Soil Science Society of America Journal 65:133-138 (2001)
© 2001 Soil Science Society of America
DIVISION S-4-SOIL FERTILITY & PLANT NUTRITION
Soil Boron Fractions and Their Relationship to Soil Properties
J.M. Xua,
K. Wanga,
R.W. Bellb,
Y.A. Yanga and
L.B. Huangb
a Department of Resource Sciences, College of Resource and Environmental Sciences, Zhejiang University, Hangzhou, Zhejiang 310029, PR China
b School of Environmental Science, Murdoch University, Murdoch, WA 6150, Australia
Corresponding author (rbell{at}central.murdoch.edu.au)
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ABSTRACT
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An understanding of soil nutrient pools and their relationship to soil properties and to soil test values should underpin soil tests, but few studies of this type have been conducted for soil B. Boron was fractionated by sequential extraction in 13 soils collected from north (47°N) to south (20°N) in eastern China. The nonspecifically adsorbed B (NSA-B) and specifically adsorbed B (SPA-B) comprised <1% of total B. By contrast, B occluded in Mn oxyhydroxide (MOH-B), in amorphous Fe and Al oxides (AMO-B) and in crystalline Fe and Al oxides (CRO-B) comprised from 0.01 to 7.6% of total B. The content of the NSA-B fraction significantly decreased with increasing mean annual rainfall of the site and increased with increasing soil pH and exchangeable Ca. The MOH-B fraction was positively correlated with soil pH and cation-exchange capacity (CEC), and negatively with rainfall and temperature. The AMO-B fraction was significantly related to amorphous Fe2O3 and rainfall. The CRO-B fraction was positively correlated with pH and exchangeable Ca, but not with crystalline Fe2O3. The SPA-B fraction was not correlated with any soil properties or climate factors. These results emphasize that the forms of B in Chinese soils were distinctly different from those in soils of southeast USA and Greece.
Abbreviations: AMO-B, B in amorphous Fe and Al oxides • CEC, cation-exchange capacity • CRO-B, B in crystalline Fe and Al oxides • CR-Al2O3, crystalline Al2O3 • CR-Fe2O3, crystalline Fe2O3 • Ex-Ca, exchangeable Ca • ICP-AES, inductively coupled plasma atomic emission spectrometry • MOH-B, B occluded in Mn oxyhydroxide • NSA-B, nonspecifically adsorbed B • OM, organic matter • RES-B, residual B • SPA-B, specifically adsorbed B
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INTRODUCTION
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BORON is distributed in various soil components, including soil solution, organic matter (OM), and minerals. Boron in soil solution is readily available for plant uptake, but this pool constitutes <3% of total soil B (Jin et al., 1987; Tsalidas et al., 1994). Maintaining B in the soil solution is important for plant nutrition (Keren and Bingham, 1985a, 1985b), and it is controlled by the pools of B in other soil fractions and their equilibration with the soil solution.
A variety of factors such as pH, OM, clay minerals, Fe and Al oxides, carbonates, and tillage management may change the content of extractable B, and transformations among different soil B fractions (Jin et al., 1987; Mandal et al., 1993; Hou et al., 1994; Tsalidas et al., 1994; Yermiyahu et al., 1995). The content of water-soluble B in soils tends to increase with soil pH, but not always in a consistent manner (Tsalidas et al., 1994), probably because B adsorption by soil components also increases with the increase of pH and reaches a maximum in the alkaline pH range (Gu and Lowe, 1990; Goldberg et al., 1993, 1996). In practice, liming soils may result in a significant decrease of B uptake presumably because of increased B sorption (Lehto and Malkonen, 1994). In China, soils developed on loessial materials generally contain moderate amounts of total B, a very high content of acid insoluble B, and a very low content of water-soluble B (Liu et al., 1990). The clay soils rich in OM were low in plant-available B in eastern Canada (Simard et al., 1996). Such investigations emphasize that the B distribution among different fractions and transformations among them induced by soil amendment and management will affect B availability to plants.
Various fractionation techniques have been developed for soil B using methods originally developed for selective dissolution of trace metals, in which a particular fraction of the element might be removed by a specific extractant (Jin et al., 1987; Tsalidas et al., 1994; Hou et al., 1994, 1996). In these previous schemes for soil B fractionation, soil B was generally differentiated into water-soluble forms, nonspecifically and specifically adsorbed forms, Fe–Al and Mn oxide–bound forms, and residual fractions. Only Hou et al. (1994)(1996) have considered it necessary to include OM-bound forms in their B fractionation procedure, and their results showed no correlation between humic acid content of the synthetic soil and either organically bound B or other fractions extracted.
Hot water-extractable B has been regarded as a suitable index of plant-available B (Bingham, 1982), but in some studies, levels of hot water-extractable B have not been correlated with plant response (Sims and Johnson, 1991), suggesting that a better understanding is needed of the pools of soil B accessed by common soil B tests, and their relationship to plant B uptake (Bell, 1997). Moreover, the available forms of soil B vary with plant species (Tsalidas et al., 1994). Jin et al. (1987) found that the B concentration in corn tissue correlated positively with not only water-soluble B, but also nonspecifically adsorbed B, specifically adsorbed B, and Mn oxyhydroxide-occluded B. Tsalidas et al. (1994) showed that B content in olive tree (Olea europaea L.) leaves was well correlated with amorphous Fe–Al oxyhydroxide-occluded B, specifically adsorbed B, and Mn oxyhydroxide-occluded B besides water-soluble B. By contrast in barley (Hordeum vulgare L.) leaves, B content was correlated with nonspecifically adsorbed B as well as amorphous Fe–Al oxyhydroxide-occluded B, specifically adsorbed B, and water-soluble B, but not with Mn oxyhydroxide-occluded B (Tsalidas et al., 1994). Therefore, a better understanding of the distribution of B in various soil fractions and their relationships with plant response would provide a basis for assessing the availability of soil B to plants and formulating field management practices to influence B availability. This need is especially evident in southeast China where extensive areas of soils contain low levels of B and deficiency is a significant limit to crop production (Shorrocks, 1997).
The objective of this research was to investigate the B fractions in a range of Chinese soils and their relationships to soil and site properties.
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MATERIAL AND METHODS
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Soil Samples
Thirteen soils from cultivated sites ranging widely in physical and chemical properties were sampled from surface horizons (0–20 cm) of the typical zonal soils from south (20°N) to north (47°N) in eastern China. The air-dried soil samples were ground and passed through a 1-mm sieve. Selected physical and chemical properties of these soils are listed in Table 1. In the surface layer, soils were analyzed for pH, OM, clay, hot water-soluble B, and effective cation-exchange capacity (Agricultural Chemistry Committee of China, 1983).
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Table 1. Basic site characteristics and soil properties of the 13 soils tested from a north (Soil 1) to south (Soil 13) transect in eastern China
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Fractionation of Soil B
Methods and procedures for separate extraction of soil B fractions were based on the sequential fractionation procedure of Jin et al. (1987). Soil B was divided into nonspecifically adsorbed B (NSA-B), specifically adsorbed B (SPA-B), B occluded in Mn oxyhydroxides (MOH-B), B occluded in amorphous Fe and Al oxides (AMO-B), B occluded in crystalline Fe and Al oxides (CRO-B), and residual B (RES-B). The extraction methods are summarized in Table 2. Boron concentrations in the extracted solutions were determined by inductively coupled plasma–atomic emission spectrometry (ICP-AES; Varian Liberty-200, Melbourne, Australia). Boron content in each fraction was calculated by subtraction of the B extracted in previous B fractions.
Data Analysis
Simple correlation analysis was used to calculate the correlation coefficients between soil B fractions and soil properties.
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RESULTS AND DISCUSSION
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Fractionation of Soil Boron
Soil Characteristics
All 13 soils used in the study were located in the eastern monsoon region of China, but sites extended along a 3200-km transect from Hainan Island in the south to Heilongjiang Province in the north. Soils were sampled from four orders of U.S. soil taxonomy, ranging from Mollisols in the north, through Inceptisols and Alfisols, to Ultisols in the south. As shown in Table 1, soil pH ranged from 4.01 to 8.30. Organic matter content varied from 8.5 to 48.0 g kg-1, and CEC from 8.5 to 37.1 cmol(+) kg-1. Clay content ranged from 78 to 483 g kg-1, but was more generally in the range of 250 to 483 g kg-1. Hot water-soluble B varied from 0.12 to 0.87 mg kg-1.
Climate is the predominant factor affecting soil properties. The annual mean temperature and rainfall significantly decreases from south to north (Table 1). The basic properties of soils from south to north also follow a regular and gradually changing tendency. For example, sesquioxides in soils showed a decreasing trend, while soil pH, extractable cations, and CEC showed an increasing trend as one proceeds from south to north. Mean annual temperature and rainfall were positively related to crystalline Fe2O3 (CR-Fe2O3) and Al2O3 (CR-Al2O3), and negatively to pH, CEC, and exchangeable Ca (Ex-Ca) (Table 3). Among soil properties, soil pH was negatively correlated with the contents of amorphous Al oxides, crystalline Fe and Al oxides, and clay content. There was a positive relationship between the contents of crystalline Al oxides and those of amorphous Al oxides, crystalline Fe oxides, and Ex-Ca, respectively. Clay was positively correlated only with crystalline Fe2O3. Organic matter, which varied widely among the soils, was not correlated with any of the other parameters measured. Similarly, apart from the negative correlations with temperature and rainfall noted above, CEC was not correlated with any of the soil parameters.
Boron Fractions
In the 13 Chinese soils examined, 87.4 to 99.7% of soil B was in the residual fraction (RES-B), which generally does not relate well to plant-available B (Jin et al., 1987; Tsalidas et al., 1994). By contrast, the more labile pools only accounted for a very small portion of the total soil B (Table 4). Lesser proportions for the RES-B fraction were reported by Jin et al. (1987) for soils from southeastern USA. A possible reason is that the soils used in this study had higher clay contents than most of soils used by Jin et al. (1987). Hou et al. (1996) found slightly lower proportions of RES-B in soils of Ontario but presented no data on clay levels of soils. Tsalidas et al. (1994) found higher proportions of RES-B in all 20 Greek soils tested. However, these proportions are possibly overestimated since no account was made of the CRO-B fraction. Morover, Tsalidas et al. (1994) did not present the clay contents of soils studied, so comparisons with the results from that study are not possible.
Overall, the contents of B followed the order of 
. The nonspecifically adsorbed B (NSA-B) comprised 0.06 to 0.99% of total soil B, specifically adsorbed B (SPA-B) 0.01 to 0.61%, Mn oxyhydroxide-bound B (MOH-B) 0.03 to 4.98%, B occluded in amorphous Fe and Al oxides (AMO-B) < 0.01 to 2.98%, and B occluded in crystalline Fe and Al oxides (CRO-B) 0.03 to 7.57%.
In contrast with the residual B fraction, other fractions had a much wider range of variation among soils in the proportion of total B held in each fraction. Among the Chinese soils, the proportion of total soil B associated with the MOH-B fraction was more varied than reported in previous studies on soils of the southeastern USA and Greece (Jin et al., 1987; Tsalidas et al., 1994). Similarly, the range of variation in the percentage of total B in the AMO-B fraction was much greater among the Chinese soils than those from southeastern USA or Greece, but the proportion of total B in that fraction was less. The CRO-B fraction was also much less as a percentage of total soil B in Chinese soils than those in southeastern USA.
The NSA-B fraction is mainly in solution or weakly adsorbed by soil particles, and is believed to be the most readily available fraction of B for plant uptake (Keren and Bingham, 1985b). The SPA-B fraction may be specifically adsorbed onto clay surfaces or associated with OM in soil (Jin et al., 1987). The NH2OH·HCl-extractable B fraction (MOH-B) may be mainly fixed with Mn oxyhydroxides, which are relatively easily dissolved with the release of occluded elements, including possibly B (Chao, 1972). Several researchers have reported that Mn could accumulate in the surface horizons and was associated with the OM (Ellis et al., 1982; Zhang et al., 1997). Therefore, the release of OM-bound with Mn by NH2OH·HCl probably resulted in the MOH-B fraction being overestimated because of the release of B adsorbed by, and incorporated in the OM (Keren and Bingham, 1985a; Yermiyahu et al., 1988). Recently, Hou et al. (1996) developed a chemical fractionation scheme for soil B in which HNO3–H2O2 was used to extract the organically bound B. However, this fraction was not significantly related to the amount of humic acid in the synthetic soils, a result attributed to the narrow range of humic acid contents used and a low B sorption capacity of the humic acid. By contrast in A horizons of 24 Ontario soils, there was a correlation between organically bound B (HNO3–H2O2) and OM of the soils (Hou et al., 1994). It remains to be demonstrated that the Mn-reducible and organically bound B fractions can be clearly distinguished and that such a distinction is important for predicting plant response to soil B. In fact there remains considerable uncertainty about the role of OM in B availability. Organic matter adsorbs B (Goldberg, 1997), and humus extracted from soils contains B (Parks and White, 1952). Hou et al. (1994) found that up to 23% of soil B was extracted by HNO3–H2O2, and was therefore presumed to be organically bound. However, Gu and Lowe (1990) concluded that in most acid to near neutral soils, humic acids are likely to have only a minor role in B adsorption. And Marzadori et al. (1991) found that treating soils to remove OM increased B sorption by the soil. Clearly further research on the role of OM in B supply to plants is needed, and following this, a reexamination of what soil tests might be necessary to assess the availability of organically bound B to plants. The fractionation procedure of Hou et al. (1994) may turn out to be more applicable than that of Jin et al. (1987), which does not specifically extract an organically bound B fraction.
The NSA-B fraction was positively correlated with both MOH-B and CRO-B fractions (Table 5). There was also a close relationship between SPA-B and MOH-B fractions. The latter correlation is consistent with the suggestions by Hou et al. (1996) that organically bound B is probably extracted by mannitol, and by Yermiyahu et al. (1988) that NH2OH·HCl partially extracted organically bound B. By contrast, Tsalidas et al. (1994) reported no correlation between the MOH-B fraction and either SPA-B or NSA-B fractions in Greek soils. The RES-B and AMO-B fraction were not related to any other extractable B fractions (Table 5).
Boron Fractions in Relation to Soil Properties
Both total and extractable B in the soils of China tend to be decrease from north to south and from west to east with the exception of coastal saline soils (Liu et al., 1990). Since the decline in B content is correlated with increasing rainfall, this may be attributed in part to prolonged leaching loss of B in soils that receive high precipitation. However, the content of mineral fractions in the soil that adsorb B, such as the amorphous Fe–Al oxides and oxyhydroxides and the Mn-oxyhydroxides, also increase with increasing precipitation, and this will also contribute to declining extractable B levels. The distribution of B in various fractions was also related to site rainfall and temperature. The content of the NSA-B fraction significantly decreased with increasing mean annual rainfall, and increased with increasing soil pH and exchangeable Ca (Table 6). This is more obviously due to a stronger B leaching regime in the regions with more abundant rainfall. However, soil properties will also have a significant bearing on B leaching losses. Wang et al. (1997), for example, reported that B from fertilizer did leach in three soils from southeast China, but generally not below the root zone of crops. The soil on which measurable leaching below the 60-cm depth was reported was an alluvial soil from Zhejiang province, unlike any of those in Table 1.
Surprisingly, no relationship was observed between SPA-B fraction and any soil properties and climate factors. A possible explanation is that the clay content of soils—which is known to affect B sorption, was generally high for all the Chinese soils tested, and was higher than in most soils tested by Jin et al. (1987)—had a dominating effect on SPA-B. By contrast, OM levels varied considerably among the 13 Chinese soils. Although mannitol might extract part of organically bound B (Hou et al., 1996), the variation of soil OM in the present investigation apparently had no controlling effect on the specifically adsorbed B fraction. In a study of Greece soils with OM levels ranging from 2.0 to 56.6 g kg-1, Tsalidas et al. (1994) found that the SPA-B fraction was correlated with pH, and negatively correlated with Fe, Al amorphous, and crystalline oxyhydroxides, but not with OM, and they did not examine the correlation with clay content. While Jin et al. (1987) did not report the correlations between SPA-B and either clay or OM, replotting their data indicates that OM levels of the soils were unrelated to SPA-B, whereas clay content was positively correlated. However, when one examines only the sandy soils from Virginia with clay contents <160 g kg-1, there was no correlation between SPA-B and clay content for nine soils. The ability of mannitol to extract B is expected to increase with the increase in solution pH due to the formation of the mannitol–H2BO3 complex (Evans and Sparks, 1983; Vaughan and Howe, 1994); however, this was not evident in our study. Recently, Hou et al. (1996) reported that the SPA-B fraction extracted by 0.05 M KH2PO4 rather than 0.02 M mannitol in 0.02 M CaCl2 was found to be significantly correlated with the content of goethite in the synthetic soils.
Increasing solution pH would enhance the hydroxide radicals on the surface of Mn oxyhydroxides, which retain B through ligand exchange mechanisms (Bingham et al., 1971). Thus it is reasonable that the MOH-B fraction gave the positive correlation with soil pH and CEC, and negatively with rainfall and temperature. The significant correlation between rainfall and both the AMO-B fraction and amorphous Fe2O3 indicated that NH4-oxalate in the dark is a relatively specific extractant for extracting amorphous Fe2O3–bound B. The CRO-B fraction was positively correlated with pH and exchangeable Ca, but not with crystalline Fe2O3. The residual B, the main fraction of total B, existed in primary and secondary mineral structures. There were no significant correlations between RES-B and any soil components in this study. But in synthetic soils, Hou et al. (1996) found that RES-B was significantly correlated with the content of clay mica.
Boron Fractions in Relation to Boron Fertilization
This study examined fractions of native B in 13 upland Chinese soils. A significant group of soils omitted from the study were those used for paddy rice (Oryza sativa L.) cultivation. In the paddy soils, repeated annual submergence causes an increase in the content of amorphous sesquioxides due to the reduction of Fe3+ to Fe2+ and its subsequent reoxidation reactions (Hazra et al., 1987). Thus, paddy soils may well have a different distribution of B among the native soil B fractions. The reaction of fertilizer B with the Chinese soils and its incorporation into soil B fractions also needs to be investigated. Jin et al. (1988) concluded that oxyhydroxides of Al and Fe sorb B into unavailable forms. Thus the NSA-B, SPA-B, and MOH-B fractions may be most available to plants (Jin et al., 1987, 1988; Tsalidas et al., 1994), although this needs further investigation. Differential incorporation of fertilizer B in soil B fractions may also explain reported variation among soils in B leaching. For example, Wang et al. (1997) reported that leaching of B occurred in the field on an alluvial soil to depths of >60 cm, but in two other rice paddy soils from southeast China, with higher clay content, negligible leaching of B occurred from the root zone.
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CONCLUSIONS
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The results emphasize that the forms of B in Chinese soils were distinctly different from those in soils of southeast USA and Greece. Chinese soils, for example, contained a larger B reserve than the sandy soils of the southeastern USA. Yet the prevalence of B deficiency in southeast China suggests that the residual B is not plant available. Among the Chinese soils, the proportion of total soil B associated with the MOH-B fraction was more varied than reported in previous studies on soils of southeast USA and Greece. Similarly, the range of variation in the percentage of total B in the AMO-B fraction was much greater among the Chinese soils than those from southeastern USA, Ontario, or Greece, but the proportion of total B in that fraction was less. The significance of these differences in B fractions for fertilizer B reactions in the soil need further investigation, as does the relationship between these soil B fractions and plant B uptake.
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ACKNOWLEDGMENTS
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The authors are grateful for financial support from the Australian Centre for International Agricultural Research (Project 9120), the Zhejiang Provincial Government and the Chinese Ministry of Agriculture.
Received for publication February 1, 1999.
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ABSTRACT
INTRODUCTION
