Soil Water Chemistry
247
solution in the upper 15 cm of soil was not sampled. Both sites followed a
cation abundance sequence in soil solution according to their respective
cation exchange complex, indicating a strong interaction with the soil matrix.
The higher concentrations observed at Prades cannot be completely attributed to its drier climate. Other factors may contribute to the differences
in soil solution chemistry between Prades and Montseny. First, the cation exchange capacity of the soil influences the soil solution chemistry: the base
saturation at Montseny is much lower than at Prades (Chap. 2; Avila et al.
1995), thus providing less solutes to the soil solution at equilibrium.
Second, the different methods of collection used at each area sampled
different phases of the soil water. Several studies obtained lower ion concentrations in soil water flow collected with zero-tension lysimeters than in soil
solution collected with low-tension lysimeters (Swistock et al. 1990; Ranger
et al.1993; Fernandez et al. 1995). Hence, differences between the low-tension
lysimetry used at Montseny and the centrifugation method used at Prades
could be expected, since the latter extracted soil water held in micropores at
much higher tensions. Zabowski and Ugolini (1990) reported larger differences between both methods in the seasonal pattern than in ion concentrations.
Third, the mean concentration of carbon dioxide in the entire soil profile
was much higher at Prades (1.7% on the upper slopes and 1.1 % on the lower
slopes of the Avic catchment) than at Montseny (0.4%; Pinol et al. 1995).
These results suggest a major influence of pC02 on soil water chemistry at
Prades through the effect of carbonic acid on weathering processes, which is
reflected in the higher alkalinity of soil waters and streamwaters at this site.
The lower pC02 observed at Montseny suggests a higher diffusivity of the
soils. Pinol et al. (1995) found that the soil of the permanent plot at Montseny
was richer in sand and poorer in silt and clay than the soils of Avic at Prades.
These features, added to the wetter climate at Montseny, suggest that water at
this site has lower residence time in soil and weaker interaction with the soil
matrix, and consequently ion concentrations in soil water are lower than at
Prades.
17.4 Soil Water Flow and Soil Solution: Soil processes
and General Remarks
Changes occurring along the profile from all collected water types indicate a
decreasing trend with depth in ion concentration at both holm oak forests.
Gravitational water collected below the forest floor was highly enriched in
K+, Ca 2 + and N03 - compared to the concentrations found in the lower layers
(Fig. 17.1; Table 17.1). Further, although soil water at Prades had higher ion
concentrations, both sites showed similar ion relationships below the forest
247
solution in the upper 15 cm of soil was not sampled. Both sites followed a
cation abundance sequence in soil solution according to their respective
cation exchange complex, indicating a strong interaction with the soil matrix.
The higher concentrations observed at Prades cannot be completely attributed to its drier climate. Other factors may contribute to the differences
in soil solution chemistry between Prades and Montseny. First, the cation exchange capacity of the soil influences the soil solution chemistry: the base
saturation at Montseny is much lower than at Prades (Chap. 2; Avila et al.
1995), thus providing less solutes to the soil solution at equilibrium.
Second, the different methods of collection used at each area sampled
different phases of the soil water. Several studies obtained lower ion concentrations in soil water flow collected with zero-tension lysimeters than in soil
solution collected with low-tension lysimeters (Swistock et al. 1990; Ranger
et al.1993; Fernandez et al. 1995). Hence, differences between the low-tension
lysimetry used at Montseny and the centrifugation method used at Prades
could be expected, since the latter extracted soil water held in micropores at
much higher tensions. Zabowski and Ugolini (1990) reported larger differences between both methods in the seasonal pattern than in ion concentrations.
Third, the mean concentration of carbon dioxide in the entire soil profile
was much higher at Prades (1.7% on the upper slopes and 1.1 % on the lower
slopes of the Avic catchment) than at Montseny (0.4%; Pinol et al. 1995).
These results suggest a major influence of pC02 on soil water chemistry at
Prades through the effect of carbonic acid on weathering processes, which is
reflected in the higher alkalinity of soil waters and streamwaters at this site.
The lower pC02 observed at Montseny suggests a higher diffusivity of the
soils. Pinol et al. (1995) found that the soil of the permanent plot at Montseny
was richer in sand and poorer in silt and clay than the soils of Avic at Prades.
These features, added to the wetter climate at Montseny, suggest that water at
this site has lower residence time in soil and weaker interaction with the soil
matrix, and consequently ion concentrations in soil water are lower than at
Prades.
17.4 Soil Water Flow and Soil Solution: Soil processes
and General Remarks
Changes occurring along the profile from all collected water types indicate a
decreasing trend with depth in ion concentration at both holm oak forests.
Gravitational water collected below the forest floor was highly enriched in
K+, Ca 2 + and N03 - compared to the concentrations found in the lower layers
(Fig. 17.1; Table 17.1). Further, although soil water at Prades had higher ion
concentrations, both sites showed similar ion relationships below the forest