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Anna Avila, Juan Bellot and Josep Pinol
However, estimates of rock weathering are subjected to methodological
uncertainties concerning the rock composition, bedrock heterogeneity, incongruent dissolution of minerals, element retention in the weathered residuum, and interaction with the soil exchange complex. A more straightforward concept has been recently proposed, the net soil release (Likens and
Bormann 1995). Net soil release for cations is the sum of the net outputs of
cation equivalents (either in the draining waters or in the growing biomass),
and results from the interaction of various processes: atmospheric inputs,
mineral weathering, accumulation/depletion of the cation exchange complex,
secondary mineral formation, storage in soil organic matter, and plant nutrient uptake (Likens and Bormann 1995). The absolute field rates of some of
these processes are difficult to ascertain in catchment studies. From data in
Tables 18.6 and 20.2, net soil cation release averaged 2.3 keq ha- l yearl at
Montseny (TM9) and 1.8 keq ha- l yearl at Prades (Avic). These fluxes testify
to the high cation supply capacity of these silicate soils. Cation accumulation
in the aggrading biomass accounted for 47% (Montseny) and 40% (Prades)
of the net soil release; the rest being contributed by net outputs in streamwater. Net soil release was not computed for the TMO catchment, since nutrient retention in biomass is only known for its holm oak forest, which covers
60% of the catchment.
Net cation export in streamwater (also termed cation denudation rate) averaged 1.2 keq ha- l yearl at TM9, 0.93 at TMO and 1.1 at Avic. Cation denudation rates for 21 temperate and boreal forested catchments ranged from -
0.1 to 4.4 keq ha- l yearl with a mean value of 1.03 keq ha- l year l (.Avila
1988). The figures for Prades and Montseny are close to this mean. Since water is a major vehicle for the export of soil elements, it is surprising that the
drier Prades catchment (Avic), whose annual drainage is only 10% of that of
both Montseny catchments (Chap. 19), had similar cation denudation rates to
the latter. However, at Prades, Ca 2 + was the main contributor to cation denudation, accounting for 76% of it. This is linked to the higher content of calcium carbonate in the Prades soils (mean carbonate content: 0.10-0.20% in
lower and upper slope soils; J. Pinol, unpubl. data) as compared to Montseny
(average 0.05% in the permanent plot at the outlet of TMO). The reason for
the relatively high carbonate content in the Prades soils, developed on carbonate-free silicate rocks, is intriguing. A possibility is that atmospheric
deposition of calcite-containing particles at Prades was more frequent in the
past than today (for example, due to an increased transport from North Africa under a drier climate). These deposited carbonates would accumulate in
some catchment soils not connected at that time to the drainage contributing
area, but which would be presently washed by percolating water as suggested
by Pinol (1990).
Anna Avila, Juan Bellot and Josep Pinol
However, estimates of rock weathering are subjected to methodological
uncertainties concerning the rock composition, bedrock heterogeneity, incongruent dissolution of minerals, element retention in the weathered residuum, and interaction with the soil exchange complex. A more straightforward concept has been recently proposed, the net soil release (Likens and
Bormann 1995). Net soil release for cations is the sum of the net outputs of
cation equivalents (either in the draining waters or in the growing biomass),
and results from the interaction of various processes: atmospheric inputs,
mineral weathering, accumulation/depletion of the cation exchange complex,
secondary mineral formation, storage in soil organic matter, and plant nutrient uptake (Likens and Bormann 1995). The absolute field rates of some of
these processes are difficult to ascertain in catchment studies. From data in
Tables 18.6 and 20.2, net soil cation release averaged 2.3 keq ha- l yearl at
Montseny (TM9) and 1.8 keq ha- l yearl at Prades (Avic). These fluxes testify
to the high cation supply capacity of these silicate soils. Cation accumulation
in the aggrading biomass accounted for 47% (Montseny) and 40% (Prades)
of the net soil release; the rest being contributed by net outputs in streamwater. Net soil release was not computed for the TMO catchment, since nutrient retention in biomass is only known for its holm oak forest, which covers
60% of the catchment.
Net cation export in streamwater (also termed cation denudation rate) averaged 1.2 keq ha- l yearl at TM9, 0.93 at TMO and 1.1 at Avic. Cation denudation rates for 21 temperate and boreal forested catchments ranged from -
0.1 to 4.4 keq ha- l yearl with a mean value of 1.03 keq ha- l year l (.Avila
1988). The figures for Prades and Montseny are close to this mean. Since water is a major vehicle for the export of soil elements, it is surprising that the
drier Prades catchment (Avic), whose annual drainage is only 10% of that of
both Montseny catchments (Chap. 19), had similar cation denudation rates to
the latter. However, at Prades, Ca 2 + was the main contributor to cation denudation, accounting for 76% of it. This is linked to the higher content of calcium carbonate in the Prades soils (mean carbonate content: 0.10-0.20% in
lower and upper slope soils; J. Pinol, unpubl. data) as compared to Montseny
(average 0.05% in the permanent plot at the outlet of TMO). The reason for
the relatively high carbonate content in the Prades soils, developed on carbonate-free silicate rocks, is intriguing. A possibility is that atmospheric
deposition of calcite-containing particles at Prades was more frequent in the
past than today (for example, due to an increased transport from North Africa under a drier climate). These deposited carbonates would accumulate in
some catchment soils not connected at that time to the drainage contributing
area, but which would be presently washed by percolating water as suggested
by Pinol (1990).