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Testing a new method for sequential silicon extraction on soils of a temperate--humid climate.


Silicon (Si) has important functions for biogeochcmical processes, for marine and terrestrial biota, as a buffer against soil acidification, and as a regulator of atmospheric carbon dioxide (Derry et al. 2005). Chemical and biological processes in soils lead to the formation of various pedogenic Si phases. However, their contribution to the global Si cycle has not been quantified (Sommer et al. 2006). Knowledge about crystalline minerals is well advanced, but quantitative data on other Si compounds in soils (mobile and adsorbed Si, Si in organic matter and pedogenic oxides, amorphous silica) arc rare (Sauer et al. 2006).

Investigations of silica in soils have focused mainly on climatic regions where soil horizons cemented by silica (silcretes) occur, such as in Australia (Langford-Smith 1978; Milnes and Twidale 1983; Summerfield 1983; Milnes et al. 1991) and across the Mediterranean (Meyer and Pena Dos Reis 1985; Rodas et al. 1994). Silica accumulations have also been studied in soils on volcanic ash, for example in central Mexico (Miehlich 1991; Elsass et al. 2000). In these soils, the short-range-order mineral allophane can also be found (Wada 1989). Imogolite, a fibrous mineral with a slightly higher degree of order, occurs primarily in Andosols of the humid tropics (Wada et al. 1972; Gcbhardt 1976) but also in B horizons of Podzols (Farmer et al. 1980; Dahlgren and Ugolini 1989; Lundstrom et al. 2000). Most silica studies have been carried out on soils of the tropics and subtropics (Pfisterer 1991; Alexandre et al. 1997), whereas Si enrichment and dynamics in soils of humid-temperate climate have been investigated less extensively.

Recently, we developed a sequential method for quantifying different Si phases in soils, based on a series of extraction experiments on well-characterised, isolated soil compounds and selected soil samples (Gcorgiadis et al. 2013). The method extracts, stepwise, various Si fractions, from the most mobile to the most immobile fraction. The sequentially extracted Si phases include: (I) mobile Si (extracted by Ca[Cl.sub.2] solution); (2) adsorbed Si (extracted by acetic acid); (3) Si bound to soil organic matter (released by [H.sub.2][O.sub.2]); (4) Si included in pedogenic oxides and hydroxides (released by NH4-oxalate under UV radiation); (5) Si in biogenic amorphous silica (extracted by density separation and subsequent NaOH extraction of the light fraction); (6) Si in minerogenic amorphous silica, precipitated by processes other than biological (calculated as NaOH-extractable Si in whole fine-earth sample minus NaOH-extractablc Si in light fraction after density fractionation). The share of Si in crystalline silicates is calculated as the difference between total Si (as determined by fusion of subsamples with lithium borate, dissolution in nitric acid and inductively coupled plasmaoptical emission spectroscopy (1CP-OES) analysis or by X-ray fluorescence analysis) and the sum of the Si fractions extracted in steps 1-6 above.

Details of the extraction procedure are described by Georgiadis et al. (2013). In the present paper, we present the results of the first application of this sequential extraction method to complete soil profiles.

Materials and methods

Six soil profiles from four petrographically different areas in south-west Germany, a region characterised by a humid-temperate climate, were selected for the first runs of the new sequential extraction procedure, both to evaluate the method and to quantify the different Si-pools in the soil profiles. Soils exhibiting a wide range of properties differing in relief position, lithology, mineral composition, and soil pH were selected for this test. In total, 42 mineral horizons and six organic horizons were analysed. The six soil profiles included (FAO Guidelines for Soil Description 2006):

* Two Podzols from the northern Black Forest (Seebach area), located on the same slope (~15[degrees]) on Triassic sandstone, under fir and spruce forest. One of the Podzols is on the upper slope and has a dominant E horizon; the other is on the footslope and has a dominant spodic horizon. Mean air temperature is 6.5[degrees]C, and mean annual precipitation is 1935 mm (Sommer 2002).

* Two Cambisols in the southern Black Forest (Wildmooswald area), located on the same slope on gneiss, under spruce forest. One of the Cambisols is on the upper slope (~3.5[degrees]); the other is on the lower slope and is a Cambisol (Chromic), characterised by lateral enrichment in iron oxides. Mean air temperature is 6[degrees]C, and mean annual precipitation is 1600 mm (Fiedler et al. 2002).

* A Luvisol on loess in the Kraichgau region, ~2 km south-west of Hclmsheim, located on a plateau, under woodruff-beech forest; mean air temperature is 9[degrees]C, and mean annual precipitation is 700 mm (Bleich et al. 1990).

* A Stagnosol on periglacially reworked loess, ~5 km south-east of Pforzheim (Lettenbach area), located on a plateau, under a mixed forest of spruce, fir and beech; mean air temperature is 7[degrees]C, and mean annual precipitation is 987 mm (Ehmann 1989).

The soil samples (Table 1) were air-dried and passed through a 2-mm sieve. Particle-size distribution was determined by wet sieving (sand fractions) and pipette method (silt and clay) (Schlichting et al. 1995). The clay mineral composition (<2 [micro]m) was determined by X-ray diffraction analysis with Cu Ka radiation (Siemens D 500-Difffactometer; Siemens AG, Munich). Clay mineral percentages (Fig. 1) were calculated by use of the software Diffrac AT 3.3, integrating the peak areas (program EVA 2.3; Socabim, Paris). The different Si fractions were separated by the sequential Si extraction method developed by Georgiadis et al. (2013), and Si, aluminium (Al) and iron (Fe) were analysed in the extracts by ICP-OES. All analyses were executed on two replicates. Selected samples were additionally analysed by optical microscope, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) after extraction steps 5 and 6 to evaluate the extractants with regard to their efficiency in dissolving bio-opal and their aggressiveness against crystalline minerals.

The software products SPSS 13.0 (SPSS Inc., Chicago) and SigmaPlot 11.0 (Systat Software Inc., San Jose, CA) were used for statistical analyses, which included correlation and regression analysis and Spearman rank correlation coefficient ([r.sub.s]) for data with non-linear distribution.


Relative contribution of the different Si fractions to total Si

Most of the Si (~86-99%) in the soils was bound in primary and secondary silicates (residual Si). In the mineral soil horizons, Si in the minerogcnic amorphous phase contributed the second largest amount of Si to the total Si, making up ~0.1-1.5% of total Si (Fig. 2). In some organic horizons, this phase was exceeded by Si from the biogenic amorphous phase. Silicon from minerogcnic amorphous silica exhibited maxima in the organic layers, in or below the lower Bs horizon of the Podzols, in the Bt horizons of the Luvisol, and in the Cg horizons of the Stagnosol. Silicon from biogenic amorphous silica reached maximum values in the organic layers. Silicon in pedogenic oxides and hydroxides and Si in soil organic matter each exhibited values usually amounting to 0.1-0.2% of total Si. The E horizon of the Luvisol showed a slight minimum and its Bt horizons a slight maximum of Si occluded in pedogenic oxides and hydroxides. The depth curve of Si in soil organic matter (SOM) in the Podzols was similar to that of Si in mincrogenic amorphous silica, but at an overall lower level of Si content. The proportions of mobile and adsorbed Si fractions both made up <0.01% of the total Si in most samples. These two fractions reached their highest values in the Bt horizons of the Luvisol and the Cg horizons of the Stagnosol.

Mobile and adsorbed Si

The amounts of mobile and adsorbed Si (extracted with Ca[Cl.sub.2] and acetic acid, respectively) were very low in the six soil profiles, tending to increase with depth (Table 2). Mobile Si contents varied between 0.3 and 28[micro]g [g.sup.-1] fine earth, whereas contents of adsorbed Si ranged from 1.2 to 39 [micro]g [g.sup.-1]. In the Podzols and Cambisols, Al extracted by Ca[Cl.sub.2] and acetic acid was usually considerably higher than Si, whereas in the Luvisol and Stagnosol it was lower than that Si.

Silicon in SOM

Silicon in SOM ranged between 0.04 and 0.9 mg [g.sup.-1] fine earth. Greater amounts of SOM-bound Si were found in the Oa horizons of the Podzols and Cambisols (0.2-0.9 mg [g.sup.-1] fine earth) and in the Ah horizon of the Luvisol (0.7 mg [g.sup.-1]). The Podzols and the Luvisol exhibited a minimum of SOM-bound Si in their E horizons and an increase below. In the Stagnosol, this pattern was less pronounced. No trend with soil depth was observed in the Cambisols.

Silicon occluded in pedogenic oxides and hydroxides Amounts of oxalate-extractable Si ranged between 0.01 and 1.7 mg g 1 fine earth, and tended to increase with depth in all soil profiles. Oxalate-extractable Al showed the same trend with depth. The highest A1 content (7.2 mg [g.sup.-1]) was found in the CB horizon of the Cambisol, with the lowest A1 in the E horizons of the Podzols (~0.3-0.7 mg [g.sup.-1]) and the Oa horizon of the Cambisol (0.09 mg [g.sup.-1]). Depth curves for Si and A1 ran parallel to each other in almost all soil profiles. The lowest Si content was found in the Podzol E horizons (upper slope 0.01-0.04mgg footslope 0.02-0.04 mg [g.sup.-1]). In the Stagnosol, contents of oxalate-extractable Si increased with depth from 0.1 to 0.3 mg [g.sup.-1] whereas in the Cambisols and Luvisol, most horizons contained 0.3-0.7 mg [g.sup.-1]. The Cambisols exhibited the highest content of oxalate-extractable Fe, 10-12 mg [g.sup.-1] in the A and B horizons of the Cambisol on the upper slope and 19-35 mg g 1 in the A and B horizons of the Cambisol (Chromic) on the lower slope.

Silicon in biogenic amorphous silica

Amounts of Si in biogenic amorphous silica were in the range 0.1-4.7mg [g.sup.-1] fine earth in the O horizons, 0.02-0.7 mg [g.sup.-1] in the mineral topsoil horizons (A and E horizons), and 0.002-0.08 mg [g.sup.-1] in the B horizons. The greatest amounts of Si in biogenic amorphous silica were generally found in the Oa horizons, with the maximum (4.7 mg [g.sup.-1], corresponding to 14% of total Si) occurring in the Oa horizon of the Cambisol on the upper slope. Aluminium content extracted in this step was very low in all samples (0.3 [micro]g [g.sup.-1] -0.25 mg [g.sup.-1]), confirming that silicates did not significantly contribute to extracted Si.

Silicon in minerogenic amorphous silica

Silicon in minerogenic amorphous silica was extracted in the last step of the sequential extraction, using NaOH at room temperature. It is the largest fraction of extractable Si in all mineral soil horizons and in almost all organic horizons. The highest content was measured in the Oa horizons of the Cambisols (~10 mg [g.sup.-1]), whereas the lowest was found in the mineral soil horizons of the Podzols, ranging from 0.2 to 1.2 mg [g.sup.-1], and tending to increase with depth. The amount of this fraction in the mineral soil horizons of the other soils ranged between 2 and 5 mg [g.sup.-1] (except for the Ck horizon of the Luvisol with 1.3 mg [g.sup.-1]-). Aluminium content extracted by NaOH was usually somewhat lower than Si content; however, in the B horizons of the Cambisols, it reached the same order of magnitude as Si content.


Mobile and adsorbed Si

A close relationship ([r.sub.s] = 0.812**) between the amounts of mobile Si (step I: Ca[Cl.sub.2]) and adsorbed Si (step 2: acetic acid) and the very similar depth curves of these two fractions indicate a dynamic equilibrium and possible overlap between them. It seems likely that some loosely adsorbed Si is already desorbed in the first step of the sequential extraction. This is not specifically a problem of the extraction method; in soils themselves, there is no sharp boundary between mobile and loosely adsorbed Si. Depending on the research question, there are two options to handle this problem: (;) mobile Si can be defined as Ca[Cl.sub.2]-extractable Si, and adsorbed Si can be defined as acetic-acid-extractable Si, keeping in mind that the boundary is somewhat artificial, and that acetic acid does not completely desorb strongly adsorbed Si (shown in previous tests; Georgiadis et al. 2013); (ii) both fractions together can be defined as easy-to-mobilise Si in soils if the research question docs not require separation of the two fractions.

The Podzols and Cambisols exhibited a positive correlation between soil pH and amounts of mobile (Ca[Cl.sub.2]-extractable) Si ([r.sub.s] = 0.761 **). The interpretation of the effect of pH on mobile Si in these soils is problematic, however, because of their narrow pH range (Table 1). The soils on loess (Luvisol and Stagnosol) with both higher pH and wider pH range than the Cambisols and Podzols did not exhibit this correlation. Schachtschabel and Heinemann (1967) did not observe a clear relationship between water-soluble Si and pH in soils on loess; they reported a positive correlation for water-soluble Si and soil organic carbon (SOC). This correlation was not observed in this study; only mobile Al and SOC were positively correlated ([r.sub.s] = 0.765**).

The mobile and adsorbed Si fractions were both positively correlated with clay content ([r.sub.s] = 0.705** and 0.468**, respectively). Maximum amounts of mobile and adsorbed Si occurred in the Bt horizons of the Luvisol and in the dense subsoil horizons of the Stagnosol. This could not have been due to Si release from clay minerals, because previous extraction tests indicated that clay minerals are not attacked by extraction with 0.01 m acetic acid. Also, previous sorption-desorption tests have shown that clay minerals (mainly smectite) adsorb considerable amounts of monomeric silicic acid (Georgiadis et al. 2013). Hence, based on the results of previous tests and the clear relationship between easy-to-mobilise Si and clay content observed in this study, we conclude that the clay fraction acts as the main adsorbent for silica in soils.

Several authors have demonstrated that frequent changes in redox conditions may increase Si-release (Morris and Fletcher 1987; Sommer 2002). Indeed, in this study also, the Stagnosol showed the highest content of Ca[Cl.sub.2]-extractable Si. Silicon itself is not redox-sensitive, but Si adsorbed to or occluded in iron oxides may be released when iron oxides arc dissolved by iron reduction. Significant positive relationships were observed between mobile Si and oxalate-extractable Si ([r.sub.s] = 0.746**), Al ([r.sub.s] = 0.315*), and Fe ([r.sub.s] = 0.455**), and between adsorbed Si and oxalate-extractable Fe ([r.sub.s] = 0604**). These results are in agreement with those of several authors who reported adsorption of silica to sesquioxides in soils (McKeague and Cline 1963a, 1963ft; Beckwith and Reeve 1964). In addition, correlations of mobile and adsorbed Si with Si from minerogenic amorphous silica ([r.sub.s] = 0.814** and 0.609**) suggest that amorphous silica represents another important source for casy-to-mobilise Si, possibly not only through dissolution of silica but also through desorption of silicic acid that is reversibly adsorbed onto the surface of amorphous silica.

Alexandre et al. (1997) considered bio-opal as an important source for dissolved silica in soil. However, in our study no correlation between dissolved or adsorbed Si with Si from biogenic amorphous silica was found.

The increase in casy-to-mobilise (mobile + adsorbed) Si with depth indicates a vertical transport of this Si fraction downwards, or a decrease in the weathering front. The Podzols from the Secbach area show the lowest amounts of easy-to-mobilise Si compared with the other soils. This may be because sandstone contains only small proportions of easily weatherable silicates, whereas gneiss contains much higher proportions. Moreover, the Seebach area is characterised by higher amounts of rainfall and runoff than the other areas. Sommer (2002) showed that the Si depletion in the area is positively correlated with runoff.

Silicon in SOM

Amounts of Si released by [H.sub.2][O.sub.2] treatment exhibited no relationship to SOM content, in contrast to Al, which was positively correlated with SOM ([r.sub.s] = 0.715**). There was also no relationship between Si and Al released by [H.sub.2][O.sub.2] treatment. These results suggest that Al is dominantly released from SOM, whereas some of the Si is extracted from another source. Previous extraction tests demonstrated that [H.sub.2][O.sub.2] treatment may lead to a release of considerable amounts of Si from smectite; somewhat less Si is released from kaolin and microcline. In addition, some amorphous silica may be dissolved (Georgiadis et al. 2013).

The observed maxima of [H.sub.2][O.sub.2]-extractable Si in the O and A horizons suggest that biogenic amorphous silica can be another important source of Si release in addition to SOM and clay minerals. This notion is confirmed by the positive correlation of [H.sub.2][O.sub.2]-extractable Si and Si from both biogenic and minerogcnic amorphous silica ([r.sub.s] = 0.826** and 0.536**, respectively) and clay content ([r.sub.s] = 0.640**). Previous extraction tests demonstrated that hot [H.sub.2][O.sub.2] also attacks sesquioxides (Georgiadis et al. 2013). Silicon released by [H.sub.2][O.sub.2] and oxalate-extractable Si were positively correlated ([r.sub.s] = 0.505**), suggesting an overlap of the extracted fractions. Considerable amounts of [H.sub.2][O.sub.2]-extractable Si in the C horizons of the studied soils also indicate Si release from silicates as an effect of [H.sub.2][O.sub.2] treatment.

Silicon extracted in step 3 of the sequential extraction ([H.sub.2][O.sub.2]) thus cannot be entirely interpreted as Si bound to SOM, since [H.sub.2][O.sub.2] also attacks clay minerals, sesquioxides and amorphous silica. This may, moreover, lead to underestimation of Si in the latter fractions because they are extracted later in the sequential extraction procedure. Hence, extraction step 3 needs to be improved in order to destroy all SOM without dissolving any of the subsequent Si fractions.

Silicon occluded in pedogenic oxides and hydroxides

Oxalate-extractable Si and Al in the Podzols on sandstone and Cambisols on gneiss exhibited a strong positive correlation to each other (Fig. 3) ([r.sub.s] = 0.924** and [r.sub.s] = 0.909**, respectively); this correlation was not found in the soils on loess, however. A positive correlation ([r.sub.s] = 0.564**) between oxalate-extractable Si and Fe existed only for the Podzols on sandstone. These results suggest that silica is occluded or co-precipitated in or with Al oxides/hydroxides rather than Fe oxides/hydroxides. Allophane- or imogolite-like compounds in the Podzols might contribute to this fraction as well, since it is well known that oxalate extracts allophane and imogolite from soils (Wada 1989; Kodama and Ross 1991). However, these short-range-order minerals are characterised by Si: Al molar ratios between 1 : 1 and 1 : 3 (Wada and Greenland 1970; Wada 1989; Breuer 1994), whereas in this study, Si: Al molar ratios in the oxalate extracts ranged from 0.02 to 0.5. It is concluded, therefore, that the positive relationships between the Si, Al and Fe contents in the Podzols reflect the extraction of both allophane-like compounds and Si-Al-Fe-oxyhydroxide co-precipitates (Veerhoff and Briimmcr 1993).

Silicon from biogenic amorphous silica

Silicon extracted from bio-opal decreased with depth in the soil profiles and was positively correlated with SOM content ([r.sub.s] = 0.479**). Similar patterns were reported by Saccone et al (2008) from soils of the Hubbard Brook Experimental Forest in the United States and by Alexandre et al. (1997) from soils under tropical rain forests. In the Podzols and Cambisols, most bio-opal-Si is restricted to the organic layers and mineral topsoil horizons; however, considerable amounts of bio-opal Si were found as deep as 60 cm in the Luvisol on loess and in the Stagnosol on loess-like sediments, indicating vertical transport of bio-opal, as described by Fishkis et al. (2010). Bio-opal may be transported downwards, for example in vertical macropores such as epigeic earthworm channels, like clay particles during lessivation. Illuviation, bioturbation, and wind-throw are probably the main processes that influence the vertical distribution of bio-opal in soils (Saccone et al. 2008).

The SEM micrographs of samples after density separation with sodium polytungstate ([rho] = 2.3 g [cm.sup.-3]) demonstrated a problem that occurred in this step of the sequential extraction: The light fraction containing the biogenic amorphous silica was contaminated with silt- and clay-size minerals, e.g. clay minerals, quartz, and mica (Fig. 4). Nevertheless, when studying biogeochemical Si cycling, it does not seem appropriate to exclude the clay fraction and obtain biogenic amorphous silica only from a certain particle-size fraction (a common procedure if phytoliths are separated for the purpose of identification for vegetation reconstruction), because bio-opal in the clay fraction, with its high surface: volume ratio, is thought to play an important role in biogeochemical Si cycling (Conley et al. 2006). Therefore, further efforts are needed to develop a method for proper separation of biogenic amorphous silica, including the portion associated with the clay-sized fraction.

Optical and scanning electron microscopy showed that biogenic amorphous silica is not completely dissolved during extraction step 5 with 0.2 M NaOH. Some partly dissolved phytoliths were still visible after the extraction (Fig. 5b). in addition, EDS analyses indicated considerable amounts of Si remaining (Fig. 5d). This observation implies that the amounts of Si from biogenic amorphous silica are underestimated, whereas those of minerogenic amorphous silica are overestimated, because the latter are calculated by subtracting Si in biogenic amorphous silica from total amorphous silica Si (obtained by NaOH-extraction applied to the whole fine-earth sample in step 6). This problem apparently affected the results of this study. In general, plant opal is supposed to accumulate in the topsoils (Golyeva 1999), whereas the contents of minerogenic amorphous silica should increase in the subsoils. The results of the sequential extraction, however, suggested that minerogenic amorphous silica prevailed over biogenic amorphous silica in several topsoils (Table 2), which is not realistic. We conclude that the sample (g) to solution (mL) ratio of 1 : 400, as used in this study for steps 5 and 6 of the sequential Si-extraction method, was not great enough to completely dissolve biogenic amorphous silica. The relevant criterion is the ratio of extractable Si to extractant, which is close to 1 : 400 in a more-or-less pure sample of biogenic amorphous silica (after density fractionation), but much greater than 1 : 400 in a whole-soil sample to which a soil: extractant ratio of 1 : 400 is applied. Therefore, this ratio is too low to completely dissolve biogenic amorphous silica from the light fraction (step 5 of the sequential extraction), whereas it is suitable for extracting Si from amorphous silica from the whole fine-earth sample (step 6).

The amounts of bio-opal Si were positively correlated with amounts of Si from minerogenic amorphous silica ([r.sub.s] = 0.599**). The Si: Al molar ratios of the two fractions were very different in the subsoils (~60 for the biogenic fraction and 1-3 for the minerogenic fraction) but similar to each other in the topsoils (10-90 for the biogenic fraction and 10-60 for the minerogenic fraction). This result confirms that the amounts of Si extracted from the topsoils in steps 5 and 6 of the sequential Si extraction were both mainly from biogenic amorphous silica. In the subsoils, different Si: Al ratios of both fractions indicate a more complete separation of bio-opal (or its absence) and minerogenic amorphous silica.

The maximum amounts of total amorphous silica (sum of steps 5 and 6) were found in the topsoils of the Cambisols in the Wildmooswald area. This result most likely reflects the dense ground vegetation in this area, including abundant horsetail (Equisetum), a well-known Si-accumulator plant (Marschner 1995). The smallest amounts of amorphous silica were found in the Podzols of the Seebach area, which is covered by spruce-fir forest. Silicon cycling through conifers is low compared with that through deciduous trees (Geis 1973; Klein and Geis 1978; Bartoli 1985; Hodson et al. 2005; Comclis et al. 2010); conifers also dominate in the Lettenbach area. These differences underline the importance of vegetation for understanding and characterising Si dynamics in landscapes.

Minerogenic amorphous silica

The efficiency of extraction step 6 (NaOH extraction of the whole fine-earth sample) was tested as follows. After NaOH extraction, the topsoil samples were subjected to density separation in order to test for remaining biogenic amorphous silica (Georgiadis 2011). Very small amounts of light fraction were obtained, indicating that amorphous silica was almost completely dissolved by extraction step 6. Very few strongly etched phytolith remnants were identified under SEM. Thus, dissolution of biogenic amorphous silica was incomplete in step 5 but complete in step 6, which implies that some of the Si from biogenic amorphous silica was measured in the minerogenic amorphous silica fraction.

Characterisation of the minerogenic amorphous silica fraction is more difficult than of biogenic amorphous silica. X-Ray amorphous siliceous substances occur especially in the fine clay fraction of many soils, for example as an intermediate product during transformation of clay minerals (Gebhardt 1976). SEM examination of selected samples after NaOH extraction was also used to check for possible etching of silicates. Visual inspection by SEM suggested that 0.2 m NaOH applied at room temperature (25[degrees]C) seems not to attack crystalline minerals in the samples; for example, weathered feldspar (albite) and clay mineral ncoformations (kaolinite) showed no visible effect (Fig. 6).

Si and A1 were released at different rates during the extraction. It was observed that A1 release may still continue after Si release has come to an end (Georgiadis 2011). This observation suggests that some proportion of these two elements is not released from the same source. For example, NaOH may additionally extract Al bound to SOM, if the latter was not completely dissolved during step 3 of the sequential extraction. It may also extract allophane-like substances that were not completely extracted by the oxalate extraction step (Wada 1989; Breuer 1994). These potential additional sources of NaOH-extractable Al in soils demonstrate that the Al content in NaOH extracts of soil samples cannot be used to reliably estimate silicate dissolution by NaOH. This is only possible in sediments containing no potential Al sources other than silicates (Foster 1953).

Silicon release during NaOH extraction of all loess soil samples ceased after 120h of extraction. Ratios of Si: Al in the extracts of mineral horizons were usually ~2.5, with Si and Al contents closely related to each other ([r.sub.s] = 0.856**, n = 16) and to the clay fraction ([r.sub.s] = 0.893** for Si and [r.sub.s] = 0.851** for Al). A maximum in NaOH-extractable Si was observed in the Bt horizons of the Luvisol. Saccone et al. (2007) reported similar results for a Luvisol on loess from south-west Germany. Because our previous tests confirmed that 0.2 m NaOH at room temperature does not attack crystalline silicates, we conclude that mainly amorphous and poorly crystalline clay compounds were extracted from the Bt horizons of the Luvisol in this study.

Residual fraction

The Si of the residual fraction comprises Si in primary and secondary minerals. A major part of residual Si was related to the sand fraction ([r.sub.s] = 0.577**), whereas Al of the residual fraction originated mainly from the clay ([r.sub.s] = 0.693**) and fine silt ([r.sub.s] = 0.670**) fractions.

Maximum amounts of residual Si were found in the E horizons of the Podzols, followed by the E horizon of the Luvisol. Podzol E horizons are characterised by residual enrichment of quartz (and thus Si), because other minerals (containing Al and other elements) are generally strongly weathered in these horizons. Luvisol E horizons arc also characterised by residual enrichment of quartz, clay minerals having migrated downwards.


A sequential extraction method for Si in soils developed by Georgiadis et al. (2013) was tested on six typical soil types from south-west Germany. These results lead to the following conclusions:

* As expected for the comparatively young soils investigated here (developed since the Pleistocene), most of the Si is still bound in primary and secondary silicates.

* The second-highest amounts of Si are found in precipitates of minerogenic amorphous silica; in the organic layers, however, biogenic amorphous silica may contribute the second-greatest proportion of Si. The amount of Si in the form of biogenic amorphous silica depends on the vegetation present and can be up to 14% of the total Si content in Oa horizons of the studied soils. Clayey horizons (subsoil horizons of a Luvisol and a Stagnosol) accumulate minerogenic amorphous silica.

* The lowest Si amounts are found in the mobile and adsorbed Si fractions. They depend on the contents of clay, sesquioxides and amorphous silica in the soil. Correlations of these fractions with soil pH and SOC were not observed. Rainfall and soil hydrology appear to have a strong influence on the mobile and adsorbed Si fractions.

* Oxalate-extractable Si comprises Si occluded in Fe and Al oxides/hydroxides, Si from Si-Al-Fe-oxyhydroxidc precipitates, and some Si from short-range order silicates.

* The Si fraction extracted by [H.sub.2][O.sub.2] in step 3 of the sequential Si extraction cannot be clearly attributed to Si bound in SOM, since [H.sub.2][O.sub.2] may lead to some Si release from clay minerals, sesquioxides and amorphous silica as well.

Despite some limitations of extraction steps 3 (aggressiveness of [H.sub.2][O.sub.2]) and 5 (contamination of the light fraction by minerals and incomplete dissolution of bio-opal due to a too-low bio-opal: solution ratio), the sequential Si extraction method enabled separation of important Si fractions in soils of a temperate-humid climate.


This study was funded by the German Research Foundation (DFG) as project STA 146/48-1 'Development of a method for fractionating analysis of Si in soils', which was project 1 of the project package 'Multiscale analysis of Si cycling in terrestrial biogeosystems'. We are also grateful to the Alfred Toepfer Stiftung F.V.S. for financial support. Special thanks go to Beate Podtschaske for her diligent work for this project in the field and laboratory and to Kathleen Regan for correcting the English of the manuscript.

Received 20 January 2014, accepted 29 May 2014, published online 10 October 2014


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Anna Ceorgiadis (A,D), Daniela Sauer (B), Ludger Herrmann (A), Jorn Breuer (C), Mehdi Zarei (A), and Karl Stahr (A)

A Institute of Soil Science and Land Evaluation, University of Hohenheim, Emil-Wolff-Str. 27, D-70599 Stuttgart, Germany.

B Institute of Geography, Dresden University of Technology, Helmholtz-Str. 10, D-01069 Dresden, Germany.

C Agricultural Technology Centre Augustenberg, Section 12 Agroecology, Nesslerstr. 23-31, D-76227 Karlsruhe, Germany.

D Corresponding author. Email:

Table 1. Main properties of analysed soils
n.d., Not determined

Horizon     Depth        Bulk density     []
             (cm)      (g [cm.sup.-3])        (%)

 Podzol on sandstone, upper slope (Seebach area,
              northern Black Forest)

Oi/Oe      +11/+18           0.40            33.45
Oa         +10/+17           0.78            15.00
AE          -8/-15           1.38             2.28
EA         -34/-40           1.52             0.66
E          -411-52           n.d.             0.23
Eg          -56/64           n.d.             0.50
EB          -68/73           n.d.             1.32
Bsl        -78/-83           n.d.             0.54
Bs2        -941-99           n.d.             0.73
BC        -120/-130          n.d.             0.66
2C          -140>            n.d.             0.31

  Podzol on sandstone, footslope (Seebach area,
             northern Black Forest)

Oi/Oe        +17             n.d.            11.50
Oa           +15             0.69             5.93
EA         -18/-23           1.18             1.90
EA/Bsl       -33             1.33             1.40
Bsl        -60/-66           1.24             1.30
Bs2        -75/-85           1.39             1.00
C          -90/-94           1.60             0.67

  Cambisol on gneiss, upper slope (Wildmooswald
           area, southern Black Forest)

Oi           +11             n.d.            46.10
Oe            +9             0.21            43.35
Oa            +6             n.d.            33.00
Ah            -4             0.69            10.70
AB/B         -24             0.80             4.91
Bw           -40             0.97             2.93
BC           -55             1.11             1.54
CB           -74             1.42             0.67

    Cambisol (Chromic) on gneiss, lower slope
    (Wildmooswald area, southern Black Forest)

Oi          +6/+13           0.17            43.80
Oe          +5/+12           n.b.            35.40
Oa          +3/+5            n.b.            18.90
Ah          -3/-9            0.66             7.99
BAg         -11/14           1.03             3.97
Bg           -38             0.79             2.76
BCg        -58/-63           1.19             0.96
CBg          -79             n.b.             1.00

    Luvisol on loess, plateau (Helmsheim area)

Ah            -5             0.87             7.66
E            -24             1.47             0.68
Btl        -50/-65           1.66             0.20
Bt2          -77             1.62             0.14
Bt3          -100            1.61             0.18
Ck           -150            1.4              0.11

    Stagnosol on periglacially reworked loess,
            plateau (Lettenbach area)

Ah         -10/-14           1.08             2.08
El         -23/-33           1.42             0.57
E2           -58             1.55             0.22
E3           -81             1.59             0.17
Cgl          -98             1.64             0.15
Cg2          -120            1.62             0.14
Cg3          -140            1.66             0.19
Cg4          -150            n.d.             0.17

Horizon        pH         Sand    Silt    Clay
          Ca[Cl.sub.2]             (%)

  Podzol on sandstone, upper slope (Seebach
        area, northern Black Forest)

Oi/Oe         n.d.        n.d.    n.d.    n.d.
Oa             3.1        73.0    19.7     7.3
AE             3.1        84.9    11.2     3.9
EA             3.2        87.1    10.8     2.1
E              3.3        83.3    13.7     3.0
Eg             3.3        75.1    12.7    12.2
EB             3.5        78.4     9.1    12.5
Bsl            4.0        88.4     8.0     3.6
Bs2            3.9        81.1    14.6     4.3
BC             4.0        73.4    17.3     9.3
2C             4.2        77.7    14.2     8.0

 Podzol on sandstone, footslope (Seebach area,
            northern Black Forest)

Oi/Oe         n.d.        n.d.    n.d.    n.d.
Oa             3.1        78.2    16.3     5.5
EA             3.2        74.4    16.3     9.3
EA/Bsl         3.3        69.7    15.4    14.9
Bsl            3.8        68.2    17.8    14.0
Bs2            4.2        64.7    18.8    16.4
C              4.1        69.6    17.4    13.0

Cambisol on gneiss, upper slope (Wildmooswald
         area, southern Black Forest)

Oi            n.d.        n.d.    n.d.    n.d.
Oe            n.d.        n.d.    n.d.    n.d.
Oa             3.1        36.1    25.2    38.7
Ah             3.2        51.3    24.3    24.4
AB/B           3.6        54.0    24.5    21.5
Bw             4.1        54.1    25.9    20.0
BC             4.3        52.2    30.3    17.5
CB             4.3        60.8    26.1    13.0

  Cambisol (Chromic) on gneiss, lower slope
  (Wildmooswald area, southern Black Forest)

Oi            n.d.        n.d.    n.d.    n.d.
Oe            n.d.        n.d.    n.d.    n.d.
Oa             3.4        36.3    29.7    34.0
Ah             3.4        38.5    33.4    28.0
BAg            3.7        44.5    33.1    22.5
Bg             3.8        43.0    31.3    25.7
BCg            3.9        49.3    31.1    19.6
CBg            3.9        54.6    28.6    16.8

  Luvisol on loess, plateau (Helmsheim area)

Ah             5.3         6.0    82.3    11.7
E              3.9         5.9    78.0    16.1
Btl            4.1         5.2    65.7    29.1
Bt2            4.4         4.4    69.9    25.7
Bt3            4.8         3.9    74.7    21.3
Ck             7.5         3.7    83.9    12.4

  Stagnosol on periglacially reworked loess,
          plateau (Lettenbach area)

Ah             3.7         6.9    79.3    13.8
El             3.8         7.7    78.1    14.2
E2             4.0         5.9    75.9    18.2
E3             4.3         4.0    70.5    25.5
Cgl            4.5         3.3    73.4    23.3
Cg2            4.7         3.5    75.3    21.2
Cg3            5.0         3.5    75.4    21.1
Cg4            5.1         3.2    74.5    22.3

Table 2. Results of the sequential silicon (Si), aluminium (A1), and
iron (Fe) extraction of analysed soils

[Si.sub.1], [Al.sub.1]: Mobile Si, Al; [], []:
adsorbed Si, Al; [], []: Si, Al in soil organic
matter; [Si.sub.occ], [A1.sub.occ], [Fe.sub.occ]: Si, Al, Fe occluded
in pedogenic oxides; [], []: Si, Al from biogenic
amorphous silica; [], []: Si, Al from minerogenic
amorphous silica; [Si.sub.res], [Al.sub.res]: Si, Al in the residual
soil sample after the sequential extraction; [Si.sub.t], [Al.sub.t]:
total Si, Al

Horizon    Depth     [Si.sub.l]   [Al.sub.l]   []
            (cm)                   ([micro]g [g.sup.-1])

 Podzol on sandstone, upper slope (Seebach area, northern
                      Black Forest)

Oi/Oe     +11/+18       n.d.         n.d.         n.d.
Oa        +10/+17       2.23        38.78          3.02
AE         -8/-15       0.98        18.19          3.25
EA        -34/-40       0.34         9.96          4.70
E         -47/-52       0.36        10.84         15.33
Eg        -56/-64       1.55        31.63         10.42
EB        -68/-73       2.71        49.14          1.23
Bsl       -78/-83       2.90        17.40          3.57
Bs2       -94/-99       3.38        20.51          1.57
BC        120/-130      5.04        20.49          5.88
2C         -140>        6.41        10.75         13.59

 Podzol on sandstone, footslope (Seebach area, northern
                       Black Forest)

Oi/Oe       +17         n.d.         n.d.         n.d.
Oa          +15         2.33        66.30          2.25
EA        -18/-23       2.69        42.33          2.57
EA/Bsl      -33         5.61        65.34          2.20
Bsl       -60/-66       8.94        47.96          5.73
Bs2       -75/-85      12.24        18.19         17.35
C         -90/-94       9.98        18.65         17.03

  Cambisol on gneiss, upper slope (Wildmooswald area,
                 southern Black Forest)

Oi          +11         n.d.         n.d.          n.d.
Oe           +9         n.d.         n.d.          n.d.
Oa           +6         9.73         31.63        10.15
Ah           -4         5.53        154.28         6.25
AB/B        -24         7.73         99.68         5.10
Bw          -40        12.83         25.68        16.75
BC          -55        12.93          8.63        20.55
CB          -74        13.66          9.74        20.94

Cambisol (Chromic) on gneiss, lower slope (Wildmooswald
              area, southern Black Forest)

Oi         +6/+13       n.d.         n.d.         n.d.
Oe         +5/+12       n.d.         n.d.         n.d.
Oa         +3/+5       11.84        117.68        11.03
Ah         -3/-9        9.54        115.56         5.15
BAg       -11/-14       8.95         75.45         4.53
Bg          -38        11.23         57.44         7.31
BCg       -58/-63       8.88         53.43        10.51
CBg         -79        11.96         44.48        16.09

       Luvisol on loess, plateau (Helmsheim area)

Ah           -5        12.95         5.39         13.91
E           -24        12.45        49.39          6.34
Btl       -50/-65      28.24        12.74         19.50
Bt2         -77        25.98         2.58         22.32
Bt3         -100       23.83         0.84         25.29
Ck          -150       11.99         0.63         14.99

  Stagnosol on periglacially reworked loess, plateau
                  (Lettenbach area)

Ah        -10/-14       7.00        27.68          5.80
El        -23/-33       9.38        29.78          6.00
E2          -58        21.55         9.55         15.00
E3          -81        26.08         0.38         21.50
Cgl         -98        25.18         0.00         23.70
Cg2         -120       24.50         0.00         28.50
Cg3         -140       24.80         0.00         34.55
Cg4         -150       26.35         0.38         39.60

Horizon   []   []   []   [Si.sub.occ]

   Podzol on sandstone, upper slope (Seebach area, northern
                          Black Forest)

Oi/Oe        n.d.           n.d.           n.d.           n.d.
Oa           14.95          0.23           0              0.07
AE            2.40          0.06           0.19           0.01
EA            2.65          0.04           0.02           0.01
E            11.21          0.05           0.02           0.01
Eg           12.31          0.10           0.04           0.02
EB           21.27          0.07           0.25           0.04
Bsl          42.20          0.10           0.00           0.04
Bs2          14.62          0.12           0.03           0.08
BC           29.63          0.17           0.08           0.16
2C           77.64          0.24           0.01           0.21

    Podzol on sandstone, footslope (Seebach area, northern
                           Black Forest)

Oi/Oe        n.d.           n.d.           n.d.           n.d.
Oa            8.86          0.29           0.04           0.04
EA            4.15          0.12           0.11           0.02
EA/Bsl        6.56          0.29           0.30           0.04
Bsl          14.50          0.25           0.42           0.07
Bs2          49.76          0.29           0.14           0.43
C            71.94          0.25           0.07           0.19

 Cambisol on gneiss, upper slope (Wildmooswald area, southern
                          Black Forest)

Oi           n.d.           n.d.           n.d.           n.d.
Oe           n.d.           n.d.           n.d.           n.d.
Oa           15.90          0.76           0.76           0.03
Ah           25.90          0.28           0.28           0.14
AB/B         17.70          0.12           0.12           0.35
Bw           33.60          0.36           0.36           0.56
BC           42.15          0.42           0.42           0.99
CB           26.52          0.40           0.40           1.75

 Cambisol (Chromic) on gneiss, lower slope (Wildmooswald area,
                    southern Black Forest)

Oi           n.d.           n.d.           n.d.           n.d.
Oe           n.d.           n.d.           n.d.           n.d.
Oa           28.25          0.91           0.91           0.27
Ah           14.69          0.36           0.36           0.33
BAg           9.03          0.19           0.19           0.35
Bg            8.73          0.14           0.14           0.32
BCg           6.17          0.21           0.21           0.51
CBg          13.24          0.29           0.29           0.64

          Luvisol on loess, plateau (Helmsheim area)

Ah           9.66           0.70           0.36           0.29
E            2.92           0.37           0.17           0.35
Btl          1.86           0.60           0.07           0.66
Bt2          1.48           0.51           0.05           0.68
Bt3          1.19           0.41           0.04           0.64
Ck           0.43           0.07           0.00           0.36

      Stagnosol on periglacially reworked loess, plateau
                       (Lettenbach area)

Ah           7.65           0.39           0.60           0.11
El           3.80           0.33           0.24           0.13
E2           1.10           0.34           0.06           0.22
E3           1.20           0.40           0.04           0.26
Cgl          1.90           0.39           0.03           0.28
Cg2          2.05           0.35           0.02           0.30
Cg3          1.55           0.33           0.01           0.28
Cg4          1.10           0.42           0.02           0.30

Horizon   [Al.sub.occ]   [Fe.sub.occ]   []   []
                                                      (mg [g.sup.-1])

  Podzol on sandstone, upper slope (Seebach area, northern
                        Black Forest)

Oi/Oe         n.d.           n.d.          0.56          0.02
Oa            2.93           1.97          1.81          0.16
AE            0.74           0.42          0.058         0.002
EA            0.26           0.09          0.021         0.001
E             0.26           0.28          0.006         0.001
Eg            0.82           1.05          0.004         0.001
EB            2.00           1.22          0.002         0.001
Bsl           1.66           2.12          0.002         0.001
Bs2           2.05           1.57          0.004         0.005
BC            2.62           2.10          0.004         0.007
2C            2.30           1.17          0.003         0.004

    Podzol on sandstone, footslope (Seebach area, northern
                         Black Forest)

Oi/Oe         n.d.           n.d.          0.18          0.002
Oa            1.56           2.14          0.12          0.004
EA            0.94           1.63          0.016         0.000
EA/Bsl        1.66           7.17          0.002         0.000
Bsl           2.90           6.52          0.002         0.001
Bs2           4.57           4.32          0.007         0.014
C             3.11           2.88          0.002         0.002

 Cambisol on gneiss, upper slope (Wildmooswald area, southern
                         Black Forest)

Oi            n.d.           n.d.          3.52          0.17
Oe            n.d.           n.d.          1.00          0.04
Oa            0.09           0.36          4.72          0.17
Ah            1.64          11.08          0.69          0.13
AB/B          3.18          11.70          0.01          0.002
Bw            5.65           9.64          0.007         0.001
BC            7.09           8.10          0.008         0.002
CB            7.21           4.78          0.014         0.004

 Cambisol (Chromic) on gneiss, lower slope (Wildmooswald area,
                      southern Black Forest)

Oi            n.d.           n.d.          0.49          0.01
Oe            n.d.           n.d.          0.30          0.02
Oa            2.04          19.10          2.18          0.25
Ah            2.55          24.36          0.08          0.01
BAg           3.14          31.76          0.007         0.001
Bg            4.95          35.40          0.011         0.008
BCg           4.11          22.45          0.008         0.001
CBg           4.73          14.52          0.009         0.0003

           Luvisol on loess, plateau (Helmsheim area)

Ah            1.47           4.51          0.61          0.05
E             2.30           4.93          0.07          0.005
Btl           3.73           6.79          0.08          0.012
Bt2           3.05           5.92          0.02          0.003
Bt3           2.18           5.23          0.05          0.008
Ck            0.70           1.61          0.02          0.0003

       Stagnosol on periglacially reworked loess, plateau
                        (Lettenbach area)

Ah            1.02           4.36          0.16          0.006
El            1.31           4.87          0.09          0.008
E2            1.69           5.09          0.08          0.014
E3            1.76           4.30          0.02          0.001
Cgl           1.57           4.11          0.02          0.001
Cg2           1.34           3.72          0.02          0.003
Cg3           1.16           3.19          0.02          0.001
Cg4           1.40           3.28          0.04          0.006

Horizon   []   []   [Si.sub.res]   [Al.sub.res]

   Podzol on sandstone, upper slope (Seebach area, northern
                          Black Forest)

Oi/Oe        n.d.          n.d.           n.d.           n.d.
Oa           1.18          0.00           289             11
AE           1.46          0.05           418             ii
EA           0.36          0.04           428             ii
E            0.23          0.02           419             20
Eg           0.56          0.22           400             31
EB           0.57          0.27           382             32
Bsl          0.38          0.10           411             20
Bs2          0.39          0.17           408             22
BC           0.72          0.41           391             30
2C           0.84          0.52           397             27

   Podzol on sandstone, footslope (Seebach area, northern
                         Black Forest)

Oi/Oe        n.d.          n.d.           n.d.           n.d.
Oa           1.67          0.08           380             22
EA           1.01          0.06           400             26
EA/Bsl       0.73          0.36           385             35
Bsl          0.64          0.93           376             37
Bs2          1.20          1.11           366             45
C            1.04          0.64           378             40

Cambisol on gneiss, upper slope (Wildmooswald area, southern
                         Black Forest)

Oi           n.d.          n.d.           n.d.           n.d.
Oe           n.d.          n.d.           n.d.           n.d.
Oa           10.15         0.36           110             27
Ah            2.62         0.89           233             53
AB/B          2.86         2.53           252             61
Bw            3.01         2.98           276             62
BC            2.98         2.86           279             62
CB            3.02         2.49           279             74

  Cambisol (Chromic) on gneiss, lower slope (Wildmooswald
                area, southern Black Forest)

Oi           n.d.          n.d.           n.d.           n.d.
Oe           n.d.          n.d.           n.d.           n.d.
Oa           7.96          0.94           186             36
Ah           3.91          1.55           239             56
BAg          2.74          2.17           261             60
Bg           2.30          2.63           271             66
BCg          2.81          2.32           284             69
CBg          3.07          2.39           282             77

Luvisol on loess, plateau (Helmsheim area)

Ah           2.96          0.14           284             33
E            2.26          0.49           329             42
Btl          4.77          1.82           299             54
Bt2          4.17          1.50           303             56
Bt3          3.32          1.05           305             55
Ck           1.32          0.29           212             34

Stagnosol on periglacially reworked loess, plateau (Lettenbach area)

Ah           2.86          0.51           275             39
El           2.43          0.84           335             42
E2           3.57          1.59           308             51
E3           4.43          1.90           315             58
Cgl          3.97          1.68           329             58
Cg2          3.78          1.38           325             57
Cg3          3.69          1.58           307             57
Cg4          3.69          1.60           333             58

Horizon   [Si.sub.t]   [Al.sub.t]   [Fe.sub.t]

  Podzol on sandstone, upper slope (Seebach
         area, northern Black Forest)

Oi/Oe        152           6.0         1.4
Oa           293           14          2.8
AE           420           12          1.5
EA           429           11          1.1
E            419           21          2.1
Eg           400           32          5.6
EB           383           34          5.8
Bsl          412           22          5.2
Bs2          409           25          5.6
BC           392           33          8.0
2C           399           30          8.5

  Podzol on sandstone, footslope (Seebach
       area, northern Black Forest)

Oi/Oe        344           16           2.6
Oa           382           23           4.8
EA           401           27           5.1
EA/Bsl       386           37          13.2
Bsl          377           41          13.8
Bs2          368           50          14.3
C            380           44          10.9

      Cambisol on gneiss, upper slope
(Wildmooswald area, southern Black Forest)

Oi            24.8         4.7          2.5
Oe            30.6         6.1          3.4
Oa           125          29.1         14.9
Ah           237          58.5         35.6
AB/B         255          69.5         39.3
Bw           280          72.6         35.1
BC           283          72.6         34.8
CB           285          76.8         37.3

 Cambisol (Chromic) on gneiss, lower slope
 (Wildmooswald area, southern Black Forest)

Oi            27.7         6.2          4.6
Oe            76.2        20.0         11.5
Oa           197          45.4         28.2
Ah           244          61.8         42.1
BAg          264          64.3         52.0
Bg           274          70.2         59.1
BCg          288          72.3         51.9
CBg          286          80.3         37.6

Luvisol on loess, plateau (Helmsheim area)

Ah           289          35.0         13.5
E            332          44.9         18.8
Btl          305          59.3         30.1
Bt2          308          60.2         30.7
Bt3          309          58.7         29.3
Ck           214          35.2         16.2

 Stagnosol on periglacially reworked loess,
         plateau (Lettenbach area)

Ah           278          40.8         18.1
El           338          44.3         18.9
E2           312          54.1         28.5
E3           321          62.0         35.7
Cgl          334          61.6         33.8
Cg2          330          60.0         33.4
Cg3          312          59.7         33.8
Cg4          338          61.5         34.2
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Author:Georgiadis, Anna; Sauer, Daniela; Herrmann, Ludger; Breuer, Jorn; Zarei, Mehdi; Stahr, Karl
Publication:Soil Research
Article Type:Report
Date:Oct 1, 2014
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