244
Noria Melia. Juan Bellot and V. Ramon Vallejo
a high percentage of the total charge if compared with that from the deep
subsurface flow (Fig. 17.1, Table 17.1).
17.3.1.1 TEMPORAL VARIABILITY
Ion concentrations in the soil solution varied seasonally (Avila et al. 1995):
they were highest at both depths in the first samples collected after the summer drought, tended to decrease gradually through the winter, and remained
low or recovered somewhat through the spring until the lysimeters dried out
in summer. This pattern applied to all ions at both depths, except by K+ and
N03 - whose variations were more erratic.
17.3.2 Centrifuge Solution at Prades
The soil solutions obtained by centrifugation at Prades were highly concentrated (Table 17.1). The solution chemistry was dominated by Ca 2 + which, averaged across soil horizons, made up 620/0· of the total cation charge, and by
S04 2 - and CI-, with 38% and 390/0 of the total analyzed anion concentrations,
respectively. The cation sequence, in order of abundance, was Ca 2 + > Mg2+ >
K+ > Na+ > NH4 +, and, similar to Montseny paralelled the abundances observed in the cation exchange complex of the soil matrix (Chap. 2). The positive charge imbalance found in soil solution modified the anion abundance
sequence with respect to the total anionic charge. This anion charge deficit
contributed to 270/0 of the total charge, as opposed to 280/0 of CI- and 280/0 of
SOl-, and showed a strong linear relationship with the analyzed dissolved
organic carbon (r = 0.81, P < 0.0001).
17.3.2.1 CHANGES ALONG THE PROFILE
Consistent changes in ion concentration and ion dominance occurred with
depth. All ion concentrations except Na+ decreased significantly along the
profIle (P < 0.0001; Table 17.1). pH was also significantly higher in the upper
horizon. This decreasing pattern with depth modified the distribution of the
anion dominance in the horizons studied. The anion charge deficit, which we
assume is mostly due to DOC charge, made the major contribution (330/0) to
the total anion charge in the upper horizon. This 'DOC buffer capacity',
added to the 170/0 bicarbonate alkalinity, made up nearly 500/0 of the total
anion charge in this horizon. The anion charge deficit declined in the deeper
horizons, where the CI- and S04 2 - codominance arose.
17.3.2.2 SPATIAL VARIABILITY
Few differences were observed between the two altitudinal extremes of the
catchment. Only Mg2+ concentrations were significantly lower on the upper
slopes, with mean values of 698, 549 and 267 ~eq L- 1 at 0-2.5 cm, 2.5-15 cm
and 15-30 cm depths, respectively, than in the valley bottom (1125,717 and
410 ~eq L- 1 at the same three depths). This finding agrees with the higher
Mg2+ concentration in holm oak leaves found in the valley bottom of the
catchment (Sabate 1993).
Noria Melia. Juan Bellot and V. Ramon Vallejo
a high percentage of the total charge if compared with that from the deep
subsurface flow (Fig. 17.1, Table 17.1).
17.3.1.1 TEMPORAL VARIABILITY
Ion concentrations in the soil solution varied seasonally (Avila et al. 1995):
they were highest at both depths in the first samples collected after the summer drought, tended to decrease gradually through the winter, and remained
low or recovered somewhat through the spring until the lysimeters dried out
in summer. This pattern applied to all ions at both depths, except by K+ and
N03 - whose variations were more erratic.
17.3.2 Centrifuge Solution at Prades
The soil solutions obtained by centrifugation at Prades were highly concentrated (Table 17.1). The solution chemistry was dominated by Ca 2 + which, averaged across soil horizons, made up 620/0· of the total cation charge, and by
S04 2 - and CI-, with 38% and 390/0 of the total analyzed anion concentrations,
respectively. The cation sequence, in order of abundance, was Ca 2 + > Mg2+ >
K+ > Na+ > NH4 +, and, similar to Montseny paralelled the abundances observed in the cation exchange complex of the soil matrix (Chap. 2). The positive charge imbalance found in soil solution modified the anion abundance
sequence with respect to the total anionic charge. This anion charge deficit
contributed to 270/0 of the total charge, as opposed to 280/0 of CI- and 280/0 of
SOl-, and showed a strong linear relationship with the analyzed dissolved
organic carbon (r = 0.81, P < 0.0001).
17.3.2.1 CHANGES ALONG THE PROFILE
Consistent changes in ion concentration and ion dominance occurred with
depth. All ion concentrations except Na+ decreased significantly along the
profIle (P < 0.0001; Table 17.1). pH was also significantly higher in the upper
horizon. This decreasing pattern with depth modified the distribution of the
anion dominance in the horizons studied. The anion charge deficit, which we
assume is mostly due to DOC charge, made the major contribution (330/0) to
the total anion charge in the upper horizon. This 'DOC buffer capacity',
added to the 170/0 bicarbonate alkalinity, made up nearly 500/0 of the total
anion charge in this horizon. The anion charge deficit declined in the deeper
horizons, where the CI- and S04 2 - codominance arose.
17.3.2.2 SPATIAL VARIABILITY
Few differences were observed between the two altitudinal extremes of the
catchment. Only Mg2+ concentrations were significantly lower on the upper
slopes, with mean values of 698, 549 and 267 ~eq L- 1 at 0-2.5 cm, 2.5-15 cm
and 15-30 cm depths, respectively, than in the valley bottom (1125,717 and
410 ~eq L- 1 at the same three depths). This finding agrees with the higher
Mg2+ concentration in holm oak leaves found in the valley bottom of the
catchment (Sabate 1993).