Effects of soil properties on heavy metal bioavailability and accumulation in crop grains under different farmland use patterns

Soil physicochemical properties

Table 1 shows the basic chemical characteristics of the 81 soil samples. The pH values of most of the soil samples from the rape fields and the paddy fields ranged between 6.5 and 7.5, while the pH values of most of the soil samples from the wheat fields were less than 6.0. The relatively low pH values for soils from the wheat fields could be due to traditional farming practices adopted in the farms, including continuous cropping. In addition, acidic soil is conducive for wheat growth. Organic matter contents were generally low in all the soils, and soils from the rape fields had the lowest organic matter contents. Generally, the mean soil total N contents in the fields were in the order of rape fields (756 mg/kg) < wheat fields (973 mg/kg) < paddy fields (1034 mg/kg), while the mean soil available K contents in the fields were in the order of paddy fields < wheat fields < rape fields. The mean soil total P and available P contents both exhibited the order of wheat fields < paddy fields < rape fields. Considering the relatively low SOM, TN, TP, AP, and AK contents, the studied soils should be fertilized regularly. Furthermore, the considerable variations in the parameters (pH, SOM, TN, TP, AP, and AK) could be a attributed to the varied farming practices of small-holder farmers, which are suitable for studying how soil properties influence heavy metal bioavailability and accumulation in different soil-cropping systems.

Total concentrations and spatial distribution characteristics of metals in soils

Figure 2 illustrates the total concentrations of the tested metals in agricultural soil samples under different farmland use patterns. Total concentrations of metals in the 81 soil samples exhibited broad variation. Among the metals, Fe had the highest concentration, and Cu, Mn, and Zn had higher concentrations than Cd and Pb. In addition, there were differences in metal concentrations among the different farmland use patterns (Fig. 2). Generally, total Cu, Zn, Pb, Cd, and Mn concentrations in soils collected from the wheat fields were lower than those from the other two fields, and the differences in total Zn, Mn, and Pb concentrations between the rape and wheat fields were significant (p < 0.05).

According to GB 15618-2018³¹, the risk screening values (RSVs) of Cu, Zn, Pb, and Cd are 50, 200, 70, and 0.3 mg/kg, respectively, for pH ≤ 5.5; 50, 200, 90, and 0.3 mg/kg, respectively, for 5.5 < PH ≤ 6.5; 100, 250, 120, and 0.3 mg/kg, respectively, for 6.5 < PH ≤ 7.5; and 100, 300, 170, and 0.6 mg/kg, respectively, for pH > 7.5. In the paddy fields, the RSVs of Pb and Cd are 80 and 0.3 mg/kg, respectively, for pH ≤ 5.5; 100 and 0.4 mg/kg, respectively, for 5.5 < PH ≤ 6.5; 140 and 0.6 mg/kg, respectively, for 6.5 < PH ≤ 7.5; and 240 and 0.8 mg/kg, respectively, for pH > 7.5. Therefore, in the rape fields, 44%, 50%, 39%, and 92% of the soil samples exceeded RSVs of Cu, Zn, Pb, and Cd, respectively. In the wheat fields, 20%, 16%, 12%, and 88% of the soil samples exceeded RSVs of Cu, Zn, Pb, and Cd, respectively. In the paddy fields, 45%, 30%, 15%, and 75% of the soil samples exceeded RSVs of Cu, Zn, Pb, and Cd, respectively. Furthermore, 67%, 32%, and 55% of soil samples in the rape, wheat, and paddy fields exceeded 1 mg Cd kg⁻¹ soil, and the highest value was 24.8 mg/kg. According to the results, mining operations in the region have led to soil Cd contamination under the three farmland use patterns. In addition, the rape fields had Cu, Zn, and Pb contamination, and paddy fields had Cu and Zn contamination. The results are similar to those reported in previous studies on heavy metal contamination trends in soils in the Tongling area^8,32. However, notably, the Zn, Pb, and Cd concentration ranges in soils from the three studied fields were almost all above the local geochemical background levels; similarly, the Cu and Mn concentration ranges in most of the soil samples were also above the local geochemical background levels²⁶. The average concentrations of Cu, Zn, Pb, Cd, and Mn were 2.86, 2.77, 3.43, 10.54, and 1.12-fold higher than the local geochemical background values, respectively.

In order to research spatial distribution characteristics of metals in soils, spatial distribution maps of Cu, Zn, Pb, and Cd were generated using the ordinary kriging interpolation method (Fig. 3). The spatial patterns of the four metals were similar, with high concentrations mainly focused around Shunan and Tongguanshan, where the Xinqiao mine, Fenghuangshan mine, Linchong, Xiangsigu, and Yangshanchong tailing pond are found. The sites with Cu concentrations exceeding 120 mg/kg were primarily located near the above-mentioned mine sites (Fig. 3a). The sites with Zn (> 350 mg/kg), Pb (> 140 mg/kg), and Cd (> 4.0 mg/kg) were also primarily located near the above mine sites (Fig. 3b,c,d). Metal concentrations increased obviously with a decrease in distance from the above mine sites, consistent with the reports of a previous study in the Tongling mine³². For example, soil samples from the rape field at site 32 (R32) had extremely higher Cu, Zn, and Cd concentrations than those from other sites, which could be due to site R32 being the nearest to the Xinqiao mine (Fig. 1). Similarly, soil samples from the paddy field at site 8(P8) had extremely higher Cd concentrations than those of the other sites, which could be due to long-term irrigation using water from a river in the vicinity (Fig. 1).

Identification of sources of soil heavy metals based on the principal component analysis

Loadings of heavy metals and soil physicochemical properties based on principal component analysis (PCA) for the 81 soil samples are shown in Table 2. When the PCA was performed based only on the data of six heavy metal concentrations, two principal components (PCs) were found (eigenvalues > 1). These two components could explain 76.67% of the total variance. The first PC (PC1) had high factor loadings for Cu, Zn, Pb, Cd, and Mn, which could explain 59.80% of the total variance. Correlation analysis also showed that Cu, Zn, Pb, Cd, and Mn were highly correlated with each other (data table not presented), indicating that they originated from similar sources. The second PC (PC2) explained 16.87% of the total variance and characterized by a positive Fe loading. In addition, Fe exhibited poor correlations with Cu, Zn, Pb, Cd, and Mn. In the present study, Fe had the highest concentration in all soil samples, considering Fe is the fourth most abundant element in the Earth’s crust; nevertheless, the Fe contents were similar to soil Fe contents in other parts of southern China³³. Therefore, Fe concentrations were mainly influenced by natural factors, and PC2 could largely be attributed to non-anthropogenic sources. In addition, mean Cu, Zn, Pb, Cd, and Mn concentrations were all above the local geochemical background levels, so that PC1 may be better attributed to anthropogenic sources, since mining activities are undertaken in the study area.

We also conducted a PCA based on the heavy metals and soil physicochemical property data (Table 2). According to the results, the eigenvalues of the first four PCs were all > 1, which could explain 74.56% of the total variance. PC1 had strong positive Cu, Zn, Pb, Cd, Mn, pH, and TP loadings, explaining 37.68% of the total variance. SOM and TN were associated with PC2, indicating that PC2 reflected natural factors related to soil occurrence³⁴. PC3 was strongly correlated with AP, whereas PC4 was strongly correlated with AK, indicating that PC3 and PC4 mainly reflected the impacts of land use³⁴. Considering Cu, Zn, Pb, Cd, and Mn were attributed to PC1, and the metals exhibited poor correlations with natural and land use factors, the concentrations of the metals were considered to be mainly influenced by mining activities.

Extractable metal concentrations in soils

The concentrations of the metals extracted using five methods are summarized in Table 3. The concentrations of extractable metals were influenced significantly by the farmland use patterns and the types of extractants used. In the rape fields, concentrations of all the metals extracted using EDTA were higher than those extracted with the other four types of extractants. Similarly, in the wheat and paddy fields, Cu, Pb, and Mn concentrations extracted using EDTA were higher than those extracted with the other four types of extractants, whereas Zn, Cd, and Fe concentrations extracted using 0.1 mol/L HCl were higher than those extracted using the other extractants. Following extraction with DTPA and EDTA, the concentrations of the metals in most soil samples collected from the rape fields were in the order of Mn > Fe > Pb > Cu > Zn > Cd, and the concentrations of metals in most soil samples collected from the wheat and paddy fields were in the order of Fe > Mn > Pb > Cu > Zn > Cd. However, independent of the farmland use patterns, the concentrations of all the metals extracted using NH₄OAC and NH₄NO₃ were much lower than those extracted with the other three types of extractants.

The concentrations of 0.1 mol/L HCl-extracted heavy metals in soils from the rape fields at the R21, R23, and R32 sites, in the wheat fields at the W21 site, and in the paddy fields at the P1 and P2 sites were extremely low, especially in the cases of Fe and Pb. For example, 0.1 mol/L HCl-extracted Fe concentrations at the R21, R23, and R32 sites were 0.2 mg/kg, 0.6 mg/kg, and below the detection limit, respectively, whereas the range of 0.1 mol/L HCl-extracted Fe concentration in the other sites in the rape fields was 22.4–533 mg/kg. Therefore, 0.1 mol/L HCl-extracted heavy metal concentrations at the R21, R23, R32, W21, P1, and P2 sites were omitted from subsequent analyses to ensure homogeneity of variance. The basic pH values at the sites (pH values at R21, R23, R32, W21, P1, and P2 were 8.0, 8.0, 7.7, 8.0, 8.0, and 8.0, respectively) could explain this low HCl-extractability and reduced mobility^35,36. Although HCl is considered a universal extractant, it may not be suitable under alkaline soil conditions.

Heavy metal concentrations in rape, wheat and rice grains

Figure 4 illustrates the concentrations of six heavy metals in rape, wheat, and rice grains. Similar to the order of the soil metal concentrations, the Fe, Mn, Zn, and Cu concentrations in the grains were much higher than the Pb and Cd concentrations.

The results are similar to those reported in previous studies with regard to heavy metal concentration trends in rice grains^16,37. The reason could be Fe, Mn, Zn, and Cu are all essential for crop growth as micronutrients, leading to the higher levels in soils. Among the studied crops, rice grains accumulated relatively lower amounts of Fe, Mn, and Zn than rape and wheat grains, where Pb and Cd concentrations exhibited opposite trends. The trends are consistent with the findings of Liu et al.³⁸ and Du et al.¹, and indicate that rice has a stronger Cd uptake capacity from soil. Williams et al.³⁹ also reported higher rice Cd concentrations than wheat, barley, and maize Cd concentrations. The results could also be attributed to water management and soil oxidation–reduction status in soil-rice systems. Paddy fields are irrigated considerably more than wheat and rape fields. In addition, the water in the study area was contaminated by Cd, Cu, As, and Zn²⁷. Previous studies have also reported that water management practices influence Cd uptake by rice and its bioavailability in soils^19,20,40. In the cases of Zn and Pb, according to Feng et al.²³, rice grains accumulated lower amounts of Zn than wheat grains, whereas rice grains accumulated higher Pb amounts than wheat grains. Although soil Cd concentrations were generally high in the rape fields, the concentrations of Cd in rape grains were lower than those in the wheat and rice grains.

On the other hand, Cu concentrations in grains in all the three crops were below the maximum allowable Cu levels in food (10 mg/kg (GB 15199-94)). Similarly, Zn concentrations in rice grains were below the maximum allowable Zn levels in food (50 mg/kg (GB 13106-91)), while 25% of the rape grain samples and 20% of the wheat grain samples exceeded the threshold value. Although total Pb concentrations in soils were generally low in the three farmland use patterns, Pb concentrations in 70% of the rice grain samples exceeded the maximum allowable Pb levels in food (0.2 mg/kg (GB 2762-2005)). Only four rape and two wheat grain samples exceeded the Pb threshold values, respectively. The varying Pb trends are potentially linked to physical contamination from direct atmospheric deposition⁸, differences in physiological activities among the crops, and the fruit structures of the studied crops²³. Similar to the soil Cd contamination, 96% and 60% of wheat and rice grain samples, respectively exceeded the maximum allowable Cd levels in food (0.1 mg/kg and 0.2 mg/kg for wheat and rice, respectively (GB 2762-2005)). Furthermore, Cd concentrations in approximately 10% of the rice grain samples exceeded 1.0 mg/kg. Conversely, only 14% of rape grain samples exceeded the maximum allowable Cd levels in food (Cd: 0.1 mg/kg for rape (GB 2762-2005)), and the maximum Cd concentration in rape grains was 0.18 mg/kg. According to the results, rape grains were generally safe for consumption whereas wheat and rice grains posed health threats in the study area.

Soil to grain bioaccumulation factors

BAF values have been used widely to evaluate the capacity of crop grains to accumulate metals from soil^30,41. Similar to a previous study on food crops³⁷, Fe and Pb had the lowest BAF values (Fig. 5). Generally, metal accumulation in crop grains did not increase considerably with an increase in total concentrations of metals in soil. Heavy metal accumulation could have been regulated by crops, so that only low amounts were accumulated into grains. In addition, there were significant differences in some BAF values of the same metal across different crop species (Fig. 5). Different crop species have different accumulation capacities for the same metal⁴². Overall, the average BAF values of Cu (0.22), Zn (0.37), and Mn (0.14) in wheat grains were significantly higher than those in rape and rice grains. The average BAF value of Pb (0.005) in rice grains was significantly higher than those in rape and wheat grains, and the average BAF values of Cu (0.07) and Cd (0.06) in rape grains were significantly lower than those in wheat and rice grains. The results indicated that rape grains have lower heavy metal accumulation capacity than wheat and rice grains, except in the cases of Zn and Fe. However, the finding is not consistent with the results of a previous study⁴³, which report that grasses have lower accumulation capacity than dicotyledonous plants. Nevertheless, as mentioned above, numerous factors could influence the accumulation capacity of metals in crop grains.

Source: Ecology - nature.com