3.3. Correlation between Zn measured by SEP and 0.005 M DTPA
Extraction with DTPA provides a pragmatic estimate of ‘readilyavailable’
trace metals in soils (Lindsay and Norvell, 1978; Singh et al.,
1998) while avoiding complete dissolution of CaCO3 in calcareous
soils (F2 of the SEP) which would release occluded micronutrients
(Saffari et al., 2009). Fig. 3 shows the correlation between Zn concentration
measured in SEP fractions and by extraction with DTPA (ZnDTPA). The most coherent trend shown, with a relationship that is close to
the 1:1 line, was ZnDTPA against Zn-carbonate (Zn-F2) (Fig. 3a). It is
clear that oxide-bound Zn (Zn-F3) was far greater than ZnDTPA
(Fig. 3b) suggesting that most of the Zn-F3 was occluded within Fe
and Mn oxides. Similarly, Maria et al. (2008) studied Zn fractionation
using SEP in seven contaminated soils collected from Paulínia, São
Paulo state, Brazil; they found a non-significant correlation for Zn-DTPA
and Zn-F3. It might be expected that humus-bound Zn would be labile.
However, a plot of Zn-DTPA against humus-bound Zn (Zn-F4; Fig. 3c)
shows a high degree of scatter and much of the data suggest that Zn-
F4 N ZnDTPA suggesting strong organic binding in such high pH soils
which resists DTPA extraction, or possibly exhaustion of the DTPA extractant
by Ca-complex formation. However, Maria et al. (2008) found
highly significant correlations between Zn-DTPA and Zn-F4 in contaminated
acidic soils. Therefore, the sum of the humus-bound Zn (Zn-F4)
and carbonate-bound Zn (Zn-F2) greatly exceeded Zn-DTPA (Fig. 3d).
Nevertheless, ZnDTPA was slightly greater than Zn-F2 (Fig. 3a) and so a
tentative conclusion may be that the DTPA extractant dissolves most
Zn fromthe surface of a Ca-carbonate phase and also dissolves a smaller
proportion of humus-bound Zn. It is also important to acknowledge that
extractions are not necessarily intended to estimate the entire labile
fraction in soils. Thus, DTPA has been successfully applied as an
empirical prediction of plant uptake but its extraction capacity is
particularly limited in calcareous systems. DTPA is used at a fairly low
concentration compared to the popular EDTA extractant and it would
be expected that mainly ‘surface adsorbed’ metal would be solubilised
by DTPA. This is supported by the fact that DTPA dissolved only 1.09%
total soil Ca and 2.56% of total Fe. Thus it appears reasonable to conclude
that most ‘available’ Zn in these soils is surface-bound on CaCO3, rather
than occluded within the majority of the CaCO3 or present as a mixed
solid-solution (Ca1 − xZnxCO3).
3.3. Correlation between Zn measured by SEP and 0.005 M DTPA Extraction with DTPA provides a pragmatic estimate of ‘readilyavailable’trace metals in soils (Lindsay and Norvell, 1978; Singh et al.,1998) while avoiding complete dissolution of CaCO3 in calcareoussoils (F2 of the SEP) which would release occluded micronutrients(Saffari et al., 2009). Fig. 3 shows the correlation between Zn concentrationmeasured in SEP fractions and by extraction with DTPA (ZnDTPA). The most coherent trend shown, with a relationship that is close tothe 1:1 line, was ZnDTPA against Zn-carbonate (Zn-F2) (Fig. 3a). It isclear that oxide-bound Zn (Zn-F3) was far greater than ZnDTPA(Fig. 3b) suggesting that most of the Zn-F3 was occluded within Feand Mn oxides. Similarly, Maria et al. (2008) studied Zn fractionationusing SEP in seven contaminated soils collected from Paulínia, SãoPaulo state, Brazil; they found a non-significant correlation for Zn-DTPAand Zn-F3. It might be expected that humus-bound Zn would be labile.However, a plot of Zn-DTPA against humus-bound Zn (Zn-F4; Fig. 3c)shows a high degree of scatter and much of the data suggest that Zn-F4 N ZnDTPA suggesting strong organic binding in such high pH soilswhich resists DTPA extraction, or possibly exhaustion of the DTPA extractantby Ca-complex formation. However, Maria et al. (2008) foundhighly significant correlations between Zn-DTPA and Zn-F4 in contaminatedacidic soils. Therefore, the sum of the humus-bound Zn (Zn-F4)and carbonate-bound Zn (Zn-F2) greatly exceeded Zn-DTPA (Fig. 3d).Nevertheless, ZnDTPA was slightly greater than Zn-F2 (Fig. 3a) and so atentative conclusion may be that the DTPA extractant dissolves mostZn fromthe surface of a Ca-carbonate phase and also dissolves a smallerproportion of humus-bound Zn. It is also important to acknowledge thatextractions are not necessarily intended to estimate the entire labilefraction in soils. Thus, DTPA has been successfully applied as anempirical prediction of plant uptake but its extraction capacity isparticularly limited in calcareous systems. DTPA is used at a fairly lowconcentration compared to the popular EDTA extractant and it wouldbe expected that mainly ‘surface adsorbed’ metal would be solubilisedby DTPA. This is supported by the fact that DTPA dissolved only 1.09%total soil Ca and 2.56% of total Fe. Thus it appears reasonable to concludethat most ‘available’ Zn in these soils is surface-bound on CaCO3, ratherthan occluded within the majority of the CaCO3 or present as a mixedsolid-solution (Ca1 − xZnxCO3).
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