Agriculture Reference
In-Depth Information
At T3 (fifteen months after PF amendment), the reducible and residual fractions were
dominant. The change in Cd speciation over the study period can be attributed to phosphate
fertilizer application decreasing soil pH in parcels P1 and P2 at the surface layer (0-20 cm). It
is well reported that under acid conditions, Cd was found accumulated in the exchangeable
fraction and acid soluble fraction (F1 and F2) (Castillo-Carrion et al. 2007; Li and Thornton
2001). In the end of our study, under alkaline soil conditions, Cd was adsorbed to iron and
manganese oxides, organic matter, and clays structure.
Pb Speciation
Concerning PG application, Pb revealed a higher mobility in the amended plot P1 (5
months following PG application) than the reference plot R where Pb appeared in the
exchangeable fraction F1 in all depths (2.29 mg kg -1 at 0-20 cm; 1.87 mg kg -1 at 20-35 cm;
0.91 mg kg -1 at 35-55 cm) (Figure 5). At the surface layer (0-20 cm), the amount of Pb in the
mobile fractions (F1 and F2) were 20 and 40% in R and P1, respectively. This increase was in
the detriment of Fe and Mn oxides (45 and 22 %). This represents a shift of 20% to the
mobile fractions. Pb became in a soluble phase and easily translocated down the profile as pH
decreased in parcel P1. Pb associated to the mobile fractions (F1 and F2) predominated in
parcel P2 (12 months following PG amendment). In parcel P2 (after the wet period), and
comparing to P1, this continuous increase of Pb content in the mobile fractions (14, 12 and
20% in 0-20, 20-35, 35-55 cm, respectively), compared to clays and iron oxides fractions,
was due to Pb release associated with acid soluble fraction of PG (30%) during rainy periods.
At T3 (16 months after PG amendment, summer period), Pb was found related to reducible
and clays fractions (F3 and F5) however the amount of exchangeable Pb fraction (F1) in
parcel P3 was significant at 0-20 cm and 35-55 cm depth (4.84 and 4.32 mg kg -1 ,
respectively) while Pb was found accumulated (4.02 mg kg -1 ) in the organic matter and sulfur
phase (F4) at 20-35 cm layer. In deed, 18% related to oxidizable fraction was released from
PG (in dry condition), due to the high porosity of PG (Rabi and Mohamad 2006) and
adsorbed to soil constituents.
On the other hand, when phosphate fertilizers was applied, 81% of Pb was mainly related
to the acid soluble fraction (F2) in layer 20-35 cm and 88% to the exchangeable fraction (F1)
in layer 35-55 cm at the end of the study (15 months following Pf application) (Figure 6).
Hence, Pb solubility increased in depth layers 20-55 cm. However, it is remarkable the
appearance of Pb in exchangeable (F1) and oxidizable fractions (F4) in parcels P1 and P2 (4
and 11 months respectively following PF application) comparing to reference plot R. The
appearance of Pb in the exchangeable fraction F1 in parcels P1 and P2 (acid pH) is in
agreement with different studies (Castillo-Carrion et al. 2007). It can be ascribed to the input
of phosphate fertilizer with speciation results showing that 38% of Pb occurred in the
exchangeable fraction F1. Pb binding to oxidizable fraction F4 has been reported in the
literature (Chaney et al. 1988), generally in the detriment of the residual fraction (F5). The
presence of complexing and organic ligands and competing cations, decreasing Pb sorption,
could substantially enhance its mobility (Kotuby-Amacher and Gambrell 1988; Puls et al.
1991), which may lead to reinstating, to some extent, the exchangeable fraction F1 (P3 layer
35-55 cm).
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