INTRODUCTION

Rare earth elements (REEs) are a group of elements, which includes scandium, yttrium, lanthanum, and lanthanides (Ln). One of the methods for determining the anthropogenic load on soils consists in their comparison with the background (undisturbed soils). There are data on the content of REEs in Swedish Haplic Podzol [13, 24] and in soils of Germany [17, 18] and China [11]. Unfortunately, published data on the content of REEs in soils of the southern taiga are insufficient. There is little information about the content of particular REE fractions, which will enable the assessment of the contribution of particular soil components to REE fixation.

Many researchers point out that the behavior of REEs in soils depends on the content of iron, manganese, and aluminum oxides [9]; these are among the most important REE carrier phases [1, 7], which affect their fractionation in soils. Surface sorption of REEs by non-silicate forms of iron and manganese is proved in [8, 10]. Nevertheless, the mechanisms of REE fractionation in soils are still insufficiently studied.

In this work, we have analyzed the distribution of total contents of REEs in the profiles of four undisturbed soils of the Central Forest State Nature Biosphere Reserve (CFSNBR) and of oxalate-soluble REEs related to non-silicate iron and manganese compounds.

OBJECTS AND METHODS

The objects of the study were represented by samples of the main horizons from four soil pits taken from 2008 to 2019 in the area of the CFSNBR. The reserve is located in the southern taiga subzone. The climate is moderately continental, the moisture coefficient is >1, and soils are regularly waterlogged.

Soils of pits 3-2019, 2-2008, and 4-2017 are formed on two-layered deposits (light loamy mantle loam underlain by heavy loamy moraine). The horizons O(L–F–H)–AEL–ELf–IIBD are distinguished in the soil profile of pit 3-2019, and the soil is determined as pale-podzolic [5] or Dystric Albic Retisol according to the WRB (2014) [26]. The profile in pit 2-2008 comprises horizons T1–T2–H–ELhi,g–EL–ELnn,g–IIBDG and is classified as peat-podzolic-gley concretionary soil or Albic Gleyic Folic Retisol (Raptic). There are numerous brownish-ocherous iron-manganic nodules from 1 to 5 mm in diameter in the ELnn,g horizon. Soil of pit 4-2017 is identified as typical agrosoddy-podzolic soil (Albic Retisol (Aric, Raptic) according to the WRB) with the set of horizons: AYg–P–EL–BEL–IIBD. Pit 1-2019 of gray-humus gleyic soil (Gleyic Umbrisols according to the WRB) has the O–AY–AYB–BMg horizons.

We determined the following parameters: \({\text{p}}{{{\text{H}}}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) and the content of organic carbon for mineral horizons. Chemical analyses were performed according to [2]. Solution by Tamm (H2C2O4∙2H2O and (NH4)2C2O4∙H2O with pH 3.3) was used to extract the REE fractions related to oxalate-soluble iron and manganese. The REE content in the obtained extracts was determined by the ICP-MS method on an Agilent 7500a ICP-MS device. REEs were extracted from the soil by aqua regia in an Ethos One microwave oven (Milestone) [4]. The REE concentration in the obtained solutions was determined by the ICP-MS method on an Agilent 7500a ICP-MS device. The total content of REEs was determined in three replications.

All the results obtained were statistically processed in the MS Excel program.

The normalizing is traditionally used to study the effect of pedogenesis on the distribution of substances in the soil profile: the content of a substance in particular horizon is divided by its content in the soil-forming rock. As a result of a lack of data on the content of REEs in the soil-forming rock and due to the two-layered composition of the soil profile, an approach adopted in the geochemistry of REEs was used in this work: the total REE content in soils and the content of REEs related to oxalate-soluble iron and manganese were normalized to the clay of the Russian Platform (CRP) [4]. This principle does not take into account the effect of heterogeneity of soil-forming rocks, but enables us to identify regularities in the behavior of different elements relative to the universal standard of the content of REEs (CRP).

Some natural objects, soils in particular, are characterized by anomalies in the distribution of normalized REEs, which are seen on the elemental spectra as positive or negative peaks. Among all REEs, anomalies in soils are the most typical for europium and cerium, because these elements may change their oxidation rate, depending on pH and Eh. The values of anomalies are usually calculated, using empirical formulas, which are based on differences between the normalized content of neighbor elements [8].

Cerium and europium anomalies (Cean and Euan) were calculated by the formulas:

$$\begin{gathered} {\text{C}}{{{\text{e}}}_{{{\text{an}}}}} = \frac{{3{\text{C}}{{{\text{e}}}_{{{\text{norm}}}}}}}{{\left( {2{\text{L}}{{{\text{a}}}_{{{\text{norm}}}}} + {\text{N}}{{{\text{d}}}_{{{\text{norm}}}}}} \right)}}, \\ {\text{E}}{{{\text{u}}}_{{{\text{an}}}}} = \frac{{3{\text{E}}{{{\text{u}}}_{{{\text{norm}}}}}}}{{\sqrt {\left( {{\text{S}}{{{\text{m}}}_{{{\text{norm}}}}}\;{\text{G}}{{{\text{d}}}_{{{\text{norm}}}}}} \right)} }}\,\,\,[18], \\ \end{gathered} $$

where Cenorm, Lanorm, Ndnorm, Eunorm, Smnorm, and Gdnorm are normalized contents of cerium, lanthanum, neodymium, europium, samarium, and gadolinium, respectively.

RESULTS AND DISCUSSION

In all the studied soils, \({\text{p}}{{{\text{H}}}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) in mineral horizons increases from top to bottom of the profile (Table 1).

Table 1.   Cerium and europium anomalies, pH, and the content of organic carbon and oxalate-soluble iron and manganese in soils
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The content of organic carbon in mineral horizons of podzolic, agrosoddy-podzolic, and gray-humus gleyic soils decreases down the soil profile. The distribution of organic carbon in the peat-podzolic-gley soil is more complicated: it decreases from the ELhi,g horizon to the ELnn,g horizon and increases in the IIBDG layer.

The content of oxalate-soluble REEs in soils. The content of the compounds of lanthanum, yttrium, and lanthanides related to oxalate-soluble iron and manganese decreases in peat-podzolic-gley soil in the following sequence of horizons (Table 2): IIBDG > ELnn,g > ELhi,g > EL > H > T2 > T1. The distribution of scandium is different: ELnn,g > ELhi,g > IIBDG > H > T2 > EL > T1. Podzolic soil is characterized by the following sequences of horizons with respect to the increase in the content of elements: L < F < EL < AEL < H < IIBD for lanthanum compounds and lanthanides, L < AEL < F < EL < H < IIBD for yttrium compounds, and L < EL < F < AEL < H < IIBD for scandium compounds. The content of these REEs compounds (for all elements except for scandium) in gray humus and agrosoddy-podzolic soils increases down the soil profile.

Table 2.   Content of oxalate-soluble REE fraction in soils, mg/kg
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In order to characterize the behavior of oxalate-soluble compounds of particular REE, their content was normalized by the CRP. Diagrams of distribution of REEs of this fraction normalized by CRP [4] are shown in Fig. 1. The plot for peat- podzolic-gley soil shows the relative enrichment of the ELnn,g and IIBDG horizons with compounds of some REEs (with cerium in both horizons, with europium in the ELnn,g horizon, and with elements of the series Gd–Ho in the IIBDG horizon). This trend may be explained by the change in the soil-forming material and by the higher content of non-silicate manganese in these horizons. It is known that manganese compounds may absorb cerium on their surface [7]. The ELnn,g horizon is enriched with europium, while the IIBDG horizon is characterized by a negative europium anomaly.

Fig. 1.
figure 1

The contents of oxalate-soluble compounds of REEs in soils of the CFSNBR normalized according to the average clay content in sediments of the Russian Platform [4]. Soils: (a) peat-podzolic-gley, (b) podzolic, (c), agrosoddy-podzolic, and (d) gray-humus soils.

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The plot of the normalized content of this fraction in podzolic soil is characterized by specific features: among all light lanthanides (La–Sm) [8], the cerium content is the highest in all horizons, and the amount of europium is the highest among medium lanthanides in the O(L) and O(F) horizons, which indicates positive cerium and positive europium anomaly. This may be related to the variable oxidation rate of these elements under conditions of periodic waterlogging.

The shapes of the curves on the normalized plots for gray-humus gleyic soil show that all horizons are enriched with medium and heavy (except Yb and Lu) lanthanides of this fraction. There is a positive cerium anomaly in all horizons, and there is a small positive europium anomaly in the O horizon. The AYB and BMg horizons are characterized by a small negative europium anomaly.

The normalized plots for agrosoddy-podzolic soil show that all horizons are also enriched with medium and heavy lanthanides of this fraction. There is a positive cerium anomaly in all the horizons studied.

Fig. 2.
figure 2

Total contents of REEs in soils of the CFSNBR normalized according to the average clay content in sediments of the Russian Platform [4]. Soils: (a) peat-podzolic-gley, (b) podzolic, (c), agrosoddy-podzolic, and (d) gray-humus soils.

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The highest cerium anomalies in peat-podzolic-gley soil are typical for the ELnn,g, IIBDG, and T2 horizons. The normalized cerium content in them is the greatest as compared to the neighbor lanthanides. This feature may be explained by the high content of oxalate-soluble manganese in the ELnn,g and IIBDG horizons. There is a positive europium anomaly in all horizons, except for IIBDG: it is maximal for organic horizons and minimal for the eluvial ones.

The greatest positive cerium anomalies in podzolic soil are seen in the O(L), O(F), and ELf horizons. There is a negative europium anomaly in the EL and IIBD horizons of podzolic soil, which is typical for mineral soil layers and may be related to the mineralogical composition of the soil-forming rock. There is a significant (α = 5%) correlation between the cerium and europium anomalies and the content of oxalate-soluble manganese (r = 0.73 and 0.88, respectively). The cerium and europium anomalies closely correlate with each other (r = 0.89). This fact does not exclude their common origin.

The gray-humus soil is characterized by a positive cerium anomaly in all horizons, a small positive europium anomaly in the O horizon, and a small negative one in the AYB and BMg horizons. The cerium and europium anomalies correlate with each other (r = 0.76).

Positive cerium anomalies are typical for all horizons in the agrosoddy-podzolic soil, and there are also insignificant negative europium anomalies in the EL and BEL horizons (Euan = 0.95 and 0.94, respectively).

Thus, a positive cerium anomaly is present in all horizons of the studied soils: this is the regularity for these compounds. Gleying is seasonal due to the specific features of the climate and water regime of soils, that is, a change in the oxidation-reduction conditions is possible. It is known that under particular oxidation-reduction conditions, which may exist as a result of periodic waterlogging typical for soils of the CFSNBR, manganese oxides will oxidize Ce3+ to CeO2 [22]. Cerium may be absorbed by manganese oxides and hydroxides and accumulated in them, while groundwater is depleted of this element [7, 23].

The normalized diagrams show that almost all horizons contain more oxalate-soluble heavy and medium lanthanides, which indicates their fractionation during pedogenesis. That is, different REEs, which were initially the components of the crystalline lattices of primary minerals, gradually become parts of iron and manganese oxides and hydroxides in different ratios. According to some data, there is a greater affinity of heavy lanthanides for iron oxides and hydroxides and of light lanthanides for manganese compounds, which has been experimentally confirmed [15].

The reason of the positive europium anomaly in some horizons is not completely clear.

An inter-element correlation analysis was performed for scandium, yttrium, lanthanum, and lanthanides to reveal the similarity of the behavior of oxalate-soluble REEs in soils. Peat-podzolic-gley soil is characterized by a weak correlation of scandium content with all lanthanides and by a close correlation of yttrium (correlation coefficient r > 0.97) with heavy and medium lanthanides (except for europium, r = 0.91) and of cerium with light lanthanides and europium (r = 1.00) and with gadolinium, terbium, and dysprosium (r = 0.99, 0.98, 0.97, respectively). In general, the compounds of light and heavy lanthanides are interrelated, and the compounds of the cerium and yttrium subgroups of lanthanides migrate into the Tamm extract according to their own laws.

The regularities revealed in podzolic soil include low correlation coefficients of iron compounds with all REEs (r < 0.29) and a weak negative correlation (r < –0.29) of manganese compounds. Scandium correlates less than other REEs with all lanthanides.

Iron compounds in gray-humus soil strongly correlate with all compounds of REEs (r < 0.99), and scandium ones are characterized by the weakest correlation with all REE compounds (r < 0.70). The rest REEs of this fraction are mainly characterized by very strong correlation.

The correlation of scandium compounds with other REEs is low, which may be related to the manifestation of individual chemical properties of this element, affecting its fractionation during soil formation. The correlation analysis shows a strong relationship between oxalate-soluble compounds of all lanthanides, which indicates that they are inherited from soil-forming rocks and that their behavior in soils is similar. Oxalate-soluble yttrium compounds are in a close correlation with the corresponding compounds of heavy lanthanides, and cerium compounds correlate with the corresponding compounds of light lanthanides, which is confirmed by the traditional specification of lanthanides into cerium and yttrium subgroups [1].

The total content of REEs in peat-podzolic-gley soil. With respect to a decrease in the content of particular elements, soil horizons may be organized in the following sequences: ELnn,g > EL > IIBDG > ELhi,g > H > T2 > T1 for light REEs, IIBDG > ELnn,g > EL > ELhi,g > H > T2 > T1 for heavy REEs, and EL > ELnn,g > IIBDG > ELhi,g > H > T2 > T1 for lanthanum.

The normalization according to the CRP shows the relative depletion of the EL, ELnn,g, IIBDG, ELhi,g, and H horizons of europium, which indicates its anomaly. The greatest depletion is typical for the EL horizon (Euan = 0.61). It may be related to the removal of the clay fraction with clay minerals, containing REEs, down the soil profile. The relative soil depletion of this element decreases in the series of horizons: EL, ELhi,g≈ELnn,g, IIBDG, H. The europium anomaly is not seen in the T1 and T2 horizons, that is, the conditions for europium fixation in peat horizons are better than for other REEs as compared to the lower part of the soil profile. There is a small positive cerium anomaly in the ELnn,g horizon, and 22.5% of the total cerium content in it is represented by oxalate-soluble compounds. Such accumulation may be related to the effect of alternating oxidation-reduction regime and the presence of iron-manganic concretions. The portion of oxalate-soluble compounds of the total content of heavy lanthanides is greater as compared to the light ones. The contribution of light REEs related to oxalate-soluble iron increases in the ELnn,g horizon.

The total REE content in podzolic soil. The REE content increases down the soil profile. The total REE normalized by the CRP is characterized by the following features: the EL, AEL, and H horizons are depleted of heavy and medium lanthanides and europium relative to their content in the CRP.

The calculated europium anomalies show that the greatest depletion is typical for the EL and AEL horizons (Euan = 0.64 and 0.66, respectively), which may be related to the podzolization process. Negative europium anomalies decrease in the sequence of horizons EL > AEL > H > IIBD > F. There is a positive europium anomaly in the L horizon, which indicates its accumulation.

The portion of heavy and medium oxalate-soluble lanthanides in their total content is greater than the portion of light lanthanides. The percentage of cerium is greater than that of other light lanthanides in the F, H, AEL, and EL horizons.

Table 3.   Total REE content in the studied soils, mg/kg (mean of three replications)
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The total content of REEs in agrosoddy-podzolic soil. The REE content increases down the soil profile. The total REE content normalized by the CRP reflects a relative depletion of Eu (in the form of an anomaly) and heavy lanthanides (Tb–Yb) in all horizons. Depletion of light lanthanides (La–Nd) and a slight enrichment with cerium are recorded in the EL and BEL horizons.

The calculated europium anomalies show that the greatest depletion is typical for the E and BEL horizons (Euan = 0.74 and 0.73, respectively). In the P horizon, the negative europium anomaly is smaller.

The total content of REEs in gray-humus soil. The REE content increases down the soil profile. The total REE content normalized by the CRP reflects a relative depletion of europium in all horizons.

The calculation of europium anomalies shows that the greatest depletion of this element is typical for the AY and AYB horizons (Euan = 0.67), and it is minimal in the O horizon.

A positive cerium anomaly is typical for the AYB and BMg horizons (Cean = 1.06 and 1.12, respectively). Non-silicate iron and manganese may be the sources of cerium in these horizons, because they contain more cerium than other light lanthanides.

The portion of heavy and medium oxalate-soluble REEs in their total content is greater than the similar portion of light REEs.

The main features of the distribution of REE compounds in the profiles of four soils of the CFSNBR. The total contents of heavy REEs normalized by the CRP in the mineral horizons of all four soils are in general lower as compared to light and medium REEs. This may be explained by the fact that ions of heavy REEs are more capable for complex formation than ions of light REEs [9, 14], which results in more intensive leaching of heavy REEs from the soil profile [14, 16, 21] under humid climate.

The eluvial horizons of the studied soils are characterized by the strongest depletion of heavy lanthanides as compared to the CRP and by high europium anomalies. It is known that REEs may be adsorbed on the surfaces of clay minerals and may also enter the inter-packet space [14, 25], replacing ions of alkaline and alkaline earth metals [14, 20]. The adsorption of REEs increases parallel to pH of the solution [12, 14]. The composition of soil minerals and their resistance to destruction exert a particular effect [1]. Soil chlorite, vermiculite, kaolinite, and poorly crystallized illite dominate the clay fraction of the EL horizon of podzolic and peat-podzolic-gley soils of the CFSNBR. According to published data [12, 14], these minerals are enriched with light REEs. One of the reasons of the lower content of heavy REEs in eluvial horizons as compared to the underlying ones under humid conditions may be the difference in the mineralogical composition of fine fractions of clay minerals related to local soil-forming processes. More detailed studies are required to show the ability of clay minerals to absorb REEs during pedogenesis. The greatest negative europium anomaly in the eluvial horizons also requires additional research.

Negative europium anomaly is less pronounced in organic horizons, and there is a positive anomaly of this element in podzolic soil (in the L horizon). It is not completely clear, why the distribution of europium in these horizons differs from that in mineral horizons. In organic horizons, this may be related to the substitution of Ca2+ ion in plants for Eu3+ having a close ionic radius [6, 19].

CONCLUSIONS

The studied soils of the CFSNBR are more enriched with light lanthanides than with heavy ones, which correspond to the concepts of REE fractionation as a result of transformation of rocks during hypergenesis. Normalization of the total contents of REEs by the CRP shows that they undergo additional fractionation during pedogenesis in the studied soils, which consists in the depletion of heavy lanthanides and europium (forming a negative anomaly), eluvial horizons, specifically. This process is accompanied by the change in the soil-forming rock at the conventional EL–IIBD boundary. The negative europium anomaly is smaller for organic horizons. The total content of REEs increases down the soil profile, along with the depletion in the eluvial horizons.

The fraction related to oxalate-soluble compounds of iron and manganese comprises from 1 to 67% of the total amount. Its content is the lowest in the EL, ELhi,g, and H horizons of (1–7%) of peat-podzolic-gley soil; in ELf and AEL horizons (2–9%) of podzolic soil; in O, AY, and AYB horizons (2–10%) of gray-humus soil; as well as in all horizons of agrosoddy-podzolic soil for Pr, Nd, Sm, Eu, and Tm (0.4–11%). The percentage is the largest in the ELnn,g and IIBDG horizons (12–28%) of peat-podzolic-gley soil; in the IIBD horizon (13–26%) of podzolic soil; in the BMg horizon (8–23%) of gray-humus soil; and in the EL and BEL horizons for La and Ce (52–67%) and for Ho, Yb, and Lu (22–30%) of agrosoddy-podzolic soil. This REE fraction exerts a significant effect on the redistribution of heavy lanthanides and cerium, and also produces a positive cerium anomaly in some horizons. The REE distributions of this fraction normalized by the CRP are characterized by positive cerium anomalies in all studied soils.

Close correlations between the content of oxalate-soluble iron and manganese and the content of REEs extracted by Tamm solution are not revealed in podzolic and peat-podzolic-gley soils. This is probably related to the fact that the content of REEs in these fractions depends not only on the number of carrier phases (oxalate-soluble iron and manganese), but also on the mechanisms of REE sorption, which may differ for these phases. However, a correlation between the content of oxalate-soluble iron and the amount of REEs in the Tamm extract has been revealed in gray-humus and agrosoddy-podzolic soils.