Improvement of the NH 2 OH-HCl-HOAc method for extracting manganese and iron oxides in Japanese Andisols and other soil types in Japan

Soil Science and Plant Nutrition ISSN: 0038-0768 (Print) 1747-0765 (Online) Journal homepage: http://www.tandfonline.com/loi/tssp20 Improvement of the NH 2 OH-HCl-HOAc method for extracting manganese and iron oxides in Japanese Andisols and other soil types in Japan Aomi Suda, Tomoyuki Makino & Teruo Higashi To cite this article: Aomi Suda, Tomoyuki Makino & Teruo Higashi (2013) Improvement of the NH 2 OH-HCl-HOAc method for extracting manganese and iron oxides in Japanese Andisols and other soil types in Japan, Soil Science and Plant Nutrition, 59:5, 700-714, DOI: 10.1080/00380768.2013.834242 To link to this article: https://doi.org/10.1080/00380768.2013.834242 Published online: 28 Nov 2013. Submit your article to this journal Article views: 731 View related articles Citing articles: 2 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalinformation?journalcode=tssp20

Soil Science and Plant Nutrition (2013), 59, 700 714 http://dx.doi.org/10.1080/00380768.2013.834242 ORIGINAL ARTICLE Improvement of the NH 2 OH-HCl-HOAc method for extracting manganese and iron oxides in Japanese Andisols and other soil types in Japan Aomi SUDA 1, Tomoyuki MAKINO 1 and Teruo HIGASHI 2 1 National Institute for Agro-Environmental Sciences, Kannondai 3-1-3, Tsukuba, Ibaraki 305-8604, Japan, 2 Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennoudai 1-1-1, Tsukuba, Ibaraki 305-8572, Japan Abstract We evaluated the validity of Tessier s method as applied to the extraction of manganese (Mn) and iron (Fe) oxides in Japanese Andisols and other soil types in Japan. Using the original Tessier s extractant mixture, 0.04 mol L 1 hydroxylamine hydrochloride in 25% acetic acid (0.04 mol L 1 NH 2 OH-HCl in 25% HOAc), we found that substantial amounts of short-range-ordered Fe oxides were not extracted from allophanic Andisol samples and that considerable amounts of total Fe oxides were not extracted from all soil types. Relatively high extraction ph and large amounts of short-range-ordered Fe oxides in the Andisol samples might be responsible for incomplete extraction. Stoichiometric calculation indicated that the concentration of NH 2 OH-HCl might be insufficient for complete extraction of Fe oxides. The extracted amounts of Mn and Fe increased with increasing concentration of NH 2 OH-HCl in the extractant, and most of the Mn and Fe oxides in the soil samples, including samples with as much as 5.6% Fe, were extracted with 0.6 mol L 1 NH 2 OH HCl in 25% HOAc. As judged from the simultaneous dissolution of aluminum (Al) and silicon (Si) minerals, extraction selectivity of Fe oxides with 0.6 mol L 1 NH 2 OH-HCl in 25% HOAc was comparable to that of the original Tessier s method and better than that of a modified Community Bureau of Reference (BCR) sequential extraction procedure or a method using an extractant consisting of a mixture of oxalate and ascorbate, especially for Andisol samples. Key words: Fe oxide, heavy metals, hydroxylamine hydrochloride, Tessier. INTRODUCTION Because of the importance of manganese (Mn) and iron (Fe) oxides to the dynamics of heavy metals in the environment, many methods for extracting Mn and Fe oxides and occluded heavy metals have been developed. A common method is extraction with acidified 0.5 mol L 1 hydroxylamine hydrochloride (NH 2 OH-HCl), the second-step fraction in the modified Community Bureau of Reference (BCR) sequential extraction procedure (SEP) (Sahuquillo et al. 1999). However, Suda et al.(2012) reported that this method was not suitable for Japanese Andisols, especially Correspondence: A. Suda, National Institute for Agro- Environmental Sciences, Kannondai 3-1-3, Tsukuba, Ibaraki 305-8604, Japan. Tel.: +81-29-838-8314. Fax: +81-29-838-8199. Email: suda_aomi@yahoo.co.jp Received 28 April 2013. Accepted for publication 9 August 2013. allophanic Andisols, which showed poor extraction selectivity and low rates of extraction of short-range-ordered Fe oxides such as ferrihydrite. Dithionite-citrate has been used to extract both short-range ordered and well-crystalline Fe oxides in soil samples (McKeague and Day 1966). However, commercial dithionite could be contaminated with zinc; in addition, target metals would be precipitated as sulfide or sulfate (Gleyzes et al. 2002). Oxalate solutions and mixtures of oxalate solution and ascorbic acid have also been used for extracting heavy metals occluded in short-range ordered and total Fe oxides, respectively (Miller et al. 1986; Wilcke et al. 1998). One of the critical disadvantages of these oxalate-containing solutions is precipitation of some metals [cadmium (Cd) and lead (Pb)] as oxalate salts (Benitez and Dubois 1999). Many researchers have used Tessier s SEP (Tessier et al. 1979) for fractionation of heavy metals in soil samples. 2013 Japanese Society of Soil Science and Plant Nutrition

Extraction of Fe and Mn oxides in soils 701 Tessier et al. (1979) used sediment samples, not soil samples, but most researchers have used Tessier s procedure for the analysis of heavy metals in soil samples without any significant modifications. In the original Tessier s method, sediment samples with heavy metals bound to Mn and Fe oxides are extracted with 0.04 mol L 1 NH 2 OH-HCl in 25% acetic acid (HOAc) in boiling water for 6 h with occasional agitation (HH 004 -A extraction). Six hours was sufficient to extract all Fe oxides from the sediment samples in Tessier s study. Therefore, the target phase of the Tessier s method is generally considered to be heavy metals occluded in all Mn and Fe oxides, including short-range ordered to highly crystalline phases (e.g., Tack et al. 2006). However, on the basis of selective sequential extraction data and X-ray adsorption near-edge structure spectroscopy, La Force and Fendorf (2000) pointed out that part of the crystalline Fe oxide was not extracted from sediments contaminated with mine waste. This result indicates that the extractability of Fe oxides with HH 004 -A extraction depends on the properties of the sediment or soil samples, especially their Fe oxide content and crystallinity. Despite its widespread use, Tessier s SEP has not been evaluated and sufficiently modified for various soil types. In Japanese agricultural fields, Andisols are among the most common soil types containing high amounts of Mn and Fe (Takeda et al. 2004). Mn oxides and short-range-ordered Fe oxides in Andisol samples are hardly extracted by 0.1 mol L 1 NH 2 OH-HCl (ph 2.0) and 0.5 mol L 1 NH 2 OH-HCl (ph 1.5), respectively (Suda et al. 2011, 2012). Inceptisols, classified as Brown Forest Soils in the Japanese soil classification system, are also widely distributed in Japan and generally contain abundant highly crystalline Fe oxides such as goethite. Fe oxides in these common Japanese soils might be incompletely extracted with HH 004 -A extraction, and better extractants might be needed for these soils. Here we evaluate the original Tessier shh 004 -A extraction method as applied to samples of Japanese Andisols and other major soil types in Japan and improve the method to completely extract Mn and Fe oxides in these soils. MATERIALS AND METHODS Soil samples and their properties Basic information about the three Entisols (E), three Inceptisols (I), five allophanic Andisols (AA), and three non-allophanic Andisols (NA) used in this study is compiled in Table 1. The soil samples were air-dried, passed through 2-mm mesh sieves, and stored at room temperature. Soil ph was measured in H 2 O or 1 mol L 1 potassium chloride (KCl) with a glass electrode ph meter. The ratio of soil sample to H 2 O or soil sample to 1 mol L 1 KCl was 1:2.5 (weight/volume). Total carbon content of the soil sample was measured by a dry combustion method (Sumigraph NC-22F, Shimadzu, Kyoto, Japan). Dithionite-citrate extractable Fe (Fe d ) and Mn (Mn d ), and ammonium oxalate-extractable Al, Fe, Mn, and Si (Al o,fe o,mn o, and Si o ) were extracted as described by Blakemore et al. (1987). Then the extract was centrifuged and filtrated by a 0.2-µm membrane filter (Merck Millipore, Burlington, MA, USA). Detailed extraction procedures for Fe d,fe o, and Mn o are described in subsequent paragraphs. Pyrophosphate-extractable Al and Fe (Al p and Fe p ) were extracted and centrifuged as Table 1 Brief description of soil samples Soil classification Site Sample Soil Taxonomy Japanese system Horizon (city, prefecture) Land use E-1 Aquents Gray Fluvic soils Ap Suzaka, Nagano Paddy Field E-2 Aquents Gray Fluvic soils Ap Tsukubamirai, Ibaraki Paddy Field E-3 Aquents Gray Fluvic soils Ap Tochigi, Tochigi Upland Field I-1 Udepts Haplic Brown Forest soils Ap Natori, Miyagi Upland Field I-2 Udepts Haplic Brown Forest soils Ap Zentuji, Kagawa Upland Field I-3 Udepts Haplic Brown Forest soils Ap Kanuma, Tochigi Upland Field AA-1 Udands Allophanic Kuroboku soils Ap Tsukuba, Ibaraki Upland Field AA-2 Udands Allophanic Kuroboku soils Ap Shimono, Tochigi Upland Field AA-3 Udands Allophanic Kuroboku soils Bw Tateyama, Toyama Forest AA-4 Udands Allophanic Kuroboku soils Bw Tsukuba, Ibaraki Upland Field AA-5 Udands Allophanic Kuroboku soils Bw Chichibu, Saitama Forest NA-1 Udands Non-allophanic Kuroboku soils A Oshaki, Miyagi Forest NA-2 Udands Non-allophanic Kuroboku soils A Tateyama, Toyama Forest NA-3 Udands Non-allophanic Kuroboku soils O/A Chichibu, Saitama Forest According to USDA Soil Taxonomy (Soil Survey Staff 1998). According to Unified Soil Classification System of Japan 2nd Approximation (The Japanese Sociaty of Pedology 2003). E, Entisols; I, Inceptisols; AA, allophanic Andisols; NA, non-allophanic Andisols.

702 A. Suda et al. described by Blakemore et al. (1987), and the supernatant was filtered through a 0.025-µm filter (Merck Millipore) to minimize contamination of Fe from microcrystalline minerals (Schuppli et al. 1983). The concentrations of these elements in the filtrate were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (Vista-Pro, Varian, Palo Alto, CA, USA). Clay content was determined by a pipette method (Gee and Bauder 1986). Experiment 1: Effects of increasing NH 2 OH-HCl concentration in NH 2 OH-HCl-HOAc extractant on the rates of extraction of Mn and Fe oxides Extraction with 0.04, 0.2, 0.4, and 0.6 mol L 1 NH 2 OH-HCl in 25% HOAc solution The extractants used in this experiment are NH 2 OH-HCl in 25% HOAc (HH-A) solutions. To determine the optimal NH 2 OH-HCl concentration for extraction of Mn and Fe oxides in soil samples, we evaluated the effect of increasing NH 2 OH-HCl concentration in NH 2 OH-HCl-HOAc extractant. For this purpose, we used NH 2 OH-HCl concentrations of 0.04, 0.2, 0.4, and 0.6 mol L 1 (HH 004 -A, HH 02 -A, HH 04 -A, and HH 06 -A). An air-dried soil sample (1 g) was placed in a 50-mL polyethylene centrifuge tube, and 20 ml of HH-A solution was added. The mixture was then heated in boiling water for 6 h with occasional agitation. After extraction, the tube was cooled to approximately room temperature with running water for 10 min. After centrifugation at 3500 rpm (1700 g) for 10 min, the supernatant was filtered through a 0.2-μm mesh membrane filter, and the filtrate was diluted with 1% nitric acid. The ph of the residual supernatant was measured with a glass electrode ph meter. Fe, Mn, Al, and Si concentrations in the diluted solution were determined by ICP-OES. All extractions and measurements were carried out in triplicate. The extraction with original Tessier s extractant of HH 004 -A aims to extract both short-range-ordered and crystalline Mn and Fe oxides as well as metals occluded in these oxides. Extraction of exchangeable and specifically adsorbed Mn and Fe Exchangeable and specifically adsorbed Mn and Fe were extracted with a mixture of 1 mol L 1 acetic acid and ammonium acetate (ph 5.0) (hereafter acetate solution). Air-dried soil (0.8 g) was placed in a 50-mL polyethylene centrifuge tube, 16 ml of acetate solution was added, and the mixture was mechanically shaken at room temperature for 2 h. After centrifugation at 3500 rpm (1700 g) for 10 min, the supernatant was filtered through a 0.2-μm mesh membrane filter, and the filtrate was diluted with 1% nitric acid. Mn and Fe concentrations in the diluted solution were determined by ICP-OES. All extractions and measurements were carried out in duplicate. Experimental conditions were based on those of Salbu et al. (1998). Extraction of total Mn oxides The amount of total Mn oxides was approximated using the value of oxalate-extractable Mn (Mn o ) according to Jarvis (1984). An air-dried soil sample (0.4 g) was placed in a 50-mL polyethylene centrifuge tube, and 40 ml of oxalate solution (mixture of 0.2 mol L 1 oxalic acid and 0.2 mol L 1 ammonium oxalate solution, ph 3.0) was added. The mixture was then mechanically shaken at 25 C for 4 h in the dark, a drop of polymeric flocculants (Superfloc, Kemira Co., Ltd., Helsinki, Finland) was added to an aliquot (about 25 ml) of the extract, and the resulting mixture was manually shaken. After centrifugation at 3500 rpm (1700 g) for 10 min, the supernatant was filtered through a 0.2-μm mesh membrane filter, and the filtrate was diluted with 1% nitric acid (HNO 3 ). Oxalate is likely to extract Mn bound to organic matter in addition to oxide-forming Mn. Therefore, the amount of total Mn oxides should be estimated by the difference between the amount of oxalate-extractable Mn and the amount of organically bound Mn. Organically bound Mn is extracted with pyrophosphate (e.g., Iu et al. 1982) or hydrogen peroxide (e.g., Shuman 1979), but pyrophosphate extracts not only organically bound Mn but also oxide-forming Mn (Manh et al. 2010). Moreover, oxalate is produced by hydrogen peroxide treatment (Harada and Inoko 1977), resulting in dissolution of Mn oxides. Therefore, quantifying organically bound Mn is difficult. McBride (1982), however, showed that strong binding of Mn to organic matter is unlikely at ph < 6.0. Because the soil samples that are rich in organic matter in this study (i.e., the non-allophanic Andisols) are either moderately or strongly acidic (ph 4.43 to 5.67), the amount of organically bound Mn extracted is negligible. Thus, we approximated the amount of total Mn oxides as the Mn o value. Extraction of short-range-ordered Fe oxides Oxalate extraction was used to approximate the amounts of short-range-ordered Fe oxides according to the method described by Blakemore et al. (1987). An air-dried soil sample (0.4 g) was placed in a 50-mL polyethylene centrifuge tube, and 40 ml of oxalate

Extraction of Fe and Mn oxides in soils 703 solution (0.2 mol L 1 oxalic acid and ammonium oxalate solution, ph 3.0) was added. The mixture was mechanically shaken at 25 C for 4 h in the dark, a drop of the polymeric flocculants was added to an aliquot (about 25 ml) of the extract, and the resulting mixture was manually shaken. After centrifugation at 3500 rpm (1700 g) for 10 min, the supernatant was filtered through a 0.2-μm mesh membrane filter, and the filtrate was diluted with 1% HNO 3. Extraction of total Fe oxides Total Fe oxides were extracted with a dithionite-sodium citrate reagent. A ground, air-dried soil sample (0.4 g, < 212 μm) and dithionite powder (0.4 g) were placed in a 50-mL polyethylene centrifuge tube, 20 ml of 0.75 mol L 1 sodium citrate solution was added, and the mixture was mechanically shaken at 25 C for 16 h. Ultrapure water (20 ml) and a drop of the polymeric flocculants were added, and the resulting mixture was manually shaken. After centrifugation at 3500 rpm (1700 g) for 10 min, the supernatant was filtered through a 0.2-μm mesh membrane filter, and the filtrate was diluted with 1% nitric acid. Fe concentration in the diluted solution was measured by ICP-OES. All extractions and measurements were carried out in triplicate. The experimental conditions were based on those of Blakemore et al. (1987). Both short-range-ordered and crystalline Fe oxides should be extracted under these conditions. Therefore, in this study, we consider dithionite-citrate extractable Fe as total Fe oxides. Experiment 2: Comparison of extraction selectivity for Mn and Fe oxides for some major methods for extracting heavy metals occluded in Mn and Fe oxides Extraction with 0.5 mol L 1 NH 2 OH-HCl (ph 1.5) An air-dried soil sample (1 g) was placed in a 50-mL polyethylene centrifuge tube, 40 ml of 0.5 mol L 1 NH 2 OH-HCl (ph 1.5) solution was added, and the mixture was mechanically shaken at 25 C for 16 h. After centrifugation at 3500 rpm (1700 g) for 10 min, the supernatant was filtered through a 0.2-μm mesh membrane filter, and the filtrate was diluted with 1% nitric acid. The ph of the extract was measured with a glass electrode ph meter, and the Mn and Fe concentrations in the diluted solution were determined by ICP- OES. All extractions and measurements were carried out in triplicate. The experimental conditions were based on those used for the step 2 fraction (reducible fraction) in the modified BCR method (Sahuquillo et al. 1999). Hereafter, this NH 2 OH-HCl extractant is designated HH mbcr. Extraction with a mixture of acid oxalate and ascorbic acid The extraction reagent is acid oxalate (0.1 mol L 1 oxalic acid and 0.175 mol L 1 ammonium oxalate) plus 0.2 mol L 1 ascorbic acid (hereafter Oxa-Asc). An air-dried soil sample (1 g) was placed in a 50-mL polyethylene centrifuge tube, and 30 ml of Oxa-Asc was added. The mixture was heated in boiling water for 1 h with occasional agitation and then cooled to approximately room temperature with running water for 10 min. After centrifugation at 3500 rpm (1700 g) for 10 min, the supernatant was filtered through a 0.2-μm mesh membrane filter, and the filtrate was diluted with 1% nitric acid. The Mn and Fe concentrations in the diluted solution were determined by ICP-OES. All extractions and measurements were carried out in triplicate. The experimental conditions were based on those reported by Shuman (1985), but slightly modified as described by Sadamoto et al. (1994). The modifications involved changes to the soil: solution ratio (from 1:50 to 1:30) and the extraction time (from 30 min to 1 h). The Oxa-Asc extraction method aims to extract both short-range-ordered and crystalline Mn and Fe oxides as well as the metals occluded in these oxides. Calculation of the rates of extraction of Mn oxides and short-range ordered Fe oxides We obtained the rates of extraction of Mn and Fe oxides for the acetate, HH-A, HH mbcr, and Oxa-Asc extractants as follows: Rate of extraction of Mn oxides ¼ Mn EX Mn s 100% Mn o Mn s Rate of extraction of short-range ordered Fe oxides ¼ Fe EX 100 ð% Þ Fe o Rate of extraction of total Fe oxides ¼ Fe EX 100 ð% Þ Fe d where Mn Ex and Fe Ex are the amounts of Mn and Fe extracted by the HH-A, HH mbcr and Oxa-Asc extractants. Mn s is the amount of exchangeable and specifically adsorbed Mn extracted by the acetate solution, and Mn o and Fe o are the amounts of Mn and Fe extracted by oxalate. Fe d is the amount of Fe extracted by dithionite-citrate. Exchangeable and specifically adsorbed metals do not form oxides, and Mn s was likely extracted by the HH-A, HH mbcr, and Oxa-Asc extractants. Therefore, Mn s is subtracted from Mn extracted by the HH-A, HH mbcr,and Oxa-Asc extractants. Because the amounts of exchangeable and specifically adsorbed Fe (Fe s )werefarlessthanthe amounts of Fe extracted by the HH-A, HH mbcr and Oxa-

704 A. Suda et al. Table 2 Selected properties of soil samples Sample ph TC Fe d Al o Fe o Mn d Mn o Si o Al p Fe p Clay (H 2 O) (KCl) g kg 1 E-1 6.27 4.74 10.4 16.5 2.19 11.6 0.248 0.202 0.402 0.428 2.04 291 E-2 5.79 4.69 32.7 27.5 5.07 4.65 0.581 0.544 1.57 0.733 1.88 378 E-3 5.91 4.62 17.8 13.9 9.54 6.81 0.667 0.644 2.36 2.40 2.25 219 I-1 6.02 4.55 11.7 19.8 1.72 4.33 0.544 0.516 0.683 0.385 0.818 258 I-2 4.65 3.72 11.6 19.4 1.45 2.12 0.694 0.609 0.151 0.663 0.835 240 I-3 6.66 5.36 30.8 20.2 15.3 5.66 1.35 1.30 3.82 3.03 1.32 76.5 AA-1 7.06 5.75 42.0 55.5 52.1 18.6 1.20 0.918 24.5 3.12 0.479 279 AA-2 6.18 5.23 45.8 30.7 61.6 14.9 0.727 0.640 26.8 3.76 0.761 198 AA-3 4.71 4.22 23.5 23.9 23.0 15.6 0.644 0.581 6.18 4.49 1.60 87.0 AA-4 6.06 5.48 14.0 31.7 72.1 20.1 0.783 0.767 27.6 2.53 0.298 430 AA-5 5.28 4.89 30.0 32.4 42.4 24.0 0.551 0.535 15.2 3.12 0.355 162 NA-1 5.67 4.56 94.2 13.7 19.4 7.76 0.523 0.473 1.84 10.9 4.35 192 NA-2 4.86 3.86 183 17.5 16.3 9.82 0.170 0.141 1.45 8.67 5.85 289 NA-3 3.80 3.47 296 20.1 20.0 17.5 0.118 0.0865 2.83 11.7 9.90 219 Some data are from Suda et al. (2012). Subscripts o, d, and p indicate oxalate-, dithionite-citrate-, and pyrophosphate-extractable elements, respectively. H 2 O, water; KCl, potassium chloride; TC, total carbon; Fe, iron; Al, aluminum; Mn, manganese; Si, silicon; E, Entisols; I, Inceptisols; AA, allophanic Andisols; NA, non-allophanic Andisols. Asc extractants, as well as much less than Fe o and Fe d,we calculated the rate of extraction of Fe oxides by simply using the values of Fe extracted by the HH mbcr and Oxa-Asc extractants. RESULTS Soil properties Selected properties of the soil samples are listed in Table 2. The ph (H 2 O) values of the soil samples ranged from acidic to neutral (ph 3.80 to 7.06). Total carbon (TC) ranged from 10.4 to 296 g kg 1, and clay contents varied from 76.5 to 430 g kg 1. The amounts of Fe d ranged from 13.7 to 55.5 g kg 1, and the amounts of Fe o varied from 2.12 to 24.0 g kg 1. Dithionite-citrate extractable Mn (Mn d ) was slightly higher than Mn o for each sample. Al p and Fe p were abundant in the nonallophanic Andisol samples than other soil types. Experiment 1: Effects of increasing NH 2 OH-HCl concentration in NH 2 OH-HCl-HOAc extractant on the extractability of Mn and Fe oxides Extraction of exchangeable and specifically adsorbed Mn and Fe The amount of exchangeable and specifically adsorbed Mn was 4.66 10 2 gkg 1 on average, ranging from 2.73 10 3 to 0.243 g kg 1. The amount of exchangeable and specifically adsorbed Fe ranged from 1.98 10 3 to 0.136 g kg 1, with an average value of 3.53 10 2 gkg 1. Extracted amounts and rates of extraction of Mn and Fe for the HH 004 -A, HH 02 -A, HH 04 -A, and HH 06 -A extraction methods The amounts of Mn extracted with HH-A (Mn HHA ) were comparable to or larger than those for oxalate extraction (Table 3). Mn HHA and rates of extraction of Mn oxides increased slightly with increasing concentration of NH 2 OH-HCl (Table 4). The rates of extraction of Mn oxides for the HH 004 -A, HH 02 -A, HH 04 -A, and HH 06 -A extraction methods from all samples averaged 117%, 130%, 136%, and 150%, respectively (Table 4). The amounts of Fe extracted with the HH-A method (Fe HHA ) were highly dependent on the NH 2 OH-HCl concentration (Table 5). The amounts of Fe extracted with the HH 004 -A, HH 02 -A, HH 04 -A, and HH 06 -A methods ranged from 4.69 to 20.9 g kg 1, 9.21 to 31.3 g kg 1, 11.9 to 37.0 g kg 1 and16.9to46.4g kg 1, respectively. For non-andisol samples (except E- 1 and E-3), the amounts of Fe extracted with HH 004 -A extraction were generally greater than those for oxalate extraction but less than those for dithionite-citrate extraction. For Andisol samples, the amounts of Fe extracted with HH 004 -A extraction were less than those with both oxalate extraction and dithionitecitrate extraction. Rates of extraction of short-rangeordered Fe oxides from the Entisols, Inceptisols, allophanic Andisols and non-allophanic Andisol samples with HH 004 -A extraction averaged 133%, 158%, 74.4% and 80.0% (Table 6). Rates of extraction of total Fe oxides for the Entisols, Inceptisols, allophanic

Extraction of Fe and Mn oxides in soils 705 Table 3 Amounts of manganese (Mn) extracted with hydroxylamine hydrochloride (NH 2 OH-HCl) in acetic acid (HOAc) extractants Extracted amounts of Mn (g kg 1 ) Soil type Sample Mn o NH 2 OH-HCl concentration 0.04 mol L 1 0.2 mol L 1 0.4 mol L 1 0.6 mol L 1 Entisols E-1 0.202 0.203 0.249 0.285 0.322 E-2 0.544 0.710 0.778 0.845 0.935 E-3 0.644 0.732 0.788 0.769 0.877 Average 0.463 0.548 0.605 0.633 0.711 SD 0.232 0.300 0.309 0.303 0.339 Inceptisols I-1 0.516 0.508 0.545 0.591 0.627 I-2 0.609 0.774 0.787 0.824 0.831 I-3 1.30 1.45 1.55 1.64 1.56 Average 0.810 0.909 0.960 1.02 1.01 SD 0.431 0.484 0.523 0.549 0.490 Allophanic Andisols AA-1 0.918 0.985 1.06 1.07 1.17 AA-2 0.640 0.650 0.722 0.746 0.820 AA-3 0.581 0.591 0.656 0.685 0.742 AA-4 0.767 1.01 1.17 1.19 1.27 AA-5 0.535 0.601 0.682 0.715 0.801 Average 0.688 0.767 0.859 0.882 0.962 SD 0.155 0.211 0.240 0.233 0.244 Non-allophanic Andisols NA-1 0.473 0.531 0.564 0.561 0.664 NA-2 0.141 0.150 0.165 0.181 0.227 NA-3 0.0865 0.0895 0.127 0.137 0.175 Average 0.233 0.257 0.285 0.293 0.356 SD 0.209 0.239 0.242 0.233 0.269 Mn o denotes oxalate-extractable Mn. SD, standard deviation. Table 4 Rates of extraction of manganese (Mn) oxides with hydroxylamine hydrochloride (NH 2 OH-HCl) in acetic acid (HOAc) extractants Extraction rate of Mn oxides (%) Soil type Sample NH 2 OH-HCl concentration 0.04 mol L 1 0.2 mol L 1 0.4 mol L 1 0.6 mol L 1 Entisols E-1 100 134 160 187 E-2 140 156 172 193 E-3 114 123 120 138 Average 118 138 151 173 SD 19.9 16.6 27.1 30.4 Inceptisols I-1 139 144 141 161 I-2 96.9 111 127 140 I-3 128 130 136 137 Average 121 128 135 146 SD 21.6 16.6 6.94 12.8 Allophanic Andisols AA-1 113 122 129 114 AA-2 107 116 117 128 AA-3 102 113 117 129 AA-4 102 113 118 128 AA-5 132 153 155 167 Average 111 123 127 133 SD 12.5 16.8 16.6 19.6 (continued )

706 A. Suda et al. Table 4. Continued. Soil type Sample Extraction rate of Mn oxides (%) NH 2 OH-HCl concentration 0.04 mol L 1 0.2 mol L 1 0.4 mol L 1 0.6 mol L 1 Non-allophanic Andisols NA-1 141 162 157 165 NA-2 114 121 120 145 NA-3 108 120 132 169 Average 121 134 137 160 SD 18.0 24.2 19.0 13.2 All Average 117 130 136 150 SD 15.9 17.3 18.4 23.8 SD, standard deviation. Table 5 Amounts of iron (Fe) extracted with hydroxylamine-acetic acid (NH 2 OH-HOAc) extractants Extracted amounts of Fe (g kg 1 ) Soil type Sample Fe HHA Fe o Fe d 0.04 mol L 1 0.2 mol L 1 0.4 mol L 1 0.6 mol L 1 Entisols E-1 11.6 16.5 11.0 16.8 20.4 25.9 E-2 4.65 27.5 10.3 16.9 22.5 32.4 E-3 6.81 13.9 5.62 9.21 12.3 17.4 Average 7.67 19.3 8.97 14.3 18.4 25.2 SD 3.54 7.25 2.92 4.42 5.35 7.54 Inceptisols I-1 4.33 19.8 6.17 13.3 17.6 22.5 I-2 2.12 19.4 4.69 10.7 15.0 18.7 I-3 5.66 20.2 6.32 10.2 13.2 18.3 Average 4.04 19.8 5.73 11.4 15.2 19.8 SD 1.79 0.427 0.903 1.63 2.21 2.29 Allophanic Andisols AA-1 18.6 55.5 14.9 24.3 31.6 42.8 AA-2 14.9 30.7 9.44 15.7 19.6 26.1 AA-3 15.6 23.9 9.35 15.6 20.0 26.8 AA-4 20.1 31.7 16.4 28.1 35.7 45.5 AA-5 24.0 32.4 20.9 31.3 37.0 46.4 Average 18.7 34.8 14.2 23.0 28.8 21.2 SD 3.70 12.0 4.91 7.14 8.45 10.2 Non-allophanic Andisols NA-1 7.76 13.7 6.34 9.95 11.9 16.9 NA-2 9.82 17.5 8.89 14.7 19.5 26.4 NA-3 17.5 20.1 11.9 17.3 19.7 25.8 Average 11.7 17.1 9.03 14.0 17.0 23.0 SD 5.14 3.20 2.77 3.72 4.42 5.33 Fe o and Fe d denote oxalate-extractable Fe and dithionite-citrate-extractable Fe, respectively. SD, standard deviation. Andisols and non-allophanic Andisol samples averaged 48.2%, 28.9%, 42.6% and 52.1%. Rates of extraction of both short-range-ordered Fe oxides and total Fe oxides increased with increasing NH 2 OH-HCl concentration. When the NH 2 OH-HCl concentration was 0.6 mol L 1, the rates of extraction of total Fe oxides from the Entisols, Inceptisols, allophanic Andisols and non-allophanic Andisol samples averaged 133%, 100%, 112% and 134%. Thus, the amounts of Fe extracted with HH 06 -A extraction were comparable to those with dithionitecitrate extraction. The ph values of the extracts decreased with increasing NH 2 OH-HCl concentration (Fig. 1). Because ph values decreased during heated

Extraction of Fe and Mn oxides in soils 707 Table 6 Rates of extraction of iron (Fe) oxides with hydroxylamine hydrochloride (NH 2 OH-HCl) in acetic acid (HOAc) extractants Extraction rate of short-range ordered Fe oxides (%) Extraction rate of total Fe oxides (%) Soil type Sample NH 2 OH-HCl concentration NH 2 OH-HCl concentration 0.04 mol L 1 0.2 mol L 1 0.4 mol L 1 0.6 mol L 1 0.04 mol L 1 0.2 mol L 1 0.4 mol L 1 0.6 mol L 1 Entisols E-1 95.1 146 176 224 66.6 102 123 157 E-2 222 364 484 698 37.4 61.5 81.7 118 E-3 82.4 135 181 255 40.5 66.4 88.9 126 Average 133 215 280 392 48.2 76.6 98.0 133 SD 77.0 129 176 265 16.1 22.1 22.3 20.7 Inceptisols I-1 143 306 405 519 31.1 66.9 88.6 113 I-2 221 505 705 883 24.2 55.3 77.4 96.8 I-3 112 181 233 323 31.3 50.7 65.1 90.6 Average 158 331 448 575 28.9 57.6 77.0 100 SD 56.3 163 239 284 4.03 8.37 11.7 11.8 Allophanic Andisols AA-1 80.0 131 170 230 26.8 43.8 57.0 77.1 AA-2 63.3 106 131 175 30.8 51.3 63.8 85.1 AA-3 60.0 100 128 172 39.2 65.3 83.8 112 AA-4 81.5 139 177 226 51.8 88.7 112.8 144 AA-5 87.0 130 154 193 64.4 96.4 114.0 143 Average 74.4 121 152 199 42.6 69.1 86.3 112 SD 11.9 17.3 22.1 27.6 15.5 22.9 26.6 31.2 Non-allophanic Andisols NA-1 81.7 128 154 218 46.2 72.4 86.9 123 NA-2 90.5 149 198 269 50.9 83.9 112 151 NA-3 67.7 98.7 112 147 59.1 86.1 98.0 128 Average 80.0 125 155 211 52.1 80.8 98.8 134 SD 11.5 25.4 43.0 61.2 6.52 7.33 12.3 15.0 All Average 106 187 244 324 42.9 70.8 89.5 119 SD 53.0 120 170 220 13.9 18.1 20.4 25.0 SD, standard deviation. extraction (data not shown), the ph values after extraction were lower than before extraction in some cases. The average ph values after extraction were much higher for the allophanic Andisol samples than for the other soil types, and those for the non-allophanic Andisol samples were slightly higher than those for the Entisols and Inceptisols samples. Experiment 2: Comparison of extraction selectivity for Mn and Fe oxides for some major methods for extracting occluded heavy metals Amounts of Mn and Fe extracted with HH mbcr and Oxa-Asc Figure 1 Relationship between hydroxylamine hydrochloride (NH 2 OH-HCl) concentration and ph after extraction. The amounts of Mn extracted with HH mbcr were comparable to those extracted with oxalate (Table 7). Oxa-Asc extracted smaller amounts of Mn than oxalate did from allophanic Andisol samples.

708 A. Suda et al. Table 7 Extracted amounts and rates of extraction of iron (Fe) and manganese (Mn) with 0.5 mol L 1 hydroxylamine hydrochloride (NH 2 OH-HCl) (ph 1.5) and with acid oxalate-ascorbic acid solution Extracted amounts (g kg 1 ) Extraction rate of oxides (%) Sample Mn o Fe o Fe d Mn Fe Mn Fe (SRO ) Fe (total) gkg 1 HH mbcr Oxa-Asc HH mbcr Oxa-Asc HH mbcr Oxa-Asc HH mbcr Oxa-Asc HH mbcr Oxa-Asc E-1 0.202 11.6 16.5 0.178 0.204 8.93 22.7 82.5 101 77.2 196 54.1 138 E-2 0.544 4.65 27.5 0.662 0.457 6.76 30.8 128 79.2 145 662 24.6 112 E-3 0.644 6.81 13.9 0.658 0.541 3.82 18.1 102 83.4 56.1 266 27.6 131 Average 0.463 7.67 19.3 0.499 0.401 6.50 23.9 104 87.9 92.9 375 35.4 127 SD 0.232 3.54 7.25 0.278 0.176 2.56 6.39 22.8 11.6 46.7 251 16.3 13.4 I-1 0.516 4.33 19.8 0.473 0.388 3.33 22.4 90.6 72.0 76.8 518 16.8 113 I-2 0.609 2.12 19.4 0.545 0.451 1.22 21.4 82.5 57.1 57.4 1008 6.29 111 I-3 1.30 5.66 20.2 1.37 1.03 3.10 20.0 105 78.5 54.7 354 15.3 99.2 Average 0.810 4.04 19.8 0.797 0.622 2.55 21.3 92.8 69.2 63.0 627 12.8 108 SD 0.431 1.79 0.427 0.500 0.352 1.16 1.20 11.6 11.0 12.1 340 5.68 7.44 AA-1 0.918 18.6 55.5 0.722 0.158 1.65 15.8 78.2 15.5 8.88 84.7 2.97 28.4 AA-2 0.640 14.9 30.7 0.470 0.430 1.38 18.1 73.1 66.8 9.27 121 4.50 58.9 AA-3 0.581 15.6 23.9 0.518 0.352 0.837 20.8 89.0 59.9 5.37 134 3.50 87.2 AA-4 0.767 20.1 31.7 0.818 0.397 2.71 14.9 107 51.6 13.5 74.1 8.56 47.1 AA-5 0.535 24.0 32.4 0.448 0.266 1.81 17.4 83.6 49.2 7.53 72.3 5.58 53.6 Average 0.688 18.7 34.8 0.595 0.321 1.68 17.4 86.1 48.6 8.90 97.2 5.02 55.0 SD 0.155 3.70 12.0 0.165 0.110 0.686 2.29 12.9 19.8 2.97 28.3 2.21 21.4 NA-1 0.473 7.76 13.7 0.437 0.523 1.42 18.0 92.1 111 18.3 232 10.3 131 NA-2 0.141 9.82 17.5 0.0730 0.156 1.73 26.3 31.1 115 17.6 267 9.91 150 NA-3 0.086 17.5 20.1 0.0701 0.121 2.08 22.4 76.7 149 11.8 128 10.3 111 Average 0.233 11.7 6.50 0.193 0.266 1.74 22.2 66.6 125 15.9 209 10.2 131 SD 0.209 5.14 2.56 0.211 0.222 0.328 4.15 31.7 20.9 3.54 72.5 0.246 19.5 Subscripts o and d indicate oxalate- ad dithionite-citrate- extractable elements, respectively. HHmBCR: 0.5 mol L 1 NH2OH-HCl (ph 1.5) extraction. Oxa-Asc: acid oxalate-ascorbic acid extraction. SRO: Short-range ordered Fe oxides. Mn, manganese; Fe, iron; SD, standard deviation.

Extraction of Fe and Mn oxides in soils 709 The amounts of Fe extracted from all samples (except E-2), especially Andisol samples, were smaller with HH mbcr than with oxalate. Oxa-Asc extracted much greater amounts of Fe from non-andisol samples than oxalate did; for some samples, the amount of Fe extracted was comparable to Fe d. However, the amounts of Fe extracted from allophanic Andisol samples were comparable to Fe o and much less than Fe d. Oxa-Asc extracted smaller amounts of Fe from allophanic Andisol samples than from other soil samples, despite an abundance of Fe o and Fe d in the allophanic Andisol samples. Amounts of Al and Si extracted with HH 004,HH 06, HH mbcr, and Oxa-Asc The amounts of Al extracted from the soil samples with the four extractants followed the order Oxa-Asc > HH 06 -A > HH 004 -A HH mbcr (Fig. 2). The amounts of Al extracted from the Andisol samples were greater than those extracted from the non-andisol samples. The order for Si was Oxa-Asc > HH 06 -A > HH 004 -A = HH mbcr (Fig. 3). Greater amounts of Si were extracted from the allophanic Andisol samples than from the other soil samples. DISCUSSION Relationships between extractability of Mn and Fe and NH 2 OH-HCl concentration The amounts of Mn extracted with HH-A was comparable to or greater than those extracted with oxalate and gradually increased with increasing NH 2 OH-HCl concentration. Because Mn ions can be incorporated into crystalline Fe oxides (Ford et al. 1997), the small increase in the amounts of extracted Mn might be caused by the release of isomorphous-substituted Mn from the crystal lattice of these oxides. More Mn was extracted by HH 06 -A than even Mn d from most of the soil samples. This indicated that Mn was released from Fe oxides, such as coarse magnetite, which were relatively insoluble in dithionite-citrate. Most of the oxalate-extractable Mn is considered to form oxides. Therefore, we believe that the HH-A method extracted total Mn oxides from all soil samples used in this study. For the non-andisol samples, the amounts of Fe extracted with HH 004 -A were the same as or greater than those extracted with oxalate. Most of the oxalate-extractable Fe originates from short-range ordered Fe oxides (Parfitt and Childs 1988). Therefore, HH-A completely extracted short-range-ordered Fe oxides from the non-andisol samples, assuming that Figure 2 Amount of aluminum (Al) extracted from soil samples with different extraction methods. HH mbcr,hh 004 -A, HH 06 -A, and Oxa-Asc denote extraction with 0.5 mol L -1 hydroxylamine hydrochloride (NH 2 OH-HCl) (ph 1.5), 0.04 mol L -1 NH 2 OH-HCl in 25% acetic acid (HOAc), 0.6 mol L -1 NH 2 OH-HCl in 25% HOAc, and a mixture of acid oxalate and ascorbic acid, respectively. Bars represent means ± standard deviation (SD) (n = 3).

710 A. Suda et al. Figure 3 Amount of silicon (Si) extracted from soil samples with different extraction methods. HH mbcr,hh 004 -A, HH 06 -A, and Oxa-Asc denote extraction with 0.5 mol L -1 hydroxylamine hydrochloride (NH 2 OH-HCl) (ph 1.5), 0.04 mol L -1 NH 2 OH-HCl in 25% acetic acid (HOAc), 0.6 mol L -1 NH 2 OH-HCl in 25% HOAc, and a mixture of acid oxalate and ascorbic acid, respectively. Bars represent means ± standard deviation (SD) (n = 3). short-range ordered Fe oxides were extracted prior to most of crystalline Fe oxides. The smaller amounts of Fe were extracted from the Andisol samples with HH 004 -A than with oxalate (Tables 5 and 6). A substantial percentage of short-range ordered Fe oxide (allophanic samples: 13 40%, non-allophanic samples: 9 32%) was not extracted with HH 004 -A. Because volcanic ashes contain magnetite, and oxalate extracts part of the oxide, the amounts of short-range ordered Fe oxides in samples of slightly weathered (i.e. low Fe o ) volcanic ash soils might be overestimated (Shoji et al. 1987). However, in a preliminary experiment, we found that more Fe was extracted from synthesized magnetite (Kanto Chemical Co., Inc., Tokyo, Japan) with HH 004 -A than with oxalate. Therefore, we believe that, for our Andisol samples, some short-range ordered Fe oxides remained after extraction, with or without magnetite in the samples. La Force and Fendorf (2000) reported that HH 004 -A extraction was insufficient for complete dissolution of crystalline Fe oxides in some polluted Fe-rich sediments. However, our study revealed that HH 004 -A extraction would not extract a relatively large percentage of short-range ordered Fe oxides from non-fe-polluted allophanic Andisol samples. For non-allophanic Andisol samples, Fe o and Fe d may include a measurable amount of Fe extracted from Fe humus complex, as indicated by the high content of Fe p (Table 2). This additional amount of Fe possibly led to underestimation of the rates of extraction of short-range ordered and total Fe oxides of the non-allophanic Andisol samples. Extraction of Fe oxides by NH 2 OH-HCl is highly dependent on extraction ph (Chao 1972). The ph values for the Andisol samples (allophanic samples: 2.43, non-allophanic samples: 2.15, on average) were higher than those for the non-andisol samples (Entisols: 2.04, Inceptisols: 1.97, on average). Dissolution of abundant allophane-like minerals or amorphous Al oxides in allophanic Andisol samples might consume great amounts of protons during extraction. A decrease in proton concentration resulted in a decrease in the rates of both reduction and acid dissolution of Fe oxides. In addition, short-range ordered Fe oxides are more abundant in the Andisol samples than in the other soil samples. Therefore, larger amount of Fe oxides might result in a shortage of NH 2 OH-HCl as a reducing agent during extraction. Stoichiometrically, when proton consumption by reduction of Fe 3+ is compensated by hydrolysis of NH 2 OH-HCl, 0.8 mmol of NH 2 OH-HCl (i.e., 20 ml of 0.04 mol L 1 solution) can reduce only 0.4 mmol (22.3 mg) of Fe 3+. The same solution can reduce double the amounts (0.8 mmol,

Extraction of Fe and Mn oxides in soils 711 44.7 mg) of Fe 3+, assuming that all proton consumption is compensated by H + from HOAc. The amount of NH 2 OH-HCl in the extractant was stoichiometrically sufficient to reduce all short-range-ordered Fe oxides. However, because 4.22 to 19.1 mg of Fe was extracted by 20 ml of 0.04 mol L 1 extractant, NH 2 OH-HCl concentration during extraction should be remarkably decreased. Stoichiometric calculation also indicates that Tessier s condition (20 ml of 0.04 mol L 1 NH 2 OH-HCl for 1 g of sample) cannot reduce all Fe oxides from relatively Fe-rich, non-polluted soil samples (e.g., AA-1 contains greater than 44.7 mg of Fe d per gram of air-dried soil sample). Moreover, since hydroxylamine (NH 2 OH) itself reacts with O 2 and is oxidized on heating, the net decrease in NH 2 OH-HCl during extraction would be greater than that calculated stoichiometrically. It might, however, be significantly important and not surprising that the original Tessier s condition potentially has a severe shortage of NH 2 OH-HCl to extract Fe oxides from soil samples in Japan. On the basis of the foregoing discussion, the concentration of NH 2 OH-HCl should be increased. Greater amounts of Fe were extracted with HH 02 -A, HH 04 -A, and HH 06 -A than with HH 004 -A (Table 5). Because NH 2 OH was oxidized on heating and HCl remained in solution, extraction ph decreased with increasing NH 2 OH-HCl concentration (Fig. 1). Therefore, this acceleration of Fe extraction was caused by the increase in both proton concentration and amount of reducing agent. On average, more Fe was extracted with HH 02 -A and HH 04 -A than with HH 004 -A, and less Fe was extracted with HH 02 - A and HH 04 -A than with dithionite-citrate. However, the amounts of Fe extracted with HH 06 -A were comparable to those extracted with dithionite-citrate. Therefore, most of the Fe oxides in the soil samples were extracted with HH 06 -A. The amount of Fe extracted with HH 06 -A was larger than that with dithionite-citrate. This indicates that HH 06 -A extracts even from hardly-soluble Fe phases, such as coarse particles of crystalline Fe oxides. Among our soil samples, AA-1 contained the largest amount of Fe oxides (Table 2). A dataset of 151 Andisol samples (24 soil profiles) studied by Kurobokudo Co-operative Research Group (1986) indicates that about 85% of Andisol samples contained smaller amounts of Fe oxides than AA-1. Moreover, in general, Andisols (Andosols) contain much greater amounts of Fe than other soil types in Japan (Takeda et al. 2004). Hence, 0.6 mol L 1 NH 2 OH-HCl seems to be a sufficiently high concentration of NH 2 OH- HCl to extract almost all Fe oxides in most Japanese soils. Lower ph is also favorable for suppressing re-adsorption of metals during extraction (Gómez-Ariza et al. 1999). However, lower ph may accelerate the extraction of non-target phases such as Al and Si minerals. Therefore, we discuss the extraction selectivity of the HH 06 -A method for Mn and Fe oxides in the next section. Selectivities of HH 004,HH 06,HH mbcr, and Oxa-Asc extractions for Fe oxides The amounts of Al and Si extracted with the four extractants followed the order Oxa-Asc > HH 06 -A > HH 004 -A HH mbcr for Al (Fig. 2) and Oxa-Asc > HH 06 -A > HH 004 - A=HH mbcr for Si (Fig. 3). Oxa-Asc-extractable Al seems to include Al from the Al-humus complex, especially for the non-allophanic Andisol samples. For the other soil samples, much more Al was extracted with Oxa-Asc than with the other extractants, although Al p values were small. Hence, we concluded that Oxa-Asc is a more destructive extractant for the Al and Si minerals than the other extractants. Less Al and Si were extracted with HH 06 -A than with Oxa-Asc, and more Al and Si were extracted with HH 06 -A than with HH 004 and HH mbcr. Heavy metal concentrations in Mn and Fe oxides are much higher than those in bulk soils (McKenzie 1989; Siposet al. 2011). Therefore, the concentrations of heavy metals in Al and Si minerals are probably much lower than those in Mn and Fe oxides. However, Al and Si minerals more or less contain heavy metals in their lattices (Kitagawa 1973; Churakov and Dähn 2012), and the amounts of the metals derived from these non-target phases would increase as their dissolution proceeds during extraction. Hence, extraction selectivity for heavy metals in Fe oxides over Al and Si minerals is important if extracted metals are defined as a reducible or Mn and Fe oxide occluded fraction. However, this point has not been studied in detail. Because it is difficult to determine the amounts of heavy metals extracted from Fe oxides or from Al and Si minerals, we used Al/Fe and Si/Fe ratios as indicators for extraction selectivity. Because the amounts of Mn oxides were much less than those of Fe oxides, and most of the Mn oxides were extracted with each extraction method, Mn is ignored in the following discussion. As judged by the selectivities indicated by the Al/Fe and Si/Fe ratios, HH 004 -A and HH 06 -A were better extractants than HH mbcr and Oxa-Asc (Table 8). Because of its relatively strong acidity, HH mbcr extracts large amounts of Al. The results for the nonallophanic Andisol samples indicate that substantial amounts of Al were extracted from the Al-humus complex, although this extraction has been carried out before organically bound heavy metals are extracted in the modified BCR SEP. In contrast, HH mbcr extracted very small amounts of Fe from the Andisol samples, resulting in remarkably high Al/Fe ratios in these samples, being consistent with the results of

712 A. Suda et al. Table 8 Comparison of rates of extraction of iron (Fe) and manganese (Mn) oxides, and aluminum (Al)/Fe and silicon (Si)/Fe mole ratios, for four extraction methods Extraction methods Extraction rate of oxides (%) Mole ratio Soils Mn Fe (SRO ) Fe (total) Al/Fe Si/Fe HH mbcr Entisols 104 92.9 35.4 1.42 0.341 Inceptisols 92.8 63.0 12.8 2.98 0.766 Allophanic 86.1 8.90 50.2 17.7 4.86 Non-allophanic 66.6 15.9 10.2 12.5 0.967 Average 87.5 45.2 27.2 8.65 1.73 SD 15.8 39.9 19.1 7.77 2.10 Oxa-Asc Entisols 87.9 375 127 1.23 0.425 Inceptisols 69.2 627 108 1.35 0.450 Allophanic 48.6 93.3 62.6 5.31 1.43 Non-allophanic 125 209 131 2.34 0.392 Average 82.7 326 107 2.56 0.673 SD 32.5 231 31.3 1.90 0.503 HH 004 -A Entisols 118 133 48.2 0.644 0.354 Inceptisols 121 158 28.9 1.06 0.619 Allophanic 111 74.4 42.6 3.00 0.493 Non-allophanic 121 80.0 52.1 2.18 0.294 Average 118 111 43.0 1.72 0.440 SD 4.69 40.8 10.1 1.07 0.146 HH 06 -A Entisols 173 392 133 1.02 0.280 Inceptisols 146 575 100 1.16 0.339 Allophanic 133 199 112 2.27 0.268 Non-allophanic 160 211 134 2.14 0.308 Average 153 345 120 1.65 0.299 SD 17.1 177 16.7 0.647 0.0315 SRO: Short-range ordered Fe oxides. HH mbcr : 0.5 mol L 1 hydroxylamine hydrochloride (NH 2 OH-HCl) (ph 1.5) extraction. Oxa-Asc: acid oxalate-ascorbic acid extraction. HH 004 -A, HH 06 -A: Extraction with 0.04 mol L 1 and 0.6 mol L 1 NH 2 OH-HCl in 25% acetic acid (HOAc), respectively. SD, standard deviation. Suda et al. (2012). Oxa-Asc extracted large amounts of Fe except for the allophanic Andisol samples. Unexpectedly, Oxa-Asc extracted much smaller amounts of Fe than Fe d from the allophanic Andisol samples. This result may be due to a lack of oxalate; however, more detailed studies might be needed. The Al/Fe and Si/Fe ratios for HH 06 -A extraction, as well as for HH 004 -A extraction, were lower than those for HH mbcr and Oxa-Asc extraction, especially for Andisol samples. Hence, we concluded that the selectivity of HH 06 -A extraction was comparable to that of Tessier s HH 004 -A extraction and better than that of HH mbcr and Oxa-Asc extractions. CONCLUSION Tessier s HH 004 -A extraction method might be unsuitable for allophanic Andisol samples because substantial amounts of short-range ordered Fe oxides are not extracted by this method. Higher extraction ph and relatively large amounts of short-range ordered Fe oxides might be responsible for incomplete extraction. For all soil samples in this study, a considerable amount of crystalline Fe oxides remained after HH 004 -A extraction. However, increasing the concentration of NH 2 OH-HCl increased the amounts of extracted Mn and Fe, and 0.6 mol L 1 of NH 2 OH-HCl was sufficient to extract almost all Mn and Fe oxides from the soil samples. The selectivity of the HH 06 -A method for Fe oxides over Al and Si minerals was comparable to or better than that of the original Tessier s method (HH 004 -A). In contrast, the selectivities of the HH mbcr and Ox-Asc methods were much lower than those for the HH-A extractions (Tessier s extraction, HH 06 -A extraction), especially when the HH mbcr method was applied to both allophanic and non-allophanic Andisol samples and when the

Extraction of Fe and Mn oxides in soils 713 Oxa-Asc method was applied to allophanic Andisol samples. Therefore, the HH 06 -A extraction method seems to be better than the HH 004 -A, HH mbcr, and Oxa-Asc extraction methods, especially for Japanese Andisol samples. ACKNOWLEDGMENTS We acknowledge Dr. S. Ono, Dr. T. Ito (Tohoku University), Dr. K. Kamewada, Dr. A. Aoki (Tochigi Prefecture Agricultural Experiment Station) and Dr. Y. Nagasaki (National Agricultural Research Center for the Western Region) for soil sampling. We also thank Dr. S. Wakabayashi (Agricultural Research Center) and Dr. J. Takahashi (University of Tsukuba) for providing some of the soil samples used in this study and for providing physicochemical data for some of the samples. We also thank Dr. Y. Maejima and Dr. I. Akahane (National Institute for Agro- Environmental Sciences) and Dr. K. Tamura (University of Tsukuba) for helpful comments. 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