Element Budgets in Catchments
291
NH4 + were always below the analytical detection limit (2 Jleq L -I) during
baseflow.
On average, K+ was also retained at Prades and Montseny. Inputs of K+ in
bulk deposition were very moderate (Table 20.2), but weathering constituted
an important source of K+ for the system: the K+ release by weathering at
Montseny would amount to 50 kg ha- I year l if congruent dissolution of the
rock occurred. However, musc9vite - the major K-bearing mineral at our
Montseny site - weathers at a lower rate than sodium plagioclase - the major
Na-bearing mineral-, whose content in bedrock was used together with Na+
net output fluxes for computing congruent dissolution rates (Sect. 20.5).
Nonetheless, the actual weathering rate of K+, jointly with atmospheric deposition (1.4 kg ha- I year l ), probably far exceeds the outputs from the soil,
which include 5.0 kg ha- I yearl in aboveground biomass retention (Table
18.6) and 1.2 kg ha- I yearl in draining waters. Equilibrium reactions in the
soil between the soil solution and the exchange complex must exist to account for the constancy of K+ concentrations in the streamwaters at low flows
(Avila et al. 1992). The low export in the streamwaters could result from the
formation of secondary minerals in the soil, tightly controlling K+ mobility
and circulation. However, the net retention of atmospheric inputs at both
sites indicates the tendency to incorporate all available K+ from bulk deposition into ecosystem compartments, either in the vegetation or the soils.
20.4.2 Elements in Balance: Chloride and Sulphur
Nutrients in balance (exports similar to inputs) are assumed to circulate
quantitatively through the system. For CI- this is the expected behaviour because there are no important CI- sources or sinks in the soils or vegetation of
the catchments, and CI- budgets have been widely used as a test for the water-tightness of the catchment (Juang and Johnson 1967). At our sites, the average CI- budget, which included an estimation of dry deposition onto forest
canopies, was close to zero for TMO and somewhat negative for Avic (Table
20.2; Figs. 20.1 and 20.2). However, the deviation from zero was small, at most
20% of the input or output fluxes. These small deviations could result from
an inaccurate estimation of CI- input in dry deposition and, for Prades, from
CI- retention during dry years. The CI- balance was slightly positive for TM9,
but it must be taken into account that, as explained in Chapter 2, the computed surface area of this catchment was adjusted to give a balanced CIbudget.
Sulphur presented a pattern similar to that of CI-. At Montseny, there was
an S net export of around 3 kg ha- I year- I (averaged for TM9 and TMO; Table
20.2), but at Prades it was close to zero. Because the inputs do not include S
dry deposition, and because the phillytes at Montseny are not a significant
source of S (Casas 1979), at Montseny this balance could represent the unmeasured dry deposition S flux. Measurements at Montseny indicate a net S
flux in throughfall and stemflow of 1.2 kg S ha- I year l (Chap. 15). Part of this
291
NH4 + were always below the analytical detection limit (2 Jleq L -I) during
baseflow.
On average, K+ was also retained at Prades and Montseny. Inputs of K+ in
bulk deposition were very moderate (Table 20.2), but weathering constituted
an important source of K+ for the system: the K+ release by weathering at
Montseny would amount to 50 kg ha- I year l if congruent dissolution of the
rock occurred. However, musc9vite - the major K-bearing mineral at our
Montseny site - weathers at a lower rate than sodium plagioclase - the major
Na-bearing mineral-, whose content in bedrock was used together with Na+
net output fluxes for computing congruent dissolution rates (Sect. 20.5).
Nonetheless, the actual weathering rate of K+, jointly with atmospheric deposition (1.4 kg ha- I year l ), probably far exceeds the outputs from the soil,
which include 5.0 kg ha- I yearl in aboveground biomass retention (Table
18.6) and 1.2 kg ha- I yearl in draining waters. Equilibrium reactions in the
soil between the soil solution and the exchange complex must exist to account for the constancy of K+ concentrations in the streamwaters at low flows
(Avila et al. 1992). The low export in the streamwaters could result from the
formation of secondary minerals in the soil, tightly controlling K+ mobility
and circulation. However, the net retention of atmospheric inputs at both
sites indicates the tendency to incorporate all available K+ from bulk deposition into ecosystem compartments, either in the vegetation or the soils.
20.4.2 Elements in Balance: Chloride and Sulphur
Nutrients in balance (exports similar to inputs) are assumed to circulate
quantitatively through the system. For CI- this is the expected behaviour because there are no important CI- sources or sinks in the soils or vegetation of
the catchments, and CI- budgets have been widely used as a test for the water-tightness of the catchment (Juang and Johnson 1967). At our sites, the average CI- budget, which included an estimation of dry deposition onto forest
canopies, was close to zero for TMO and somewhat negative for Avic (Table
20.2; Figs. 20.1 and 20.2). However, the deviation from zero was small, at most
20% of the input or output fluxes. These small deviations could result from
an inaccurate estimation of CI- input in dry deposition and, for Prades, from
CI- retention during dry years. The CI- balance was slightly positive for TM9,
but it must be taken into account that, as explained in Chapter 2, the computed surface area of this catchment was adjusted to give a balanced CIbudget.
Sulphur presented a pattern similar to that of CI-. At Montseny, there was
an S net export of around 3 kg ha- I year- I (averaged for TM9 and TMO; Table
20.2), but at Prades it was close to zero. Because the inputs do not include S
dry deposition, and because the phillytes at Montseny are not a significant
source of S (Casas 1979), at Montseny this balance could represent the unmeasured dry deposition S flux. Measurements at Montseny indicate a net S
flux in throughfall and stemflow of 1.2 kg S ha- I year l (Chap. 15). Part of this