Transformation of Calcium Phosphates in Alkaline Vertisols by Acidified Incubation.

Acid-soluble soil phosphorus (P) is a potential resource in P-limited agricultural systems that may become critical as global P sources decrease in the future. The fate of P in three alkaline Vertisols, a major agricultural soil type, after acidic incubation was investigated using synchrotron-based K-edge X-ray absorption near-edge structure (XANES) spectroscopy, geochemical modeling, wet chemistry soil extraction, and a P sorption index. Increases in labile P generally coincided with decreased stability and dissolution of calcium phosphate (CaP) minerals. However, only a minor proportion of the CaP dissolved in each soil was labile. In two moderate-P soils (800 mg P kg-1), XANES indicated that approximately 160 mg kg-1 was repartitioned to sorbed phases at pH 5.1 of one soil and at pH 4.4 of the second; however, only 40 and 28% were labile, respectively. In a high-P soil (8900 mg P kg-1), XANES indicated a decrease in P of 1170 mg kg-1 from CaP minerals at pH 3.8, of which approximately only 33% was labile. Phosphorus mobilized by agricultural practices without concurrent uptake by plants may be repartitioned to sorbed forms that are not as plant-available as prior to acidification.

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.

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.

Non-destructive quantification of cereal roots in soil using high-resolution X-ray tomography.

One key constraint to further understanding plant root development is the inability to observe root growth in situ due to the opaque nature of soil. Of the present non-destructive techniques, computed tomography (CT) is best able to capture the complexities of the edaphic environment. This study compared the accuracy and impact of X-ray CT measurement of in situ root systems with standard technology (soil core washing and WinRhizo analysis) in the context of treatments that differed in the vertical placement of phosphorus fertilizers within the soil profile. Although root lengths quantified using WinRhizo were 8% higher than that observed in the same plants using CT, measurements of root length by the two methodologies were highly correlated. Comparison of scanned and unscanned plants revealed no effect of repeated scanning on plant growth and CT was not able to detect any changes in roots between phosphorus treatments that was observed using WinRhizo. Overall, the CT technique was found to be fast, safe, and able to detect roots at high spatial resolutions. The potential drawbacks of CT relate to the software to digitally segment roots from soil and air, which will improve significantly as automated segmentation algorithms are developed. The combination of very fast scans and automated segmentation will allow CT methodology to realize its potential as a high-throughput technique for the quantification of roots in soils.