322
Isabel Serrasolses and V. Ramon Vallejo
period of N immobilization, mostly occurring in the L layer, than the control
plot, where N increased in both Land F layers. Sauras et al. (1991) found a
similar trend when they incubated green and senesced holm oak leaves in
the forest of Prades: higher mineralization of N and greater weight loss rates
were found for the green litter. In our study, P and Mg concentrations increased with time in the slash L, whereas K decreased rapidly through
leaching and accumulated in the F layer.
The burned plot (Fig. 22.2) initially showed high nutrient concentrations
in the ashbed. These progressively decreased and were incorporated into the
H or deeper layers, becoming stabilized during the second post-fire year. K,
and to a lesser extent Ca, showed a rapid washout from the ashbed. This was
also observed in the drainage water (Poch 1992; Gazquez 1994). P, Sand Mg
presented a more gradual decrease. N concentration remained stable until
the heavy rains in autumn 1989 when it decreased (the same was found to
occur for C). These rains led to the mixing of the ashbed and H layers with
the mineral soil, thereby reducing C and N concentrations by dilution, as detected by the increase.in AI content, and probably enhanced organic matter
and N mineralization.
22.6 Phosphorus and Nitrogen Dynamics
22.6.1 Soluble Phosphorus
Soluble P (mostly inorganic) in the H layer was sensitive to the disturbances
applied (Fig. 22.3). In the control plot, both total and organic soluble P fluctuated without a clear trend. In the clearcut plot, total soluble P slowly increased in the first year after clearing and then decreased to reach initial levels in just a few months (Fig. 22.3). In the burnt plot, the ashbed and the H
layer showed similar soluble P contents to the control H layer, just after
burning. However, 2 months later the ashbed showed a three-fold increase in
its total and organic soluble P content, probably because of the increase of P
solubility due to the pH raise. Organic soluble P was rapidly mineralized
while total soluble P remained constant until the heavy rains in autumn 1989.
These rains washed out the soluble P from the ashbed, and both the ashbed
and the H layer achieved lower P concentrations than the control H.
22.6.2 Mineral Nitrogen
In the H layer of the control plot (Fig. 22.4), N03 - -N content was correlated
positively with the rainfall in the 5 days prior to sampling (r = 0.60, P < 0.05).
Therefore, nitrification was especially active in spring and autumn when the
nitrate concentration was higher than that of ammonium (Chap. 16). In the
Isabel Serrasolses and V. Ramon Vallejo
period of N immobilization, mostly occurring in the L layer, than the control
plot, where N increased in both Land F layers. Sauras et al. (1991) found a
similar trend when they incubated green and senesced holm oak leaves in
the forest of Prades: higher mineralization of N and greater weight loss rates
were found for the green litter. In our study, P and Mg concentrations increased with time in the slash L, whereas K decreased rapidly through
leaching and accumulated in the F layer.
The burned plot (Fig. 22.2) initially showed high nutrient concentrations
in the ashbed. These progressively decreased and were incorporated into the
H or deeper layers, becoming stabilized during the second post-fire year. K,
and to a lesser extent Ca, showed a rapid washout from the ashbed. This was
also observed in the drainage water (Poch 1992; Gazquez 1994). P, Sand Mg
presented a more gradual decrease. N concentration remained stable until
the heavy rains in autumn 1989 when it decreased (the same was found to
occur for C). These rains led to the mixing of the ashbed and H layers with
the mineral soil, thereby reducing C and N concentrations by dilution, as detected by the increase.in AI content, and probably enhanced organic matter
and N mineralization.
22.6 Phosphorus and Nitrogen Dynamics
22.6.1 Soluble Phosphorus
Soluble P (mostly inorganic) in the H layer was sensitive to the disturbances
applied (Fig. 22.3). In the control plot, both total and organic soluble P fluctuated without a clear trend. In the clearcut plot, total soluble P slowly increased in the first year after clearing and then decreased to reach initial levels in just a few months (Fig. 22.3). In the burnt plot, the ashbed and the H
layer showed similar soluble P contents to the control H layer, just after
burning. However, 2 months later the ashbed showed a three-fold increase in
its total and organic soluble P content, probably because of the increase of P
solubility due to the pH raise. Organic soluble P was rapidly mineralized
while total soluble P remained constant until the heavy rains in autumn 1989.
These rains washed out the soluble P from the ashbed, and both the ashbed
and the H layer achieved lower P concentrations than the control H.
22.6.2 Mineral Nitrogen
In the H layer of the control plot (Fig. 22.4), N03 - -N content was correlated
positively with the rainfall in the 5 days prior to sampling (r = 0.60, P < 0.05).
Therefore, nitrification was especially active in spring and autumn when the
nitrate concentration was higher than that of ammonium (Chap. 16). In the