290
Anna Avila. Juan Bellot and Josep Pinol
and N against annual runoff are shown in Figs. 20.1 and 20.2, and they illustrate the importance of the amount of water circulating through the catchment as a vector for the export of nutrients from the catchments.
This relationship with water discharge has been widely reported for cooltemperate sites (Likens et al. 1977), but the Mediterranean climate is particularly affected by a high variation of the precipitation regime which will
determine a great variability in yearly net exports, as observed. Therefore,
measurements taken over a short range of the hydrologic variability would
give poor estimates of the average conditions. In consequence, there is a special need for long-term studies in order to obtain representative average balances. We present here 12 and 10 years of nutrient budgets for Prades and
Montseny, respectively. The recorded period comprised dry and wet years
and we consider the figures provided to be quite representative.
In Figs. 20.1 and 20.2, the patterns of variation of net exports with annual
runoff are presented for (a) litophilic, (b) atmophilic and (c) biophilic ions at
the Montseny and Prades catchments. The behaviour of some ions was similar at Prades and Montseny: (1) HC03- and Mg2+ always had net exports; (2)
N always presented net retention; (3) Cl- and S oscillated around the zero
line, with positive exports during wet years and net retention during dry
years; and (4) Ca 2 + was usually exported, but it was retained in years of particularly important inputs in red rains (1984-1985, 1988-1989), a pattern
more clearly defined at Montseny than at Prades. On the other hand, K+ and
Na+ behaved differently at Prades and Montseny. Potassium was always retained at Prades, but had a positive export in 3 years at Montseny. The
amounts exported, however, were small (lower than 1 kg ha- I yearl) and on
average there was a net retention. Sodium presented a contrasting behaviour
at Prades and Montseny: it was always exported from the Montseny catchment, but it was retained during dry years at Prades.
20.4.1 Elements with Net Retention: Nitrogen and Potassium
Nearly all inorganic nitrogen entering the Montseny and Prades catchments
in bulk deposition was retained. At both sites, the relationship between net
output and runoff was negative (although non-significant; Figs. 20.1 and 20.2),
related to the fact that years with more precipitation (and runoff) received
higher amounts of inorganic N, but outputs were little affected by increased
runoff, resulting in higher retention in the catchment.
It has been proposed that ecosystems in an aggrading phase would efficiently retain those elements necessary for the construction of growing biomass (Vitousek and Reiners 1975). These atmospheric N inputs, in fact, constitute a low-level fertilization for the aggrading holm oak forests (Roda et al.
1990). Inorganic N inputs in bulk precipitation were similar to the N amounts
annually stored in the net growth of aboveground biomass in the holm oak
forests of Prades and Montseny (Tables 18.6 and 20.2). The efficiency of
N retention in these catchments is demonstrated by the fact that N03 - and
Anna Avila. Juan Bellot and Josep Pinol
and N against annual runoff are shown in Figs. 20.1 and 20.2, and they illustrate the importance of the amount of water circulating through the catchment as a vector for the export of nutrients from the catchments.
This relationship with water discharge has been widely reported for cooltemperate sites (Likens et al. 1977), but the Mediterranean climate is particularly affected by a high variation of the precipitation regime which will
determine a great variability in yearly net exports, as observed. Therefore,
measurements taken over a short range of the hydrologic variability would
give poor estimates of the average conditions. In consequence, there is a special need for long-term studies in order to obtain representative average balances. We present here 12 and 10 years of nutrient budgets for Prades and
Montseny, respectively. The recorded period comprised dry and wet years
and we consider the figures provided to be quite representative.
In Figs. 20.1 and 20.2, the patterns of variation of net exports with annual
runoff are presented for (a) litophilic, (b) atmophilic and (c) biophilic ions at
the Montseny and Prades catchments. The behaviour of some ions was similar at Prades and Montseny: (1) HC03- and Mg2+ always had net exports; (2)
N always presented net retention; (3) Cl- and S oscillated around the zero
line, with positive exports during wet years and net retention during dry
years; and (4) Ca 2 + was usually exported, but it was retained in years of particularly important inputs in red rains (1984-1985, 1988-1989), a pattern
more clearly defined at Montseny than at Prades. On the other hand, K+ and
Na+ behaved differently at Prades and Montseny. Potassium was always retained at Prades, but had a positive export in 3 years at Montseny. The
amounts exported, however, were small (lower than 1 kg ha- I yearl) and on
average there was a net retention. Sodium presented a contrasting behaviour
at Prades and Montseny: it was always exported from the Montseny catchment, but it was retained during dry years at Prades.
20.4.1 Elements with Net Retention: Nitrogen and Potassium
Nearly all inorganic nitrogen entering the Montseny and Prades catchments
in bulk deposition was retained. At both sites, the relationship between net
output and runoff was negative (although non-significant; Figs. 20.1 and 20.2),
related to the fact that years with more precipitation (and runoff) received
higher amounts of inorganic N, but outputs were little affected by increased
runoff, resulting in higher retention in the catchment.
It has been proposed that ecosystems in an aggrading phase would efficiently retain those elements necessary for the construction of growing biomass (Vitousek and Reiners 1975). These atmospheric N inputs, in fact, constitute a low-level fertilization for the aggrading holm oak forests (Roda et al.
1990). Inorganic N inputs in bulk precipitation were similar to the N amounts
annually stored in the net growth of aboveground biomass in the holm oak
forests of Prades and Montseny (Tables 18.6 and 20.2). The efficiency of
N retention in these catchments is demonstrated by the fact that N03 - and