Soil Nitrogen Dynamics
225
rate (Fig. 16.1} and, consequently, the C:N ratios of the H horizon and the
upper part of the mineral soil remain approximately constant, with values
between 20 and 16. These C:N ratios are in the range 15-25 commonly found
in surface (0-20 cm) forest soils (Attiwill and Leeper 1987). In deeper mineral
soil at our sites, the C:N ratio decreases to 6-8 below 50-cm depth. Most of
the organic matter with a low C:N ratio in deep soil is assumed to be humified and it is N recalcitrant (Vallejo and Hereter 1986; Vallejo 1993).
The forest floor of our holm oak forests contains 531-622 kg N ha- I
(Table 18.5). Using the litter decomposition model of Jenny et al. (1949), the
calculated annual amount of N released from the forest floor is around 35 kg
N ha- I in both holm oak forests, with a half-life of N in the forest floor of 11
years. N turnover in Mediterranean forest floors varies widely, from 2.5 years
in a calcareous site with a high decomposition rate (Le Rouquet, southern
France; Lossaint and Rapp 1978) to 18.2 years in a dry site (Vallejo and Rosich
1990). This range overlaps with the lower range for temperate forests (Cole
and Rapp 1981}.
16.3 Soil Mineral Nitrogen
Concentrations of soil mineral N and rates of microbial transformations of N
were studied at Prades on the upper, SE-facing slope of Torners (Chap. 2) in
the context of the fertilization and irrigation experiment (Chap. 13) and the
burning and clear-cutting experiment (Chap. 22). At Montseny, they were
studied in a midslope position on the N-facing slope adjacent to the TM9
catchment at La Castanya (Chap. 2), with the purpose of characterizing the
soil N dynamics in an undisturbed mesic holm oak forest (Bonilla and Roda
1992) and of assessing the effects of root uptake through a trenching experiment (Bonilla and Roda 1989, 1990). Unless otherwise stated, all results
on soil N in this chapter refer to untreated (control) holm oak plots.
At Prades, mineral N concentrations were high in the H horizon (Fig 16.2),
probably due to the high mineralization and nitrification rates of this horizon, with a mean of 119 (SE 16) mg N kg-I. On average, this mineral N was
65% ammonium and 35% nitrate, though these proportions varied seasonally. The amount of mineral N in the H horizon was 2.8 ± 0.5 kg N ha-I, less
than 1 % of the total N is this layer. Similar amounts of mineral N were found
in the litter layer of the chaparral (2-6 kg N ha- I ; DeBano et al. 1979), and in
the H horizon of a Scots pine forest near Montseny (3-6 kg N ha-I, Casals
et al. 1995). The H horizon was not sampled at Montseny because it was
practically absent in the studied slope. However, since a high concentration
of nitrate was leached out of the forest floor in the permanent plot (see
Sect. 16.5), high mineralization and nitrification rates probably also occurred
at Montseny in sites where the H horizon was well developed.
225
rate (Fig. 16.1} and, consequently, the C:N ratios of the H horizon and the
upper part of the mineral soil remain approximately constant, with values
between 20 and 16. These C:N ratios are in the range 15-25 commonly found
in surface (0-20 cm) forest soils (Attiwill and Leeper 1987). In deeper mineral
soil at our sites, the C:N ratio decreases to 6-8 below 50-cm depth. Most of
the organic matter with a low C:N ratio in deep soil is assumed to be humified and it is N recalcitrant (Vallejo and Hereter 1986; Vallejo 1993).
The forest floor of our holm oak forests contains 531-622 kg N ha- I
(Table 18.5). Using the litter decomposition model of Jenny et al. (1949), the
calculated annual amount of N released from the forest floor is around 35 kg
N ha- I in both holm oak forests, with a half-life of N in the forest floor of 11
years. N turnover in Mediterranean forest floors varies widely, from 2.5 years
in a calcareous site with a high decomposition rate (Le Rouquet, southern
France; Lossaint and Rapp 1978) to 18.2 years in a dry site (Vallejo and Rosich
1990). This range overlaps with the lower range for temperate forests (Cole
and Rapp 1981}.
16.3 Soil Mineral Nitrogen
Concentrations of soil mineral N and rates of microbial transformations of N
were studied at Prades on the upper, SE-facing slope of Torners (Chap. 2) in
the context of the fertilization and irrigation experiment (Chap. 13) and the
burning and clear-cutting experiment (Chap. 22). At Montseny, they were
studied in a midslope position on the N-facing slope adjacent to the TM9
catchment at La Castanya (Chap. 2), with the purpose of characterizing the
soil N dynamics in an undisturbed mesic holm oak forest (Bonilla and Roda
1992) and of assessing the effects of root uptake through a trenching experiment (Bonilla and Roda 1989, 1990). Unless otherwise stated, all results
on soil N in this chapter refer to untreated (control) holm oak plots.
At Prades, mineral N concentrations were high in the H horizon (Fig 16.2),
probably due to the high mineralization and nitrification rates of this horizon, with a mean of 119 (SE 16) mg N kg-I. On average, this mineral N was
65% ammonium and 35% nitrate, though these proportions varied seasonally. The amount of mineral N in the H horizon was 2.8 ± 0.5 kg N ha-I, less
than 1 % of the total N is this layer. Similar amounts of mineral N were found
in the litter layer of the chaparral (2-6 kg N ha- I ; DeBano et al. 1979), and in
the H horizon of a Scots pine forest near Montseny (3-6 kg N ha-I, Casals
et al. 1995). The H horizon was not sampled at Montseny because it was
practically absent in the studied slope. However, since a high concentration
of nitrate was leached out of the forest floor in the permanent plot (see
Sect. 16.5), high mineralization and nitrification rates probably also occurred
at Montseny in sites where the H horizon was well developed.