232
Isabel Serrasolses, Victoria Diego and David Bonilla
16.6 Ecosystem Nitrogen Availability
On average, the soil content of mineral N and the rates of net N mineralization and nitrification were higher at Montseny than at Prades, probably due
to its wetter climate and shorter summer water stress. At Montseny, net N
mineralization in the 0-20 cm soil yielded a flux of 80 kg of plant available N
ha- I year-I, similar to the mean N mineralization rate in temperate deciduous
forests (Bonilla and Roda 1992; Reich et al. 1997). This is a relatively high
rate, indicating that N availability is quite high in this holm oak forest, a fact
that would not be evident from its moderate N flux in litterfall (37 kg N ha- I
year-I; Table 18.6).
At Prades, estimates of the annual rates of soil N transformations are uncertain since only four monthly incubations were conducted (Fig. 16.3).
However, the low monthly rates of net N mineralization measured at Prades
(Fig. 16.3) may indicate that N availability is much lower than at Montseny,
and that N could limit primary production. In fact, when the Prades holm
oak forest was fertilized with ammonium nitrate, the net N mineralization
during the first year after treatment increased four times over that of unfertilized plots (V, Diego, unpubl. data). In this experiment, aboveground net
primary production was enhanced by N fertilization (Chap. 13), demonstrating the role of N availability in this dry Mediterranean forest.
Nitrification in the 0-20 cm soil at Montseny amounted to 7.5 kg N ha- I
year-I, i.e. only 9% of the N mineralized annually. This result shows that in
this mesic holm oak forest, growing on an acid soil, nitrification is of limited
importance in soil N dynamics. It also means that the mineral N nutrition of
holm oak at this site is based on ammonium rather than nitrate. Low nitrification is a key ecosystem property involved in maintaining a tight N cycle
and in keeping low the leaching losses of nitrate (Sect. 16.5).
In the drier Prades forest, growing on a circumneutral soil, nitrification
seems to playa somewhat major role since on average 14% of the mineralized N was nitrified in the 0-20 cm mineral soil (Fig. 16.3). When the H layer
is included, this percentage raised to 28%. However, nitrification pulses can
be very localized in space and time, and thus the role of nitrate in the nutrition of holm oak can be very variable. For example, a further single field incubation at Prades (Serrasolsas 1994) showed that in the H horizon N was
taken up by trees as nitrate, whereas in the mineral soil both nitrate (57%)
and ammonium (43%) were taken up. In particular, fast nitrification may occur when the soils are moist and warm. High nitrate concentrations are often
found in the soil solution after the first rainfall following the summer
drought at both Prades and Montseny (Chap. 17). Microbial N is one of the
main sources of mineralized N after soil drying and rewetting (Okano 1990;
Serrasolsas and Khanna 1995). At Prades, microbial N is a labile reserve of
easily mineralizable N, corresponding to 3% of the total N in the H horizon,
and between 1 and 2% in the mineral soil (0-5 cm), thus exceeding mineral
N (data not shown).
Isabel Serrasolses, Victoria Diego and David Bonilla
16.6 Ecosystem Nitrogen Availability
On average, the soil content of mineral N and the rates of net N mineralization and nitrification were higher at Montseny than at Prades, probably due
to its wetter climate and shorter summer water stress. At Montseny, net N
mineralization in the 0-20 cm soil yielded a flux of 80 kg of plant available N
ha- I year-I, similar to the mean N mineralization rate in temperate deciduous
forests (Bonilla and Roda 1992; Reich et al. 1997). This is a relatively high
rate, indicating that N availability is quite high in this holm oak forest, a fact
that would not be evident from its moderate N flux in litterfall (37 kg N ha- I
year-I; Table 18.6).
At Prades, estimates of the annual rates of soil N transformations are uncertain since only four monthly incubations were conducted (Fig. 16.3).
However, the low monthly rates of net N mineralization measured at Prades
(Fig. 16.3) may indicate that N availability is much lower than at Montseny,
and that N could limit primary production. In fact, when the Prades holm
oak forest was fertilized with ammonium nitrate, the net N mineralization
during the first year after treatment increased four times over that of unfertilized plots (V, Diego, unpubl. data). In this experiment, aboveground net
primary production was enhanced by N fertilization (Chap. 13), demonstrating the role of N availability in this dry Mediterranean forest.
Nitrification in the 0-20 cm soil at Montseny amounted to 7.5 kg N ha- I
year-I, i.e. only 9% of the N mineralized annually. This result shows that in
this mesic holm oak forest, growing on an acid soil, nitrification is of limited
importance in soil N dynamics. It also means that the mineral N nutrition of
holm oak at this site is based on ammonium rather than nitrate. Low nitrification is a key ecosystem property involved in maintaining a tight N cycle
and in keeping low the leaching losses of nitrate (Sect. 16.5).
In the drier Prades forest, growing on a circumneutral soil, nitrification
seems to playa somewhat major role since on average 14% of the mineralized N was nitrified in the 0-20 cm mineral soil (Fig. 16.3). When the H layer
is included, this percentage raised to 28%. However, nitrification pulses can
be very localized in space and time, and thus the role of nitrate in the nutrition of holm oak can be very variable. For example, a further single field incubation at Prades (Serrasolsas 1994) showed that in the H horizon N was
taken up by trees as nitrate, whereas in the mineral soil both nitrate (57%)
and ammonium (43%) were taken up. In particular, fast nitrification may occur when the soils are moist and warm. High nitrate concentrations are often
found in the soil solution after the first rainfall following the summer
drought at both Prades and Montseny (Chap. 17). Microbial N is one of the
main sources of mineralized N after soil drying and rewetting (Okano 1990;
Serrasolsas and Khanna 1995). At Prades, microbial N is a labile reserve of
easily mineralizable N, corresponding to 3% of the total N in the H horizon,
and between 1 and 2% in the mineral soil (0-5 cm), thus exceeding mineral
N (data not shown).