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Published online 11 January 2008
Published in Soil Sci Soc Am J 72:98-110 (2008)
DOI: 10.2136/sssaj2007.0058
© 2008 Soil Science Society of America
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SOIL CHEMISTRY

Evidence for Different Reaction Pathways for Liquid and Granular Micronutrients in a Calcareous Soil

Ganga M. Hettiarachchia,*, Mike J. McLaughlinb, Kirk G. Scheckelc, David J. Chittleborougha, Mathew Newvilled, Steve Suttond and Enzo Lombib

a Soil and Land Systems, School of Earth and Environmental Sciences, Univ. of Adelaide, Waite Campus, Glen Osmond, SA 5064, Australia
b CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia
c National Risk Management Research Lab., U.S. Environmental Protection Agency, 5995 Center Hill Ave., Cincinnati, OH 45224
d GSECARS, Univ. of Chicago, Chicago, IL 60637

* Corresponding author (ganga.hettiarachchi{at}adelaide.edu.au).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The benefits of Mn and Zn fluid fertilizers over conventional granular products in calcareous sandy loam soils have been agronomically demonstrated. We hypothesized that the differences in the effectiveness between granular and fluid Mn and Zn fertilizers is due to different Mn and Zn reaction processes in and around fertilizer granules and fluid fertilizer bands. We used a combination of several synchrotron-based x-ray techniques, namely, spatially resolved micro-x-ray fluorescence (µ-XRF), micro-x-ray absorption near edge structure spectroscopy (µ-XANES), and bulk-XANES and -extended x-ray absorption fine structure (EXAFS) spectroscopy, along with several laboratory-based x-ray techniques to speciate different fertilizer-derived Mn and Zn species in highly calcareous soils to understand the chemistry underlying the observed differential behavior of fluid and granular micronutrient forms. Micro-XRF mapping of soil–fertilizer reactions zones indicated that the mobility of Mn and Zn from liquid fertilizer was greater than that observed for equivalent granular sources of these micronutrients in soil. After application of these micronutrient fertilizers to soil, Mn and Zn from liquid fertilizers were found to remain in comparatively more soluble solid forms, such as hydrated Mn phosphate-like, Mn calcite-like, adsorbed Zn-like, and Zn silicate-like phases, whereas Mn and Zn from equivalent granular sources tended to transform into comparatively less soluble solid forms such as Mn oxide-like, Mn carbonate-like, and Zn phosphate-like phases.

Abbreviations: EDAX, energy dispersive x-ray analysis • EXAFS, extended x-ray absorption fine structure • LCF, linear combination fitting • MAP, monoammonium phosphate • SEM, scanning electron microscopy • TGMAP, technical-grade monoammonium phosphate • XANES, x-ray absorption near-edge structure • XAS, x-ray absorption spectroscopy • XRD, x-ray diffraction • XRF, x-ray fluorescence


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Millions of hectares of arable land worldwide, particularly in arid and semiarid regions, are deficient in plant-available Mn and Zn, necessitating the use of various organic and inorganic Mn and Zn fertilizers. Inclusion of Mn and Zn as well as other micronutrients in commercial macronutrient fertilizers is a common practice throughout the world because it is much more practicable than applying micronutrients separately. Despite the economic and human health importance of micronutrient fertilizer behavior, our understanding of the chemistry of fertilizer Mn and Zn is far from complete.

Recent field studies conducted in South Australia have shown increased response, as measured by Mn and Zn concentrations in grain, to fluid sources of fertilizer Mn and Zn injected into the soil at planting, compared with granular sources (Holloway et al., 2002, 2006). It is not economically feasible, however, to conduct detailed investigations for each soil type to fully determine under which circumstances soil-injected fluid fertilizers offer the potential to significantly increase agricultural productivity. This limitation can be overcome through an improved understanding of the fundamental mechanisms responsible for the enhanced bioavailability of Mn and Zn observed when fluid fertilizers are used.

Recent experiments (Hettiarachchi et al., unpublished data, 2006) using isotopic dilution techniques to investigate the solubility, diffusion, and lability (potential availability) of Mn and Zn from different granular and fluid fertilizer sources in alkaline soils revealed that most fertilizer Mn diffused out of the fertilizer granule but was retained or precipitated in the soil in close proximity to the granule (<4 mm). In contrast, fluid Mn diffused farther away (>7.5 mm) from the point of application. Furthermore, granular Mn did not remain as isotopically exchangeable or labile as the fluid form. Most of the Zn in the granular Zn fertilizer source remained in the granule after 5 wk of incubation. Incomplete dissolution of granular Zn or precipitation of dissolved Zn in the granule appeared to be the reasons for restricted movement of Zn. Although the movement of fluid fertilizer Zn was mainly restricted to <7.5 mm from the point of fluid Zn injection, the lability and solubility of Zn from fluid fertilizers were significantly higher than the granular fertilizer Zn, indicating that more Zn from fluid fertilizers remained in more soluble forms (Hettiarachchi et al., unpublished data, 2006).

Although the above study provided indirect evidence that fluid fertilizer micronutrients behave differently at the point of soil–fertilizer contact, there is no direct evidence for differences in reaction products or chemical changes in and around granules or fluid bands in these soils. This information is also lacking in the international literature.

Synchrotron-based x-ray absorption spectroscopy (XAS) techniques provide detailed chemical and structural information of an absorbing element in situ without any pretreatment or processing (e.g., Bertsch and Hunter, 1998). Although XAS would provide useful information on dominant chemical bonding mode(s) of a particular element in soil regardless of its crystallinity, form, etc., one of the major obstacles in using XAS is its lack of species selectivity, meaning that the resultant spectra are an abundance-weighted average of all forms of the element of interest in the system under study (Manceau et al., 2002). This disadvantage becomes progressively greater when using "bulk" XAS techniques to speciate an element in highly heterogeneous systems such as soil because a volume of several cubic millimeters is probed. Fortunately, with the high-brilliance synchrotron sources, it is now possible to speciate elements in heterogeneous systems using micro-focused XAS with high spatial resolution. When probing a small area of a sample, typically a few tens to hundreds of square micrometers, we can minimize the number of minerals or solid phases contributing to the overall spectrum of the element of interest (Nachtegaal et al., 2005). Micro-focused XAS alone, however, may not be sufficient to assess the "average" molecular environment surrounding the element of interest in soil because of the limited analytical volume. Therefore, the combined use of "bulk" and microscale synchrotron-based techniques has been proposed to enhance the understanding of the chemistry of nutrient elements in fertilized soils (Lombi et al., 2006).

The aims of this research were to investigate the distribution of fertilizer Mn and Zn in soil in situ and to understand whether the reaction products and primary Mn and Zn sinks around granular fertilizers and in fluid fertilizer bands were similar or not. This required a different methodology whereby bulk soil analyses were complemented by microscale spatially resolved studies using laboratory x-ray and synchrotron technologies. We used a combination of several bulk and focused x-ray techniques, namely, laboratory-based x-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive x-ray analysis (EDAX), and synchrotron-based x-ray fluorescence (XRF), x-ray absorption near-edge structure (XANES), and extended x-ray absorption fine structure (EXAFS) spectroscopies.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A gray calcareous sandy loam soil (Calcixerollic Xerochrept, Soil Survey Staff, 1992), collected from upper Eyre Peninsula, South Australia, was used for all experiments. This soil had a pH (in H2O) of 8.5, a clay content of 50 g kg–1, an organic C content of 12 g kg–1, and contained 630 g CaCO3 kg–1. Total Mn and Zn concentrations (digested with aqua regia [1:3 v/v conc. HNO3/conc. HCl]) were 73.5 and 8.3 mg kg–1, respectively. Soil pH was measured in 1:5 soil/water extract (Rayment and Higginson, 1992) and soil texture was determined by the procedure described in Soil Survey Staff (1984). Soil organic C and CaCO3 content were measured following the procedures described by Rayment and Higginson (1992). See Table 1 for details.


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  		Table 1. Selected properties of the soil used in this study.

 
Sample Preparation for Different X-Ray Techniques
The laboratory incubation of soils treated with granular or liquid fertilizers for synchrotron-based bulk XANES, EXAFS, and XRD and laboratory-based XRD and SEM-EDAX analyses was set up in a manner similar to that described by Hettiarachchi et al. (unpublished data, 2006). Petri dishes (8.7 cm in diameter and 1.1 cm high) were filled with moist soil (at 60% of its water holding capacity as measured by the procedure described by Jenkinson and Powlson, 1976) to obtain approximately 1.2 Mg m–3 bulk density. The four treatments included two granular and two liquid treatments as follows:
  1. Granular Mn: commercially available Mn-incorporated monocalcium phosphate (MCP) granules containing 9.3% P, 10.6% S, 13.4% Ca, and 8.7% Mn.
  2. Granular Zn: commercially available Zn-incorporated monoammonium phosphate (MAP) granules containing 15.9% N, 9.1% P, 13.8% S, 0.5% Ca, and 1.2% Zn.
  3. TGMAP+MnZn: 100 µL of dissolved technical-grade MAP containing 12% N, 26% P, and 0% K, Zn as ZnSO4•7H2O, and Mn as MnSO4•2H2O.
  4. H3PO4+Mn: 100 µL of a mixture of H3PO4 (45% P w/v) containing Mn as MnSO4•2H2O. This treatment was included as an additional liquid Mn treatment because H3PO4 may represent more closely the dissolution products of MCP than MAP.

All treatments received the same amounts of P (9.1 mg per petri dish), Mn (3.6 mg per petri dish), and Zn (0.5 mg per petri dish), and the mass of soil per petri dish was 78 g (dry). Analytical-grade (NH4)2SO4 was used to balance the P/N ratio in all treatments. The granules were placed in the center of the petri dishes, whereas the liquid treatments were injected into a similar position using a needle. Two replicates of each treatment were prepared for the bulk XANES and EXAFS, while six replicates of each treatment were prepared for the XRD and SEM-EDAX analyses. After the treatments were introduced, the petri dishes were closed, sealed with Parafilm (Structure Probe, West Chester, PA), and incubated in the dark for 4 wk (for bulk-XANES and -EXAFS) or 5 wk (for XRD and SEM-EDAX) in a controlled environment (25/20°C day/night temperature, 16-h day period). At the end of the incubation period, the petri dishes were opened and concentric sections of soils from the fertilizer placement point (0–4, 4–7.5, 7.5–13.5, 13.5–25.5, and 25.5–43 mm) were removed using a series of plastic cylinders as described by Lombi et al. (2004). Incubated granules were carefully extracted from the center section and adhering soil particles were removed using a magnifying glass and tweezers. Removed adhering soil particles were placed back with the rest of the soils from the center section. Incubated granules and soils were dried rapidly in an oven maintained at 40°C.

For synchrotron-based µ-XRF and µ-XANES, only the first three treatments (granular Mn, granular Zn, and TGMAP+MnZn) were used. Moist soils for each treatment (at 60% of water holding capacity) were placed in Plexiglas boxes (with a circular Kapton film window with 25-mm diameter at the top) about 5 cm long by 5 cm wide by 0.5 cm high to obtain a soil bulk density of ~1.2 g cm–3. Fertilizer treatments were placed in the center close to the Kapton film window (center top) and were incubated for 4 wk in a controlled-temperature environment (25°C).

Laboratory-Based XRD and SEM-EDAX Data Collection and Analyses
For the XRD analysis, three each of the unexposed (original granules not incubated in soil) and incubated granules were ground in an agate mortar and pestle and lightly pressed into aluminum sample holders. X-ray diffraction patterns were collected with a Philips PW1800 microprocessor-controlled diffractometer (PANalytical B.V., Almelo, the Netherlands) using Co K{alpha} radiation, variable divergence slit, and graphite monochromator. The diffraction patterns were recorded in steps of 0.05° 2{theta} with a 3-s counting time per step. The XRD patterns were analyzed using in-house developed XPLOT software. For SEM and EDAX analyses, three each of unexposed and incubated granules were cross-sectioned manually using a stainless steel knife, then oriented and mounted onto aluminum specimen holders using Araldite. The samples were subsequently placed in an oven at 105°C for 15 min to aid polymerization and coated with 30 nm of Cu by evaporation to provide electrical conductivity and to maximize the backscattered electron signal. The specimens were loaded in a Philips XL30 FEG-SEM (PANalytical B.V., Almelo, the Netherlands), with an attached EDAX DX4 energy-dispersive x-ray system (EDAX, Mahwah, NJ) and examined using primary electron beam energy of 15 keV. Spectra were analyzed using the ZAF correction software (Armstrong, 1995; CITZAF Version 3.03, J.T. Armstrong, download available at www.cstl.nist.gov/div837/Division/outputs/CITZAF%20GUI/CITZAF%20GUI.htm, verified 30 Oct. 2007). Semiquantitative elemental analysis was performed using selected spots of about 200 by 100 µm and spectra were collected for 120 s.

Synchrotron-Based "Bulk" XANES–EXAFS Data Collection and Analyses
Bulk Zn and Mn XANES and EXAFS data were collected at the BL-20B (Australian National Beamline Facility [ANBF], Photon Factory, Tsukuba, Japan). The Photon Factory electron storage ring operated at 2.5 GeV and BL-20B is a bending magnet beam line equipped with a water-jet cooled Si(111) channel-cut monochromator with detune capability. The monochromator was detuned by ~50% to reject the higher order harmonics during data collection. The incident beam intensity, I0, was measured using an ionization chamber containing a He–N2 mixture for Mn, and N2 at atmospheric pressure for Zn. Manganese bulk EXAFS for Mn and Zn were collected at ambient temperature in fluorescence mode with a Canberra-Eurisys 36-element pixel array detector. For each sample, duplicate or triplicate scans covered the range from 230 eV below to 580 eV above the x-ray absorption edge of Mn (~6550 eV) and Zn (~9675 eV). A Cr(III) filter was used during Mn scans and a Cu(II) filter during Zn EXAFS data collection to suppress the Fe fluorescence from the samples. The EXAFS spectra of standard Mn and Zn compounds were also collected in the same mode (fluorescence) and scan conditions after diluting standard compounds using BN to bring the concentration of Mn or Zn in each sample to approximately 0.5%. Standard Mn compounds included MnSO4 • H2O, rhodochrosite (MnCO3), hureaulite [(Mn2+)5(PO3OH)2(PO4)2 · 4H2O], switzerite [(Mn2+,Fe2+)3(PO4)2 · 7H2O], and high Mn-calcite (CaCO3) as Mn2+ standards; hausmannite (Mn2+Mn23+O4) as a Mn2+ and Mn3+ mixed standard; Mn2O3 as a Mn3+ standard; and Na-birnessite (NaMn7O14 · 2.8H2O) as a Mn4+standard. Standard Zn compounds included ZnSO4 • 7H2O, smithsonite (ZnCO3), zincite (ZnO), gahnite (ZnAl2O4), hydrozincite [Zn5(CO3)2(OH)6], scholzite [CaZn2(PO4)2·2H2O], hopeite [Zn3(PO4)2 · 4H2O], willemite [Zn2(SiO4)], franklinite (Zn0.6Mn0.32+Fe0.12+Fe1.53+Mn0.53+O4), and ferrihydrite (Fe23+O3 · 0.5H2O)-adsorbed Zn (source: South Australian Museum and CSIRO Land & Water mineral collection).

The Mn EXAFS spectra for a particular soil and fertilizer sample or standard were averaged. The edge energy was calibrated, the pre-edge subtracted (by a linear function), and the spectrum was normalized to the second-order polynomial to be equal to 1 (Ravel and Newville, 2005). Reduced spectra for the samples were analyzed by linear combination fitting (LCF) using IFEFFIT software (Newville, 2001). Spectra for the model compounds were reduced and normalized as for the spectra of the soil and fertilizer samples. The region of 6525 to 6600 eV of each merged Mn scan was isolated for XANES analysis. The linear combination XANES fitting procedure attempted to reconstruct the soil–fertilizer spectra using all combinations of the eight model spectra (MnSO4, rhodochrosite, hureaulite, switzerite, Mn-calcite, hausmannite, Mn2O3, and birnessite). For each soil–fertilizer sample, the combination with the lowest reduced {chi}2 was chosen as the most likely set of components, with reduced {chi}2 defined as the sum of squares of the fit residual divided by the estimated uncertainty in the data, and normalized by the number of degrees of freedom in the fit (i.e., the number of data points minus the number of components used). For all normalized XANES spectra for soils or fertilizers, the uncertainty in the data was estimated to be 0.01 by statistical rendering within IFEFFIT software (Newville, 2001). Standards that had partial contributions <5% were removed; however, this did not result in a significantly larger reduced {chi}2. The accuracy of this fitting procedure depends on data quality and how well the reference standards represent the components in the samples (Roberts et al., 2002). A reduced {chi}2 near 1 indicates a reliable fit. Because there are a limited number of standard spectra and problems with the representativeness of the standards used in the fits, the best-fit compositions may not give the true composition but the results can be used to describe the differences between samples.

The Zn EXAFS spectra for the samples and the standards were reduced and normalized as for the Mn EXAFS spectra. The data were then converted to k space (k is the photoelectron wavenumber), weighted with k3 to compensate for the dampening of the XAFS amplitude with increasing k, windowed, and Fourier transformed to convert to R space. The linear combination of the k3–weighted EXAFS fitting procedure attempted to reconstruct the soil–fertilizer spectra using all combinations of the 10 model spectra (ZnSO4, smithsonite, zincite, gahnite, hydrozincite, scholzite, hopeite, willemite, franklinite, and ferrihydrite-adsorbed Zn). Fitting was done over the 2 to 10 k-space. The combination with the lowest reduced {chi}2 was chosen as the most likely set of components, as described above for XANES fitting. It should be noted that LCF becomes more difficult when bulk soils are fitted, because potentially more species combine to give the final spectrum.

The Zn EXAFS spectra were also analyzed in a two-step procedure using the Labview software at beamline 10.3.2 at the Advanced Light Source for comparison, as described by Manceau et al. (2002). The first step consisted of a principal component analysis (PCA) of the different sets of experimental (soil) spectra and the second step consisted of linear least squares fitting of experimental EXAFS spectra to the combination of reference spectra identified by PCA. This was only performed to test the consistency of the fitting procedure and, therefore, results will not be presented in detail. The number of significant components determined by PCA was three or four based either on the minimum indicator (IND) value of each component or on the weight of each component, which is directly related to how much of the signal it represents. This was consistent with the number of standards used in the LCF analysis using IFEFFIT software. Moreover, the targeted standards were the same as those included in the reported data.

Synchrotron-based µ-XRF and µ-XANES Data Collection and Analyses
Micro-XRF maps and µ-XANES were collected at beamline 13-BM (GeoSoilEnviro Consortium of Advanced Radiation Sources) at the Advanced Photon Source at Argonne National Laboratory, Argonne, IL. The electron storage ring operated at 7 GeV with a top-up fill status. Plexiglas boxes containing fertilizer-treated soils were mounted onto the sample stage with the side containing the Kapton x-ray window facing the beam. Mapping data (µ-XRF) and µ-XANES spectra were collected at ambient temperature in fluorescence mode with a Ge solid-state 13-element detector (Canberra Industries). The µ-XRF microprobe at APS beamline 13-BM is capable of collecting fluorescence data with a 15- to 30-µm beam spot size range (<20-µm resolution) and about 10 mg kg–1 sensitivity, allowing study of elements at very low concentrations in complex environmental samples. The samples were mounted on the rotation axis of an x–y–{theta} stepping-motor stage. Fluorescence data were collected at ambient temperature for a 10,000- by 500-µm area for the granular Mn-treated soil sample, an 8000- by 5000-µm area for the granular Zn-treated soil sample, and a 10,000- by 5000-µm area for the liquid Mn- and Zn-treated soil sample with a step size of 50 µm using a solid-state energy-dispersive x-ray detector that allowed simultaneous detection of fluorescence signals from multiple elements. At each position, the fluorescence signal from a given element was proportional to the integrated number of atoms of that element along the transect of the synchrotron beam. The sample thickness was about 5000 µm, i.e., greater than the absorption lengths for the fluorescence x-rays of interest, and only the upper ~100 µm of sample contributed significant fluorescence signal. In addition, XRF spectra for two thin-film NIST multielement standard reference materials (SRM 1832 and SRM 1833 for x-ray fluorescence spectrometry) were also collected (mounted at the same distance) for conversion of XRF signals to relative elemental concentrations.

The NRLXRF program (Criss, 1977) was used to predict the relative sensitivities for these elements in the thin films and thick soils. Elemental maps for the unknown soils were quantified to obtain the element concentration in each pixel Cel,soil using the following equation:

Formula
where Cel,std is the concentration of the element in the thin film, Iel,soil is the count rate from the soil, Iel,std is the count rate from the thin film, and Sel is the relative sensitivity in the thin film compared with the soil. Concentrations determined in this way are in units of micrograms per square centimeter, the units provided for the thin film standards. Conversion to parts-per-million-weight was obtained by assuming the sample thickness was 1 g cm–2 (5 mm thick, 2.0 g cm–3), the same thickness used in determining the relative sensitivities, Sel.

Average radial distributions were computed for each element in each map in the following way. First, the injection point within each map was defined (center of granule or center of liquid introduction). For the granules, this position was determined from optical images or the element maps themselves. In the case of the liquid, however, the injection point was not within the element maps. In this case, the center of introduction was estimated using the curvature of the Zn map and assuming that the diffusion in two dimensions was circular. This indicated that the injection point was along the left edge of the map but vertically higher by 4 mm. The radial distance of each pixel from the injection point was computed, the concentrations were binned in distance, divided by the number of pixels in each bin to obtain the average concentration, and average concentrations were normalized to the mean concentration in each profile (mean value = 1) to produce a plot of normalized concentrations as a function of distance.

From all three samples, three replicates of µ-XANES spectra of Mn and Zn were collected across the range of –200 to 600 eV above the K-edge of Mn (~6550 eV) and the K-edge of Zn (~9675 eV) in fluorescence mode. Spectra were obtained from four to six hotspots (high relative concentrations) of Mn and Zn moving outward from the point of fertilizer application. The µ-XANES spectra for all the Mn and Zn standards outlined above were also collected in duplicate. In addition, the Zn-XANES spectra of Zn-containing calcite were collected.

The Mn and Zn µ-XANES spectra for a particular hotspot or a standard were reduced and normalized. The reduced spectra for the each hotspot of Mn or Zn were analyzed by LCF using IFEFFIT software (Newville, 2001) as described above.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Structure and Speciation of Incubated and Unexposed Manganese and Zinc Granules
X-ray diffraction analyses of the unexposed granular Mn fertilizer mainly contained anhydrite (CaSO4), bassanite (2CaSO4·H2O), and szmikite (MnSO4·H2O). In addition, some MnHPO4 and hydroxylapatite [Ca5(PO4)3(OH)] were also originally present in these Mn-containing granules. The alterations that can occur during incorporation of micronutrients into macronutrient fertilizers have been discussed in detail by Lehr et al. (1959) for the MnO–P2O5–H2O system and MnHPO4 has been identified as a possible reaction product. The solubility of Mn phosphates ranges from fairly soluble to extremely insoluble, depending on both the Mn phosphate species and the soil conditions (Boyle and Lindsay, 1985, 1986). The unexposed MAP+Zn granules showed the presence of biphosphammite [(NH4,K)H2PO4], mascgnite [(NH4)2SO4], arkanite (K2SO4), zincite (ZnO), and some metallic Zn in the granules. Backscattered electron microscopy images of cross-sectioned Mn and Zn granules before and after incubation in soil are shown in Fig. 1 . The EDAX spectra of the circular area observed in the backscattered electron image for the original granular Mn (Fig. 1a) showed that the circular area contained mainly Mn, S, O, significant amounts of Fe, and some P. The EDAX spectra of the bright spots observed in the backscattered electron microscopy image for the granular Zn (Fig. 1c) showed that these areas contained almost exclusively Zn and O, indicating that ZnO is distributed heterogeneously throughout the granules.


Figure 1
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  		Fig. 1. Backscattered electron micrographs of cross-sections of granular Mn (a) unexposed and (b) incubated for 5 wk in soil, and granular Zn (c) unexposed and (d) incubated for 5 wk in soil.

 
It was evident from the electron images that both of the granules became more porous following their incubation in soil. Structural changes were probably caused by the partial dissolution of the granules and the diffusion of soluble nutrients into the surrounding soil.

In contrast to the original unexposed Mn granules, XRD analysis of incubated Mn granules did not show significant concentrations of any Mn-containing minerals, except for only traces of szmikite (MnSO4·H2O) and MnHPO4. This supports data from previous experiments with the same granular Mn fertilizer incubated under similar conditions, where nearly 90% of the Mn in the granule dissolved and diffused out of the granule but was retained within 4 mm from the point of granule placement (Hettiarachchi et al., unpublished data, 2006). One of the main residual forms of P remaining in the incubated Mn granules was hydroxyapatite (data not shown). X-ray diffraction analysis of incubated Zn granules in soils showed dominating and more intense peaks for zincite, indicating that the concentration of Zn species in the incubated granules increased following incubation in soil, possibly due to the decreased concentration of major nutrient elements (i.e., N and P). This observation is in agreement with our previous studies, which showed that ~85% of the Zn in the granular Zn fertilizer source remained in the granule after 4 wk of incubation in the same soil (Hettiarachchi et al., unpublished data, 2006). In addition, most of the remaining P phases in the incubated granules either appeared to lack crystallinity (i.e., they demonstrated peak broadening) or were x-ray amorphous (no diffraction pattern was observed). Although peaks were shifted a little (<0.02 Å) and broader, some broader peaks appeared in incubated granular Zn that closely matched those of crandallite [CaAl3(PO4)2], suggesting that one of the dominant residual phases of P in incubated Zn granules could be crandallite-like minerals (e.g., crandallite incorporated with other metals such as Zn) (data not shown). The presence of crandallite-like minerals in exposed P granules is in agreement with the findings of Lombi et al. (2004).

Energy-dispersive x-ray analysis of randomly chosen spots of Mn fertilizer granules indicated that the concentrations of both P and Mn decreased following their incubation in soil, whereas concentrations of Al, Ca, Fe, and S increased. In contrast, EDAX analysis of randomly chosen spots of the incubated Zn fertilizer granules revealed that the concentrations of both P and N decreased following their incubation in soil, whereas concentrations of Zn, Al, Ca, and Fe increased. These observations indicate that most of the Mn originally present in the granules moved into the soil, unlike Zn, which remained within the granules.

Distribution of Manganese and Zinc from the Point of Application
Figure 2 shows the XRF maps (10,000 by 500 µm) of selected elements for soil treated with granular Mn (Fig. 2a) and granular Zn (Fig. 2b). White or yellow color indicates high concentrations and blue or black, low concentrations. The µ-XRF map of Mn revealed that most of the fertilizer Mn diffused out of the granule but was retained around the granule. In our previous experiments with the same granular Mn source in this soil, ≤11% of the Mn remained in the granule after 4 wks of incubation, the remainder being localized in the soil just outside the granule (Hettiarachchi et al., unpublished data, 2006). We also found that the separation of incubated Mn granules from soil was extremely difficult because of a 1- to 2-mm cementitious shell around the granule (Hettiarachchi et al., unpublished data, 2006), which suggested a Mn deposition zone adjacent to the granule. The µ-XRF map of Zn distribution around the granular Zn-treated soil (Fig. 2b) clearly showed that most of the fertilizer Zn was retained in the granule, and its distribution in the granule was heterogeneous. Distribution of Zn within the granule correlated well with Fe distribution (Fig. 2b). This observation was in agreement with our previous finding that about 90% of the fertilizer Zn remained in the granule after 4 wk of incubation in the same calcareous soil.


Figure 2
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  		Fig. 2. Micro-x-ray fluorescence maps of Mn, Ca, Fe, and K or Zn for soil incubated with (a) granular Mn and (b) granular Zn. Area of a single map is 10,000 by 500 µm for granular Mn added to soil and 8000 by 5000 µm for granular Zn added to soil. The color scheme used ranges from white or yellow for high-fluorescence signal to blue or black for low-fluorescence signal. Shading is relative across each map. The markers noted as P1 to P5 and P36 to P39 represent locations for which µ-x-ray absorption near-edge structure (XANES) analyses were conducted. The XANES spectra are presented in Fig. 6 and 8 and the XANES fitting is presented in Tables 3 and 5.

 

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  		Table 3. Percentages of Mn species in selected "Mn hotspots" (points of interests, POI) in the soil treated with granular and liquid Mn determined by linear combination fitting of µ-x-ray absorption near-edge structure (XANES) spectra.

 

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  		Table 5. Percentages of Zn species in selected "Zn hotspots" (points of interests, POI) in soil treated with granular and liquid Zn fertilizer determined by linear combination fitting of µ-x-ray absorption near-edge structure (XANES) spectra.

 
In contrast to the soil treated with granular micronutrients, soil treated with liquid Mn and Zn showed very different nutrient distributions. Both Mn and Zn diffused farther away from the point of application (Fig. 3 ). Figure 4 shows the average concentrations (normalized to the mean concentration in each profile [mean value = 1]) generated using a histogram of concentration vs. the radial distance (starting from the center of the granule or point of liquid injection) for Mn and Zn calculated from the µ-XRF maps shown in Fig. 2 and 3. In the soil treated with liquid Mn and Zn, Mn and Zn concentrations showed a broader distribution, indicating that (i) both Mn and Zn diffused farther away from the point of application compared with the soil treated with granular Mn or Zn; (ii) most of the granular Mn precipitated around the granule; (iii) the diffusion of liquid Mn was greater than the diffusion of liquid Zn in the soil treated with liquid fertilizer; and (iv) most of the granular Zn was retained in the granule.


Figure 3
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  		Fig. 3. Micro-x-ray fluorescence maps of Mn, Zn, Fe, and Ca for the soil incubated with fluid Mn and Zn. Area of a single map is 10,000 by 5000 µm. The color scheme used ranges from white or yellow for high-fluorescence signal to blue or black for low-fluorescence signal. Shading is relative across each map. The markers noted as P7 to P11 and P 13 to P16 represent locations for which µ-x-ray absorption near-edge structure (XANES) analyses were conducted. The XANES spectra are presented in Fig. 6 and 8 and the XANES fitting is presented in Tables 3 and 5.

 

Figure 4
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  		Fig. 4. The radial distribution plots for Mn and Zn generated using micro-x-ray fluorescence (µ-XRF) maps for soils treated with granular Mn, granular Zn, and liquid Mn and Zn plus technical-grade monoammonium phosphate (TGMAP+MnZn).

 
The mean Fe concentrations were also calculated for the maps to test the accuracy of the µ-XRF concentrations. Mean Fe values determined were 2100, 2400, 3000 mg kg–1 (dry weight calculated by removing 22.4% water) for the Mn granule, Zn granule, and liquid, respectively. These values were 0.54, 0.62, and 0.77 times the 3900 mg kg–1 concentration determined for associated soil by aqua regia digestion followed by inductively coupled plasma atomic emission spectroscopy analysis (Table 1). These differences are indicative of soil heterogeneity but suggest that the recovery of metals using the µ-XRF determinations in these complex samples are 40% (or better) of actual.

There was almost complete removal of Mn from the granule and precipitation in a zone about 2 mm wide immediately adjacent to the granule, consistent with previous physical observations (Fig. 4). The absolute Mn concentrations, however, were lower than those observed for the liquid treatment and lower than expected based on the known initial granule Mn concentration and mass balance considerations. The most likely reason for these low concentrations is that attenuation of the Mn fluorescence has taken place due to the presence of precipitate-free soil adjacent to the kapton window. Nonetheless, normalized concentration profiles were not compromised by this effect (Fig. 4). Moreover, the trends observed from XRF calculations agreed with our previous studies and indicated that diffusion of liquid Mn and Zn was not simply a process of capillarity. Our previous laboratory-based macroscopic studies indicated that Mn added as liquid travelled >7.5 mm away from the point of injection, whereas movement of Zn was restricted to <7.5 mm from the fertilizer placement regardless of its physical form (Hettiarachchi et al., unpublished data, 2006). Even though these differences are small, their effect on nutrient accessibility by plant roots could be significant.

Speciation Changes of Manganese with Distance from the Point of Application
The Mn K-edge bulk XANES spectra of selected Mn standards and soil treatments are shown in Fig. 5 . The weak pre-edge features due to 1s -> 3d electronic transitions are common for all three major Mn oxidation states (+2, +3, and +4) and are lower in energy than the main edge by several electron volts (Schulze et al., 1995). For samples containing only Mn2+, the Mn K-edge XANES spectra are characterized by a weak pre-edge feature near 6540 eV, followed by the edge jump peaks around 6552 eV. For samples containing only Mn4+, the edge jump peaks are around 6561 eV. In a mixed-oxidation-state sample, the XANES spectra will be the convolution of spectra corresponding to each individual oxidation state and therefore the mixed-state samples show both peaks at varying intensity. Moreover, the K-edge XANES spectra of standard Mn compounds containing Mn in a single oxidation state also contain distinct features in each individual spectrum that can be used to identify the mineral species in the unknown, experimental soil spectra (XANES fingerprinting). For example, the Mn K-edge spectra for samples containing pure MnCO3 show an absorption edge that begins at about 6545 eV and peaks at about 6550 eV.


Figure 5
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  		Fig. 5. Bulk Mn x-ray absorption near-edge structure (XANES) spectra of selected Mn standards, soils, and incubated granular Mn in soil (TGMAP is technical-grade monoammonium phosphate). Dotted lines indicate the linear combination XANES fits using all the standard compounds. Vertical dashed lines represent white line peaks for Mn2+ and Mn4+, respectively. Soil Section 1 is the soil up to 4 mm from the point of fertilizer application, and Section 2 is the soil between 4 and 7.5 mm from the point of fertilizer application. Granular Mn Section 2 was similar to untreated soil and therefore not included.

 
Visual observation of bulk XANES spectra for the soil samples suggested that all the soil–granule spectra were a mixture of at least Mn2+ and Mn4+ oxidation states and the presence of minor amounts of Mn3+ cannot be ruled out (Fig. 5). The noise of the Mn XANES spectrum for the control soil was caused by the low total Mn concentration in the unfertilized soil (73.5 mg kg–1). The LCF indicated that the control soil was a mixture of approximately equal (50:50) proportions of Mn2+ and Mn4+ (Table 2 ). The spectra of the incubated granular Mn fertilizer (note that most of the fertilizer Mn had already diffused out of the granule) consisted of about 58% hureaulite and 42% Mn-calcite. The spectra for the first section (0–4 mm) of soil treated with granular Mn, where the majority of fertilizer Mn was present, was clearly different from the other treatments, and consisted of mixtures of minerals similar to Mn oxides and Mn-calcite. Moreover, this soil appeared to contain mainly minerals or solid phases composed of Mn(IV) and Mn(III,II mixed) oxides. In contrast, spectra from the soil treated with liquid fertilizers (TGMAP+Mn Zn and H3PO4+Mn) showed that they consisted mainly of hureaulite-like minerals (Table 2). It is noteworthy that in these treatments, the majority of Mn still remained as reduced Mn2+, even in an air-dried, completely aerated state. There are therefore significant differences between the chemistries of fertilizer Mn in granular or liquid form in highly calcareous soils.


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  		Table 2. Percentages of Mn species in treatments, as determined by linear combination fitting of bulk x-ray absorption near-edge structure (XANES) spectra.

 
Although "bulk" Mn XANES would provide an indication of the "average" molecular environment surrounding Mn in soil, alone it is unable to directly assess whether or how the Mn speciation changes as a function of the distance from the point of fertilizer application. Therefore, we used spatially resolved XRF and XANES to probe Mn speciation at the microscale and as a function of the distance from the point of application.

The µ-XANES spectra of selected Mn hotspots for soils treated with granular Mn and liquid Mn (TGMAP+Mn Zn) are shown in Fig. 6 . From visual observation of these spectra, the chemistry of Mn was more homogenous in soil treated with liquid Mn (i.e., spectra were similar across hotspots) than with the granular product. The LCFs of the Mn µ-XANES soil spectra for granular treatments against the spectra of the standard Mn compounds showed that there were significant amounts of MnCO3 at points in or adjacent to the granule, in combination with hureaulite and Mn-calcite (Table 3 ). The contribution of MnCO3 to the soil µ-XANES spectra appeared to be insignificant with increasing distance from the granule. In contrast, spectra for the soil treated with liquid Mn were dominated by hureaulite and Mn-calcite. Differences in the redox status of Mn in the soil samples studied using microscale XAS (in situ or moist) and bulk XAS (dried rapidly) may have been the reason for the observed difference in the proportions of MnCO3 to Mn oxides in the soils treated with granular Mn (fewer Mn oxides and more MnCO3 were detected for the granular Mn-treated soils analyzed in situ using microscale XAS data compared with the bulk XAS data).


Figure 6
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  		Fig. 6. Manganese micro-x-ray absorption near-edge structure (XANES) spectra of selected different Mn hotspots shown in Fig. 3 (granular) and 4 (liquid) for soil treated with granular and liquid Mn. Dotted lines indicate the linear combination XANES fits using all the standard compounds. Vertical dashed lines represent white line peaks for Mn2+ and Mn4+, respectively.

 
On dissolution of the soluble granular Mn fertilizer, Mn2+ would be released to soil solution and form new solid phases with other soil constituents. Although under acidic conditions fertilizer Mn would largely remain in the soil solution or in exchangeable forms on soil surfaces, at high pH Mn2+ is usually either strongly adsorbed or oxidized to fairly insoluble Mn(III, IV) (hydr)oxides (Sherman and Harmer, 1943). Schwab and Lindsay (1983) showed that in a calcareous soil with a high total Mn concentration (3800 mg kg–1), solubility of Mn appeared to be controlled by Mn oxides at high redox and by MnCO3 (rhodocrosite) under moderate and low redox conditions (pe + pH < 15). Solution Mn2+ concentrations in equilibrium with MnCO3 are dependent on CO2 partial pressure and usually decrease with increasing partial pressure of CO2, suggesting that at high CO2 partial pressure, the formation of MnCO3 can decrease Mn solubility. Moreover, previous studies have also shown divalent Mn phosphates to be important reaction products of Mn and P fertilizers in soils (Boyle and Lindsay 1985, 1986; Heintze, 1968). Under different conditions, the solubility of Mn phosphates ranges from fairly soluble to extremely insoluble depending on the Mn phosphate species (Boyle and Lindsay, 1985, 1986).

Our calculations using solubility data given by Lindsay (1979) and Boyle and Lindsay (1985, 1986) have shown that when P and Ca solubilities are controlled by hydroxyapatite and calcite, and at a log(CO2 pressure) of –2 (which is more realistic for highly calcareous soils), MnCO3 (rhodochrosite) can be comparatively less soluble than hureaulite or various other Mn phosphates. Lombi et al. (2006) using XAS showed that, in the soil surrounding fertilizer granules, P precipitation in the form of octacalcium phosphate and apatite-like compounds was the dominant mechanism responsible for decreases in P exchangeability or solubility and that this process was less prominent when liquid P fertilizer was applied to the soil. We propose that P solubility in our experiments was controlled by hydroxyapatite in the granular treatments, thus making the formation of Mn phosphates (hureaulite-like minerals) less likely and the formation of Mn carbonates more likely. In the liquid treatments, P solubility would have been significantly higher than in the granular treatments, thus making the formation of Mn phosphate minerals much more likely. This hypothesis accords with the Mn speciation determined by both the bulk and microscale XANES spectra. Although soil samples for the bulk XANES analyses were dried while soils used for µ-XANES were moist, both sets of XANES data for the liquid treatments were basically consistent because both techniques showed that the majority of Mn was in the form of comparatively more soluble Mn2+ species in these treatments. These observations are in agreement with data from previous isotopic dilution studies, viz., when Mn was added to soils in liquid form, more Mn remained in the soil in more soluble solid forms than when Mn was added in granular forms.

Speciation Changes of Zinc with Distance from the Point of Application
The k3–weighted Zn bulk EXAFS spectra and their fits from LCF (dashed lines) for treated soils and unexposed and incubated granules are shown in Fig. 7 . The Zn K-edge EXAFS spectrum of the unexposed granular Zn fertilizer was best fitted with a mixture of Zn phosphates and Zn oxide (Table 4 ). The Zn phosphate-like minerals dominated the original granular Zn fertilizer EXAFS spectra (about 92% of spectra were reconstructed using Zn phosphate minerals). According to Fig. 2 and 3, most fertilizer Zn from granular sources remained in the granule, whereas Zn from a liquid source diffused into the soil immediately around the point of injection (i.e., Section 1). The k3–weighted Zn EXAFS spectrum for the incubated granule was best fitted with a mixture of scholzite (65.8%), willemite (20.5%), and zincite (13.7%), whereas the spectrum for soil immediately adjacent to the granule (Section 1) was best fitted with a mixture of willemite (59.5%), hopeite (31.4%), and ferrihydrite-adsorbed Zn (9.1%), indicating a dominance of Zn phosphates in the granule of the soil treated with granular Zn.


Figure 7
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  		Fig. 7. The raw Zn k3–weighted (where k is the photoelectron wavenumber) extended x-ray absorption fine structure (EXAFS) spectra (solid lines) for the Zn K-edge bulk x-ray absorption spectroscopic data. Dotted lines indicate the linear combination fits using all the standard compounds.

 

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  		Table 4. Percentages of Zn species in treated soils, the unexposed granular Zn fertilizer granules, and incubated granular Zn determined by linear combination fitting of bulk extended x-ray absorption fine structure (EXAFS) spectra.

 
For the liquid treatment, adsorbed Zn was an important fertilizer reaction product, particularly as the distance from the point of injection increased. This form was absent from the spectra for the granular treatment (Table 4).

As a check on the LCF procedure, the Zn EXAFS spectra were also analyzed in a two-step procedure using the Labview software at beamline 10.3.2 at the Advanced Light Source as described by Manceau et al. (2002). The results were in agreement with the results presented in Table 4 (data not shown).

As with Mn, it appeared that the solid-phase chemistry of fertilizer Zn in soil was more homogeneous when the soil was treated with fluid fertilizer (Fig. 8 ). The Zn µ-XANES spectra collected at the different hotspots (locations shown in Fig. 2 and 3) and LCFs from pure Zn mineral spectra are given in Fig. 8 and Table 5 . Linear combination fitting of the µ-XANES spectra suggested that soils receiving different treatments contained the species indicated in Table 5. The differences between spectra for soil treated with liquid or granular Zn were (i) liquid treatments were dominated by hopeite and ferrihydrite-adsorbed Zn; (ii) granular treatments were dominated by combinations of scholzite, zincite, hopeite, gahnite, and willemite; (iii) scholzite was more dominant even though both Zn phosphates (hopeite and scholzite) were present in the granular treatment; and (iv) ferrihydrite-adsorbed Zn was absent or insignificant in the soil treated with granular Zn.


Figure 8
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  		Fig. 8. Zinc micro-x-ray absorption near-edge structure (XANES) spectra of selected point of interest (hotspots) in soil treated with liquid Zn (technical-grade monoammonium phosphate + Mn and Zn) or granular Zn (monoammonium phosphate + Zn). Dotted lines indicate the linear combination fittings.

 
Although additional Zn phases were identified in the Zn µ-XANES spectra, the spatially resolved Zn µ-XANES LCF analysis was in agreement with the bulk Zn EXAFS LCF analysis presented above. It is possible that these additional phases show up in the µ-XANES spectra because they were from Zn hotspots in the XRF maps. Similar differences between Zn bulk and µ-XANES spectra analysis were made by Nachtegaal et al. (2005).

Hopeite is generally less soluble than Zn oxides, hydroxides, and carbonates but can be expected to furnish available Zn to plants (Lindsay, 1979). Scholzite (Zn,Ca phosphate) is generally more stable (less soluble) than hopeite (Nriagu, 1984). The formation of sparingly soluble mixed-metal Zn phosphates (i.e., more stable than either hopeite or scholzite) has also been suggested as a possible mechanism restricting Zn solubility in alkaline environments (Nriagu, 1984). Our XAS data provide useful information on the dominant chemical bonding mode(s) of Zn in soil regardless of crystallinity. The spectra observed at both bulk and microscales clearly showed that the differences in chemistry between the reaction products in soil of fertilizer Zn derived from granular or liquid sources. In general, granular Zn tended to remain, or transform into, Zn phosphate species. A significant percentage of fertilizer-derived Zn from liquid sources formed adsorbed species in soil. These results accord with data for the isotopic exchangeability of Zn in soils fertilized with granular and liquid sources of Zn (Hettiarachchi et al., unpublished data, 2006) and provide an understanding of the mechanisms for the improved agronomic performance of liquid Zn fertilizers on calcareous soils.

Table 6 presents a summary of the results from all the microscopic and spectroscopic techniques used in this study. Basically, all the data seemed to be consistent and suggested that the reaction pathways for liquid and granular Mn and Zn in calcareous soil were different.


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  		Table 6. A summary of Mn and Zn speciation results with x-ray diffraction (XRD), energy dispersive x-ray analysis (EDAX), x-ray fluorescence (XRF), x-ray absorption fine structure (XAFS), and x-ray absorption near-edge structure (XANES) techniques.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
From micro-XRF mapping of soil–fertilizer reaction zones, we conclude that the mobility of Mn and Zn from fluid fertilizer was greater than that observed for equivalent granular sources of these trace elements in a calcareous sandy loam soil. Both x-ray microscopic and spectroscopic analyses agreed with our previous isotopic dilution data and provide evidence that application of fluid Mn and Zn to soil resulted in a considerably greater proportion of these micronutrients remaining in relatively more soluble forms than granular products. Following both bulk and microscale XANES analyses, we conclude that in granular Mn-treated soils, Mn may have transformed into mixtures of Mn carbonate-like and Mn oxide-like forms, whereas in fluid-treated soil, Mn existed predominantly in Mn(II) phosphate-like solid phases. Analysis of the microscale and bulk Zn-XANES and -EXAFS data indicated that for granular Zn, Ca,Zn phosphate-like, Zn oxide-like, and Zn phosphate-like compounds dominated the reaction products, whereas for fluid Zn, adsorbed Zn, Zn silicate-like, or Zn phosphate-like compounds appeared to dominate the Zn spectra. The superior agronomic effectiveness of fluid Mn and Zn fertilizers in calcareous soils under field conditions is the result of the enhanced diffusion and solubility of these micronutrients in soil when applied in fluid form. Moreover, this study confirmed that the combined use of synchrotron-based research techniques with more conventional x-ray techniques is useful for the examination of complex soil–fertilizer reactions and can aid our understanding of the reaction pathways of micronutrients applied to soil.


   ACKNOWLEDGMENTS
 
We thank Garry Foran and James Hester at ANBF for bulk XAS data collection and useful suggestions, Caroline Johnston and Sharyn Zrna for sample preparation, Mark Raven for XRD analysis, Stuart McClure for the help with SEM-EDAX analysis and Alain Pring at the SA Museum for providing mineral samples. This work was supported by the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities Program, and by a linkage grant from the Australian Research Council (LP0454086), the South Australian Grains Industry Trust, and CSBP Ltd. Ganga Hettiarachchi and Mike McLaughlin gratefully acknowledge the support of Grains Research and Development Corporation. A part of this work was performed at GeoSoilEnviro CARS (GSECARS), Sector 13, Advanced Photon Source at Argonne National Laboratory; GSECARS is supported by the National Science Foundation, Earth Sciences; Department of Energy–Geosciences; W.M. Keck Foundation; and the U.S. Department of Agriculture. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science, under Contract no. W-31-109-Eng-38.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication February 14, 2007.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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