Oxygen input controls the spatial and temporal dynamics of arsenic at the surface of a flooded paddy soil and in the rhizosphere of lowland rice (Oryza sativa L.): a microcosm study

The impact of oxygen (O₂) input at the soil surface and in the rhizosphere of rice (Oryza sativa L.) on the spatial and temporal dynamics of arsenic (As) was investigated in a flooded paddy soil. A soil microcosm and root-mat technique were designed to mimic submerged conditions of paddy fields. Water-filled containers with (planted) or without (unplanted) 27-day-old rice seedlings were fitted for 20 days on top of microcosms containing an As-affected soil (Bangladesh). After the initial establishment of strongly reduced conditions (?230 mV) in both planted and unplanted soils, the redox potential gradually increased until the day 8 to reach?+?50 mV at 2 mm from the surface of unplanted soils only. This oxidation was associated with an accumulation of NH₄-oxalate extractable As (25.7 mg kg^?1) in the 0.5-mm top layer, i.e. at levels above the initial total content of As in the soil (14 mg kg^?1) and a subsequent depletion of As in soil solution at 2 mm from soil surface. Root O₂-leakage induced the formation of an iron (Fe) plaque in root apoplast, with no evidence of outer rhizosphere oxidation. Arsenic content reached 173 mg kg^?1 in the Fe plaque. This accumulation induced a depletion of As in soil solution over several millimetres in the rhizosphere. Arsenic contents in root symplast and shoots (112 and 2.3 mg kg^?1, respectively) were significantly lower than in Fe plaque. Despite a large As concentration in soil solution, Fe plaque appeared highly efficient to sequester As and to restrict As acquisition by rice. The oxidation-mediated accumulation of As in the Fe plaque and in the oxidised layer at the top of the soil mobilised 21 and 3% of the initial amount of As in the planted and unplanted soils, respectively. Soil solution As concentration steadily decreased during the last 16 days of the soil stage, likely indicating a decrease in the ability of the soil to re-supply As from the solid-phase to the solution. The driving force of As dynamic in soil was therefore attributed to the As diffusion from reduced to oxidised soil layers. These results suggest a large mobility of As in the soil during the flooded period, controlled by the setting of oxic/anoxic interfaces at the surface of soil in contact with flooding water and in the rhizosphere of rice.     

Bacterial Community Structure and Functional arrA Gene Diversity Associated with Arsenic Reduction and Release in an Industrially Contaminated Soil

This study aimed at evaluating potential arsenic (As) mobility in an industrially contaminated soil (64 mg/kg of As) of the Meuse River basin, and at identifying key bacterial groups that drive soil As dynamics. Both speciation and release of As from this soil was followed under anaerobic conditions using a laboratory batch experiment. In the presence of exogenous carbon sources, As^V initially present in the soil matrix and/or adsorbed on synthetic hydrous ferric oxides were solubilized and mainly reduced to As^III by indigenous soil microflora. After a 1-month incubation period in these biotic conditions, As^III accounted for 80–85% of the total dissolved As and more than 60% of the solid-phase As. Bacterial community structure (i.e., 16S rDNA-based capillary electrophoresis single-strand conformation polymorphism profiles) changed with incubation time and As amendment. The detection of distantly related arsenate respiratory reductase genes (arrA), as functional markers of As^V respirers, indicates that novel dissimilatory As^V-reducing bacteria may be involved in As biotransformation and mobility in anoxic soils. Since As and iron were concomitantly released, a crucial role of indirect As-mobilizing bacteria on As behavior was also revealed. Our results show that the majority of As within the soil matrix was bioavailable and bioaccessible for heterotrophic As^V reduction to As^III, which may increase As toxicity and mobility in the contaminated soils.     

INFLUENCE OF SOIL PROPERTIES AND PHOSPHATE ADDITION ON ARSENIC UPTAKE FROM POLLUTED SOILS BY VELVETGRASS (HOLCUS LANATUS)

Four kinds of soil material were used in a pot experiment with velvetgrass (Holcus lanatus). Two unpolluted soils: sand (S) and loam (L) were spiked with sodium arsenite (As III) and arsenate (As V), to obtain total arsenic (As) concentrations of 500 mg As kg^?1. Two other soils (ZS I, ZS III), containing 3320 and 5350 mg As kg^?1, were collected from Zloty Stok where gold and arsenic ores were mined and processed for several centuries. The effects of phosphate addition on plants growth and As uptake were investigated. Phosphate was applied to soils in the form of NH₄H₂PO₄ at the rate 0.2 g P/kg. Average concentrations of arsenic in the shoots of velvetgrass grown in spiked soils S and L without P amendment were in the range 18–210 mg As kg^?1 d.wt., whereas those in plants grown on ZS I and ZS II soils were considerably lower, and varied in the range 11–52 mg As kg^?1 d.wt. The addition of phosphate caused a significant increase in plant biomass and therefore the total amounts of As taken up by plants, however, the differences in As concentrations in the shoots of velvetgrass amended and non-amended with phosphate were not statistically significant.     

Plant availability of applied and native boron in soils with diverse properties

Laboratory and greenhouse research was conducted to study effects of soil properties on the availability of native and applied B in 14 Virginia soils. Boron absorption could be described by the Langmuir equation in 12 of the 14 soils, and maximum B adsorption (Vmax) in these 12 soils ranged from 3.3 to 26.5 mg kg⁻¹. A multiple regression equation, −19.3+3.51 pH+0.048 clay content, accounted for 89.6% of the variation in Vmax for the 12 soils. Curvilinear relationships (α=0.01) occurred between B in corn (Zea mays L.) tissue from native B and hot-water soluble B, mannitol exchangeable B, and NH₄-acetate and Mehlich III extractable B. Among these four procedures, mannitol exchangeable B correlated most closely (r=0.923) with B in corn tissue from native B. From 0.4 to 13.5% of the applied B was absorbed by corn plants and translocated to shoots. Curvilinear relationships (α=0.01) occurred between B in corn tissue from applied B and soil clay content, NH₄-oxalate extractable Al and Fe, and acidified NH₂OH·HCl extractable Mn. It is evident from these relationships that soil clay and oxyhydroxides of Al, Fe, and Mn have an affinity to adsorb B in somewhat unavailable forms.     

Ferrous Iron Treatment of Soils Contaminated with Arsenic-Containing Wood-Preserving Solution

This article discusses the results of efforts to reclaim As-contaminated soil from a former timber-treating plant. The study site, commonly referred to as the Rocker Timber Framing site, is located along Silver Bow Creek approximately 7 miles west of the Butte Mining District, MT, USA. The plant operations resulted in contamination of the soils with a highly caustic solution containing 5% As (III). Contaminated soil resulted in the groundwater plumes that contained up to 25?mg L^?1 As, with As (V) being the predominant species. The objective of this study was to evaluate the effectiveness of Fe (II) treatment for remediation of As-contaminated soils. Laboratory-treatability studies were conducted on samples of saturated zone (AS1) and va-dose zone (AV1) soils. The AS1 soil was a mixture of coarse alluvium and potentially some mill tailings from adjacent mining operations. The AV1 soil consisted primarily of fill, including soil, construction debris, and timber fragments. Initial concentrations of total As in AS1 and AV1 soils were 683 and 4814?µg kg^?1, respectively. Water-soluble As concentrations were 15.4 and 554?µg L^?1, respectively, in a 20:1 solution to soil extract. Batch equilibration were performed by placing 10?g of soil into 20 vessels and adding increasing amounts of FeSO₄.7H₂O. Amendment increments were made as multiples of molar ratios of total As present in each soil. Treatability studies were run with and without a pH buffer of CaCO₃ (added at a 2:1 molar ratio to the FeSO₄.7H₂O treatment). Solution concentrations of As in the AS1 and AV1 soils (without CaCO₃) decreased from 554 to 15.4?µ L^?1 and 3802 to 0.64?µ L^?1, respectively, as the Fe:As molar ratios increased from 0 to 2, whereas for the AS1 soil the solution As concentration increased at the Fe:As molar ratios >2 and reverse trend was observed for the AV1 soils. The decrease in As solution concentration for the AS1 soil is attributable to the dramatic decrease in soil pH with increasing Fe:As molar ratios. In the case of soils treated with CaCO₃, the solution concentrations decreased from 564 to 0.65?µg L^?1 and 3790 to 0.79?µg L^?1 for the AS1 and AV1 soils, respectively,as the Fe:As molar ratios increased from 0 to 50. Generally, in both the soils, the CaCO₃-treated soil contained significantly more solution As compared with the non-CaCO₃-treated soil at the comparable Fe:As molar ratios. This is attributable to higher solution pH on CaCO₃ treatment. Our rapid engineering study indicates that treating both the soils with Fe:As molar ratio of 2 lowered the As water quality limit to <50?µL^?1, whereas treating the AS1 and AV1 soils with Fe:As molar ratio of 2 and 3, respectively, lowered the As water quality limit to ≤15?µg L^?1. The concentrations of the Cu and Zn were below the instrument detection limits for the AS1 and AV1 soils without CaCO₃ treatment. Sequential extraction of Fe-treated soils illustrated that As was relatively stable. Less than 1% of the As was extractable using a modified TCLP approach and <70% of the As was extractable using a harsh acid modified hydroxylamine hydrochloride extraction.     

Speciation of Arsenic under Dynamic Conditions

In periodically flooded soils, reductive conditions can occur, which favor the dissolution of Fe (hydr)oxides. Fe (hydr)oxides such as goethite are important sorbents for arsenate (As^V), which is the dominant As species in soils under aerobic conditions. Hence, the dissolution of Fe (hydr)oxides under reductive conditions can result in the mobilization and reduction of As^V and, thus, in an increase in the bioavailability of arsenic. The temporal dynamics of these processes and possible re‐sorption or precipitation of arsenite (As^III) formed are poorly understood. Under controlled laboratory conditions, the temporal change in the redox potential and arsenic speciation with time after a simulated flooding event in a quartz‐goethite organic matter substrate, spiked with As^V, was examined. During a period of 6 weeks, substrate solutions were sampled weekly using micro‐suction cups and analyzed for pH, As^III and As^V, Fe, Mn and P concentrations. Redox potentials and matric potentials were determined in situ in the substrate‐bearing cylinders. The redox potential and the ratio between As^III and As^V concentrations remained unchanged during the experiment without organic matter application. With organic matter applied, the redox potential decreased and the As^III concentrations in the substrate solution increased while the total As concentrations in the substrate solution strongly decreased. An addition of goethite (1 g/kg) per se led to a decrease of the total As in the substrate solution (almost 50 %). In respect to the potential As availability for plants, and consequently, the transfer into the food chain, the results are difficult to evaluate. The lower the total As concentrations in the substrate solution, determined with decreasing redox potential, the least plant As uptake will occur. This effect may however be compensated by a shift of the molar P/As^V ratio in the solution in favor of As^V which is expected to increase the As uptake.     

Effects of natural and artificially induced reduction on soil solution phosphorus in rice soils

The effect of natural and artificial reduction on P extractability from soils used for rice production and the relation of these values to response to fertilizer P were investigated. Soil solution P increased from a mean of 3.8 mg P·kg^?1 soil for naturally oxidized slurries of 28 soils to 19.8mg P·kg^?1 when the soils were naturally reduced. The mean values were further increased to 40.8 and 45.3 mg·kg^?1 when the soils were reduced with 0.1M Na₂S₂O₄ and 0.2M Na₂S₂O₄, respectively. These P-values compare with 18.2 mg kg^?1 when the dry soils were extracted with Bray No. 1 extractant. When the yields of rice were correlated with solution and extracted P, the R²'s for the quadratic relationships were 0.40^**, 0.31^*, 0.34^**, 0.30^*, and 0.55^** for the naturally oxidized, the naturally reduced, 0.1M Na₂S₂O₄, 0.2M Na₂S₂O₄ and Bray No. 1, respectively. The Cate-Nelson calculation confirmed the superiority of the weak acid Bray extractant and the critical value of 8.6 mg P·kg^?1 soil needed for satisfactory yields of rice. There was little response of rice to added fertilizer P on soils with solution P-values greater than 0.09 mg P·l^?1 in oxygenated soil slurries. Some soils with solution P of this order and high amounts of Bray No. 1 extractable P still gave modest responses to fertilizer P. Although natural or chemically induced reduction increased soil solution P, it did not improve prediction of yield response of rice to added fertilizer P.     

Light Intensity Alters the Extent of Arsenic Toxicity in Helianthus annuus L. Seedlings

The present study is aimed at assessing the extent of arsenic (As) toxicity under three different light intensities—optimum (400 μmole photon m^?2 s^?1), sub-optimum (225 μmole photon m^?2 s^?1), and low (75 μmole photon m^?2 s^?1)—exposed to Helianthus annuus L. var. DRSF-113 seedlings by examining various physiological and biochemical parameters. Irrespective of the light intensities under which H. annuus L. seedlings were grown, there was an As dose (low, i.e., 6 mg kg^?1 soil, As₁; and high, i.e., 12 mg kg^?1 soil, As₂)-dependent decrease in all the growth parameters, viz., fresh mass, shoot length, and root length. Optimum light-grown seedlings exhibited better growth performance than the sub-optimum and low light-grown seedlings; however, low light-grown plants had maximum root and shoot lengths. Accumulation of As in the plant tissues depended upon its concentration used, proximity of the plant tissue, and intensity of the light. Greater intensity of light allowed greater assimilation of photosynthates accompanied by more uptake of nutrients along with As from the medium. The levels of chlorophyll a, b, and carotenoids declined with increasing concentrations of As. Seedlings acquired maximum Chl a and b under optimum light which were more compatible to face As₁ and As₂ doses of As, also evident from the overall status of enzymatic (SOD, POD, CAT, and GST) and non-enzymatic antioxidant (Pro).     

Effects of vegetation and fertilizer on metal and Sb plant uptake in a calcareous shooting range soil

Shooting range soils frequently contain anomalous concentrations of metals (e.g. Pb, Zn, Mn) and Sb coming from bullets which may be released into the environment. In a pot experiment, we investigated metal and Sb uptake by three plant species (Plantago lanceolata, Lolium perenne and Triticum aestivum) growing on a calcareous shooting range soil (pH 7.8; 500 mg kg⁻¹ Pb, 21 mg kg⁻¹ Sb) and the uptake changes when an acidic fertilizer solution was applied to the soil. Metal and Sb solubility in the soil was determined by extraction with 0.1 M NaNO₃. In addition, we measured pH, electrical conductivity and dissolved organic carbon in drainage samples. The results showed significant increase over time of pH (from 7.8 to 8.3) and decrease of electrical conductivity and dissolved organic carbon (from 230 to ∼130 mg L⁻¹). Fertilizer application increased NaNO₃-extractable Pb and Sb and root:shoot biomass ratio but not plant metal uptake. In T. aestivum spikes accumulated more Zn, Ni and Cu than shoots and grains. Mn and Zb uptake was correlated in L. perenne shoots. P. lanceolata, a Sb-bioindicator, did not accumulate high amounts of Sb (<1 mg kg⁻¹).     

Enhanced mineralization and denitrification as a result of heterogeneous distribution of clover residues in soil

Batch studies were conducted with Mn oxides (birnessite-hausmannite mixture, BHM) and samples of four soil series from the Mid-Atlantic region of the USA to determine effects of reducing organic acids, similar to those found in the rhizosphere, on the SeO₃/SeO₄ distribution. Jackland (Typic Hapludalf), Myersville (Ultic Hapludalf), Christiana (Aeric Paleaquult), and Evesboro (Typic Quartizipsamment) A and B horizon soil samples with and without prior Mn oxide reduction were incubated aerobically for 10 d with 0.1 mmol kg^-1 SeO₃ and 0 or 25 mmol kg^-1 of ascorbic acid, gallic acid, oxalic acid, or citric acid. Selenite was also added to BHM (10 mmol kg^-1) with 0 or 0.1 mmol kg^-1 ascorbic acid. The availability of Se for plant uptake as a result of root-soil interactions was examined using growth chamber studies with barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) seedlings grown in 150-mL cone-shaped containers to maximize root-soil surface interactions and to create rhizosphere soil throughout the root zone. In the BHM system ascorbic acid increased oxidation of SeO₃ to SeO₄ to 33% of added SeO₃. In the presence of ascorbic and gallic acids and Mn oxides, oxidation of SeO₃ to SeO₄ occurred in the B horizons of all the soils and in the A horizons of Jackland and Myersville soils. Removal of Mn oxides decreased the oxidation in some samples. Wheat and barley plants were able to accumulate up to 20 mol Se kg^-1 from the Jackland soil when soluble Se was not measurable. The root-soil interactions in the Jackland soil with barley and wheat provided the plant with Se from insoluble sources. The results also indicate that Mn oxides coming in contact with reducing root exudates have a greater ability to oxidize SeO₃ to SeO₄. Thus, rhizosphere processes play an important role in the availability of Se for plant uptake.Maryland Agricultural Experiment Station Scientific Article A 6381.Maryland Agricultural Experiment Station Scientific Article A 6381.     

Assessment of Pollution-Induced Microbial Community Tolerance to Heavy Metals in Soil Using Ammonia-Oxidizing Bacteria and Biolog Assay

This study attempted to investigate if the tolerance of soil bacterial communities in general, and autotrophic ammonia-oxidizing bacteria (AOB) in particular, evolved as a result of prolonged exposure to metals, and could be used as an indigenous bioindicator for soil metal pollution. A soil contaminated with copper, chromium, and arsenic (CCA) was mixed with an uncontaminated garden soil (GS3) to make five test soils with different metal concentrations. A modified potential ammonium oxidation assay was used to determine the metal tolerance of the AOB community. Tolerance to Cr, Cu, and As was tested at the beginning and after up to 13 months of incubation. Compared with the reference GS3 soil, the five CCA soils showed significantly higher tolerance to Cr no matter which form of Cr (Cr³⁺, CrO₄ ^2?, or Cr₂O₇ ^2?) was tested, and the Cr tolerance correlated with the total soil Cr concentration. However, the tolerance to Cu²⁺, As³⁺, and As⁵⁺ did not differ significantly between the GS3 soil and the five CCA soils. Community level physiological profiles using Biolog microtiter plates were also used to examine the chromate tolerance of the bacterial communities extracted after six months of exposure. Our results showed that the bacterial community tolerance was altered and increased as the soil Cr concentration was increased, indicating that the culturable microbial community and the AOB community responded in a similar manner.     

Immobilization and phytoavailability of cadmium in variable charge soils. III. Effect of biosolid compost addition

We examined the effect of biosolid compost on the adsorption and complexation of cadmium (Cd) in two soils (Egmont and Manawatu) which varied in their organic matter content. The effect of biosolid compost on the uptake of Cd from the Manawatu soil, treated with various levels of Cd (0–10 mg Cd kg^–1 soil), was also examined using mustard (Brassica juncea L.) plants. The transformation of Cd in soil was evaluated by a chemical fractionation scheme. Addition of biosolid compost increased negative charge in soil. The effect of biosolid compost on Cd adsorption varied between the soils, with a large portion of the sorbed Cd remaining in solution as an organic complex. Increasing addition of Cd increased Cd concentration in plants, resulting in decreased plant growth at high levels of Cd (i.e., phytotoxicity). Addition of biosolid compost was effective in reducing the phytotoxicity of Cd as indicated by the decrease in the concentration of NH₄OAc extractable-Cd and soil solution-Cd. The solid-phase fractionation study indicated that the addition of biosolid compost decreased the concentration of the soluble and exchangeable Cd fraction but increased the concentration of organic-bound Cd fraction in soil. Alleviation of Cd phytotoxicity by biosolid compost can be attributed primarily to complexation of Cd by the organic matter in the biosolid compost.     

Leaf silicon content in banana (Musa spp.) reveals the weathering stage of volcanic ash soils in Guadeloupe

Several plant species accumulate silicon, which is taken up by roots in soil solution. The Si concentration in soil solution can be governed by silicate dissolution and formation, and thus soil constitution. Here, we study the Si leaf content of mature banana plants (Musa acuminata cv Grande Naine) cropped on soils derived from andesitic ash in Guadeloupe through standard foliar analysis. The soils strongly differ in weathering stage and total Si content. The most desilicated soils (Andosol–Nitisol–Ferralsol) occur in the wettest areas, on the Eastern slopes (Es) of the volcano exposed to rain bearing winds. Least weathered soils (Andosol–Cambisol) occur on Western slopes (Ws). The average leaf Si concentration ranges from 2.7 to 3.9 g kg^?1 for bananas cropped in Es soils, and from 7.7 to 9.6 g kg^?1 in Ws soils. The leaf Si concentrations are lowest for the Es gibbsite-rich Andosols and Ferralsols. The leaf Si concentration is positively correlated with soil CaCl₂-extractable Si content, soil Si content and total reserve in weatherable minerals. The silicon content of banana leaves thus reveals the weathering stage of volcanic ash soils in Guadeloupe.     

Growth and physiological activity in <Emphasis Type="Italic">Larix kaempferi</Emphasis> seedlings inoculated with ectomycorrhizae as affected by soil acidification

To evaluate the effect of ectomycorrhizal colonization on growth and physiological activity of Larix kaempferi seedlings grown under soil acidification, we grew L. kaempferi seedlings with three types of ectomycorrhizae for 180 days in acidified brown forest soil derived from granite. The soil had been treated with an acid solution (0 (control), 10, 30, 60, and 90 mmol H⁺ kg⁻¹). The water-soluble concentrations of Ca, Mg, K, Al, and Mn increased with increasing amounts of H⁺ added to the soil. Ectomycorrhizal development significantly increased in soil treated with 10 and 30 mmol H⁺ kg⁻¹ but was significantly reduced in soil treated with 60 and 90 mmol H⁺ kg⁻¹. The concentrations of Al and Mn in needles or roots increased with increasing H⁺ added to the soil. The total N in seedlings significantly increased with increasing H⁺ in soil and colonization with ectomycorrhiza. The maximum net photosynthetic rate at light and CO₂ saturation (P _max) was greater in soil treated with 10 mmol H⁺ kg⁻¹ than in controls, and was less is soils treated with greater than with 30 mmol H⁺ kg⁻¹, especially with 60 and 90 mmol H⁺ kg⁻¹. However, colonization with ectomycorrhiza significantly reduced the concentration of Al and Mn in needles or roots and increased the values of P _max and total dry mass (TDM). The relative TDM of L. kaempferi seedlings was approximately 40% at a (BC, base cation)/Al ratio of 1.0. However, ectomycorrhizal seedlings had a 100–120% greater TDM at a BC/Al ratio of 1.0 than non-ectomycorrhizal seedlings, even though the acid treatment reduced their overall growth.     

Growth,net photosynthesis and leaf nutrient status of <Emphasis Type="Italic">Fagus crenata</Emphasis> seedlings grown in brown forest soil acidified with H<Subscript>2</Subscript>SO<Subscript>4</Subscript> or HNO<Subscript>3</Subscript> solution

To obtain basic information for evaluating critical loads of acid deposition for protecting Japanese beech forests, growth, net photosynthesis and leaf nutrient status of Fagus crenata seedlings grown for two growing seasons in brown forest soil acidified with H₂SO₄ or HNO₃ solution were investigated. The whole-plant dry mass of the seedlings grown in the soil acidified by the addition of H₂SO₄ or HNO₃ solution was significantly less than that of the seedlings grown in the control soil not supplemented with H⁺ as H₂SO₄ or HNO₃ solution. However, the degrees of reduction in the whole-plant dry mass and net photosynthetic rate of the seedlings grown in the soil acidified by the addition of H⁺ as H₂SO₄ solution at 100 mg l^–1 on the basis of air-dried soil volume (S-100 treatment) were greater than those of the seedlings grown in the soil acidified by the addition of H⁺ as HNO₃ solution at 100 mg l^–1 (N-100 treatment). The concentrations of Al and Mn in the leaves of the seedlings grown in the S-100 treatment were significantly higher than those in the N-100 treatment. A positive correlation was obtained between the molar ratio of (Ca+Mg+K)/(Al+Mn) in the soil solution and the relative whole-plant dry mass of the seedlings grown in the acidified soils to that of the seedlings grown in the control soil. Based on the results, we concluded that the negative effects of soil acidification due to sulfate deposition are greater than those of soil acidification due to nitrate deposition on growth, net photosynthesis and leaf nutrient status of F. crenata, and that the molar ratio of (Ca+Mg+K)/(Al+Mn) in soil solution is a suitable soil parameter for evaluating critical loads of acid deposition in efforts to protect F. crenata forests in Japan.     

Soil nutrient supply in natural and managed forests

The effect of a laboratory addition of 10, 100 and 500 mg Cd kg_{dry soil} ^-1 on ammonification and nitrification was studied using soil samples of two unpolluted grassland soils. Calcareous and non-calcareous soil were selected for this purpose. Various parameters of nitrifying activity were investigated simulataneously: activity during long-term laboratory incubations in the presence and absence of a substrate, mineralization potentials, and potential activity of both ammonium and nitrite oxidizers during short-term incubations in soil slurries. Cadmium was added as aqueous CdCl₂.Additions of both 100 and 500 mg Cd kg_{dry soil} ^-1 doses significantly lowered the ability of both soils to nitrify 100 g added NH₄ ⁺-N g_{dry soil} ^-1 as a substrate, which was reflected in a decreased rate of nitrate formation (maximum inhibition reached 60% in the calcareous soil and 45% in the non-calcareous soil). Furthermore, these two concentrations of Cd caused an abnormal accumulation of nitrite immediately after incorporation, particularly in the calcareous soil. The addition of 10 mg Cd kg_{dry soil} ^-1 intensified N-mineralization in both soils, probably as a consequence of a higher concentration of readily metabolized substrate originating from killed bacteria or fungi. An excess of nitrate was then formed as a final step. The harmful effect of cadmium was more pronounced in calcareous soil, probably due to the higher sensitivity of nitrite-oxidizers in these soil samples.     

Priming of Soil Carbon Decomposition in Two Inner Mongolia Grassland Soils following Sheep Dung Addition: A Study Using 13C Natural Abundance Approach

To investigate the effect of sheep dung on soil carbon (C) sequestration, a 152 days incubation experiment was conducted with soils from two different Inner Mongolian grasslands, i.e. a Leymus chinensis dominated grassland representing the climax community (2.1% organic matter content) and a heavily degraded Artemisia frigida dominated community (1.3% organic matter content). Dung was collected from sheep either fed on L. chinensis (C₃ plant with δ¹³C = −26.8‰; dung δ¹³C = −26.2‰) or Cleistogenes squarrosa (C₄ plant with δ¹³C = −14.6‰; dung δ¹³C = −15.7‰). Fresh C₃ and C₄ sheep dung was mixed with the two grassland soils and incubated under controlled conditions for analysis of ¹³C-CO₂ emissions. Soil samples were taken at days 17, 43, 86, 127 and 152 after sheep dung addition to detect the δ¹³C signal in soil and dung components. Analysis revealed that 16.9% and 16.6% of the sheep dung C had decomposed, of which 3.5% and 2.8% was sequestrated in the soils of L. chinensis and A. frigida grasslands, respectively, while the remaining decomposed sheep dung was emitted as CO₂. The cumulative amounts of C respired from dung treated soils during 152 days were 7–8 times higher than in the un-amended controls. In both grassland soils, ca. 60% of the evolved CO₂ originated from the decomposing sheep dung and 40% from the native soil C. Priming effects of soil C decomposition were observed in both soils, i.e. 1.4 g and 1.6 g additional soil C kg⁻¹ dry soil had been emitted as CO₂ for the L. chinensis and A. frigida soils, respectively. Hence, the net C losses from L. chinensis and A. frigida soils were 0.6 g and 0.9 g C kg⁻¹ soil, which was 2.6% and 7.0% of the total C in L. chinensis and A. frigida grasslands soils, respectively. Our results suggest that grazing of degraded Inner Mongolian pastures may cause a net soil C loss due to the positive priming effect, thereby accelerating soil deterioration.     

Measurement and modelling of photosynthetic response of pearl millet to soil phosphorus addition

There have been no studies of the effects of soil P deficiency on pearl millet (Pennisetum glaucum (L.) R. Br.) photosynthesis, despite the fact that P deficiency is the major constraint to pearl millet production in most regions of West Africa. Because current photosynthesis-based crop simulation models do not explicitly take into account P deficiency effects on leaf photosynthesis, they cannot predict millet growth without extensive calibration. We studied the effects of soil addition on leaf P content, photosynthetic rate (A), and whole-plant dry matter production (DM) of non-water-stressed, 28 d pearl millet plants grown in pots containing 6.00 kg of a P-deficient soil. As soil P addition increased from 0 to 155.2 mg P kg^–1 soil, leaf P content increased from 0.65 to 7.0 g kg^–1. Both A and DM had maximal values near 51.7 mg P kg^–1 soil, which corresponded to a leaf P content of 3.2 g kg^–1. Within this range of soil P addition, the slope of A plotted against stomatal conductance (g_s) tripled, and mean leaf internal CO₂ concentration ([CO₂]_i) decreased from 260 to 92 L L^–1, thus indicating that P deficiency limited A through metabolic dysfunction rather than stomatal regulation. Light response curves of A, which changed markedly with P leaf content, were modelled as a single substrate, Michaelis-Menten reaction, using quantum flux as the substrate for each level of soil P addition. An Eadie-Hofstee plot of light response data revealed that both K_M, which is mathematically equivalent to quantum efficiency, and V_max, which is the light-saturated rate of photosynthesis, increased sharply from leaf P contents of 0.6 to 3 g kg^–1, with peak values between 4 and 5 g P kg^–1. Polynomial equations relating K_M and V_max, to leaf P content offered a simple and attractive way of modelling photosynthetic light response for plants of different P status, but this approach is somewhat complicated by the decrease of leaf P content with ontogeny.     

Phytotoxic doses of boron in contrasting soils depend on soil water content

Boron (B) affects plant growth in soil at B doses (mg added B kg^-1 soil) that appear in the range of natural background B concentrations. A study was set up to determine B bioavailability by testing B toxicity to plant as affected by soil properties and ageing after soil dosing. Nineteen soils (pH 4.4?C7.8) and 3 synthetic soils (sand-peat mixtures) were amended with 7 doses of H₃BO₃. Barley root elongation was determined immediately after B amendment and after 1 and 5 months ageing. Soil solution B concentrations increased linearly with added B concentrations with almost no detectable adsorption. In contrast, the ratio of aqua regia soluble B/soil solution B in unamended soils (no B added) was 10?C25 times higher than in B amended soils at similar aqua regia soluble B concentrations illustrating a much lower B availability in unamended soils. Soil solution B concentrations did not decrease by ageing. The toxic B doses or soil B concentrations that decreased barley root growth by 10% (EC10 values) varied about tenfold (respectively 3?C27 mg added B kg^-1 and 5?C52 mg B kg^-1) among soils. Corresponding thresholds in soil solution varied less than fourfold (16?C59 mg B l^-1). Soil ageing for 5 months did not significantly change EC10 and EC50 values, expressed either as total soil B or as soil solution B, unless in 1 soil. Variability in EC10 and EC50 values was explained by various soil properties (soil moisture content, background B, %clay, cation exchange capacity), but covariance of these properties with the soil moisture content suggest that B dilution is the critical factor explaining B toxicity. It is concluded that effects of B amendments do not decrease by ageing and that soil solution B or B doses corrected for soil moisture content may be used as an index for B toxicity across different soils.     

The effect of phosphate rock dissolution on soil chemical properties and wheat seedling root elongation

Soils of the Appalachian region of the United States are acidic and deficient in P. North Carolina phosphate rock (PR), a highly substituted fluoroapatite, should be quite reactive in these soils, allowing it to serve both as a source of P and a potential ameliorant of soil acidity. An experiment was conducted to evaluate the influence of PR dissolution on soil chemical properties and wheat (Triticum aestivum cv. Hart) seedling root elongation. Ten treatments including nine rates of PR (0, 12.5, 25, 50, 100, 200, 400, 800, and 1600 mg P kg^-1) and a CaCO₃ (1000 mg kg^-1) control were mixed with two acidic soils, moistened to a level corresponding to 33 kPa moisture tension and incubated for 30 days. Pregerminated wheat seedlings were grown for three days in the PR treated soils and the CaCO₃ control. Root length was significantly (P<0.05) increased both by PR treatments and CaCO₃, indicating that PR dissolution was ameliorating soil acidity. The PR treatments increased soil pH, exchangeable Ca, and soil solution Ca while lowering exchangeable Al and 0.01 M CaCl₂ extractable soil Al. Root growth in PR treatments was best described by an exponential equation (P<0.01) containing 0.01 M CaCl₂ extractable Al. The PR dissolution did not reduce total soil solution Al, but did release Al complexing anions into soil solution, which along with increased pH, shifted Al speciation from toxic to nontoxic forms. These results suggest that North Carolina PR should contribute to amelioration of soil acidity in acidic, low CEC soils of the Appalachian region.