Determination of organic carbon and nitrogen in particulate organic matter and particle size fractions of Brookston clay loam soil using infrared spectroscopy

The objective of this study was to determine whether models developed from infrared spectroscopy could be used to estimate organic carbon (C) content, total nitrogen (N) content and the C:N ratio in the particulate organic matter (POM) and particle size fraction samples of Brookston clay loam. The POM model was developed with 165 samples, and the particle size fraction models were developed using 221 samples. Soil organic C and total N contents in the POM and particle size fractions (sand, 2000–53 µm; silt, 53–2 µm; clay, <2 µm) were determined by using dry combustion techniques. The bulk soil samples were scanned from 4000 to 400 cm^?1 for mid‐infrared (MIR) spectra and from 8000 to 4000 cm^?1 for near‐infrared (NIR) spectra. Partial least squares regression (PLSR) analysis and the ‘leave‐one‐out' cross‐validation procedure were used for the model calibration and validation. Organic C and N content and C:N ratio in the POM were well predicted with both MIR‐ and NIR‐PLSR models ( = 0.84–0.92; = 0.78–0.87). The predictions of organic C content in soil particle size fractions were also very good for the model calibration ( = 0.84–0.94 for MIR and = 0.86–0.92 for NIR) and model validation ( = 0.79–0.94 for MIR and = 0.84–0.91 for NIR). The prediction of MIR‐ and NIR‐PLSR models for the N content and the C:N ratio in the sand and clay fractions was also satisfactory ( = 0.73–0.88; = 0.67–0.85). However, the predictions for the N content and C:N ratio in the silt fraction were poor ( = 0.23–0.55; = 0.20–0.40). The results indicate that both MIR and NIR methods can be used as alternative methods for estimating organic C and total N in the POM and particle size fractions of soil samples. However, the NIR model is better for estimating organic C and N in POM and sand fractions than the MIR model, whereas the MIR model is superior to the NIR model for estimating organic C in silt and clay fractions and N in clay fractions.     

A comparison of the convection-dispersion equation and transfer function model for predicting chloride leaching through an undisturbed, structured clay soil

The transfer function mode) (TFM) and convection-dispersion equation (CDE) were compared for predicting Cl ^? transport through a calcareous pelosol during steady, nearsaturated water flow. Large, undisturbed soil cores were used at constant irrigation intensities (q₀) between 0.3 and 3 cm h^?1, with a step-change in Cl^? concentration. The assumption of a lognormal distribution of travel times–characterized by the mean (μ) and variance (σ²)–permitted the flux-averaged breakthrough curves (BTCs) to be modelled very accurately by the TFM. The BTCs could be modelled equally well by the CDE when both the mean pore water velocity (v) and dispersion coefficient (D) were optimized simultaneously by the method of least squares, but not when v was put equal to q₀/_v, where _V was the mean volumetric water content. The best estimate of v was consistently > q₀/_v, which suggested that not all the pore water was effective in chloride transport. An operationally defined transport volume (θ_st) was calculated from the mean () or median (τ_m) travel times derived from the TFM. Chloride exclusion was not solely responsible for θ_st() being <_V: immobile water also contributed. The positive skewness of the travel time distributions meant that θ_st(τ_m) < θ_st(), indicating the effectiveness of macropore flow in solute transport. Dαv^1.42 (from the CDE), and σ²αv (from the TFM), confirmed that Cl^? dispersion increased as flow velocity increased. Flux-averaged concentrations were used to calculate the volume-averaged resident concentrations. They matched the measured Cl^? concentrations most closely when there was a gradual decrease in measured Cl ^? concentration with depth, but not when Cl ^? decreased sharply below c. 10 cm. Calculations assuming that all the water was effective in chloride transport gave less accurate results. Comparison of the measured and predicted concentrations of solute demonstrated that this must be a critical part of the evaluation of any model of solute transport.     

Determination of total soil organic C and hot water‐extractable C from VIS‐NIR soil reflectance with partial least squares regression and spectral feature selection techniques

The calibration of soil organic C (SOC) and hot water‐extractable C (HWE‐C) from visible and near‐infrared soil reflectance spectra is hindered by the complex spectral interaction of soil chromophores that usually varies from one soil or soil type to another. The exploitation of spectral variables from spectroradiometer data is further affected by multicollinearity and noise. In this study, a set of soil samples (Fluvisols, Podzols, Cambisols and Chernozems; n = 48) representing a wide range of properties was analysed. Spectral readings with a fibre‐optics visible to near‐infrared instrument were used to estimate SOC and HWE‐C contents by partial least squares regression (PLS). In addition to full‐spectrum PLS, spectral feature selection techniques were applied with PLS (uninformative variable elimination, UVE‐PLS, and a genetic algorithm, GA‐PLS). On the basis of normalized spectra (mean centring + vector normalization), the order of prediction accuracy was GA‐PLS ? UVE‐PLS > PLS for SOC; for HWE‐C, it was GA‐PLS > UVE‐PLS, PLS. With GA‐PLS, acceptable cross‐validated (cv) prediction accuracies were obtained for the complete dataset (SOC, , RPD_cv = 2.42; HWE‐C_cv, , RPD_cv = 2.13). Splitting the soil data into two groups with different basic properties (Podzols compared with Fluvisols/Cambisols; n = 21 and n = 23, respectively) improved SOC predictions with GA‐PLS distinctly (Podzols, , RPD_cv = 3.14; Fluvisols/Cambisols, , RPD_cv = 3.64). This demonstrates the importance of using stratified models for successful quantitative approaches after an initial rough screening. GA selection frequencies suggest that the spectral region over 1900 nm, and in particular the hydroxyl band at 2200 nm are of great importance for the spectral prediction of both SOC and HWE‐C.     

Soil wetting‐drying and water‐retention properties in a mine‐soil treated with composted and thermally‐dried sludges

The main objective of this study was to analyse how different sewage sludges influence soil wetting and drying dynamics. Three composted and three thermally‐dried municipal sludges from different wastewater plants located in Catalonia (NE Spain) were mixed with a mine‐soil obtained from a limestone quarry. Measurements of the time required to reach zero contact angle () and water holding time (WHT) provided information on the time required for a mine‐soil to reach its complete wettability and the residence time of water stored between ?0.75 and ?25 MPa of soil suction, respectively. One month after sludge amendments, one composted and one thermally‐dried sludge significantly increased . WHT was increased in the mine‐soil treated by composted sludges (50.6% by Blanes' sludge, 65.5% by Manresa's sludge and 52.5% by Vilaseca's sludge) one month after sludge amendments. The amount of water retained in the mine‐soil was increased by all composted sludges and one thermally‐dried sludge after one month (by 42.3% with Blanes' sludge, 42.3% with Manresa's sludge, 65.7% with Vilaseca's sludge and 23.9% with Mataró's sludge) and one year after sludge amendments and at a small suction. Increments in WHT corresponded with the amount of water retained so the time‐scale of soil water availability should also be considered. The value was modified mainly by increments in carbon stock and microbial biomass, while the WHT was modified mainly by increments in pH and electrical conductivity. Under similar air‐drying conditions, mine‐soil treated with composted sludges retained more water for longer compared with thermally‐dried sludges.     

THE MECHANICAL STRENGTH OF UNSATURATED POROUS GRANULAR MATERIAL

The influence of pore-water suction on the strength of a porous material is that it contributes a compressive load which increases the shear strength. When the material is unsaturated, the normal load or effective stress is due, in part to the continuous water at measured suction in unemptied pores, and in part to isolated bodies in nominally emptied pores at suctions approximating to the suction at emptying. When the material is draining from saturation, the effective stress σ is where S is the fraction of saturation, α is the fraction of the initial water content drained at the maximum suction, _Ps_d is the prevailing pore water suction, and _Ps_d is a suction passed through in reaching _pS_d at which the reduction of S is dS. When the material is rewetting, the relationship becomes where _ps_w is now the prevailing suction during wetting and f is a distribution function of the degree of saturation such that δS is the fractional saturation removed in the suction range δs_d at s_d and regained in the suction range δs_w at s_w. _ms_d is the maximum suction attained. The effective stress is revealed experimentally by unconfined compression tests on samples with imposed pore water suctions, and the dependence on this suction confirms reasonably that which is predicted by the theoretical formulas.     

How does the application of different nitrification inhibitors affect nitrous oxide emissions and nitrate leaching from cow urine in grazed pastures?

Nitrous oxide (N₂O) is a potent greenhouse gas, and nitrate () is a water contaminant. In grazed grassland, the major source of both leaching and N₂O emissions is nitrogen (N) deposited in animal excreta, particularly in the urine. The objective of this study was to determine the effectiveness of two nitrification inhibitors: (i) a solution of dicyandiamide (DCD) and (ii) a liquid formulation of 3,4‐dimethylpyrazole phosphate (DMPP) for reducing N₂O emissions and leaching from urine patch areas in two grazed pasture soils under different environmental conditions. In the Canterbury Templeton soil, the nitrification rate of ammonium from the animal urine applied at 1000 kg N/ha was significantly decreased by the application of DCD (10 kg/ha) and DMPP (5 kg/ha). N₂O emissions, measured over a 3‐month period, from dairy cow urine applied to the Canterbury Templeton soil were 1.14 kg N₂O‐N/ha, and this was reduced to 0.43 and 0.39 kg N₂O‐N/ha by DCD and the liquid DMPP, respectively. These are equivalent to 62–66% reductions in the total N₂O emissions. Nitrate leaching losses from dairy cow urine applied to the Waikato Horotiu soil lysimeters were reduced from 628.6 kg ‐N/ha to 400.6 and 451.5 kg ‐N/ha by the application of DCD (10 kg/ha) or DMPP (1 kg/ha), respectively. There was no significant difference between the DCD solution and the liquid DMPP in terms of their effectiveness in reducing N₂O emissions or leaching under the experimental conditions of this study. These results suggest that both the liquid formulations of DCD and DMPP have the potential to be used as nitrification inhibitors to reduce N₂O emissions and leaching in grazed pasture soils.     

THE MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOIL

If an exchangeable ion in soil diffuses along a liquid and solid pathway, its diffusion coefficient may be expressed as where D, v, f, C are diffusion coefficient, volume fraction, impedance factor, and concentration terms and the suffixes _l,_S refer to liquid and solid. The self-diffusion coefficient of the ion is then where D′, D′_t, and D′_s, are self-diffusion coefficients. D and D′ will vary with concentration. In diffusion out of the soil to a zero sink, the appropriate average diffusion coefficient is, approximately, the self-diffusion coefficient in the undisturbed soil. Diffusion of one ion species is influenced by other ions diffusing in the system through the diffusion potential set up. When ions are diffusing to plant roots, the diffusion potential is likely to be small. A more likely, though more complicated, expression for D than the first equation above is derived by assuming the ion to follow solid and liquid pathways in series as well as in parallel.     

QUALITY CONTROL IN SOIL SURVEY

The paper examines 66 Australian soil surveys in a variety of terrains (but not close forest), by several survey procedures, and published at a range of map scales. It relates the Survey Effort (E) of professional staff (in man-days per km²) to (I) survey procedure, (2) the kind of mapping unit, and (3) the intricacy of the soil pattern mapped. Intricacy (I), the average number of mapped soil boundaries crossed by 1 km of random linear traverse, is related to the total length of mapped boundary (km per km²). When the surveys are grouped according to survey procedure and mapping unit, the survey effort for each group may be described by a regression of the form . B could not be shown to differ significantly between groups. D varied in the ratio 0.5: I, according to whether or not surveys used air photograph interpretation, and in the ratio 0.3:0.7:1.o, according to whether they mapped land systems, other compound mapping units, or simple mapping units. Since the choice of survey procedure and mapping unit is usually governed by the intricacy of the soil pattern the effect of these factors can be summarized in a single regression for all 66 surveys: There is a significant (P≤ 0.001) log-log regression between I and map scale.     

SATURATION PERCENTAGE AS A MEASURE OF SOIL TEXTURE IN THE LOWER INDUS BASIN

The saturation percentage is related to the mechanical constituents of a soil: and is therefore a quantitative expression of soil texture. Profiles may be described in terms of the S.P.; thus Maps may be drawn showing quantitative changes in texture, and associated statements of the reliability of an S.P. estimate and the probable error of the position of a textural contour be made. Decomposition of the calcium carbonate component should be avoided in routine soil survey work.     

Toxic levels of metals in Ferralsols under natural vegetation and crops in New Caledonia

The chemistry of soil solutions and the potential toxicity of trace metals (Co, Cr, Cu, Ni and Mn) were investigated on soils formed on ultramafic rocks. Soil solutions were collected along a soil toposequence under natural vegetation and under a cropped field. In the latter, metal speciation and species activity were computed with the WHAM 6 model. Total element concentrations varied with the soil topographic position. Upslope, in well‐drained soils, they were relatively small with mean concentrations of <0.2 µmol l⁻¹ for Co and Cr and <2 µmol l⁻¹ for Ni and Mn. Downslope, in temporarily waterlogged soils, concentrations reached 37 (Mn), 5.6 (Ni), 1.9 (Co) and 0.1 (Cr) µmol l⁻¹. Under crops, Ni, Mn and Co concentrations were similar to those under natural vegetation, but Cr concentration averaged 5 µmol l⁻¹. Cu concentration was close to 1 µmol l⁻¹. Free‐ion species amounted to 53–71% of all species for Co, Ni and Mn but only 5% for Cu. Cr was almost entirely in the Cr(VI) form (CrO, HCrO. The free‐metal‐ion activities were in the range 26–81% of the corresponding free‐metal‐ion concentration. Comparing our data with levels that are toxic to crops, Ni and Cr are potentially toxic in the well‐drained and the poorly‐drained soils. In the latter, Co and Mn are also potentially toxic. Both the large concentration of metals and the chemical species in which they occur in solution could limit the use of the land for agricultural purpose.     

Dissolved organic nitrogen and carbon in pastoral soils: the New Zealand experience

Dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) in soils are increasingly recognized as important components of nutrient cycling and biological processes in soil‐plant ecosystems. The aims of this study were to: (i) quantify the pools of DON and DOC in a range of New Zealand pastoral soils; (ii) compare the effects of land use changes on these pools; and (iii) examine the seasonal variability associated with these two components of dissolved organic matter. Soil samples (0–7.5 cm depth) from 93 pastoral sites located in Northland, Waikato, Bay of Plenty and Otago/Southland, New Zealand, were collected in autumn. Adjacent sites under long‐term arable cropping or native vegetation and forestry land use were also sampled at the same time to estimate the impacts of different land use on DON and DOC in these soils. Twelve dairy and 12 sheep and or beef pastures were sampled in winter, spring, summer and autumn for a 2‐year period to study the seasonal fluctuations of DON and DOC. A field incubation study was also carried out in a grazed pasture to examine fluctuations in the concentrations of and and DON levels in soil. Other soil biological properties, such as microbial biomass‐C, biomass‐N and mineralizable N, were also measured. Pastoral soils contained the greatest amounts of DON (13–93 mg N kg⁻¹ soil, equivalent to 8–55 kg N ha⁻¹) and DOC (73–718 mg C kg⁻¹ soil, equivalent to 44–431 kg C ha⁻¹), followed by cropping and native vegetation and forestry soils. The DON concentration in soils was found to be more seasonally variable than DOC. There was approximately 80% fluctuation in the concentration of DON in winter from the annual mean concentration of DON, while DOC fluctuated between 23 and 28% at the dairy and the sheep and beef monitoring sites. Similar fluctuations in the concentrations of DON were also observed in the field incubation studies. These results indicate that DON is a dynamic pool of N in soils. There was a strong and significant positive correlation between DON and DOC in pastoral soils (r = 0.71, P < 0.01). There were also significant positive correlations between DON and total soil C (r = 0.59, P < 0.01), total soil N (r = 0.62, P < 0.01) and mineralizable N (r = 0.47, P < 0.01). The rather poor correlations between total soil C and N with DOC and DON, suggest other biogeochemical processes may be influencing concentrations of DOC and DON in these soils. Given the size of DON and DOC pools in the pastoral soils, we suggest that these pools of C and N should be taken into account when assessing the impact of pastoral land use on soil C and N enrichment of surface and groundwater.     

A LANGMUIR TWO-SURFACE EQUATION AS A MODEL FOR PHOSPHATE ADSORPTION BY SOILS

For forty-one soils (pH > 5.0) from southern England and eastern Australia, the Langmuir equation was an excellent model for describing P adsorption from solutions < 10^-3M P, if it was assumed that adsorption occurs on two types of surface of contrasting bonding energies. For most of these soils, which were relatively undersaturated with P, this equation may be written as: where x = adsorption, k = adsorption/desorption equilibrium constant, x_m= monolayer adsorption capacity, and c = equilibrium solution concentration. The relative magnitude of the parameters for each surface were approximately: x_m= 0.3 x^″_m=0.3 and k′= 100 k. More than 90 per cent of the native adsorbed P occurs on the high-energy surface in most soils.     

Relationship between equivalent salt solution series of different soils

Equivalent salt solution series have been previously defined as solutions with combinations of sodium absorption ratio (SAR) and electrolyte concentration (E_c) producing the same extent of clay swelling in a given soil. The present study shows that there is a high (r²>0.96) positive correlation between log E_c and log SAR of equivalent salt solutions series, in the equation: where a₁ and b₁ are constants for each equivalent salt solution series for a given soil. Log a₁ could also be represented as a linear function of b₁ resulting in the equation: where a₂ and b₂ are constants for a given soil. Solving this equation using any given value of b₁ yields the combinations of SAR and E_c which make up each equivalent salt solution series for a given soil. The relationship between log a₁ and b₂ for three soils from western United States, namely Waukena, Pachappa and Grangeville, was similar, with their combined data having a r² value of 0.96. This indicated that a single set of equivalent salt solution series values could be used for these three soils which have different clay contents and clay mineralogy. Prediction of hydraulic conductivity decreases with E_c reduction at given values of SAR in red-brown and alluvial soils from southern Tasmania, using the equivalent salt solution series values for Waukena soil, showed similar patterns to measured values and also to those predicted using the equivalent salt solution values applicable to the respective Tasmanian soils. Thus, available data indicate that the same set of equivalent salt series could be applied to the five soils studied. If further testing shows that a single set of equivalent salt solutions values could be applied to all or large groups of soils, this would facilitate the application of the equivalent salt solution concept to predict salt solution flow in the field.     

Morphological and physiological responses of Picea asperata to different nitrogen availability and pH

Soil nitrogen (N) availability and pH are two determinants affecting plant growth, both of which are influenced by long‐term N deposition. However, the physiological mechanism of plants response to the changes in soil N availability and pH are not fully understood. To investigate the response of Picea asperata to both factors, seedlings of P. asperata were exposed to 50 or 1000 µM NH₄NO₃ with pH 5 or pH 7. In the current study, P. asperata, regardless of N availability and pH in growth medium, exhibited invariably a preference. Lower root biomass, root : shoot mass ratio, total root length and area, and root vitality were detected in high N condition compared to those in low N supply, corresponding well to lower net influxes of and at the root surface in both pH treatments. These results indicate that P. asperata may employ an active‐forge strategy to exploit nutrient resources for growth under low N availability, probably by increased below‐ground carbon allocation and net influxes of and . Although low pH, to some extent may generate more malondialdehyde, P. asperata would enhance pH tolerance by increased detoxification, i.e., antioxidant enzymes (peroxidase), free proline and soluble protein as well as improved carbohydrate status (i.e., soluble sugar and starch).     

Accompanying ions of ammonium sources and nitrate : ammonium ratios in tomato plants

The tomato (Solanum lycopersicum L.) cultivar Micro‐Tom (MT) is widely used in physiological studies, but the effects of nitrate ( ) and ammonium ( ) ratios ( : ratios) and, in particular, the effects of the accompanying ions in sources are unknown. To determine whether the accompanying ions in sources influence toxicity, the effects of : ratios on the physiology, electrolyte leakage index, nutrition, and dry weight were studied using hydroponics. The sources were ammonium chloride (NH₄Cl) or ammonium sulfate [(NH₄)₂SO₄], and five : ratios were used: 100 : 0, 75 : 25, 50 : 50, 25 : 75, and 0 : 100. The source was calcium nitrate [Ca(NO₃)₂], and the nitrogen (N) concentration was 15 mmol L^?1. The results indicate that NH₄Cl or (NH₄)₂SO₄ can be used in studies on toxicity because the accompanying ions did not influence the tomato plants. In addition, : ratios of 100 : 0 and 75 : 25 resulted in the highest dry weight of tomato plants, whereas ratios of 25 : 75 or 0 : 100 were toxic.     

The solid solution equilibria of lead and cadmium in polluted soils

A method for the measurement of Pb and Cd in equilibrium soil solutions involving soil equilibration with a dilute Ca electrolyte, centrifugation and filtration to <0.2 μm was evaluated. The procedure was subsequently used for the analysis of 100 Pb- and 30 Cd-contaminated soils. Solutions were analysed for Pb- and Cd using graphite-furnace AAS and the concentrations of Pb²⁺ and Cd²⁺ were estimated using standard speciation calculations. The concentrations of Pb and Cd found in the soil solutions were in the range 3.5–3600 μg dmp ^?3 and 2.7–1278 μg dm ^?3 respectively; both ranges represented less than 0.1% of the total metal concentration in the soils. Depending on solution pH, Pb ⁺² accounted for between 42–78% of Pb in solution while about 65% of Cd in solution was present as Cd⁺². The concentrations of Pb²⁺ and Cd²⁺ in solution suggested that the soil solutions were undersaturated with respect to the solid phases PbC0₃ and CdC0₃ but supersaturated with respect to Pb₅(P0₄)₃Cl and, for some samples, Cd₃(P0₄)₂ respectively. However, for both metals, a good empirical relationship was obtained between the total metal concentration in soil (mol kg^?1), free metal concentration in solution (mol dm^?3) and solution pH. The relationships took the general form of a pH-dependent Freundlich adsorption equation: For both lead and cadmium relationships, the values ofn and K₁ were close to unity, so that the distribution coefficient could be estimated from pH and a single metal-dependent constant, K₂. The algorithms appeared to be valid over a metal concentration range of four logarithmic units and pH range of 3.5–7.5.     

THE VOLUME AND POROSITY OF SOIL CRUMBS

A simple and inexpensive apparatus (a test-tube, burette, and pin) is described for measuring volumes by liquid displacement to an accuracy of greater than 0.5 per cent. This has been adapted to measure soil crumb porosities, ε_c, by saturating 3–4 g samples of crumbs with kerosene, measuring the weight of kerosene retained internally, then measuring their volume by displacement. Three estimates of crumb porosity from these measurements are compared. Experimental values range from ε_c= 0.205 for the headland of an arable field to ε_c= 0.351 for a permanent pasture. Crumb porosity is proposed as a measure of structural status for soils because it assesses the degree to which soil management has succeeded in holding the constituent primary particles apart from the positions of inherent closest packing that they would ultimately assume in an unstable soil. By comparison, the inter-crumb porosity, ε_v, can be used as a measure of cultivation status. In the form expressed, these two porosities are related to the more frequently encountered total porosity ε_t by the relation      

Depletion of non‐exchangeable NH$ _4^+ $ at the root–soil and hyphae–soil interface of VA mycorrhizal maize (Zea mays) and parsley (Petroselinum sativum)

Mobilization of non‐exchangeable ammonium (NH ) by hyphae of the vesicular‐arbuscular mycorrhizal (VAM) fungus Glumus mosseae was studied under controlled experimental conditions. Maize (Zea mays) and parsley (Petroselinum sativum) were grown either alone or in symbiosis with Glomus mosseae in containers with separated compartments for roots and hyphal growth. In one experiment, ¹⁵NH was added to the soil to differentiate between the native non‐exchangeable NH and the non‐exchangeable NH derived from N fertilization. Non‐exchangeable NH was mobilized by plant growth. Plant dry weight and N uptake, however, were not significantly influenced by mycorrhizal colonization of the roots. The influence of root infection with mycorrhizal fungus on the mobilization of non‐exchangeable NH was negligible. In the hyphal compartment, hyphal uptake of N resulted in a decrease of NH in the soil solution and of exchangeable NH . However, the NH concentration was still too high to permit the release of non‐exchangeable NH . The results demonstrate that, in contrast to roots, hyphae of VAM fungi are not able to form a non‐exchangeable‐NH depletion zone in the adjacent soil. However, under conditions of a more substantial depletion of the exchangeable NH in the mycorrhizal sphere (e.g., with longer growth), an effect of mycorrhiza on the non‐exchangeable NH might be found.     

The Burns leaching equation

The simplicity and utility of Burns' leaching equation make it worthy of study. The equation may be written as where X is the fraction of initially surface-resident fertilizer leached below depth z by net rainfall I, in soil with a volumetric water content at ‘field capacity’ of θ. The equation is analysed using transfer functions. The analysis shows that Burns' equation is consistent with an ‘independent flow tube’ soil leaching model, rather than the soil solution being well-mixed at each soil depth as Burns suggested. The flux and resident soil solution soil concentration profiles are shown to be quite different. An alternative definition of θ is suggested. The behaviour of ‘a Burns soil’ for different initial and boundary conditions is discussed.     

Plant availability and leaching of (heavy) metals from ammonium‐, calcium‐, carbohydrate‐, and citric acid‐treated uranium‐mine‐dump soil

Remediation of an uranium‐mine soil from Settendorf (East Germany) includes phytoextraction under conditions which make its heavy metals more plant‐available but less leachable. A second way is active inhibition of heavy metal uptake by the plant. In a pot trial with Chinese cabbage (Brassica chinensis L.), planted and unplanted soil samples were daily irrigated with deionized water or aqueous solutions with a total of (g (kg soil)^–1) CaCl₂ (0.26 Ca), NH₄Cl (1.39), casein, sucrose, citric acid (13), and an extract of rape (B. napus L.) shoots (13 DW) in a phytotron for 26 d. Water‐irrigated plants were also treated with a 50 mM citric acid solution (10.5 g (kg soil)^–1) 6 and 7 d prior to harvesting. Total elements in plant tissue and soluble elements in aqueous extracts from control and postharvest soils were determined by ICP‐AES. Supplements of NH , and the NH ‐generating casein and rape extract reduced soil pH during nitrification, and increased plant uptake of Cd, Cu, Ni, and Zn. Citric acid at 50 mM adjusted soil to pH 4.5–6.0 and enhanced uptake of all elements. Long‐term application of sucrose and citric acid increased pH and inhibited uptake of Cd, Cr, Cu, Ni, and Zn. Contemporarily, leaching of heavy metals and humic substances was lowest with Ca and NH and highest with sucrose and citric acid amendments. It is concluded that Chinese cabbage grown for chelate‐assisted phytoextraction should be supplied with Ca and NH to obtain a high plant biomass on soil with a low hazard of leaching. Metal uptake should be stimulated by application of chelator 7 d prior to harvesting. Undesired uptake of heavy metals by Chinese cabbage determined as food should be inhibited with carbohydrate amendments. Long‐term application of NH or chelator, which reduces the solubility of certain elements but increases their uptake moderately, is recommended as a tool for continuous phytoextraction technologies.