**As transfer from soil to shoot and grain**

**Overview of total shoot, soil and grain As for ESR, SSR and WSR**

100

80

60

Cumulative distribution (%)

40

20

0

Cumulative distribution (%)

(solid), WSR (dash) and SSR (square dot).

Cumulative distribution (%)

ESR, WSR and SSR.

864 Environmental Risk Assessment of Soil Contamination

A one-way analysis of variance (ANOVA) was performed on each of the three regions studied. The ANOVA showed that that the mean As levels for grain between sites were not significantly different (*P*=0.662 ), but that those for soil and shoot were (soil: *P*<0.001; shoot: *P*<0.002). Fig.9 shows a comparison of the distribution of As concentrations in the shoot, soil and grain from

0 5 10 15 20

Soil A s (mg/kg)

0.0 0.1 0.2 0.3 0.4

Shoot As (mg/g)

13

0.0 0.1 0.2 0.3 0.4 0.5

Grain As (mg/kg)

**Figure 8 Cumulative ranked distribution of As concentrations in Libyan agricultural soil, shoot and grain from ESR (solid), WSR (dash) and SSR (square dot). Figure 9.** Cumulative ranked distribution of As concentrations in Libyan agricultural soil, shoot and grain from ESR The range of As shoot/soil transfer factors (shoot/soil TFs) was considerable. Minimum and maximum shoot/soil TFs were 0.06 for ESR, 0.04–0.29 for SSR and 0.04 for WSR, while means and medians were 0.1 and 0.02, respectively, for ESR; 0.13 and 0.12 for SSR and 0.06 and 0.06 for WSR. In SSR, the median shoot/soil TF was nearly twice as high as that in ESR and WSR. In this study, transfer of As was an order of magnitude greater from shoot to grain, despite lower rates of soil to grain transfer. The differences in these transfer ratios may be due to differences in As speciation. [18],reported that, in general, higher shoot As levels were consistent with low grain/shoot transfer factors.

#### **Estimation of bioavailable As concentrations by nitric acid extraction**

Nitric acid extraction is a widely used method of estimating the bioavailable concentration of elements. The precision of the results in the present work was controlled by analysis of CRM, blank and spike samples. The correlation factors showed good precision, with the exception of two analytical bioavailable and total results (Table 3). A summary of the bioavailable As in soils sampled from ESR, WSR and SSR are shown in Table 3. A one-way analysis of variance (ANOVA) showed that the differences in As levels between location means are highly significant (*P*<0.001, Table 3). SSR recorded the highest mean levels of bioavailable soil As (0.05 ± 0.01 mg/kg) (Fig.10). In contrast, the highest total mean As level was recorded in ESR (8.10 ± 0.48 mg/kg;.Table 3 and Fig.11b). The mean bioavailable As concentrations in the three regions studied decreased from SSR > ESR > WSR (Table 3) and was in general low, ranging from 0.02–0.07 mg/g, 0.02–0.03 mg/g and 0.01–0.14 mg/g for SSR, ESR and WSR, respectively. Bioavailable concentrations of As are 1–5 % of the total concentrations. In this study, the highest median bioavailable / median total (B/T) As was recorded in SSR (Table 3, Fig.10 and 3.11b).


a **\*** Correlation is significant at the 0.05 level

bB/T= Median bioavailable/ Median total %

**Table 3.** The descriptive statistics for total (T) and bioavailable (B) trace element concentrations in Libyan soils (mg/kg).

There was no correlation between the mean total and the bioavailable As concentrations in ESR and WSR. However, there was a negative correlation between the mean total and the bioavailable As in SSR. [16]. reported that appreciable As can move with leaching water, especially in coarse-textured soils. The mean and median bioavailable As concentration values for SSR were close to the those for WSR and ESR (Table 3). The correlation factors between the bioavailable and the total element concentrations were: 0.073, –0.419 and –0.097 (As); 0.371, – 0.349 and 0.892 (Co); 0.318, –0.124 and 0.614 (Cu); 0.24, –0.161 and –0.545 (Pb); and 0.466, 0.659 and –0.309 (Mn) for ESR, SSR and WSR, respectively (Table 3, Fig.10). Adriano (1986) reported that As mobility and phytotoxicity is usually greater in sandy than in clayey soils. The SSR samples had high B/T bioavailable As, Co, Pb and Mn contents, compared to B/T total elements (Table 3 &Fig.10). In these samples, the levels of bioavailable elements were derived from P fertilization ) [12]. The low correlation value for total Cd is because the concentrations were lower than the ICP-MS detection limit. Weathering will increase bioavailability of trace elements in SSR. [1],found that at P:As ratios of 4:1 or greater, phytotoxicity on wheat was markedly reduced and deduced phytotoxicity to be a function of P concentration.

There was no correlation between the mean total and the bioavailable As concentrations in ESR and WSR. However, there was a negative correlation between the mean total and the bioavailable As in SSR. [16]. reported that appreciable As can move with leaching water, especially in coarse-textured soils. The mean and median bioavailable As concentration values for SSR were close to the those for WSR and ESR (Table 3). The correlation factors between the bioavailable and the total element concentrations were: 0.073, –0.419 and –0.097 (As); 0.371, – 0.349 and 0.892 (Co); 0.318, –0.124 and 0.614 (Cu); 0.24, –0.161 and –0.545 (Pb); and 0.466, 0.659 and –0.309 (Mn) for ESR, SSR and WSR, respectively (Table 3, Fig.10). Adriano (1986) reported that As mobility and phytotoxicity is usually greater in sandy than in clayey soils. The SSR samples had high B/T bioavailable As, Co, Pb and Mn contents, compared to B/T total elements (Table 3 &Fig.10). In these samples, the levels of bioavailable elements were derived from P

**Table 3.** The descriptive statistics for total (T) and bioavailable (B) trace element concentrations in Libyan soils (mg/kg).

Ni ESR 0.72 0.10 4.32 SSR 0.19 0.15 0.29 WSR 0.09 0.10 0.03 P ESR 7.66 1.95 105.61 SSR 52.80 33.40 154.40 WSR 8.92 9.96 12.27

**T T T B B B (B/T)\*100**

mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg ra B/Tb Mean Median Range Mean Median Range % As ESR 7.81 8.45 24.36 0.02 0.01 0.13 0.1 0.2 SSR 4.05 2.48 6.50 0.05 0.05 0.05 –0.4 **2.0** WSR 4.42 3.96 7.16 0.02 0.02 0.02 –0.1 0.5 Co ESR 11.49 11.63 15.11 0.76 0.02 5.59 0.4 0.2 SSR 2.27 2.27 2.26 0.10 0.02 0.33 –0.4 **0.9** WSR 2.28 2.33 2.26 0.02 0.02 0.01 0.9**\*** 0.7 Cu ESR 15.39 15.32 23.12 0.29 0.06 2.30 0.3**\*** 0.4 SSR 4.28 4.22 7.21 0.08 0.05 0.33 –0.1 **1.2** WSR 4.46 3.07 7.57 0.04 0.05 0.05 0.6 1.5 Pb ESR 14.36 14.06 24.11 0.22 0.01 2.98 0.2 0.1 SSR 3.05 3.04 3.36 0.01 0.01 0.06 –0.2 **0.3** WSR 3.39 3.46 1.69 0.00 0.01 0.01 –0.6 0.3 Mn ESR 492.30 504.60 780.60 58.50 8.50 300.50 0.5**\* 1.7** SSR 117.80 111.40 241.50 14.17 1.34 72.57 0.7**\*** 1.2 WSR 95.66 96.89 82.96 1.15 0.98 2.50 –0.3 1.0

**Regions Elements**

866 Environmental Risk Assessment of Soil Contamination

a

**\*** Correlation is significant at the 0.05 level bB/T= Median bioavailable/ Median total %

**Figure 10.** Scatter diagram of total vs. bioavailable trace element concentrations in agricultural soils from ESR, SSR and WSR.

**Figure 11.** Comparison of (b) soil extractable of As with (a) total soil concentrations in Libya

#### **4. Discussion**

Total As mean concentrations in ESR are nearly 4.8 and 4.1 times higher than in SSR and WSR, respectively. Total As loads in ESR may be related to the soil parent material, which is comprised of subsequently covered Lower Palaeozoic rocks. This may be the reason why the clay agricultural soils of ESR have the highest background levels of As. This is in agreement with [8], who reported higher concentrations of trace elements in Eastern Libyan soil than in other areas. In the present study, the trace elements present in Eastern Libyan soil (Co, Zn, Cu, As, Cd, Pb and Mn) were nearly five times higher than in WSR and SSR. Trace element concentrations in agricultural soils showed positive linear correlations (Table 2).

In this study, transfer of As was an order of magnitude greater from shoot to grain, despite lower rates of soil to grain transfer. In addition, this study considered samples of soil, shoot and wheat grains from arable land with a sampling design that allowed for the consideration of intra-field variations. The differences in these transfer ratios may be due to differences in As speciation. [18]reported that, in general, higher shoot As levels were consistent with low grain/shoot transfer factors. Although this similarity in trends for plants from varying geographical locations strongly suggests the involvement of plant physiological regulation, the actual mechanisms involved are still far from clear. However, the findings from the present study indicate that As transfer to grain isgoverned by multiple factors (Fig. 7.1). The bioavail‐ ability of trace elements in soils is influenced by a wide array of biophysical factors (Fig. 7.1), including pH, redox potential, organic matter content and soil texture. These factors were, however, not considered in the experimental/sampling design since the main thrust of the field studies was to establish the contribution of elevated soil As to grain As levels.

In this study, the highest median bioavailable / median total (B/T) As was recorded in SSR (Table 3, Fig.10 and 3.11b).[15]reported that appreciable amounts of As can move with leaching water, especially in coarse-textured soils. This is in agreement with the [9], which reported that As mobility and phytotoxicity is usually greater in sandy than in clayey soils. The SSR samples had high B/T bioavailable As, Co, Pb and Mn contents, compared to B/T total elements (Table 3 &Fig.10). In these samples, the levels of bioavailable elements derived from P fertilization (Hurd-Karrer 1939) and the high pH affects the availability of many crop nutrients in the southern Libyan soil [12]. Weathering will increase bioavailability of trace elements in SSR. [10]found that at P:As ratios of 4:1 or greater, phytotoxicity on wheat was markedly reduced and deduced phytotoxicity to be a function of P concentration.

The present study agrees with [16], who reported that the available soil moisture holding capacity for silty clay loams is three times higher than for loamy sands. Therefore, in the present study, total As mean concentrations in ESR are nearly 4.8 times higher than in SSR. Total As loads in Libyan agricultural soil may be related to the soil moisture holding capacity, with different types of soil having varying capacities and also different levels of As.
