Element Uptake by Plants at Higher Soil Element Concentrations
6.3 Transfer Factors Results
6.2.3 Multivariate Analysis
For multivariate approach covariance biplots a PCA is executed on clr (centered log-ratio) transformed data. Biplots are useful tools to explore datasets qualitatively. There is no quantitative information in these plots because they are built from clr-transformed data. This means that each ray in the biplots (variables) is in relationship with all the other variables. The clr transformation is calculated via
clr(x) =ln
x
g(x)
i=1,...,D
withg(x)as the geometric mean. Dis the number of variables (in this case, elements).
The graphical representation of the results of the PCA is a covariate biplot. Biplots are useful tools to explore the structure of a dataset (Gabriel, 1971). The variables and observations are displayed in a two-dimensional plot. In the covariate biplot the vari-ables are plotted as arrows or rays from the center of the dataset after transformation.
That is, rays with a very short link between each other are likely proportional and have a quasi-constant log ratio. If the links between the rays are very long (meaning, the angle in between is more than 90◦) the log ratio of the two variables is highly variant.
If rays exhibit 90◦ between them they are likely to be uncorrelated (van den Boogaart and Tolosana-Delgado, 2013).
6.3 Transfer Factors Results
6.3.1 Transfer Factor Based on Total Soil Concentrations
In Figure 6.1 the TF from all samples of the two main field trials (Garte Nord and Sömmerling) are shown ordered by decreasing median with total soil concentration as denominator. Three fields of main nutrients, mobile trace elements and immobile elements are marked based on the TF value only and mostly corresponds to definitions in the literature (Marschner, 1995; Taiz and Zeiger, 2010). The TF over this broad range of elements spans four orders of magnitude and therefore to observe differences in the small TF as well, the data is displayed on a log scale. The first area are TFs > 1, and the second area are marked by TFs > 0.001. The second area termed "trace nutrients and mobile elements" includes all elements considered as micronutrients (Fe, Mn, Zn, Cu, Ni) and elements which are relatively mobile in the soil. Some toxic elements like Cd and As also display good mobility.
In the third area there are elements which are immobile and are only taken up by the plant in very small amounts. Most of these elements are major constituents in the soil (Ti, Al). Also the REEs fall within this range. The group of REEs is represented only by La, to improve the readability of the diagrams.
Differences in transfer factors of plant species
Figure 6.2 shows the TFs from members of the Pocaea family (maize, rye, ryegrass and triticale). For Cu, Zn, As and Fe the median points for these species congregate very close together. In the third area ("immobile elements") the variance is greater, and all
58 Chapter 6. Influence of Plant Species on Element Uptake species overlap in their 1st to 3rd quartiles. That was expected, as these elements are also mostly affected by the correction for adhering particles (refer to Chapter 4).
Ba, Mo and Na display the greatest variance. The two cereal crops rye and triticale almost overlap perfectly. In Figure 6.3 faba bean (Fabaceae/Leguminosae) and maize are shown. Faba bean plants show a greater TF in almost all elements considered, except for Cd and some immobile elements like Sc, Th, Zr, Ti and Al.
Figure 6.4 revealed distinctive TF patterns of amaranth and cup plant species com-pared to faba bean and maize. Cup plant has remarkably low TFs of Mo and Na.
Amaranth showed high TFs of potentially toxic elements Tl, Cd, and Cs. This holds also for the flower mixtures (annual and perennial, not shown). Cup plant exhibits a remarkably small TF for Mo, Cd and Na. Another possible application of the trans-fer factors and this kind of visualization is the field of phytoremediation, especially phytoextraction. If plant concentrations corrected for adhering particles are used the real uptake behavior of the plant could be investigated. Of course, then other impor-tant factors, like the plant DM yield, need to be included in a following calculation of extractable amount of elements (Sauer and Ruppert, 2013; Sauer et al., 2017).
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Figure 6.1:Transfer factor of all samples from Garte Nord and Sömmerling (n=228), Element data is ordered by decreasing medians.The blue area
represents the area from 1st to 3rd quartile.
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Figure 6.2:Median transfer factor of Poaceae plants, from Garte Nord and Sömmerling with elements ordered by decreasing median TF. The colored
area represents the 1st to 3rd quartile area (IQR) of plant species.
6.3. Transfer Factors Results 59
Figure 6.3:Median transfer factor of faba bean plants and maize from Garte Nord and Sömmerling with element ordered by decreasing median TF. The colored area represents the 1st to 3rd quartile area (IQR) of plant species.
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Figure 6.4:Median transfer factor of amaranth, cup plant, faba bean and maize plants, from Garte Nord and Sömmerling with element order by de-creasing median TF. The colored area represents the 1st to 3rd quartile area
(IQR) of plant species.
60 Chapter 6. Influence of Plant Species on Element Uptake 6.3.2 Transfer Factor based on Extraction Concentrations
The following diagrams show the TF with the soil extractions with ammonium nitrate as the denominator. In this approach only the elements which could be recovered in the extraction can be investigated (21 elements).
The TFs now result in greater maximum ratios up to a factor of 2000. The order of el-ements have also changed. The three zones –macro-nutrients, mobile/micronutrients and immobile /pedogenic elements –cannot be defined any more. Elements like Fe, Ti, Al have very large transfer factors (TFex) based on soil extractions, because their concentration in the extract is very low. This naturally leads to a very high factor of around 1000 for Fe (Fig. 6.5). High TFs between 500 and above might indicate that the plant was able to access other pools in the soil than the pool of "potentially bioavail-able" elements, which were targeted with the extraction procedure with ammonium nitrate or similar neutral salt solution extractions.
The spread between the 1st and 3rd quartile is very similar to that in Fig. 6.1. Both diagrams show larger variation in Ti, Na, Co and very small variations in K or Cu.
This is not surprising, as these pattern are totally controlled by the concentration in the plants (= numerator in Eq. 6.1 and 6.2).
The TFs of faba bean plants (summer and winter variety) and maize plants are very different (Fig. 6.6). The TFexfor faba bean plants are much greater for Co, Na, Ca, Sr and Mn, than for maize plants. This pattern was similar to the TFs based on total soil concentrations (Fig. 6.3). For cup plant samples, a small TFexwas detected for Mo and Na and a very high TFexfor Ca and Sr (Fig. 6.7). This observation corresponds to Fig.
6.4).
Figure 6.5:Transfer factor (TFex = plant conc/extr. conc) of all samples from Garte Nord and Sömmerling (n=228), element ordered by decreasing median TFex, colored area represents the 1st to 3rd quartile area (IQR).
6.3. Transfer Factors Results 61
Figure 6.6: Transfer factor (TFex = plant conc/extr. conc) of faba bean and maize plants from Garte Nord and Sömmerling, element ordered by decreasing median TFex, colored area represents the 1st to 3rd quartile area
(IQR) of plant species.
Figure 6.7: Transfer factor (TFex = plant conc/extr. conc) of amaranth, cup plant, faba bean and maize plants from Garte Nord and Sömmerling, element ordered by decreasing median TFex, colored area represents the 1st
to 3rd quartile area (IQR) of plant species.
62 Chapter 6. Influence of Plant Species on Element Uptake