Element Budgets in Catchments
293
By considering the dust amounts from three identified source regions in
North Africa (western Sahara, Moroccan Atlas and central Algeria) and their
corresponding mean Ca 2 + concentration, the Ca z + deposited with the African
dust was estimated to be 3 kg ha- 1 year-I (Avila et al. 1998). If this amount was
included as an input in the budgets, then the net Ca z + export would tend to
zero at Montseny and would be reduced to around 14 kg ha- I year- I at Prades.
The net export for the main base cations (Ca z +, Mg2+ and Na+) together
with HC03 - and SiOz net outputs indicate active weathering of the bedrock
within the catchments (Johnson et al. 1968, 1981). At Prades, Ca2+ net export
is by far the greatest export, followed by Mgz+ and Na+. At Montseny, the
greatest net export is for Na+, followed by Mgz+ and Ca z +. The differences in
the release of the different base cations have to be interpreted in view of differences in hydrology (implying the water availability for solute export), bedrock mineralogy and weathering rates at both sites.
20.5 Weathering Rates
Mass balance studies are considered a reliable method for estimating mineral
weathering rates in nature (Clayton 1979). The method assumes that the rate
of export of base cations from the system indicates the rate at which the bedrock is being weathered. For example, Na+ has been widely used to obtain
weathering estimates from catchment studies (Johnson et al. 1968; Feller
1981; Paces 1985, 1986). This approach assumes that the annual release of
Na+ by weathering equals the net export of Na+ in the streamwaters plus the
small amount of Na+ annually accumulated in the accreting biomass. Other
cations could be used as reference, but Na+ is not incorporated into secondary minerals or accumulated in the plant biomass to such an extent as K+,
Ca2+ or Mgz+. A further assumption is that Na+ is exported at the same rate as
is generated by weathering and that, in consequence, the soil exchange complex is in steady state for Na+, a reasonable approximation since the pool of
exchangeable Na+ is usually small.
The annual net Na+ export from our Montseny catchments (11.8 kg ha- I
year-I, averaged for TMO and TM9) plus the amount stored annually in new
aboveground biomass (0.06 kg ha- I year-I; Fern~s et al. 1984) produced an
annual Na+ export of 11.9 kg ha- I year-I. The mean bedrock NazO content
was 0.76% in four samples of phyllites around the experimental catchments
(Casas 1979). With these figures, 1500 kg of bedrock ha- I year- I must weather
annually to provide for the exported Na+ in streamwaters and in the growing
biomass at the site. For Prades, the weathering rates were estimated at 655 kg
ha- I year- I using Na+ net exports as done at Montseny, and 632 kg ha- I year- I
by using K+ and Mgz+ as the relevant elements (Piii.oI1990). From these values, weathering seems to proceed at a higher rate at Montseny than at Prades.
293
By considering the dust amounts from three identified source regions in
North Africa (western Sahara, Moroccan Atlas and central Algeria) and their
corresponding mean Ca 2 + concentration, the Ca z + deposited with the African
dust was estimated to be 3 kg ha- 1 year-I (Avila et al. 1998). If this amount was
included as an input in the budgets, then the net Ca z + export would tend to
zero at Montseny and would be reduced to around 14 kg ha- I year- I at Prades.
The net export for the main base cations (Ca z +, Mg2+ and Na+) together
with HC03 - and SiOz net outputs indicate active weathering of the bedrock
within the catchments (Johnson et al. 1968, 1981). At Prades, Ca2+ net export
is by far the greatest export, followed by Mgz+ and Na+. At Montseny, the
greatest net export is for Na+, followed by Mgz+ and Ca z +. The differences in
the release of the different base cations have to be interpreted in view of differences in hydrology (implying the water availability for solute export), bedrock mineralogy and weathering rates at both sites.
20.5 Weathering Rates
Mass balance studies are considered a reliable method for estimating mineral
weathering rates in nature (Clayton 1979). The method assumes that the rate
of export of base cations from the system indicates the rate at which the bedrock is being weathered. For example, Na+ has been widely used to obtain
weathering estimates from catchment studies (Johnson et al. 1968; Feller
1981; Paces 1985, 1986). This approach assumes that the annual release of
Na+ by weathering equals the net export of Na+ in the streamwaters plus the
small amount of Na+ annually accumulated in the accreting biomass. Other
cations could be used as reference, but Na+ is not incorporated into secondary minerals or accumulated in the plant biomass to such an extent as K+,
Ca2+ or Mgz+. A further assumption is that Na+ is exported at the same rate as
is generated by weathering and that, in consequence, the soil exchange complex is in steady state for Na+, a reasonable approximation since the pool of
exchangeable Na+ is usually small.
The annual net Na+ export from our Montseny catchments (11.8 kg ha- I
year-I, averaged for TMO and TM9) plus the amount stored annually in new
aboveground biomass (0.06 kg ha- I year-I; Fern~s et al. 1984) produced an
annual Na+ export of 11.9 kg ha- I year-I. The mean bedrock NazO content
was 0.76% in four samples of phyllites around the experimental catchments
(Casas 1979). With these figures, 1500 kg of bedrock ha- I year- I must weather
annually to provide for the exported Na+ in streamwaters and in the growing
biomass at the site. For Prades, the weathering rates were estimated at 655 kg
ha- I year- I using Na+ net exports as done at Montseny, and 632 kg ha- I year- I
by using K+ and Mgz+ as the relevant elements (Piii.oI1990). From these values, weathering seems to proceed at a higher rate at Montseny than at Prades.