Timothy I. McLaren

Complex Forms of Soil Organic Phosphorus–A Major Component of Soil Phosphorus

Phosphorus (P) is an essential element for life, an innate constituent of soil organic matter, and a major anthropogenic input to terrestrial ecosystems. The supply of P to living organisms is strongly dependent on the dynamics of soil organic P. However, fluxes of P through soil organic matter remain unclear because only a minority (typically <30%) of soil organic P has been identified as recognizable biomolecules of low molecular weight (e.g., inositol hexakisphosphates). Here, we use (31)P nuclear magnetic resonance spectroscopy to determine the speciation of organic P in soil extracts fractionated into two molecular weight ranges. Speciation of organic P in the high molecular weight fraction (>10 kDa) was markedly different to that of the low molecular weight fraction (<10 kDa). The former was dominated by a broad peak, which is consistent with P bound by phosphomonoester linkages of supra-/macro-molecular structures, whereas the latter contained all of the sharp peaks that were present in unfractionated extracts, along with some broad signal. Overall, phosphomonoesters in supra-/macro-molecular structures were found to account for the majority (61% to 73%) of soil organic P across the five diverse soils. These soil phosphomonoesters will need to be integrated within current models of the inorganic-organic P cycle of soil-plant terrestrial ecosystems.

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.

Organic phosphorus in the terrestrial environment: a perspective on the state of the art and future priorities

BackgroundThe dynamics of phosphorus (P) in the environment is important for regulating nutrient cycles in natural and managed ecosystems and an integral part in assessing biological resilience against environmental change. Organic P (Po) compounds play key roles in biological and ecosystems function in the terrestrial environment being critical to cell function, growth and reproduction.ScopeWe asked a group of experts to consider the global issues associated with Po in the terrestrial environment, methodological strengths and weaknesses, benefits to be gained from understanding the Po cycle, and to set priorities for Po research.ConclusionsWe identified seven key opportunities for Po research including: the need for integrated, quality controlled and functionally based methodologies; assessment of stoichiometry with other elements in organic matter; understanding the dynamics of Po in natural and managed systems; the role of microorganisms in controlling Po cycles; the implications of nanoparticles in the environment and the need for better modelling and communication of the research. Each priority is discussed and a statement of intent for the Po research community is made that highlights there are key contributions to be made toward understanding biogeochemical cycles, dynamics and function of natural ecosystems and the management of agricultural systems.

100 Years of superphosphate addition to pasture in an acid soil - Current nutrient status and future management

Pasture-based animal production systems, which occupy a significant proportion of the landscape in Victoria, Australia, have historically been nutrient-limited, with phosphorus (P) often the most limiting nutrient. The Permanent Top-Dressed (PTD) pasture experiment was established in 1914 at the Rutherglen Research Station, Victoria, to investigate the management of this deficiency. The main objective of the PTD experiment was to demonstrate the value of adding P fertiliser at two rates to increase pasture productivity for lamb and wool production. We report on the status of the PTD soils after 100 years, investigating the long-term implications of continuous grazing and fertiliser management (0, 125 and 250 kg/ha of superphosphate every second year) of non-disturbed pasture. We investigated the long-term effects of P fertiliser on the forms and distribution of P and other relevant soil parameters. In the fertilised treatments, P has accumulated in the surface soils (0–10 cm) as both orthophosphate and organic P, with an Olsen P of 16–21 mg P/kg, which is non-limiting for pasture production. In the treatment with 250 kg superphosphate, there has also been movement of P down through the soil profile, probably due to the high sand content of the surface soil and the transfer through the profile of small quantities of water-soluble P and P bound to organic ligands. Over time, the site has continued to acidify (surface 0–10 cm); the soil acidity combined with aluminium (Al) concentrations in the fertilised treatments approach a level that should impact on production and where broadcast lime would be recommended. After 100 years of non-disturbed pasture, the surface soils of these systems would be in a state of quasi-equilibrium, in which the fertilised systems have high levels of carbon (C), nitrogen, P and exchangeable Al. The continued stability of this system is likely dependent upon maintaining the high C status, which is important to nutrient cycling and the prevention of Al phytotoxicity. There are two risks to this system: (i) the declining pH; and (ii) soil disturbance, which may disrupt the equilibrium of these soils and the bio-chemical processes that maintain it.

Validation of Soil Phosphate Removal by Alkaline and Acidic Reagents in a Vertosol Soil using XANES Spectroscopy

There is a paucity of information on the soil phosphorus (P) forms removed by alkaline and acidic reagents in Vertosols. The first aim of this study was to identify which soil phosphates are removed by a two-step sequential fractionation (0.1 M NaOH and 1 M HCl) and by a dilute acid extractant (0.005 M H2SO4; Bureau of Sugar Experiment Stations (BSES) soil P test) on an “untreated” Vertosol using P K-edge x-ray absorption near-edge structure (XANES) at the Australian Synchrotron. There was supporting evidence that the 0.1 M sodium hydroxide (NaOH), 1 M hydrochloric acid (HCl), and 0.005M sulfuric acid (H2SO4) extractants remove soil phosphates according to the chemical solubility of known P minerals. The XANES spectra revealed the 1 M HCl and 0.005 M H2SO4 extractants remove calcium (Ca) phosphates from Vertosols, suggesting the latter extractant could be used as an alternative for a rapid and cost-effective measure of Ca phosphates in Vertosols.

The composition of organic phosphorus in soils of the Snowy Mountains region of south-eastern Australia

Few studies have considered the influence of climate on organic phosphorus (P) speciation in soils. We used sodium hydroxide–ethylenediaminetetra-acetic acid (NaOH–EDTA) soil extractions and solution 31P nuclear magnetic resonance spectroscopy to investigate the soil P composition of five alpine and sub-alpine soils. The aim was to compare the P speciation of this set of soils with those of soils typically reported in the literature from other cold and wet locations, as well as those of other Australian soils from warmer and drier environments. For all alpine and sub-alpine soils, the majority of P detected was in an organic form (54–66% of total NaOH–EDTA extractable P). Phosphomonoesters comprised the largest pool of extractable organic P (83–100%) with prominent peaks assigned to myo- and scyllo-inositol hexakisphosphate (IP6), although trace amounts of the neo- and d-chiro-IP6 stereoisomers were also present. Phosphonates were identified in the soils from the coldest and wettest locations; α- and β-glycerophosphate and mononucleotides were minor components of organic P in all soils. The composition of organic P in these soils contrasts with that reported previously for Australian soils from warm, dry environments where inositol phosphate (IP6) peaks were less dominant or absent and humic-P and α- and β-glycerophosphate were proportionally larger components of organic P. Instead, the soil organic P composition exhibited similarities to soils from other cold, wet environments. This provides preliminary evidence that climate is a key driver in the variation of organic P speciation in soils.

Growth and phosphorus uptake of faba bean and cotton are related to Colwell-P concentrations in the subsoil of Vertosols

Abstract. Recent studies report low and variable phosphorus (P) fertiliser use efficiency (PUE) for cotton in the northern grains region (NGR) of eastern Australia. This may be due to cotton accessing P pools that are not currently tested for in the subsoil (10–30 cm) or variation in response to P source and placement strategy. Two glasshouse studies were used to investigate this, incorporating two soil P tests to assess readily and slowly available P pools (Colwell, and a dilute acid colloquially referred to as the BSES extractant), and five different P fertiliser placement strategies in the subsoil. Eighteen Vertosols were collected across southern to central Queensland in the NGR, and then used to grow faba bean (Vicia faba L.) and cotton (Gossypium hirsutum L.) sequentially in the same 28-L pot. Readily available P pools assessed by Colwell-P were of major importance for faba bean and cotton dry matter, as well as for tissue P concentrations. Cotton was less responsive to extractable subsoil P concentrations than faba bean, suggesting either greater internal PUE or improved ability to accumulate P under conditions of limited availability. We recommend that subsoil P fertilisation should occur before sowing faba bean to maximise PUE in a cotton–faba bean rotation. Faba bean and cotton both recovered more P when the subsoil was fertilised, but no individual P fertiliser placement strategy was superior. Phosphorus extracted using the BSES method was not correlated with faba bean or cotton dry matter or tissue P concentration over the single crop cycle. We also recommend that Colwell-P be measured in the topsoil and subsoil to understand the quantity of plant-available P in Vertosols of the NGR, and that further research is needed to describe the resupply of the readily available P pool from slowly available P pools during a single crop cycle.

Effect on larval growth of adding finely ground silicon‐bearing minerals (wollastonite or olivine) to artificial diets for Helicoverpa spp. (Lepidoptera: Noctuidae)

The uptake of silicon (Si) by plants is known to reinforce plant tissues against invertebrate herbivores, but whether there is also direct antibiosis from ingesting Si remains in question. To investigate for antibiotic effects, Si‐bearing minerals, wollastonite (CaSiO3) or olivine (Mg/FeSiO3), were added to artificial diets as finely ground powders (40–50 μm) before or after acidulation to determine whether Si‐reduced larval growth. Newly hatched Helicoverpa armigera (Hübner, 1808) and H. punctigera (Wallengren, 1860) (Lepidoptera: Noctuidae) were placed onto the diets and weighed once the larvae on the control diet had completed feeding (day12 for H. armigera; day 14 for H. punctigera: 25.5°C). Acidulated olivine at rates of 0.4–1.7% Si w/dw (weight Si/dry weight of diet) reduced larval weight of H. armigera by 95–99% compared with control diets. Non‐acidulated olivine also appeared to cause a decline in larval weight. Wollastonite at rates of up to 3.3% Si had no significant effect on larval weight whether acidulated or not. A similar effect was observed for H. punctigera. Very few insects survived to emergence on diets containing the higher rates of acidulated olivine. Olivine contains quantities of heavy metals, particularly nickel, cobalt and chromium, which can be toxic to insects. Given the lack of toxicity when Si was included as wollastonite compared with similar quantities as olivine, the heavy metals are implicated as the antibiotic agents. Acidulation increased the toxicity of olivine probably by rendering the metals more biologically active. The results of this bioassay do not support the hypothesis that Si is directly antibiotic to Helicoverpa spp. via ingestion.

Correction to: Organic phosphorus in the terrestrial environment: a perspective on the state of the art and future priorities

The article “Organic phosphorus in the terrestrial environment: a perspective on the state of the art and future priorities”, written by Timothy S George et al., was originally published with incorrect affiliation information for one of the co-authors, E. Klumpp.