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Fe oxides must be taken into account to interpret these results (see Buurman et al., this
volume).
The Alp/Al0 ratio was <0.5 in most samples, indicating that the reactive A1 pool is
dominated by inorganic constituents. The content in allophane, estimated according to Mizota
& van Reeuwijk (1989), which is well-correlated with allophane contents estimated from Si0
(r2=0.93), ranged from negligible in the soils from Greece, Hungary and Italy (EUR01 and
02) to more than 20% in some horizons fforn France and Azores. In general, the higher
allophane contents occur in horizons with pHwater between 5.5-6.5. The average Al/Si molar
ratio ranged from 1.7 in the soils from Iceland to 2.7 in those from Azores, with intermediate
values in the other soils (1.9, in Italy; 2.1, in France; 2.6, in Tenerife). In spite of the
dominance of the inorganic Al-pool, Al-humus complexes are an important component in
many soils. Their proportion increases with carbon content (Alp/Al0 vs C content r2=0.76,
excluding the soils with very low Alp). In most horizons, CuCl2 extracted about of 35% of Alp
(r2=0.96), suggesting that it constitutes a specific fraction of organically-complexed A1
(García-Rodeja et al., 2004). In many mineral horizons with allophane, Alcu is highly
correlated to CEC (r2=0.76), which suggests that A1 dominates the adsorption complex.
The content in ferrihydrite (calculated from Fe0) is usually less than 2%, although it exceeds
4% in some horizons from Azores and France. As with allophane, the higher ferrihydrite
contents occur in the pH range 5.5-6.5. Calculating ferrihydrite directly from Fe0 suggests that
there is no extractable Fe in humus complexes. Because pyrophosphate extracts quantities
equal to 5-100% of Fe0, this assumption is irrealistic. Fep clearly increases with C content in
soils EUR03 and 04 from Italy (r2=0.99) and in those from Azores, Iceland and France (r2
0.73-0.75, excluding horizons with very high C contents). Extractable manganese is clearly
more abundant in the surface horizons of the soil EUR07. Citrate-dithionite extracted from
>80% (soil EUR20) to <10% (soil EUR09) of Mn,. Mn0 was close to Mnd in the soils from
Hungary or France, but clearly lower in EUR10 and 12. Good correlations of Mnt with Mn^
(r2=0.96) and of Mnd with Mn0 (r2=0.97) were found for the whole data set, but r2 decreases if
the 3 horizons richer in Mn are excluded from the regression analysis.
References
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