ENVIRONMENTAL IMPACT OF PHOSPHATE FERTILIZERS AND BY-PRODUCTS ON AGRICULTURAL SOILS - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 58

Zn, Pb, Cu versus Cd
The studied elements were mainly associated to the reducible and residual fractions in
reference soil. The application of PG or Phosphate Fertilizers induced a variation in soil
speciation. Fe/Mn oxides and clays were the most susceptible to changes that resulted in an
increase of Pb, Zn and Cu solubility in soil profile. The presence of soluble phosphates and
sulfates in PF amendment (at least 85-90% of the total P in PF is water-soluble) (Chien et al.
2011) may have had a significant impact on Pb, Zn and Cu solubility. Moreover, phosphate
fertilizers speciation showed that these elements were associated to the mobile fraction
(exchangeable fraction F1) in relative high percent. After fifteen months of amendment
(parcel P3), comparing to the reference plot, the mobile fractions and mainly the
exchangeable fraction (F1) showed increments in Cu, Pb and Zn amounts relative to the
reference (14%, 7.7%, 35.5% for Cu; 23%, 32.9%, 20% for Zn at 0-20, 20-35 and 35-55 cm
respectively; and 48%, 72.4% for Pb at 20-35 and 35-55 cm, respectively), which could
mobilize these TEs in soil profile, imposing a threat to the ground water. On the contrary, Cd
was sorbed in soil particles (clays and Fe/Mn oxides and hydroxides) over the study period.
These results were opposite to those of phosphogypsum-amended soil where Cd, after the
study period (16 months) was mostly in the exchangeable fraction, whereas Pb, Zn and Cu
were sorbed in soil particles (Kassir et al. 2012a). However, phosphogypsum application
increased the solublization of the studied elements where they were associated to the
exchangeable and acid-soluble fractions in the phosphogypsum which caused their transfer
during winter season to soil particles and increased their concentration in soil.
Trace Elements Transfer from Soil to Cichorium Intybus: Phosphate Fertilizers
The TE transfer factor (TF) is a measure of its availability to plant uptake. There are
many factors that can influence the TF, including soil properties and TE chemical phases. The
exchangeable fraction (F1) is readily available and considered to be the primary nutrient
source for plants (Narwal and Singh 1998). The fraction associated with carbonate (F2) is
potentially available for plant uptake, depending on soil pH conditions, whereas TE in the
other chemical fractions (oxides-F3, organic-F4, and residual-F5) with minimal solubility and
very low mobility are not involved with the plants biological activities (Xian 1989). Thus,
during the experiment course, the main changing factors affecting TEs bioavailability, hence
their TFs, are the soil moisture content (associated with rainfall), soil pH (influenced by PF
acidity), and the respective TEs leaching and available fraction concentrations. Cd exhibited
the highest TF (0.32 ± 0.04 — 0.52 ± 0.05) among the other TEs in all parcels, followed by
Cu (0.16 ± 0.02 — 0.27 ± 0.03). Pb and Zn had comparable TFs (0.08 ± 0.01 — 0.17 ± 0.02)
in all parcels, except in P3 where it increased to 0.20 ± 0.03 for Pb, while remained in the
same range for Zn (0.11 ± 0.01).
In comparison with the reference parcel, it can be seen from Figure 7 that the TF of Cd,
thus its bioavailability, increased in P1 and P2 by about 49% and 78%, respectively, while it
dropped back to near its reference value in P3. This variation in Cd bioavailability is in line
with its chemical fraction changes depicted in Figure 6. In fact, in the root zone (0-20 cm), the
Cd pH-susceptible mobile fraction F2 increased from 0.102 mg kg -1 in parcel R to 0.203 mg
kg -1 in P1 (Figure 6), simultaneously with a drop in the layer average pH from 8.23 ± 0.54 to
6.92 ± 0.45. This was followed by a phase transformation yielding 0.303 mg kg -1 of Cd
exchangeable fraction (F1) in P2, and the depletion of the mobile fractions in P3 (Figure 6).