Prediction of Concentrations in Plants - Bioavailability
8.3.1 Recovery Rates of the Soil Extractions
In the extraction samples with ammonium nitrate 21 elements could be measured (out of 46 elements from total soil element concentrations). It was not possible to measure:
As, Ce, Cs, Dy, Er, Eu, Hf, Ho, Li, Lu, Nb, Nd, Pb, Rb, Sb, Sm, Ta, Tb, Th, Tl, Tm, U, Y, Yb and Zn. These elements mostly belong to the group of REEs, or did not represent trace nutrient elements for plants, with the exception of Zn. Zn could be measured in some of the soil extraction samples, the majority of the samples showed values below the LOD. La was the only REE element which could be recovered. Figure 8.1 shows median recovery rates of all soil extractions and total concentration values.
(a) (b)
Figure 8.1:Median recovery rates: elements with higher (a) and with inter-mediate and lower rates (b). Rates are concentration of extractions/total soil concentration in %. Error bars are the Interquartile Range (IQR) of the
medians of all sample locations.
Most of the main nutrient elements showed high recovery rates of concentration in soil extractions vs. total concentration in soils. About 25 % of Ca could be recovered from the soil extractions; for Sr and Ba about 5 %. The extraction method was also very effective for Cd (3 %) (Fig. 8.1a). For Cd, Ba and Mn the error bar (= IQR) show a high variability in the extraction rates. This was probably caused by the different soil properties in the various locations. Co and Ni also showed large error bars, but the recovery rates were similar to P (about 0.13%). That indicates that ammonium nitrate may be useful in describing the availability of these elements to the plants.
The elements La, Cr, V, Sc, Al, Fe and Ti showed the smallest recovery rates. They generally had very small extraction concentrations leading to the smallest recovery rates (extr/total conc. soil). Most remarkably was that Fe was part of this group, although it represents a minor nutrient element for plants.
Figure 8.2a shows that the soil locations Lindau and Trögen have the highest recovery rates in soil extractions with ammonium nitrate. These two locations also had the lowest soil pH values of 4.6 and 5.4, promoting the high recovery rates in some of the elements (Co, Ni and Mn). Mo in the soil extractions could not be measured for some locations (Trögen, Bühren, Deppoldshausen and Lindau) and is missing therefore.
88 Chapter 8. Prediction of Concentrations in Plants - Bioavailability
(a) (b)
Figure 8.2: Median recovery rates per soil location: elements with higher (a) and with intermediate and lower rates (b). Rates are concentration of extraction/total soil concentration in %. Soil sample locations are
abbrevi-ated (see Table 8.1).
8.3.2 Concentration in Plants vs. Total Element Concentration in Soils Figures 8.3 to 8.6 show the concentrations of the whole aboveground plant on the y-axis vs. the corresponding soil concentration on the x-axis. The y-axis show me-dian concentration in the soils. The Figures are divided into panels according to plant species. These are: amaranth, winter faba bean, hairy vetch, ryegrass and winter triti-cale. Every point represents one plant sample. There are varying sample numbers per location. The circles indicate pot or field grown plant samples. The pots used were large pots containing approximately 20 kg of soil and were placed in the open air. The only exception were the pots with amaranth (light blue triangles on soil from Bühren) which were grown in small pots with 2.8 kg of soil.
The Figures can be used easily to check if a correlation between plant concentration and soil (total or extracted) is detectable or not. If a trend for a correlation is visible, and if there are enough data points, a regression line including the equation and the coefficient of determination (R2) is added.
The Poaceae plants (ryegrass and triticale) showed no special enrichment of Co in the plant tissue with increasing soil concentration. The greatest plant concentration was found in the sample from Lindau with the lowest soil pH of 4.6 (green triangles; Fig.
8.3). Faba bean, amaranth and ryegrass plants did exhibit a greater plant concentra-tion of Ni with higher soil concentraconcentra-tions (Fig. 8.4), but this weak trend was mostly caused by plants from Bühren alone. Intermediate Ni soil concentrations were lacking (between 40 and 60 mg/kg).
Mn showed no correlation between plant and total soil concentration (Fig. 8.5). The plant levels were very similar and ranged below 100 mg/kg. Only the pot experi-ments from Bühren (light blue triangles), Trögen (orange triangles) and Lindau (green triangles) showed elevated Mn concentrations in plant tissue. Trögen and Lindau had the lowest soil pH-values.
There was no correlation pattern for Mo in plants vs. Mo in the soil (Fig. 8.6). The highly variable Mo concentrations in plants from Garte Nord (yellow) and Sömmer-ling (blue) were striking, but was most evident for ryegrass samples (see also Chapter 7). These highly variable concentrations in the plants of the two main field trials (Garte Nord and Sömmerling) prevented a correlation between plant and soil concentrations.
8.3. Results 89
Figure 8.3:Co in plants vs. in soil, sample locations are abbreviated (Table 8.1).
Figure 8.4:Ni in plants vs. in soil, sample locations are abbreviated (Table 8.1).
90 Chapter 8. Prediction of Concentrations in Plants - Bioavailability
Figure 8.5: Mn in plants vs. concentration in soil, sample locations are abbreviated (Table 8.1).
Figure 8.6: Mo in plants vs. concentration in soil, sample locations are abbreviated (Table 8.1).
8.3. Results 91 8.3.3 Concentration in Plants vs. Soil Extraction with Ammonium Nitrate In the following two Figures of Co and Ni the samples from Lindau (triticale and faba bean, green triangles) were particularly interesting. The low soil pH likely caused the greatest soil extraction rates and plant concentrations (Figures 8.7 and 8.8). Second greatest plant concentrations were measured for plants grown on soil obtained from Bühren. A correlation was hard to detect for Co. The plants on soil from Sömmerling had a high variability in plant concentrations (dark blue). Bakkaus et al. (2008) also stated a high correlation for greater plant concentration of Co with decreasing pH, but 6 out of 8 soils in this study had been subjected to atmospheric deposition of an-thropogenic Co and had high total Co soil concentration of > 30 mg/kg. Furthermore, their plants (Triticum aestivumL.) only grew for 46 days.
There is a trend for linear correlation for Ni with faba bean, ryegrass and amaranth.
The concentration in triticale cannot be modelled with ammonium nitrate as the plant concentrations are small, even for the sample from Lindau (light green).
y = 0.15+4.3 x Radj2 = 0.44
Figure 8.7:Co concentration in plants vs. extracted with NH4NO3 in soil, sample locations are abbreviated (Table 8.1).
For Mn also the sites with the lowest pH (Lindau and Trögen) showed the greatest plant concentrations. For ryegrass, faba bean and triticale one could argue a linear correlation exists. This was possibly due to enhanced mobility of Mn in soils with pH lower than 7. Soils with intermediate Mn extraction concentrations were lacking (between 25 and 80 mg/kg).
There was the issue of low extraction rates of Mn from the soils. Most values were below the LOD. Therefore, no figure is presented. Only for soils from Sömmerling, Garte Nord and Groß Ellershausen soil extraction concentrations could be measured.
92 Chapter 8. Prediction of Concentrations in Plants - Bioavailability
y = 0.42+16 x Radj2 = 0.87
AmaranthFaba Bean (Wi)Hairy VetchRyegrassTriticale (Wi)
0.0 0.1 0.2 0.3
0 2 4 6
0 2 4 6
0 2 4 6
0 2 4 6
0 2 4 6
Ni soil extr median [mg/kg]
Ni plant [mg/kg]
Loc_short Aholf Bühr
Dep Ellieh
GN Lindau
SÖ Straub
Trögen pot.trial field pot
Figure 8.8:Ni concentration in plants vs. extracted with NH4NO3 in soil, sample locations are abbreviated (Table 8.1).
Greater Mo concentrations in the locations with lower pH was not be expected due to the lower mobility of Mo in acidic conditions.
8.4. Discussion 93
Figure 8.9:Mn concentration in plants vs. extracted with NH4NO3 in soil, sample locations are abbreviated (Table 8.1).