246
Nuria Melia, Juan Bellot and V. Ramon Vallejo
largest differences between the two upper horizons and the deepest layer for
Ca 2 +, Mg2+, SOl- and CI-. The long and severe drought of 1994 probably resulted in an accumulation of salts in the upper horizons through evapotranspiration. This concentration effect could mainly be seen for CI- and sol(Fig. 17 Ae), which showed an increasing pattern from the first to the last
sampling. In contrast, Na+ failed to present any differences between samplings or depths. The effects of the drought on soil solution chemistry must
have been higher than evidenced by Fig. 1704 since soil solution could not be
obtained by the centrifugation method during the driest months until the
September sampling, which took place 3-4 days after a 100-mm storm.
Nitrate concentrations in soil solution were extremely high in September,
at the end of the drought (Fig. 17Af). Other authors also found high peaks of
N03 - concentrations after the summer drought in forest floor leachates in
Scots pine forests (Casals et al. 1995) and in Pinus radiata plantations (Cortina et al. 1995). Microbial processes occurring after the rewetting of dried
soils increase nitrogen mineralization (van Gestel et al. 1993), especially at
high temperatures (Chap. 16). In addition, the drought of 1994 produced a
high mortality of trees and fine roots, and nitrogen uptake was probably reduced. The role of nutrient demand by trees in keeping low levels of N03 - in
the soil solution of holm oak forests was clearly demonstrated by a trenching
experiment at Montseny: after severing all tree roots entering otherwise undisturbed 2 x 2-m plots, N03 - concentrations in soil solution skyrocketed
(Bonilla and Roda 1990).
Potassium showed a particular behaviour. The concentration effect due to
the summer drought, found for most other ions, was not apparent for K+. On
the contrary, K+ concentrations in November, when the soil was wettest, were
similar to those in September (Fig. 17Ac). These results seem to indicate that
leaching of K+ from the forest floor to deeper soil water during wet periods
could be as important as the concentration effect observed in other cations in
dry periods. Furthermore, K+ concentrations were lowest in December or
April, while those of Ca 2 + or CI- were lowest in the wettest month (November).
Those ions most affected by the biological activity in soils (K+, NHt and
N03 -) also differed from other ions in having minimal concentrations in the
deeper horizons in April (Fig. 17Ac,f). Biological demand for these ions was
probably involved in this depletion.
17.3.3 Differences in Soil Solution Between Prades and Montseny
The previous soil solution data from Prades and Montseny show that both
holm oak forests are very different in terms of the magnitude of their ion
concentrations. Soil solution at Prades was highly mineralized, with higher
ion concentrations, alkalinity and pH than at Montseny (Table 17.1). Moreover, ion concentrations decreased with depth at Prades, whilst lower differences between depths were found at Montseny, partly because here the soil
Nuria Melia, Juan Bellot and V. Ramon Vallejo
largest differences between the two upper horizons and the deepest layer for
Ca 2 +, Mg2+, SOl- and CI-. The long and severe drought of 1994 probably resulted in an accumulation of salts in the upper horizons through evapotranspiration. This concentration effect could mainly be seen for CI- and sol(Fig. 17 Ae), which showed an increasing pattern from the first to the last
sampling. In contrast, Na+ failed to present any differences between samplings or depths. The effects of the drought on soil solution chemistry must
have been higher than evidenced by Fig. 1704 since soil solution could not be
obtained by the centrifugation method during the driest months until the
September sampling, which took place 3-4 days after a 100-mm storm.
Nitrate concentrations in soil solution were extremely high in September,
at the end of the drought (Fig. 17Af). Other authors also found high peaks of
N03 - concentrations after the summer drought in forest floor leachates in
Scots pine forests (Casals et al. 1995) and in Pinus radiata plantations (Cortina et al. 1995). Microbial processes occurring after the rewetting of dried
soils increase nitrogen mineralization (van Gestel et al. 1993), especially at
high temperatures (Chap. 16). In addition, the drought of 1994 produced a
high mortality of trees and fine roots, and nitrogen uptake was probably reduced. The role of nutrient demand by trees in keeping low levels of N03 - in
the soil solution of holm oak forests was clearly demonstrated by a trenching
experiment at Montseny: after severing all tree roots entering otherwise undisturbed 2 x 2-m plots, N03 - concentrations in soil solution skyrocketed
(Bonilla and Roda 1990).
Potassium showed a particular behaviour. The concentration effect due to
the summer drought, found for most other ions, was not apparent for K+. On
the contrary, K+ concentrations in November, when the soil was wettest, were
similar to those in September (Fig. 17Ac). These results seem to indicate that
leaching of K+ from the forest floor to deeper soil water during wet periods
could be as important as the concentration effect observed in other cations in
dry periods. Furthermore, K+ concentrations were lowest in December or
April, while those of Ca 2 + or CI- were lowest in the wettest month (November).
Those ions most affected by the biological activity in soils (K+, NHt and
N03 -) also differed from other ions in having minimal concentrations in the
deeper horizons in April (Fig. 17Ac,f). Biological demand for these ions was
probably involved in this depletion.
17.3.3 Differences in Soil Solution Between Prades and Montseny
The previous soil solution data from Prades and Montseny show that both
holm oak forests are very different in terms of the magnitude of their ion
concentrations. Soil solution at Prades was highly mineralized, with higher
ion concentrations, alkalinity and pH than at Montseny (Table 17.1). Moreover, ion concentrations decreased with depth at Prades, whilst lower differences between depths were found at Montseny, partly because here the soil