Plant species and nutritional-mediated control over rhizodeposition and root decomposition

This study focuses on the influence of nitrogen (N) availability and species on rhizodeposition and on decomposition of rhizodeposits, roots and soil organic matter. Four perennial grass species were studied that are characteristic of grassland habitats that differ in nutrient availability. These perennial grass species, Holcus lanatus L., Festuca rubra L., Anthoxanthum odoratum L. and Festuca ovina L., were homogeneously labeled with ¹⁴CO₂. Plants were grown on soil without N addition and with N addition (14 g N m^–2). After 8 weeks, plants were harvested and root production and the remaining amount of rhizodeposits in the soil were measured. ¹⁴C-labelled roots were incubated in fresh soil. Decomposition was measured of 1) the labeled rhizodeposits in the soil in which the plants had been growing and 2) the labeled dead roots incubated in fresh soil, by trapping the evolved ¹⁴CO₂, over 69 days.In general, decomposability of both roots and rhizodeposits increased when nitrogen availability increased. Moreover, the species differed in their response to N. Higher N supply increased total rhizodeposition of H. lanatus and the decomposability of rhizodeposited carbon compounds of this high fertility species was greater than of the low fertility species F. ovina, but lower than of A. odoratum. The presented study gives no evidence for a relation between root decomposition rate and the nutrient availability of the habitat of the four species. Overall, we suggest on the basis of the results that species can affect nutrient cycling by differences in rates of rhizodeposition and litter production. This offers a mechanism whereby species can influence species replacement during succession.     

Effect of Different Chloride Concentrations on Nutrient Release in Wetland Soils: A Phytometer Assessment in the Botshol Wetlands,The Netherlands

Turbid water, high phosphorus (P)-loading and disappearing Chara communities forced local water authorities to carry out restoration measures in the lakes and marshes of the Botshol The Netherlands. The reduction of the external-P input could be reached by chemical treatment of the brackish suppletion water and by separating the area from nearby agricultural areas. A side effect of these measures was an increase of chloride from 400 mg l⁻¹ to 1000 mg l⁻¹ in the surface water of Botshol. Internal biogeochemical processes were investigated with phytometers and direct measurements of soil nutrient availability in greenhouse experiments. The increased chloride levels were assumed to increase soil pore water P. The first experiment showed higher P in the peat-soils treated with the highest Cl-concentration and an increased leaching of PO₄ from the lake-bottom peat-soils. No reaction of the phytometer Epilobium hirsutum was found. In the second experiment the 800 mg l⁻¹ Cl-treatment resulted in significantly higher biomass of Carex acutiformis grown on treated bank soil. N-uptake by the phytometer Carex acutiformis was significantly higher. The available-P and total-P in the bank soil did not show a treatment effect. The two studies showed under similar ‘standardized’ conditions a treatment effect of chloride on the P-availability, resulting in higher PO₄ leaching and increased plant nutrient concentrations and biomass. The field study showed higher available-P concentrations in the shore zone than in remote areas. The high chloride levels after restoration impact internal nutrient availability in the Botshol wetlands, on soil loaded with P in recent and historic times.     

Annuario della Società Botanica Italiana 1932

A sandy culture experiment was conducted to investigate the effects of exogenous CaCl₂ on the indole alkaloid accumulation in Catharanthus roseus under salt stress. One-month seedlings of C. roseus were treated with the different concentrations of NaCl (0, 50, and 100 mmol l^? 1) and 7.5 mmol l^? 1 CaCl₂. The plant samples were analyzed after 7 days of the treatments. The NaCl-stressed plants showed decrease of fresh and dry weight and increase of malondialdehyde (MDA) content compared to control. Tryptophan decarboxylase (TDC) activity increased significantly under 50 mmol l^? 1 NaCl without CaCl₂ addition, 50 mmol l^? 1 NaCl with 7.5 mmol l^? 1 CaCl₂, and CaCl₂ treatment without NaCl addition. There was a significant increase in peroxidase activity under NaCl stress compared to control. The vindoline, catharanthine, vincristine, and vinblastine contents increased under salt stress (especially with 50 mmol l^? 1 NaCl treatment with or without CaCl₂). Addition of CaCl₂ to NaCl-stressed plants increased biomass, TDC activity, vindoline, and catharanthine contents and lowered MDA and vincirstine contents compared to the plants without CaCl₂. The plants treated with CaCl₂ alone showed higher TDC activity, vindoline, catharanthine, and vinblastine content when compared to control. The results showed that exogenous CaCl₂ could promote the indole alkaloid metabolism under salt stress.     

Glasgow Lake: an early-warning sentinel of lake acidification in Cape Breton Highlands National Park (Nova Scotia,Canada)

In contrast to other lakes studied in Cape Breton Highlands National Park (Nova Scotia, Canada), our paleolimnological results indicated that Glasgow Lake has been impacted by acidic deposition starting in the early 1900s. Based on analysis of diatom assemblages, the lake experienced a decrease in diatom-inferred lakewater pH from a pre-industrial pH of ~5.8 to a current pH of 5.3 (2000–2002 measured mean pH = 5.0) as well as a decrease in diatom-inferred Gran-alkalinity. In this study, diatom-based paleolimnological techniques were used in conjunction with a dynamic biogeochemical model (MAGIC) to assess both the timing and extent of the acidification trend, as well as determine a probable explanation as to why this lake, and none of the other 15 Cape Breton Highlands lakes studied for paleolimnology thus far, acidified under a peak non-marine sulphate deposition load of 43.6 mmol_c m⁻² year⁻¹ in the mid-1970s. Steady-state models estimate that Glasgow Lake had the lowest buffering capacity of six study lakes and estimated critical sulphate loading of <1 mmol_c m⁻² year⁻¹. MAGIC also estimated a loss of charge balance alkalinity from a pre-1850 value of 38 μmol_c l⁻¹ to a low of 12 μmol_c l⁻¹. While no evidence of biological recovery has been recorded, MAGIC estimates an increase in charge balance alkalinity to 27 μmol_c l⁻¹ in 2002 in response to decreased SO₂ emissions. Of the five other lakes that were modelled, all showed trends towards more acidic states and subsequent increases in charge balance alkalinity; however, the empirical paleo-diatom approach applied to these lakes showed no evidence of acidification. Thus, Glasgow Lake has the lowest buffering capacity among the Cape Breton Highland study lakes and serves as a sentinel of potential acidification trends and recovery in this region. Handling editor: J. Saros     

The role of salinity and sodicity in the dieback of Acacia xanthophloea in Ngorongoro Caldera, Tanzania

Dieback of Acacia xanthophloea (Benth.) has opened up the once densely forested Lerai area in Ngorongoro Caldera, Tanzania. Soil samples were taken from profiles in the Ngorongoro Conservation Area and Lake Manyara National Park at sites of dieback and at sites with healthy A. xanthophloea trees. Dieback sites had significantly greater electrical conductivity (EC), water‐soluble Na⁺, K⁺, Cl^?, SO and sodium adsorption ratios (SAR) than healthy sites. The following mean values were recorded: EC (179 versus 70 mS m^?1; P < 0.001, Student's t‐test, n = 8 and 10, respectively; 40–60 cm); Na⁺ (99 versus 30 mmol_c kg^?1, P < 0.001, n = 7 and 8 respectively); K⁺ (11 versus 3 mmol_c kg^?1, P < 0.05); Cl^? (36 versus 7 mmol_c kg^?1, P < 0.01); SO (31 versus 5 mmol_c kg^?1, P < 0.01); and SAR (28 versus 8 mmol l^?1/2, P < 0.01). Water‐soluble Na⁺, Cl^? and SO concentrations in the Lerai profiles have probably resulted in toxicity and osmotic stress which contributed to dieback. Accumulation of salts may have occurred because of reduced flow of freshwater through Lerai and/or flow of water from Lake Magadi into Lerai. Forest recovery may be possible if salinity is reduced. Management strategies for reducing salinity have been implemented and included re‐establishing streams that flow through Lerai. Exclusion of elephants (Loxodonta africana) from Lerai is another management strategy presently under consideration.     

Nitrogen losses from perennial grass species

Nitrogen losses from plants may occur through a variety of pathways, but so far, most studies have only quantified losses of nutrients by above-ground litter production. We used ¹⁵N pulse labelling to quantify total nitrogen losses from above- and below-ground plant parts. Using this method we were able to include also pathways other than above-ground litter production. To test the hypothesis that species from nutrient-poor habitats lose less nitrogen than species from more fertile soils, six perennial grasses from habitats with a wide range of nutrient availability were investigated: Lolium perenne, Arrhenatherum elatius, Anthoxanthum odoratum, Festuca rubra, F. ovina and Molinia caerulea. The results of an experiment carried out in pots in a green-house at two fertility levels show that statistically significant losses occur through pathways other than above-ground litter production. In the low fertility treatment, most (70%) losses from L. perenne occurred by litter production, but in Ar. elatius, F. rubra, F. ovina and M. caerulea, more than 50% of labelled N losses took place by root turn-over, leaching or exudation from roots. When nutrient supply increased, the ¹⁵N losses in above-ground dead material increased in all species and in Ar. elatius, A. odoratum and F. rubra the ¹⁵N losses via other pathways decreased. Ranked according to decreasing turnover coefficient the sequence of species was: L. perenne, A. odoratum, F. rubra, F. ovina, Ar. elatius, M. caerulea. These results suggest that species adapted to sites with low availability of nutrients lose less nitrogen (including above- and below-ground losses) than species adapted to more fertile soils.     

EXPERIMENTAL STUDIES OF THE EVOLUTIONARY SIGNIFICANCE OF SEXUAL REPRODUCTION. VI. A GREENHOUSE TEST OF THE SIB-COMPETITION HYPOTHESES

This study tests the hypothesis that competition among groups of sexual and asexual siblings generates an advantage for sexual females. Individual tillers of Anthoxanthum odoratum were planted singly, among other siblings from the same family, and among groups of sexual and asexual siblings from different families in pots in an unheated greenhouse. Unlike previous field experiments, there was little difference between the performance of sexual and clonal tillers after two years, despite strong treatment effects and high mortality. The results demonstrate that sib competition does not generate an advantage for sexual reproduction in biotically simple environments.     

Influence of microbial activity on plant–microbial competition for organic and inorganic nitrogen

To investigate how the level of microbial activity in grassland soils affects plant–microbial competition for different nitrogen (N) forms, we established microcosms consisting of a natural soil community and a seedling of one of two co-existing grass species, Anthoxanthum odoratum or Festuca rubra. We then stimulated the soil microbial community with glucose in half of the microcosms and followed the transfer of added inorganic (¹⁵NH₄¹⁵NO₃) and organic (glycine-2-¹³C-¹⁵N) N into microbial and plant biomass. We found that microbes captured significantly more ¹⁵N in organic than in inorganic form and that glucose addition increased microbial ¹⁵N capture from the inorganic source. Shoot and root biomass, total shoot N content and shoot and root ¹⁵N contents were significantly greater for A. odoratum than F. rubra, whereas F. rubra had higher shoot and root N concentrations. Where glucose was not added, A. odoratum had higher shoot ¹⁵N content with organic than with inorganic ¹⁵N addition, whereas where glucose was added, both species had higher shoot ¹⁵N content with inorganic than with organic ¹⁵N. Glucose addition had equally negative effects on shoot growth, total shoot N content, shoot and root N concentrations and shoot and root ¹⁵N content for both species. Both N forms produced significantly more shoot biomass and higher shoot N content than the water control, but the chemical form of N had no significant effect. Our findings suggest that plant species that are better in capturing nutrients from soil are not necessarily better in tolerating increasing microbial competition for nutrients. It also appears that intense microbial competition has more adverse effects on the uptake of organic than inorganic N by plants, which may potentially have significant implications for interspecific plant–plant competition for N in ecosystems where the importance of organic N is high and some of the plant species specialize in use of organic N.     

Ethene as an auxiliary substrate for the cooxidation of cis-1,2-dichloroethene and vinyl chloride

Cultures able to dechlorinate cis-1,2-dichloroethene (cDCE) were selected with ethene (3–20%, v/v) as the sole source of carbon and energy. One mixed culture (K20) could degrade cDCE (400 μmol l^–1) or vinyl chloride (100 μmol l^–1) in the presence of ethene (≤ 80 μmol l^–1 and ≤ 210 μmol l^–1, respectively). This culture consists of at least five bacterial strains. All five strains were able to degrade cDCE cometabolically in pure culture. The mixed culture K20 was highly tolerant against cDCE (up to 6 mmol l^–1 in the liquid phase). Degradation of cDCE (200 μmol l^–1) was not affected by the presence of trichloroethene (100 μmol l^–1) or tetrachloroethene (100 μmol l^–1). Transformation yields (T_y, defined as unit mass of chloroethene degraded per unit mass of ethene consumed) of the mixed culture K20 were relatively high (0.51 and 0.61 for cDCE and vinyl chloride, respectively). The yield for cDCE with ethene as auxiliary substrate was ninefold higher than any values reported with methane or methane/formate as auxiliary substrate. The viability of the cells of the mixed culture K20 (0.3 mg of cells ml^–1) was unaffected by the transformation of ≤ 200 μmol l^–1 cDCE in 300 min. Received: 9 March 1999 / Accepted: 21 July 1999     

The effect of living plants on root decomposition of four grass species

We tested whether living plant roots of Holcus lanatus and Festuca ovina can affect the decomposition rate of dead roots of Holcus lanatus, Festuca rubra, Anthoxanthum odoratum and Festuca ovina. Moreover, we investigated whether this effect is dependent on the decomposing root species or the nitrogen supply during the growth of the roots. The selected perennial grass species are typical of grassland habitats in a range from high to low nitrogen availability: H. lanatus, F. rubra, A. odoratum and F. ovina. Seedlings of these species were homogeneously labelled with ¹⁴CO₂ for eight weeks. Plants were grown on soil at two nitrogen levels: one without additional nitrogen and one with nitrogen addition (14 g N m⁻²).
At the start of the decomposition experiment ¹⁴C labelled roots were separated from soil and incubated in litterbags (mesh width 1 mm) in fresh soil. These ¹⁴C labelled roots were left to decompose for 19 weeks in an open greenhouse in soil planted with H. lanatus or F. ovina and in unplanted soil. After the incubation period, the decomposition of the ¹⁴C labelled roots of the four species was measured. The mass and ¹⁴C losses from the dead roots were calculated and the living plant biomass and C, N and P contents of the living plants were measured.
Living plant roots of F. ovina had positive effects on the decomposition rate of F. ovina root litter, but dead A. odoratum roots from the N fertilized treatment decomposed slower in the presence of living F. ovina plants. It seems likely that living plants like F. ovina exude carbon compounds that stimulate the growth of soil microbes and thereby increase dead root decomposition and mineralization. Root decomposition rates differed among the species. We found no evidence to support our hypothesis that dead roots of high fertility species (i.e. H. lanatus and F. rubra) decompose faster than dead roots of low fertility species (i.e. A. odoratum and F. ovina). In unplanted soil, the mass loss and total ¹⁴C loss from A. odoratum dead roots were higher than those from H. lanatus, F. rubra and F. ovina dead roots. Dead roots of F. ovina lost less mass and total ¹⁴C than dead roots of H. lanatus.     

Calcium chloride penetrates plant cuticles via aqueous pores

Penetration of calcium chloride across astomatous cuticular membranes (CMs) isolated from leaves of Pyrus communis L. has been studied. Penetration was a first-order process when calcium chloride concentrations ranged from 2 g l⁻¹ to 10 g l⁻¹. Rate constants were increased 10-fold by adding wetting agents but they did not depend on temperature. The accelerators tributyl phosphate and diethyl sebacate had no effect on rates of penetration. Increasing humidity over the salt residue on the CMs from 50 to 90% increased rate constants by about 2-fold. Extracting cuticular waxes from pear leaf CMs increased rate constants by factors of 2 to 3, depending on humidity. Leaf CMs from Malus domestica Borkh., Populus alba L., Stephanotis floribunda Brongn. and Schefflera actinophylla (Endl.) Harms were also permeable to CaCl₂. Highest rate constants were observed with poplar CMs while Schefflera CMs exhibited the lowest permeability. By comparing these results with the well established transport properties of the lipophilic pathway it is concluded that calcium chloride hexahydrate penetrated cuticular membranes via aqueous pores. Received: 14 December 1999 / Accepted: 31 March 2000     

Effects of nitrate treatment on a mixed species, oil field microbial biofilm

Biofilms of bacteria, indigenous to oil field produced water, were grown in square section, glass capillary flow cells at 45 °C. Initially, in situ image analysis microscopy revealed predominantly coccoid bacteria (length-to-width ratio measurements (l _c:w _c) of bacterial cells gave a mean value of 1.1), while chemical measurements confirmed sulphate reduction and sulphide production. After nitrate ion addition at 100 and 80 mg/l, in the two repeat experiments respectively, the dominance of rod-shaped bacteria (mean l _c:w _c = 2.8) was observed. This coincided with the occurrence of nitrate reduction in the treated flow cells. Beneficially, no significant increase in biofilm cover was observed after the addition of nitrate. The dominant culturable nitrate-reducing bacterium was Marinobacter aquaeolei. The l _c:w _c ratio measured here concurs with previously reported cell dimensions for this organism. Several Marinobacter strains were also isolated from different oil fields in the North Sea where nitrate treatment has been applied to successfully treat reservoir souring, implying that this genus may play an important role in nitrate treatment.     

Saline groundwater as an aquaculture medium: physiological studies on the red drum, Sciaenops ocellatus

Physiological responses of the euryhaline red drum, Sciaenops ocellatus, to chloride salt addition, low salinity, and high sulfate concentration were measured. Survival was increased by addition of calcium chloride (CaCl₂) or magnesium chloride (MgCl₂) to dilute artificial seawater (0.2 ppt salinity). Although survival and routine metabolic rates were greater in MgCl₂ treatments, growth and feed efficiency were greater in CaCl₂ treatments. Marginal metabolic scope increased when CaCl₂ or MgCl₂ were added to dilute artificial seawater. There was a strong positive linear relationship (p=0.0001, r=0.91) between fish survival and salinity of artificial seawater dilutions over the salinity range 0.1 to 3.0 ppt. Monovalent ion concentrations in red drum plasma varied; whereas, divalent ion concentrations were relatively constant. Survival and growth were not affected by high sulfate concentrations (2000 mg l^-1) in 3.0 ppt artificial seawater supplemented with either sodium sulfate or magnesium sulfate. Routine metabolic rate and marginal metabolic scope of red drum exposed to high sulfate concentrations were slightly, but not significantly, lower than those of red drum in 3 ppt artificial seawater.     

Plant cation-anion balance as affected by the ionic composition of the growing medium

Plants that remove an excess of cations over anions may cause soil acidification. The acidification potential of plants has been evaluated using solution culture techniques, but the influence of ionic composition of the medium on the plant cation-anion balance remains unclear. Our objective was to determine how electrolyte concentration and salt type affect the cation- anion balance of two test plants [barley (Hordeum vulgare L.) and kochia (Kochia scoparia L. Schrad.)]. Seedlings were grown in sand culture and irrigated with nutrient solution (Hoagland’s solution), which was adjusted to a range of electrolyte concentrations (target electrical conductivity of 7.5, 17.5 and 27.5 dS m⁻¹) using either chloride or sulphate salts. Increase in electrolyte concentration reduced yield of kochia, a salt-tolerant plant, by up to 38%. Total cation (Ca + Mg + K + Na) equivalents in kochia exceeded those of anions (Cl + S + P + NO₃) by 250 to 280 cmol_c kg⁻¹ of dry matter. Electrolyte concentration had no effect on the cation-anion balance of kochia, but excess cation values were significantly greater in the sulphate than in the chloride system. Kochia had a large content of water-soluble oxalate (194 to 226 cmol_c kg⁻¹), which was linearly related to the excess cation content. Growth of barley was severely restricted at the intermediate and high electrolyte concentrations. Cations exceeded anions by 21 to 59 cmol_c kg⁻¹ of barley dry matter. Excess cation content was greater in the sulphate than in the chloride medium, but electrolyte concentration did not have a consistent effect on the cation-anion balance. The small amounts of oxalate found in barley (0.9 to 2.6 cmol_c kg⁻¹) were insufficient to balance the cation excess.     

Effects of addition of calcium and magnesium salts on ammonia volatilization during manure decomposition

Ammonia volatilization during aerobic decomposition of poultry manure was significantly reduced through additions of calcium and magnesium salts. The percentage reduction in ammonia loss decreased during the 48 day decomposition period from 85–100% in the first 2–3 weeks, to 23–52% at the end of the experiment. The maximum amount of ammonia which was retained (i.e. maximum reduction in ammonia loss) through addition of the chloride salts of Mg²⁺ or Ca²⁺ was independent of the type of cation. However, CaCl₂ released some of the ammonia initially retained as production of CO₂ and NH₃ from the manure decreased after 3 weeks of decomposition, whereas both MgCl₂ and MgSO₄ did not release any of the initially retained ammonia over the 7 week incubation period. Over the entire incubation period MgCl₂ therefore retained more ammonia than CaCl₂. Magnesium sulphate was considerably less effective in retaining ammonia than either chloride salts.     

The effect of different kinds of electrolyte and non‐electrolyte solutions on the survival rate and morphology of zebrafish Danio rerio embryos

The effect of electrolyte and non‐electrolyte solutions on the survival and on the morphology of zebrafish Danio rerio embryos was investigated. Embryos in different ontogenetic stages were incubated in electrolyte (NaCl, KCl, MgCl₂ and CaCl₂) and non‐electrolyte solutions [sucrose and polyvinylalcohol (PVA)] of different concentrations for 5 – 15 min. The embryos were hatched to the long‐pec stage and the effective concentrations which caused a 50% decrease in embryo development (EC₅₀) were determined. The morphometric changes, which were caused by the test solutions, were measured. Ion channel blockers were used to see if active ion transport played a role for embryo survival. Finally, dechorionated embryos were exposed to the test solutions to get indications about the importance of chorion and perivitelline space. For 12 hours post fertilization (hpf) embryos and a 15 min exposure period, EC₅₀ was highest for MgCl₂ (1·60 mol l^?1), followed by sucrose (0·73 mol l^?1), NaCl (0·49 mol l^?1), KCl (0·44 mol l^?1), CaCl₂ (0·43 mol l^?1) and PVA [0·0005 mol l^?1 (2·2%)]. EC₅₀ were lower for early embryonic stages than for advanced stages for all solutions with exception of MgCl₂ and sucrose. At the EC₅₀, MgCl₂ and CaCl₂ solutions did not induce morphometric changes. NaCl and sucrose solutions induced reversible morphometric changes, which were compensated within 10 min. Only the EC₅₀ of KCl and PVA solutions induced permanent morphometric changes, which could not be compensated. Incubation of embryos in electrolyte and non‐electrolyte solutions together with ouabain (blocker of Na⁺– K⁺ ATPase), HgCl₃ (dose‐dependent inhibition of aquaporine channels), verapamil (inhibition of calcium and magnesium uptake) and amiloride (inhibition of sodium uptake) significantly decreased the per cent of embryos developing to the long‐pec stage in comparison to the same solutions without blockers. Ouabain and HgCl₃ also induced morphometric changes. For dechorionated embryos the survival rates in water and in the different test solutions were similar to untreated embryos.     

Effect of Sodium and Magnesium on Kinetics of Phosphorus Release in Some Calcareous Soils of Western Iran

Knowledge of the rate of release of phosphorus (P) from soils resulting from poor water quality application is essential for long-term planning of crop production while minimizing the impact on groundwater quality. In this study, we examined the effect of sodium adsorption ratio (SAR) and Ca:Mg ratio of water on P release of some calcareous soils from western Iran. Nine different solutions at a total electrolyte concentration of 100 mmol_c l^?1 and three levels of SAR (5, 15, 45) each with Ca:Mg ratios of 1:3, 1:1, or 3:1, prepared using solutions of NaCl, CaCl₂, and MgCl₂, were used to extract P from the soils. The geochemical Visual MINTEQ was used to calculate saturation indices and P species at the initial and end of P release. Significantly different quantities of P were extracted by the solutions. The maximum (average of five soils) (233.6 mg kg^?1) and the minimum (162.9 mg kg^?1) P were extracted by an SAR 45 solution with a Ca:Mg ratio of 1:3 and SAR 15 solution with Ca:Mg ratio of 3:1, respectively. Elovich model adequately described P release. The release rate for SAR 15 with Ca:Mg of 3:1 and SAR 45 with Ca:Mg 1:3 ranges from 16.3 to 31.3 mg kg^?1 h^?1 and from 20.0 to 32.8 mg kg^?1 h^?1, respectively. In the initial stage of P release the solution samples in most soils were saturated with respect to hydroxyapatite, octacalcium phosphate, ß-tricalcium phosphate, and undersaturated with respect to dicalcium phosphate dihydrate, dicalcium phosphate, and mangnesium phosphates. At the end of P release, all solutions were saturated with respect to hydroxyapatite and under saturated with respect to other phosphate minerals. The results imply that P release from soils could be increased during use of saline and sodic irrigation water containing high Mg concentration and that P fertilization management may need modification.     

Effect of Immobilizing Substances and Salinity on Heavy Metals Availability to Wheat Grown on Sewage Sludge-Contaminated Soil

The objective of the investigation was to evaluate the effect of immobilizing substances and NaCl salinity on the availability of heavy metals: Zn, Cd, Cu, Ni, and Pb to wheat (Triticum aestivum L.). In greenhouse pot experiment, a sewage sludge amended soil was treated with the following immobilizing substances: three clay minerals (Na-bentonite, Ca-bentonite and zeolite), iron oxides (goethite and hematite), and phosphate fertilizers (superphosphate and Novaphos). The pots were planted with wheat and were irrigated either with deionized or saline water containing 1600 mg L^?1 NaCl. Wheat was harvested two times for shoot metal concentrations and biomass measurements. Metal species in soil solution were estimated using the software MINEQL+.

The addition of metal immobilizing substances to the soil significantly decreased metal availability to wheat. The largest reduction in metal bioavailability was found for bentonites. The irrigation with saline water (1600 mg L^?1 NaCl) resulted in a significant increase in metal chloride species (MCl⁺ and MCl₂ ⁰). The highest metal complexation with Cl occurred for Cd, which was about 53% of its total soil solution concentration. The total concentration of Cd (Cd_T) in soil solution increased by 1.6–2.8-fold due to saline water. The NaCl salinity caused a significant increase in uptake and shoot concentration of Cd for two harvests and small but significant increase in shoot Pb concentration for the second harvest. It was concluded that the use of bentonites is the most promising for the reduction of heavy metal availability to plants. Saline water containing 1600 mg L^{? 1} NaCl increased the availability of Cd and Pb to wheat and decreased the efficiency of bentonites to immobilize soluble Cd.     

Development and application of a nutrient-diffusing bioassay for large rivers

1. Laboratory and field experiments were performed to develop and then apply a nutrient-diffusing substratum (NDS) design suitable for use in large, fast-flowing rivers. 2. Initial laboratory experiments quantified diffusion of PO₄ and NO₃ from new and previously used clay pots, which were soaked in deionized distilled water. Mean release rates initially exceeded 2.4 and 725 μmol l^–1 day^–1 P and 0.22 and 18 μmol l^–1 day^–1 N from new and used pots, respectively, but declined rapidly with increasing time spent in deionized distilled water and were below detectable levels after about 18 and 29 days, respectively. 3. A phosphorus (P) dose–response experiment in a P-limited reach of the Athabasca River, Alberta, Canada showed that epilithic biomass and macroinvertebrate density on NDS increased with increasing concentrations of KH₂PO₄ up to about 0.5 m . Beyond this threshold, biomasses and densities were unaffected by initial KH₂PO₄ concentration. Coefficients of variation of epilithic biomass estimates declined with increasing KH₂PO₄ whereas invertebrate density appeared to be unaffected by KH₂PO₄ levels. 4. Release rates of both P and N from NDS filled with 0.5 m KH₂PO₄ or 0.5 m NaNO₃ declined at a log-negative rate from about 5000 μmol N-NO₃ l^–1 day^–1 and 3500 μmol P-PO₄ l^–1 day^–1 on day 2, to 200 μmol l^–1 day^–1 for both N and P on day 32. 5. After development, we used the diffusing substrata to identify spatial patterns in nutrient limitation at seven sites along a 120 km reach in the Athabasca River, that receives two known point-source nutrient inputs. NDS consisting of N, P, N + P and unenriched controls were attached to the river bottom for 22–23 days and then retrieved and sampled for epilithic chlorophyll a. Physicochemical parameters and epilithic biomasses on upper stone surfaces were also quantified when NDS were deployed and retrieved from each site. 6. Sites located immediately downstream of the two point source inputs had higher water column concentrations of PO₄ and epilithic biomasses than the site immediately upstream; epilithic biomass was positively related to PO₄ in the late autumn (r²⁼ 0.58) but not in early autumn. Sites located immediately below nutrient inputs were not nutrient-limited, whereas upstream reference sites were P-limited.     

The measurement of small-scale environmental heterogeneity using clonal transplants of Anthoxanthum odoratum and Danthonia spicata

Summary To assess the scale of micro-environmental heterogeneity perceived by two co-occurring grass species, Anthoxanthum odoratum and Danthonia spicata, cloned tillers of each species were planted into the natural habitat at a range of spacings (from 2 cm to more than 2 m apart) and measured for survival and fecundity over three years. A. odoratum responded to heterogeneity at a scale of 4–8 cm and at a scale of 2–8 m but not to intermediate scales. D. spicata did not respond significantly to heterogeneity. However one genotype infected with the systemic fungus Atkinsonella hypoxylon showed a large response to heterogeneity at both small and large spatial scales. The results showed that the scale and level of environmental heterogeneity as measured by its fitness impact depends on the species and genotype involved. The results indicate that small scale environmental heterogeneity could play a role in the maintenance of sexual reproduction in A. odoratum.