ENVIRONMENTAL IMPACT OF PHOSPHATE FERTILIZERS AND BY-PRODUCTS ON AGRICULTURAL SOILS - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 57

Zn Speciation
Phosphogypsum application varied Zn speciation only in parcels P1 and P2 (5 and 12
months following PG application) to finally be restored as reference speciation at the end of
the study (16 months following PG application) where the reducible and the residual fractions
(F3 and F5) were again dominant in the soil control section under study (Figure 5). However,
the mobile fractions (F1 and F2) increased in the soil profile of parcel P1 (5 months following
PG application). Indeed, the amount of Zn in the mobile fractions at the layer 0-20 cm is 3
and 16% for parcels R and P1, respectively. This increase was in the detriment of clays
fractions (F5). This represents a relative shift of 13% to the mobile fractions. Moreover, Zn
was found mostly associated to the oxidizable fraction (F4) in parcel P2 (12 months following
PG application). With pH increase due to soil buffering capacity, Zn was finally intercepted
by reactive negatively charged soil constituents such as phyllosilicates and iron oxides
(Fernandez et al. 2007; Van Oort et al. 2006).
Contrary, Phosphate fertilizer application varied Zn speciation in the soil horizons where
the greatest proportion of Zn resided in the oxidizable fraction (F4) in parcel P1 (4 months
following PF application, 42 and 75% in layers 0-20 cm and 20-35 cm, respectively), in the
detriment of the residual fraction (F5) (Figure 6). This can be related to the soil organic
matter's affinity for soluble Zn (Kabata-Pendias and Pendias 1984). Zn was mainly sorbed in
clays in parcel P2, 11 months following PF application. At the end of the study (15 months
after PF application), Zn mobility was modified, where Zn in the exchangeable and acid
soluble fractions (F1 and F2) was in relatively higher concentrations of 21.90 mg kg -1 -26%,
31.94 mg kg -1 -37%, and 24.40 mg kg -1 -27% at 0-20, 20-35 and 35-55 cm, respectively. Zn
solubility increased, comparing with parcel P2, in the detriment of residual fraction (at 0-20
and 35-55 cm) and reducible fraction (at 20-35 cm) (net shifts of 14%, 15% and 27% at 0-20,
20-35 and 35-55 cm, respectively, were observed from Fe/Mn oxides and clay fractions to the
mobile fractions (F1 and F2).
Cu Speciation
Cu was found in a soluble phase following phosphogypsum application in parcel P1
where Cu was associated to the mobile fractions (exchangeable and acid-soluble) in the soil
profile (till depth of 55 cm), due to pH decease (Figure 5). In fact, the presence of Cu in the
mobile fractions of parcel P1 increased with depth. This increase continued in parcel P2 only
at depth 35-55 cm during winter season to reach 11 %. At the end of the experiment (16
months), Cu was partitioned in the reducible, oxidizable and alumino-silicate fractions of the
soil in P3 as reference speciation. However, 19% of Cu content was present in the mobile
fraction (F1 and F2) in the layer 20-35 cm. Phosphate fertilizers application induced a
mobilization of Cu in the soil profile of parcel P1 (winter period) (3% to 32%, 3% to 35%,
and 3% to 23% increase in mobile fractions (F1 and F2) at 0-20, 20-35 and 35-55 cm,
respectively, comparing R to P1), with a decrease occurring mainly in the fraction associated
with clays (F5) (63% to 32%; 63% to 35%; 64% to 36% at 0-20, 20-35 and 35-55 cm,
respectively) (Figure 6). At parcel P2, (summer period), Cu concentrations decreased mainly
in the mobile fractions and particularly at layer 0-35cm, this may be due to plants absorption
at this period. At the end of the experiment in P3, Cu was partitioned in the reducible,
oxidizable and alumino-silicate fractions of the soil. However, 17% and 39% of Cu content
was present in the exchangeable fraction (F1) in layers 0-20 and 35- 55 cm.