292
Anna Avila, Juan Bellot and Josep PHiol
flux could derive from internal foliar leaching but isotopic studies in a temperate deciduous forest revealed that dry deposition accounted for 85% of
the net throughfall S flux (Lindberg and Garten 1988). Using evidence from
net throughfall fluxes and from the S budgets, it can be concluded that S dry
deposition at Montseny lies between 1 and 3 kg ha- l year-I.
As described in Chapter 14, there has been a significant decrease in S inputs in the Mediterranean area over the last 15 years. Because of these diminishing S inputs, one could expect a decreasing trend in the sol- streamwater concentrations or in the S annual exports. However, both at Prades and
Montseny, the variation of the annual S exports was more strongly associated
with variations in the annual runoff amount (r = 0.99, P < 0.001 at Montseny;
r = 0.62, P< 0.05 at Prades) than with any temporal trend.
20.4.3 Elements with Net Exports
Magnesium was always exported at Prades and Montseny. Calcium presented
net retentions for some years at both sites. For Na+, some differences existed
between sites: it was always exported at Montseny, but was retained at Prades
in two of the driest years (Fig. 20.1). As discussed above, the chemistry of
Prades streamwaters is strongly influenced by the flushing capacity of
autumn and winter rainstorms. Although Na+ was retained in some years,
there was a net Na+ export from the Avic catchment in Prades over the 12year period (Table 20.2). The Teula catchment, adjacent to Avic, also presented net Na+ exports during 2 years of record (PiiioI1990).
Calcium net exports were positive, and presented a good correlation with
runoff, except for the years with an important contribution of red rains. It is
well known that red rains contain high amounts of dissolved calcium (L6yePilot et al. 1986; Roda et al. 1993; Avila et al. 1998), so that bulk deposition
inputs are much higher in years with a number of red rains than in years
with negligible red rain. However, years with increased Ca2+ inputs due to red
rains did not show increased outputs related to this Ca2+ availability. In fact,
at Montseny it has been clearly observed that net exports were lower in years
with red rains compared with other years (Fig. 20.1), implying that the soils
effectively retained this extra input, probably increasing their base saturation. At Prades, Ca2+ net exports were much higher and the effects of red
rains on net outputs were not so evident (Fig. 20.2). Averaging over the
whole record, Ca2+ presented a clear net export at Prades of 17 kg ha- l yearl,
but at Monseny the average net output was only 3-4 kg ha- l yearl. A further
consideration is that red rains not only contribute important amounts of dissolved Ca2+, but also transport and deposit a silty-clayish dust rich in calcite
(Guerzoni and Chester 1996). The dissolution of this calcite in soils would be
another pathway of Ca 2 + input to the catchments. The amount of dust deposited at Montseny has been estimated as 5 g m- 2 year l over an II-year period (Avila et al. 1996). However, the calcite content in red rains was highly
variable, depending on the source regions providing the dust in North Africa.
Anna Avila, Juan Bellot and Josep PHiol
flux could derive from internal foliar leaching but isotopic studies in a temperate deciduous forest revealed that dry deposition accounted for 85% of
the net throughfall S flux (Lindberg and Garten 1988). Using evidence from
net throughfall fluxes and from the S budgets, it can be concluded that S dry
deposition at Montseny lies between 1 and 3 kg ha- l year-I.
As described in Chapter 14, there has been a significant decrease in S inputs in the Mediterranean area over the last 15 years. Because of these diminishing S inputs, one could expect a decreasing trend in the sol- streamwater concentrations or in the S annual exports. However, both at Prades and
Montseny, the variation of the annual S exports was more strongly associated
with variations in the annual runoff amount (r = 0.99, P < 0.001 at Montseny;
r = 0.62, P< 0.05 at Prades) than with any temporal trend.
20.4.3 Elements with Net Exports
Magnesium was always exported at Prades and Montseny. Calcium presented
net retentions for some years at both sites. For Na+, some differences existed
between sites: it was always exported at Montseny, but was retained at Prades
in two of the driest years (Fig. 20.1). As discussed above, the chemistry of
Prades streamwaters is strongly influenced by the flushing capacity of
autumn and winter rainstorms. Although Na+ was retained in some years,
there was a net Na+ export from the Avic catchment in Prades over the 12year period (Table 20.2). The Teula catchment, adjacent to Avic, also presented net Na+ exports during 2 years of record (PiiioI1990).
Calcium net exports were positive, and presented a good correlation with
runoff, except for the years with an important contribution of red rains. It is
well known that red rains contain high amounts of dissolved calcium (L6yePilot et al. 1986; Roda et al. 1993; Avila et al. 1998), so that bulk deposition
inputs are much higher in years with a number of red rains than in years
with negligible red rain. However, years with increased Ca2+ inputs due to red
rains did not show increased outputs related to this Ca2+ availability. In fact,
at Montseny it has been clearly observed that net exports were lower in years
with red rains compared with other years (Fig. 20.1), implying that the soils
effectively retained this extra input, probably increasing their base saturation. At Prades, Ca2+ net exports were much higher and the effects of red
rains on net outputs were not so evident (Fig. 20.2). Averaging over the
whole record, Ca2+ presented a clear net export at Prades of 17 kg ha- l yearl,
but at Monseny the average net output was only 3-4 kg ha- l yearl. A further
consideration is that red rains not only contribute important amounts of dissolved Ca2+, but also transport and deposit a silty-clayish dust rich in calcite
(Guerzoni and Chester 1996). The dissolution of this calcite in soils would be
another pathway of Ca 2 + input to the catchments. The amount of dust deposited at Montseny has been estimated as 5 g m- 2 year l over an II-year period (Avila et al. 1996). However, the calcite content in red rains was highly
variable, depending on the source regions providing the dust in North Africa.