB-Fe(III) only did so in wild type plants. Easily reducible Fe(III)-chelates such as TAGE-Fe(III) can be a better source of Fe for rice plants than most natural microbial siderophores-Fe(III). Our study also demonstrated that rice plants have the ability to utilize microbial siderophores-Fe(III) as the Fe source through the Strategy I-like Fe acquisition system.