Paul J. Milham

Effects of humic material on the precipitation of calcium phosphate

Abstract Soil organic acids such as humic and fulvic acids can play an important role in influencing inorganic phosphate availability in P-fertilized soils by inhibiting formation of thermodynamically stable calcium phosphates. Calcium phosphate phases which are important in these systems may include amorphous calcium phosphate (Ca9(PO4)6·nH2O; ACP), dicalcium phosphate dihydrate (CaHPO4·2H2O; DCPD, also known as brushite), octacalcium phosphate (Ca8H2(PO4)6·5H2O; OCP) and the thermodynamically most stable phase, hydroxyapatite (Ca5(PO4)3OH; HAp). In this study, the formation of these phases in the presence of soil humic acids derived from the Sydney Basin in New South Wales, Australia has been examined using the combined techniques of pH-stat autotitration, Fourier transform infrared (FTIR) and laser Raman spectroscopy, as well as X-ray diffraction (XRD) and elemental analyses. Under conditions of high supersaturation at a pH of 7.4 and a temperature of 25 °C, it was found that these soil humics delay the transformation of unstable ACP to thermodynamically more stable OCP and thence to an apatitic phase resembling poorly crystalline HAp. At the lower pH of 5.7, and in the presence of humic acids, ACP was also precipitated initially. However, this was in contrast to the humic-free solutions which produced DCPD. ACP produced in the presence of humic materials persisted longer than DCPD in their absence, before ultimately hydrolyzing to OCP. Modes of humic-calcium phosphate interaction are discussed. It has been concluded that humic materials are geologically relevant inhibitors of calcium phosphate transformation and that they may modify the availability of phosphorus in soils by changing crystallisation behaviour from solution.

EFFECT OF pH AND PHOSPHATE ON SOLUBLE SOIL ALUMINIUM AND ON GROWTH AND COMPOSITION OF KIKUYU GRASS

SummaryKikuyu (Pennisetum clandestinium Hochst) grew relatively poorly on the Wollongbar krasnozem at soil pH values below 4.36. At these low pH values dry matter yields were increased by raising the pH or by application of high rates of phosphate. Both treatments decreased the concentration of soluble soil-Al on which the concentration of Al in tops was linearly dependent (r=0.95).The inverse relationship found between plant growth and Al concentration, when present in excess of ∼1.5 μg/g soil and ∼90 μg/g tops, is suggestive of Al toxicity. However, at Al concentrations causing severe yield reductions, the Ca concentration in kikuyu tops was approaching deficiency levels. The Al-Ca antagonism was further demonstrated by the reduction in Ca-uptake caused by increased concentrations of soluble soil-Al under constant conditions of exchangeable Ca and of pH. The yield-reducing effects of Al toxicity per se and Al-induced Ca deficiency are therefore confounded.

Phosphorus Fertilizer and Grazing Management Effects on Phosphorus in Runoff from Dairy Pastures

Fertilizer phosphorus (P) and grazing-related factors can influence runoff P concentrations from grazed pastures. To investigate these effects, we monitored the concentrations of P in surface runoff from grazed dairy pasture plots (50 x 25 m) treated with four fertilizer P rates (0, 20, 40, and 80 kg ha(-1) yr(-1)) for 3.5 yr at Camden, New South Wales. Total P concentrations in runoff were high (0.86-11.13 mg L(-1)) even from the control plot (average 1.94 mg L(-1)). Phosphorus fertilizer significantly (P < 0.001) increased runoff P concentrations (average runoff P concentrations from the P(20), P(40), and P(80) treatments were 2.78, 3.32, and 5.57 mg L(-1), respectively). However, the magnitude of the effect of P fertilizer varied between runoff events (P < 0.01). Further analysis revealed the combined effects on runoff P concentration of P rate, P rate x number of applications (P < 0.001), P rate x time since fertilizer (P < 0.001), dung P (P < 0.001), time since grazing (P < 0.05), and pasture biomass (P < 0.001). A conceptual model of the sources of P in runoff comprising three components is proposed to explain the mobilization of P in runoff and to identify strategies to reduce runoff P concentrations. Our data suggest that the principal strategy for minimizing runoff P concentrations from grazed dairy pastures should be the maintenance of soil P at or near the agronomic optimum by the use of appropriate rates of P fertilizer.

In vitro uptake of minerals by Gypsophila paniculata and hybrid eucalypts, and relevance to media mineral formulation

Despite the importance of mineral nutrition for plantlet growth in vitro, there have been few studies on mineral uptake from growth media or on optimising the media used in tissue culture. As plants in vitro experience abnormal growth conditions and may not possess roots, they may use different mechanisms of mineral uptake than plants growing ex vitro. To examine this possibility, plantlets of Gypsophila paniculata were grown on media in which the K or Ca concentration was varied. Mineral analysis showed a linear relationship between concentrations of K or Ca in the growth medium and plantlet tissues, suggesting uptake is by passive diffusion. However, interactions occurred between K, Ca and Mg uptake; therefore, other mechanisms are also likely to be involved in regulating mineral concentrations in tissue. The study also demonstrated that critical mineral concentrations could be estimated by using tissue-culture systems, as the concentration ranges of K and Ca in vitro correlated well with data for a related species ex vitro. This knowledge of critical concentrations, in conjunction with tissue analysis and ion speciation modelling, can be used to optimise in vitro mineral formulations through cycles of culture, tissue analysis and medium reformulation. To test this proposal, plantlets of Eucalyptus europhylla × grandis were grown on a proprietary medium formulation (SEM) and one modified as a result of tissue analysis (MEM). Plantlets cultured on SEM had chlorotic leaves and serious mineral imbalances. In contrast, plantlets cultured on MEM were not chlorotic, had more uniform growth and a more balanced mineral content. However, modification of mineral concentrations in the culture medium did not always result in similar changes in plant tissues. These differences in the proportions of minerals in the medium and those in the plant indicate that there are interactions between minerals in the medium and/or between minerals and the agar matrix that influence mineral availability and uptake.

Relationship between phosphorus concentration in surface runoff and a novel soil phosphorus test procedure (DGT) under simulated rainfall

There is a need to be able to identify soils with the potential to generate high concentrations of phosphorus (P) in runoff, and a need to predict these concentrations for modelling and risk-assessment purposes. Attempts to use agronomic soil tests such as Colwell P for such purposes have met with limited success. In this research, we examined the relationships between a novel soil P test (diffuse gradients in thin films, DGT), Colwell P, P buffering index (PBI), and runoff P concentrations. Soils were collected from six sites with a diverse range of soil P buffering properties, incubated for 9 months with a wide range of P additions, and then subjected to rainfall simulation in repacked trays growing pasture. For all soil and P treatment combinations, the relationship between DGT (0–10 mm) and runoff P was highly significant (P < 0.001, r2 = 0.84). Although there were significant curvilinear relationships between Colwell P and runoff P for individual soils, there were large differences in these relationships between soils. However, the inclusion of a P buffering measure (PBI) as an explanatory variable resulted in a highly significant model (P < 0.001, R2 = 0.82) that explained between-soil variability. We conclude that either DGT, or Colwell P and PBI, can be used to provide a relative measure of runoff P concentration.

XANES Demonstrates the Release of Calcium Phosphates from Alkaline Vertisols to Moderately Acidified Solution.

Calcium phosphate (CaP) minerals may comprise the main phosphorus (P) reserve in alkaline soils, with solubility dependent on pH and the concentration of Ca and/or P in solution. Combining several techniques in a novel way, we studied these phenomena by progressively depleting P from suspensions of two soils (low P) using an anion-exchange membrane (AEM) and from a third soil (high P) with AEM together with a cation-exchange membrane. Depletions commenced on untreated soil, then continued as pH was manipulated and maintained at three constant pH levels: the initial pH (pHi) and pH 6.5 and 5.5. Bulk P K-edge X-ray absorption near-edge structure (XANES) spectroscopy revealed that the main forms of inorganic P in each soil were apatite, a second more soluble CaP mineral, and smectite-sorbed P. With moderate depletion of P at pHi or pH 6.5, CaP minerals became more prominent in the spectra compared to sorbed species. The more soluble CaP minerals were depleted at pH 6.5, and all CaP minerals were exhausted at pH 5.5, showing that the CaP species present in these alkaline soils are soluble with decreases of pH in the range achievable by rhizosphere acidification.

Improving Rice Zinc Biofortification Success Rates Through Genetic and Crop Management Approaches in a Changing Environment

Though rice is the predominant source of energy and micronutrients for more than half of the world population, it does not provide enough zinc (Zn) to match human nutritional requirements. Moreover, climate change, particularly rising atmospheric carbon dioxide concentration, reduces the grain Zn concentration. Therefore, rice biofortification has been recognized as a key target to increase the grain Zn concentration to address global Zn malnutrition. Major bottlenecks for Zn biofortification in rice are identified as low Zn uptake, transport and loading into the grain; however, environmental and genetic contributions to grain Zn accumulation in rice have not been fully explored. In this review, we critically analyze the key genetic, physiological and environmental factors that determine Zn uptake, transport and utilization in rice. We also explore the genetic diversity of rice germplasm to develop new genetic tools for Zn biofortification. Lastly, we discuss the strategic use of Zn fertilizer for developing biofortified rice.

Analysis of oxygen-18 in orthophosphate by electrospray ionisation mass spectrometry

Abstract Isotope distributions in phosphate are useful measurements in geochemical systems since they give information on origin and environmental oxygen exchange processes. It is often necessary to perform measurements rapidly at the parts per million level on milligram quantities of sample. However, current methodologies require conversion to gas and/or time consuming multiple precipitation steps or insensitive nuclear magnetic resonance (NMR) techniques. In the electrospray method developed here, lengthy analyses requiring precipitation of phosphate are unnecessary. Moreover, there is no requirement for conversion to a simple gas and indirect evaluation. Accuracy is demonstrated by good agreement with NMR measurements on concentrated laboratory prepared chemical solutions. After dilution, electrospray analyses gave similar results.

Can synchrotron micro-x-ray fluorescence spectroscopy be used to map the distribution of cadmium in soil particles?

Plants take up cadmium (Cd) from the soil, and the concentration of Cd in some plant products is a health concern. Plant uptake of Cd is poorly predicted by its concentration in soils; consequently, there is interest in the binding and distribution of Cd in soil. Synchrotron micro-X-ray fluorescence spectroscopy (micro-XRFS) is the most sensitive method of observing this distribution. We used beam-line 2-ID-D of the Advanced Photon Source (APS), Argonne, to test whether this technique could map the Cd distribution in 5 soils from Greater Sydney that contained 0.3-6.4 mg Cd/kg. A subsample of one soil was spiked to contain ∼100 mg Cd/kg. Cadmium was readily mapped in the Cd-enriched subsample, whereas in the unamended soils, only one Cd-rich particle was found; that is, sensitivity generally limited Cd mapping. We also examined a sample of Nauru phosphorite, which was a primary source of much of the Cd in farm soils on the peri-urban fringe of Greater Sydney. The phosphorite contained ∼100 mg Cd/kg and the Cd was relatively uniformly distributed, supporting the findings of an earlier study on an apatite from Africa. The micro-XRFS at beam-line 2-ID-D of the APS can be reconfigured to increase the sensitivity at least 10-fold, which may allow the distribution of Cd and its elemental associations to be mapped in particles of most agricultural soils and facilitate other spectroscopic investigations.

Modelling the effects of soil properties on the concentration of Cd extracted by 10 mm CaCl2 from soils of the Sydney Basin

Undisturbed surface soils on the peri-urban fringe of the Sydney Basin are mostly acidic, with low concentrations of total cadmium (Cdt) and a wide range of other properties. In taxonomically similar soils on vegetable farms, Cdt is typically about 20-fold greater and undesirably large concentrations of Cd occur in some of the vegetables. We used a sequence of multiple regression models to evaluate how soil properties, taxonomy, and farming affected the behaviour of Cd in surface composites (0–15 cm) from 29 farmed and 12 unfarmed sites in the Basin. The dependent variable was the log10 transformation of the concentration of Cd extracted in 1 h by 10 mm CaCl2 solution (log10 CdCa). The range of values of CdCa was 0.2–60 μg/L. The independent variables in the base model were pHCa and log10 Cdt (R2 = 0.885, r.s.d. = 0.245). The final model contained 2 additional log10-transformed soil properties: effective cation exchange capacity (ECEC) and oxalate-extractable Fe (Feox) (R2 = 0.974, r.s.d. = 0.121). The effect of log10 Feox was significant (P 5.6, the approximate sorption edge for Cd on goethite. The effects of other soil properties could not be adequately tested because the residual variation was too small. The measured values of CdCa had a median error of ±17% and a maximum error of ±58% relative to the back-transformed fitted values from the final model. The coefficient of log10 Cdt in the final model was 1.33 ± 0.05 for the farmed soils and 1.01 ± 0.04 for the unfarmed soils, i.e. log10 CdCa was more sensitive to changes in log10 Cdt in the farmed than the unfarmed soils (P < 0.01). This difference is consistent with the effects of the greater load and briefer duration of contact for Cd in the farmed soils. The coefficients of pHCa and of log10 ECEC in our final model had values of 0.49 (± 0.03) and 0.69 (± 0.08). These values are remarkably similar to those obtained when we fitted our data, using a model that had been used to describe the effects of soil properties on a compilation of Cd-sorption data, and to those in a partitioning model based on the desorption of Cd from contaminated soils. That is, the behaviour of Cd may be influenced by the duration of contact between Cd and the soil, and perhaps the load; however, neither factor appears to change the effects of pHCa and log10 ECEC.