284
Anna Avila, Juan BeJlot and Josep Pinol
Chap. 18). A representation of the conceptual model for nutrient budgets in
catchments is given in Fig. 21.1.
Atmospheric inputs to the ecosystem are in wet and dry form, but quantification of wet inputs does not pose as many problems as the measurement
of dry deposition. In fact, the flux of some nutrients with a gaseous phase
(e.g. Nand S) is quite difficult to evaluate. Approaches to the estimation of
dry deposition inputs include micrometeorological techniques (Fowler et al.
1991; Erisman and Baldocchi 1994), measurement of throughfall under natural or surrogate surfaces (Lovett 1994; Butler and Likens 1995) and mass balances in catchments (Eaton et al. 1978; Likens et al. 1990; Hultberg and Grennfelt 1992).
Besides the considerable contribution from atmospheric inputs, a major
supply of many mineral nutrients to the soils and the vegetation comes from
the weathering of the bedrock. Base cations released to the soil by weathering
are either incorporated into the net biomass increment, removed from the
soils with the percolating soil water, incorporated into secondary minerals, or
retained in the soil exchange complex. By formulating appropriate hypotheses regarding the situation in the soil exchange complex, the net output of
base cations from catchments has been used to estimate the rate of weathering of the bedrock under natural conditions (Johnson et al. 1968, 1981; Paces
1985,1986; VelbelI985).
Here, we use the nutrient budgets for two Mediterranean holm oak forested sites in the experimental areas of Prades and Montseny (NE Spain;
Chap. 2) with similar vegetation and bedrock to understand the role of climate, vegetation and soils in regulating the circulation of nutrients. Also,
some inferences on the dry deposition flux for S and on weathering rates are
provided.
20.2 Input Fluxes
The atmospheric inputs of nutrients were sampled with bulk deposition collectors. These collectors are permanently open to the atmosphere so they
catch a fraction of the dry deposition, the gravitational sedimentation. For
base cations, bulk deposition was estimated to increase wet inputs at Montseny by 10% due to gravitational sedimentation (Avila 1988).
Input fluxes were calculated by multiplying the measured concentrations
of each ion in the weekly bulk deposition sample by the weekly volume of
precipitation. These weekly amounts were then summed up for the year.
Since dry deposition represents an important nutrient input to forested ecosystems, it has been suggested that throughfall gross fluxes are the relevant
input fluxes for nutrient cycling studies (Matzner 1986; Lindberg and Lovett
1992). However, nutrient leaching from plants confuses the distinction between the external dry deposition flux and the intrasystem flux (Chap. 15).
Anna Avila, Juan BeJlot and Josep Pinol
Chap. 18). A representation of the conceptual model for nutrient budgets in
catchments is given in Fig. 21.1.
Atmospheric inputs to the ecosystem are in wet and dry form, but quantification of wet inputs does not pose as many problems as the measurement
of dry deposition. In fact, the flux of some nutrients with a gaseous phase
(e.g. Nand S) is quite difficult to evaluate. Approaches to the estimation of
dry deposition inputs include micrometeorological techniques (Fowler et al.
1991; Erisman and Baldocchi 1994), measurement of throughfall under natural or surrogate surfaces (Lovett 1994; Butler and Likens 1995) and mass balances in catchments (Eaton et al. 1978; Likens et al. 1990; Hultberg and Grennfelt 1992).
Besides the considerable contribution from atmospheric inputs, a major
supply of many mineral nutrients to the soils and the vegetation comes from
the weathering of the bedrock. Base cations released to the soil by weathering
are either incorporated into the net biomass increment, removed from the
soils with the percolating soil water, incorporated into secondary minerals, or
retained in the soil exchange complex. By formulating appropriate hypotheses regarding the situation in the soil exchange complex, the net output of
base cations from catchments has been used to estimate the rate of weathering of the bedrock under natural conditions (Johnson et al. 1968, 1981; Paces
1985,1986; VelbelI985).
Here, we use the nutrient budgets for two Mediterranean holm oak forested sites in the experimental areas of Prades and Montseny (NE Spain;
Chap. 2) with similar vegetation and bedrock to understand the role of climate, vegetation and soils in regulating the circulation of nutrients. Also,
some inferences on the dry deposition flux for S and on weathering rates are
provided.
20.2 Input Fluxes
The atmospheric inputs of nutrients were sampled with bulk deposition collectors. These collectors are permanently open to the atmosphere so they
catch a fraction of the dry deposition, the gravitational sedimentation. For
base cations, bulk deposition was estimated to increase wet inputs at Montseny by 10% due to gravitational sedimentation (Avila 1988).
Input fluxes were calculated by multiplying the measured concentrations
of each ion in the weekly bulk deposition sample by the weekly volume of
precipitation. These weekly amounts were then summed up for the year.
Since dry deposition represents an important nutrient input to forested ecosystems, it has been suggested that throughfall gross fluxes are the relevant
input fluxes for nutrient cycling studies (Matzner 1986; Lindberg and Lovett
1992). However, nutrient leaching from plants confuses the distinction between the external dry deposition flux and the intrasystem flux (Chap. 15).