Simultaneous extraction of soil phytoavailable cadmium,copper, and lead by chelating resin membrane

Cadmium, copper, and lead were extracted from suspensions of contaminated soils using metal chelating exchange resin membranes. Nine soils with widely varying properties and Cd, Cu and Pb levels were tested. Soil suspensions made up with 4 g in 40 mL deionized water were equilibrated with 5 cm² Ca-saturated Chelex exchange resin membrane which was retained inside a polypropylene bag and shaken at 150 rpm for 24 hrs. Resin membrane extractable Cd, Cu and Pb of the soils were correlated with Cd, Cu, and Pb uptake by young wheat seedlings grown in these soils and compared with soil Cd, Cu, and Pb extracted by 0.1 M HCl, 0.01 M CaCl₂, and 0.005 M Diethylenetriamine pentaacetic acid (DTPA). The amounts of Cd, Cu and Pb extracted by the Ca-saturated Chelex membrane from all tested soils correlated well with those absorbed by young wheat seedlings. The Ca-saturated Chelex membrane extractable Cd, Cu and Pb of the soil had the strongest correlation with plant uptake Cd, Cu and Pb among the extraction methods we tested. It was demonstrated that the Ca-saturated Chelex membrane extraction is an appropriate method in simultaneously estimating Cd, Cu and Pb phytoavailability of soil and is applicable to a wide range of soils.     

Analysis of field-moist Cd contaminated paddy soils during rice grain fill allows reliable prediction of grain Cd levels

Research undertaken over the last 40 years has confirmed that the long-term consumption of cadmium (Cd) contaminated rice contributes to human Cd disease. Rice is the staple of millions throughout South and Southeast Asia. Therefore, the ability to accurately assess the risk of rice grain Cd uptake in areas of elevated soil Cd would be a pre-requisite to protecting public health and regional export security. During 2001–2002, 308 concomitant soil and rice grain samples were collected from a Cd/Zn co-contaminated site in Western Thailand and determined for aqua regia digested soil Cd and rice grain Cd. No significant relationship was observed between total soil Cd and rice grain Cd (r ² = 0.117). This intuitively is to be expected since total soil Cd bears no relationship to phyto-available Cd. Similarly no relationship was observed between 0.005 M DTPA extractable soil (air-dry) Cd and rice grain Cd (r ² = 0.165). Again this result could have been predicted as the phyto-availability of Cd in paddy soils is a function of the complex interaction between soil pH, redox conditions and the presence of competing ions. Consequently, in 2003 a further study was undertaken to assess the effectiveness of commonly utilized soil extractants namely, 0.1, 0.05 and 0.01 M CaCl₂ solutions at a soil extractant ratio of 1:5 and 1 M NH₄NO₃ for 2 h or 4 h extractions times at a soil/extractant ratio of 1:2.5. Soil samples were collected at the critical rice grain fill stage and sub-divided into Portion A which was subjected to conventional air-drying and sample preparation procedures and Portion B which was maintained at Field Condition (FC) and stored at <4°C until extractions were undertaken. Concomitant rice grain samples were collected at maturity. The results indicate that air-dried soil samples subjected to conventional soil preparation procedures were totally ineffective at predicting the uptake of Cd by rice stem, leaf or grain, regardless of extractant. Further, the results indicate that the Stepwise Regression model incorporating 0.1 M CaCl₂ extractable Cd and soil pH_w determined on field moist samples accounts for 63.8% of the variability in rice grain Cd.     

Cadmium Uptake by Lettuce (Lactuca sativa L.) as Basis for Derivation of Risk Limits in Soils

The availability and uptake of Cd by lettuce (Lactuca sativa L.) in two common tropical soils (before and after liming) were studied in order to derive human health-based risk soil concentration. Cadmium concentrations ranging from 1 to 12 mg kg^?1 were added to samples from a clayey Oxisol and a sandy-loam Ultisol under glasshouse conditions. After incubation, a soil sample was taken from each pot, the concentration of Cd in the soil was determined, lettuce was grown during 36 d, and the edible parts were harvested and analyzed for Cd. A positive linear correlation was observed between total soil Cd and the Cd concentration in lettuce. The amount of Cd absorbed by lettuce grown in the Ultisol was about twice the amount absorbed in the Oxisol. Liming increased the soil pH and slightly reduced Cd availability and uptake. CaCl₂ extraction was better than DTPA to reflect differences in binding strength of Cd between limed and unlimed soils. Risk Cd concentrations in the Ultisol were lower than in the Oxisol, reflecting the greater degree of uptake from the Ultisol. The derived risk Cd values were dependent on soil type and the exposure scenario.     

Soil pH Effects on Uptake of Cd and Zn by Thlaspi caerulescens

For phytoextraction to be successful and viable in environmental remediation, strategies that can optimize plant uptake must be identified. Thlaspi caerulescens is an important hyperaccumulator of Cd and Zn, whether adjusting soil pH is an efficient way to enhance metal uptake by T. caerulescens must by clarified. This study used two soils differing in levels of Cd and Zn, which were adjusted to six different pH levels. Thlaspi caerulescens tissue metal concentrations and 0.1 M Sr(NO₃)₂ extractable soil metal concentrations were measured. The soluble metal form of both Cd and Zn was greatly increased with decreasing pH. Lowering pH significantly influenced plant metal uptake. For the high metal soil, highest plant biomass was at the lowest soil pH (4.74). The highest shoot metal concentration was at the second lowest pH (5.27). For low metal soil, due to low pH induced Al and Mn toxicity, both plant growth and metal uptake was greatest at intermediate pH levels. The extraordinary Cd phytoextraction ability of T. caerulescens was further demonstrated in this experiment. In the optimum pH treatments, Thlaspi caerulescens extracted 40% and 36% of total Cd in the low and high metal soils, respectively, with just one planting. Overall, decreasing pH is an effective strategy to enhance phytoextraction. But different soils had various responses to acidification treatment and a different optimum pH may exist. This pH should be identified to avoid unnecessarily extreme acidification of soils.     

Effects of soil acidification on the chemical extractability of Fe,Mn, Zn and Cu and the growth and micronutrient uptake of highbush blueberry plants

Summary The effects of soil acidification and micronutrient addition on levels of extractable Fe, Mn, Zn and Cu in a soil, and on the growth and micronutrient uptake of young highbush blueberry plants (Vaccinium corymbosum L. cv. Blueray) was investigated in a greenhouse study.Levels of 0.05M CaCl₂-extractable Fe, Mn, Zn and Cu increased as the pH was lowered from 7.0 to 3.8. However, the solubility (CaCl₂-extractability) of Fe and Cu was considerably less pH-dependent than that of Mn and Zn. With the exception of HCl-and DTPA-extractable Mn, micronutrients extractable with 0.1M HCl, 0.005M DTPA and 0.04M EDTA were unaffected or raised only slightly as the pH was lowered from 6.0 to 3.8. Quantities of Mn and Zn extractable with CaCl₂ were similar in magnitude to those extractable with HCl, DTPA and EDTA whilst, in contrast, the latter reagents extracted considerably more Cu and Fe than did CaCl₂. A fractionation of soil Zn and Cu revealed that soil acidification resulted in an increase in the CaCl₂- and pyrophosphate-extractable fractions and a smaller decrease in the oxalate-extractable fraction.Plant dry matter production increased consistently when the soil pH was lowered from 7.0 to 4.6 but there was a slight decline in dry matter as the pH was lowered to 3.8. Micronutrient additions had no influence on plant biomass although plant uptake was increased. As the pH was lowered, concentrations of plant Fe first decreased and then increased whilst those of Mn, and to a lesser extent Zn and Cu, increased markedly.     

Influence of pH,exchangeable aluminium and 0.02M CaCl2-extractable aluminium on the growth and nitrogen-fixing activity of white clover (Trifolium repens) in some New Zealand soils

The effects of soil acidity on the growth and N₂-fixing activity of white clover in seven acid topsoils and subsoils of New Zealand were investigated using a glasshouse experiment.The application of phosphate (Ca(H₂PO₄)₂) to the soils resulted in very large increases in white clover growth on all soils. The application of phosphate, as well as increasing P supply, also decreased 0.02M CaCl₂-extractable Al levels, but had little effect on exchangeable Al levels.Where adequate phosphate was applied, increasing rates of lime (CaCO₃) resulted in increased plant growth on most soils. N₂[C₂H₂]-fixing activity was increased by the first level of lime for one soil, but generally remained approximately constant or declined slightly at higher rates of lime. Up to the point of maximum yield, white clover top weight was more highly correlated with 0.02M CaCl₂-extractable soil Al than with exchangeable Al or pH. At pH values greater than 5.5, plant yield declined on some soils, apparently because of Zn deficiency. The data suggest that white clover is unlikely to be affected by Al toxicity at 0.02M CaCl₂-extractable Al levels of less than about 3.3 g g^–1. However, there were differences between soils in apparent plant tolerance to 0.02M CaCl₂-extractable Al, which appeared to be caused by differing C levels in the 0.02M CaCl₂ extracts.     

Distribution, movement and availability of Cd and Zn in a dredged sediment cultivated with Salix alba

Willows occur as volunteer vegetation on sediment-derived soils, such as dredged sediments, landfill cover or stockpile deposits, and are used as phytoremediators on such soils. The present study evaluates growth and metal uptake by Salix alba grown on a contaminated dredge sediment for 209 days under greenhouse conditions. At the end of the study, the aerial parts of the S. alba plants had grown to heights of between 80 and 117 cm. Biomass and Cd and Zn concentration in the roots, stems and leaves, at 70, 112 and 209 days, showed that Cd and Zn had been bioaccumulated, especially in the leaves.At the three sampling dates, Cd and Zn extractability and pH measurements were also carried out on samples of two soil layers (0–15 and 15–30 cm) from both the planted and the control pots. Cd and Zn extractability were assessed using single extraction procedures (0.01 M CaCl₂; 1 M HNO₃; CaCl₂–TEA–DTPA). The two metals showed similar variations in CaCl₂ and HNO₃ extractabilities, but this was not the case for DTPA extractability. The greatest variations were observed in the upper soil layers of the control pots. In the planted pots, the CaCl₂ extractability of Zn decreased in the upper layer, and the HNO₃ extractability of Zn increased in the lower layer. The pH of the upper soil layer was always higher than the pH of the bottom layer. In addition, we monitored several parameters of the percolates from both the planted and the control sediments, including pH, Eh, conductivity, dissolved organic carbon, Zn and Cd concentrations, and presence of certain cations/anions. Dissolved organic carbon, and Cd and Zn concentrations increased steadily over time. There were no significant differences between the planted and the control pots. After 209 days, the characteristics of the control sediment reflected the effects of ageing in that the CaCl₂-extractable Cd and Zn concentrations had decreased compared with the initial concentrations. Conversely, the concentrations of HNO₃-extractable Cd and Zn had increased. A fraction of the metal initially extracted by CaCl₂ (considered as exchangeable) became less available with time. After 112 days, the plants had extracted approximately 2.8 mg of Zn. At the same time, the CaCl₂ extractability of Zn in the upper, rooted layer decreased by 2.6 mg. We can assume that S. alba extracted Zn from the pool of CaCl₂-extractable Zn.     

Phosphate potential and phosphate capacity of soils

Summary The relationship between the phosphate potential (I) and the amount of phosphate (Q), added to the soil has been examined by equilibrating soil samples with 0.001M or 0.01M CaCl₂ solutions containing various amounts of phosphate. For one neutral and two alkaline soils the Q/I relationship depends on the CaCl₂ concentration and the pH in such a way that the apparent values of I decrease when the CaCl₂ concentration increases from 0.001 M to 0.01M. The difference between the two values increases when the pH increases. When correction is made for the formation of the soluble calcium phosphate complex, CaHPO₄, the Q/I relationship becomes independent of the CaCl₂ concentration. The initial phosphate potential (I₀) determined by interpolation, is also found to be independent of the CaCl₂ concentration. The necessary amount of phosphate to be added or removed per gram of soil in order to obtain a certain alteration of the phosphate potential is designated the differential phosphate potential buffering capacity, DPBC. For ten soils DPBC-values are determined on the basis of the Q/I relationships, (ΔQ/ΔI)_Io, and found to be independent of the CaCl₂ concentration. The content of colloids and of inorganic phosphate accounts for a significant part of the variation in the DPBC for different soils. The importance of the DPBC for characterization of the phosphate status of soils in respect to phosphate supply to plants is briefly discussed. The author is indebted to professor, Dr. H. C. Aslyng, head of the department for his suggestions and helpful criticism during the progress of this work.     

Chemical characterization of two extracts used in the determination of available soil nitrogen

Summary Two soil extracts used for chemical indexes for N availability, 0.01M NaHCO₃ and boiling 0.01M CaCl₂, were analyzed in effort to learn more about the nature of the extracted organic matter (O.M.). The two extracts appeared to remove different fractions of the soil O.M. A study of five soils showed that the C/N value of the NaHCO₃ extract (following decarbonation) was significantly higher than that of the total soil O.M.; while the C/N value in the boiling CaCl₂ extract was not significantly different from that in the soil O.M. There was also significant variation in C/N values among soils for the boiling CaCl₂ extract. The extracts of three soils were analyzed for apparent molecular weight distribution using gel filtration and the results compared to those for base-extracted humic substances. Almost all the molecules in the extracts had apparent molecular weights less than 21,000 daltons while 21 to 47% of the humic substances from the same soils (extracted with 0.5M NaOH) had molecular weights greater than 21,000 daltons. In the boiling CaCl₂ extract, 78 to 87% of the humic substances had apparent molecular weights less than 1,000 daltons, whereas with the NaHCO₃ extract, 42 to 83% of the humic substances were in the 1,000 to 21,000 dalton range. Forty-three to 92% of the N extracted by the NaHCO₃ was in protein form, and 8 to 30% was ninhydrin-detectable. In the boiling CaCl₂ extract 25 to 30% of the extracted N was ninhydrin-detectable. For the same 10 soils, ninhydrin-detectable N values of the boiling CaCl₂ extract appeared closely related to greenhouse and field relative N uptake, while the ninhydrin-detectable N values of the NaHCO₃ extract appeared unrelated to both. The protein N and protein in plus ninhydrin-detectable N values of the NaHCO₃ extract were closely related to greenhouse relative N uptake only. The results of this study indicated that specific fractions of the soil O.M. were being extracted by the two solutions and that significant differences existed in the chemical nature of the two extracts. Paper No. 6175 of the J. Ser. of the Pennsylvania Agric. Exp. Stn. Authorized for publication Jan. 26, 1981.     

Selenium-rich dissolved organic matter determines selenium uptake in wheat grown on Low-selenium arable land soils

Aims

The study aimed to find soil parameters that are best related to Se plant uptake for low Se soils with predominantly organic Se, and to explore the mechanisms that control Se bioavailability in the soils under study.

Methods

A pot experiment using nineteen soil samples taken from different fields of arable land (potato fields) in the Netherlands was conducted on summer wheat (Triticum aestivum L.). Selenium in wheat shoots and soil parameters, including basic soil properties, C:N ratio, inorganic selenite content, and Se and organic C in different soil extractions (0.01 M CaCl₂, 0.43 M HNO₃, hot water, ammonium oxalate, aqua regia) were analysed. Regression analysis was performed to identify soil parameters that determine Se content in wheat shoots.

Results

The regression model shows that Se:DOC ratio in 0.01 M CaCl₂ soil extraction explained about 88 % of the variability of Se uptake in wheat shoots. Selenium uptake increased with Se:DOC ratio in CaCl₂ extraction, which can be interpreted as a measure of the content of soluble Se-rich organic molecules. Selenium:DOC ratio in CaCl₂ extraction and Se uptake increased towards higher soil pH and lower soil C:N ratio. The soil C:N ratio is also negatively correlated to Se:organic C ratio in other extractions (0.43 M HNO₃, hot water, ammonium oxalate, aqua regia), indicating that at low soil C:N ratio soil organic matter is richer in Se. Contrarily, the soil pH is positively and strongly correlated to Se:organic C ratio in CaCl₂ and hot water extractions, but only weakly correlated to Se:organic C ratio in other extractions.

Conclusions

Selenium-rich dissolved organic matter is the source of bioavailable Se in low Se soils with predominantly organic Se. The soil pH and quality of soil organic matter (i.e. soil C:N ratio) are the main soil properties determining Se bioavailability in these soil types.

     

The extraction by soil and absorption by plants of applied zinc and cadmium

In five consecutive years lettuce, spinach, spring wheat, endive and maize were grown in pots and the effects of native and soil-applied Zn and Cd on plant Zn and Cd concentrations were studied. The normal interactive pattern was antagonistic, Zn reducing plant Cd uptake, and conversely, but less so. Only in loam soil Zn and Cd were synergistic to some extent, plant Zn uptake increasing with applied Cd.When relating total soil Cd/Zn to plant Cd/Zn separate sets of data could be distinguished for loam and sandy soil, each fitting a straight line. The use of 0.1 M CaCl₂ instead of total extractable soil Cd/Zn makes the two sets of data to coalesce around a single straight line. All crops were found to show a positive linear relationship between 0.1 M CaCl₂-extractable soil Cd/Zn and plant Cd/Zn.     

Predicting zinc bioavailability to wheat improves by integrating pH dependent nonlinear root surface adsorption

Aim

Our aim was to improve the prediction of Zn bioavailability to wheat grown on low-Zn soils. The classical approach that directly relates Zn in a certain soil extract to Zn uptake has been shown to be inadequate in many cases. We tested a stepwise approach where the steps of the uptake process are characterized with, respectively, Zn solid-solution distribution, adsorption of Zn to root surface, Zn uptake into root and Zn translocation to shoot.

Methods

Two pot experiments were done with wheat grown on nine low-Zn soils varying widely in pH, clay and organic matter content. Soluble Zn concentrations in two soil extracts (DTPA and CaCl₂) were measured. Free Zn ion concentrations in CaCl₂ soil extracts were determined with the Donnan Membrane Technique. These Zn concentrations were then related to plant Zn uptake following both the direct and the stepwise approach.

Results

In the direct approach, Zn in the DTPA extract was a better predictor for shoot Zn uptake than Zn in the CaCl₂ extract. In the stepwise approach, the relationship between Zn in CaCl₂ extracts and the root surface adsorbed Zn was pH-dependent and nonlinear. Root surface adsorbed Zn was linearly related to root Zn uptake, and the latter was linearly related to the shoot Zn uptake. The stepwise approach improved the Zn uptake prediction compared to the direct approach and was also validated for different wheat cultivars.

Conclusions

The adsorption of Zn on the root surface is pH dependent and nonlinear with respect to the soil Zn concentration, and a useful proxy for bioavailable Zn over a wide range of soils.     

Influence of root-induced pH on the solubility of soil aluminium in the rhizosphere

Increase in solubility of soil aluminium (Al) as a result of root-induced decrease of soil pH was studied. Soil samples of known distances from the roots of NH₄-N fertilized Ryegrass were analyzed for pH and aluminium extractable with 0.01 M CaCl₂. Results showed that though no Al was found in bulk soil (pH 6.8), its concentration in the vicinity of roots increased to 0.023 mM with a concomitant decrease of soil pH from 6.8 to 4.4.     

Interactions between aluminium,phosphorus and pH in the response of barley to soil acidity

Summary Two acid soils showing different Al solubility as a function of pH were limed to a range of pH values (in 10^–2 M CaCl₂) between 4.1 and 5.6. The apparent critical pH for the growth of barley in pots was 0.25 lower in the soil showing lower Al solubility. The addition of phosphate reduced exchangeable and soluble Al in the soils, and lowered the apparent critical pH by 0.35 while maintaining the difference between the soils. The Al concentration at the critical pH, measured after cropping to take account of the treatment effects on soil Al, also varied with soil and with phosphate addition. These apparent critical values of both pH and soluble Al varied linearly with available phosphate, over the range 18 to 73 mg P/kg soil, as follows: pH from 4.9 to 4.3; soluble Al, from 0.010 mM to 0.056 mM; and the soluble Ca/Al mole ratio, from 1270 to 214.     

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.     

Models for relating pH measurements in water and calcium chloride for a wide range of pH,soil types and depths

Soil pH is commonly measured in water (pHw) or 0.01 M CaCl₂ (pH_Ca). The need to convert between these methods has led to the publication of linear, quadratic and cubic polynomial relationships for limited suites of soils. Concerns over the applicability of such relationships when mapping a wide range of soils and pH led to the establishment of a database of pH_W and pH_Ca values on each of 7894 samples from soil survey and field experimental sites in Queensland. The relationship between pH_W and pH_Ca across all soils was investigated and preliminary results examining the effect of soil depth and soil type on the relationship are presented.For all soils and depths, a linear regression accounted for 93.2% of the variation but did not predict pH_Ca well at very high or low pH_W values. The inclusion of second and third powers of pH_W accounted for significantly more of the variation (R²=0.94) in pH_Ca and the resultant curve matched the data better at high and low pH.Analysis of surface, sub-surface and subsoil groupings did not reveal any appreciable differences in the relationship between pH_W and pH_Ca attributable to depth. In contrast, differences in the relationship were evident between soil types. Generally, the mildly leached soils had linear relationships, while the weathered soils were distinctly curvilinear at low pH.     

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

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

Assessment of phosphorus leaching losses from arable land

Phosphorus (P) losses from soil to water by erosion and surface runoff have been much studied and quantified. However, P losses by leaching have received much less attention, mainly because, until recently, the quantities involved were not considered to be of environmental significance. Furthermore, P leaching losses, unlike P losses from erosion or surface runoff were not believed to be related to rates of P addition, as inorganic fertilizer or manures. Here we report results from a number of field and laboratory experiments, designed to assess the significance of P leaching losses from soil to water. Annual cumulative total P losses in drainage waters from four UK field sites ranged from about 0.03 to 5 kg P ha⁻¹ during 2001–2002. Molybdate reactive P ranged from 45–57%, soluble organic P from 10–13% and particulate P from 29–45% of total P on the two sites (Broadbalk and Woburn) where they could be accurately measured. The proportions of these different P forms were comparable in all treatments, including drainage waters from the unfertilised soils and soils receiving long-term applications of farmyard manure or inorganic fertilizer. In all soils, there was indication of an Olsen- (0.5 M NaHC0₃-, pH 8.5) extractable P concentration, (termed the Change-Point), where P measured in field drainage waters or in laboratory soil extracts of 0.01 M CaCl₂ began to increase linearly as Olsen-P increased. There was also some agreement between drainage water-P or CaCl₂-P and the Olsen-P concentration where the Change-Point occurred. This suggests that CaCl₂-extractable P may provide an approximate indicator of soil P concentrations above which significant quantities of P may be lost by leaching under field conditions. There were positive linear relationships between soil dithionate-extractable Al and soil organic C with the Change-Point: [Change Point = [(0.049)[Al3+] minus (9.2)(% organic C)] accounting for 93% of the variance in the data. If this relationship holds under further testing it could well be a useful predictor of Change-Points in different soils. Phosphate sorption isotherms were used to study the soil P concentrations above which P was at risk of moving from soil to water. They showed that soil solution P concentrations were significantly lower between pH 6.9–7.2 than between pH 7.7–8.1, with implications for P loss from soil to water.     

Selected bioavailability assays to test the efficacy of amendment-induced immobilization of lead in soils

Lead immobilization in 10 soils contaminated with Pb from different origin was examined using lime (CaCO₃), a mix of cyclonic ash and steelshots (CA+ST), and a North Carolina phosphate rock. The immobilization efficacy of the three amendments was evaluated using single (CaCl₂solution) and sequential (BCR method) chemical extractions in tandem with a microbiological Pb biosensor (BIOMET), a Pb phytotoxicity test, Pb plant uptake, and a Physiologically Based Extraction Test (PBET) mimicking Pb bioavailability in the human gastro-intestinal tract. The results demonstrated the necessity of using a diverse suite of bioavailability methodology when in situ metal immobilization is assessed. Sequential (BCR) extractions and PBET analysis indicated that PR was the most effective amendment. PR however, proved ineffective in totally preventing Pb phytotoxicity and Pb uptake on all soils tested. On the contrary, CA+ST and lime decreased BIOMET Pb, Pb phytotoxicity, and Pb uptake to a far greater extent than did PR. BIOMET detectable Pb and Pb uptake, however, were strongly related to Pb in soluble or exchangeable soil fractions (i.e., CaCl₂ extractable). By combining these fractions with the acid-extractable Pb, accomplished by using acetic acid extractant, the recently developed BCR sequential extraction scheme appeared to have lost some valuable information on judging Pb bioavailability. The data show that different amendments do not behave consistently across different soils with different sources of contamination. Different indices for measuring Pb bioavailability are also not necessarily consistent within particular soil and amendment combinations.     

Chickpea and white lupin rhizosphere carboxylates vary with soil properties and enhance phosphorus uptake

Chickpea and white lupin roots are able to exude large amounts of carboxylates, but the resulting concentrations in the rhizosphere vary widely. We grew chickpea in pots in eleven different Western Australian soils, all with low phosphorus concentrations. While final plant mass varied more than two-fold and phosphorus content almost five-fold, there were only minor changes in root morphological traits that potentially enhance phosphorus uptake (e.g., the proportion of plant mass allocated to roots, or the length of roots per unit root mass). In contrast, the concentration of carboxylates (mainly malonate, citrate and malate, extracted using a 0.2 mM CaCl₂ solution) varied ten-fold (averaging 2.3 mol g^–1 dry rhizosphere soil, approximately equivalent to a soil solution concentration of 23 mM). Plant phosphorus uptake was positively correlated with the concentration of carboxylates in the rhizosphere, and it was consistently higher in soils with a smaller capacity to sorb phosphorus. Phosphorus content was not correlated with bicarbonate-extractable phosphorus or any other single soil trait. These results suggest that exuded carboxylates increased the availability of phosphorus to the plant, however, the factors that affected root exudation rates are not known. When grown in the same six soils, three commonly used Western Australian chickpea cultivars had very similar rhizosphere carboxylate concentrations (extracted using a 0.2 mM CaCl₂ solution), suggesting that there is little genetic variation for this trait in chickpea. Variation in the concentration of carboxylates in the rhizosphere of white lupin did not parallel that of chickpea across the six soils. However, in both species the proportion of citrate decreased and that of malate increased at lower soil pH. We conclude that patterns of variation in root exudates need to be understood to optimise the use of this trait in enhancing crop phosphorus uptake.