Principles of terrestrial ecosystem ecology.pdf

has been observed in developmental sequences
in radically different climates, such as those
found in New Zealand, Australia, and Hawaii
(Vitousek and Farrington 1997).
In contrast to phosphorus, fixed nitrogen is
nearly absent from most nonsedimentary rocks,
so young soils must accumulate it from the
atmosphere. Although sedimentary rocks contribute
some nitrogen by weathering (0.01 to
2gm -2 yr -1 ) (Holloway et al. 1998), much of this
may be lost in groundwater rather than entering
ecosystems. The combination of substantial
inputs of phosphorus and other elements
with no nitrogen give nitrogen fixers a strong
advantage early in soil and ecosystem development.
Indeed, many early successional systems
are dominated by symbiotic nitrogen fixers
(Chapin et al. 1994). Where nitrogen fixers
occur, nitrogen accumulates relatively quickly.
Where nitrogen fixers are sparse or absent,
nitrogen enters from deposition and accumulates
slowly. Nitrogen continues to accumulate
in ecosystems until nitrogen availability comes
into approximate equilibrium with other
resources, including phosphorus (Walker and
Syers 1976). Nitrogen limits forest growth on
young substrates in Hawaii, for example; phosphorus
limits growth on old substrates; and
nitrogen and phosphorus are both relatively
available in intermediate-aged sites (Vitousek
and Farrington 1997).
Geochemical tracers have been used to identify
dust and to determine its rate of input to
the Hawaiian Islands. Hawaiian rocks are
derived from Earth’s mantle, whereas Asian
dust comes from the crust.These two sources
differ in the ratio of two isotopes of neodynium,
in the ratio of europium to other
lanthanide elements, and in the ratio of
thorium to halfnium. All of these elements
are relatively immobile in soils, so changes
over time in the isotopic or elemental ratios
can be used to calculate time-integrated
inputs of Asian dust. Knowing the phospho-
Other Element Cycles 221
Why does phosphorus rather than other
rock-derived elements limit biological processes
in the long run? The major cations,
especially calcium, are absorbed by organisms
in much larger quantities than is phosphorus
and are more readily leached from soils. On
the Hawaiian sequence, rock-derived calcium,
magnesium, and potassium virtually disappear
within 100,000 years but do not limit forest production
anywhere on the sequence (Vitousek
and Farrington 1997). Atmospheric inputs of
cations prevent these elements from becoming
limiting. Marine-derived aerosols containing
calcium, magnesium, and potassium are
deposited on forests in Hawaii through rain and
cloud droplets. Phosphorus concentrations in
marine aerosols are low, however, because high
phosphorus demands by marine organisms
maintain a low concentration in surface waters.
The atmospheric inputs of calcium are 10-fold
less than weathering inputs in young sites but
are more than 1000-fold greater than weathering
inputs in older sites (Chadwick et al. 1999).
In continental interiors, dust from semiarid and
other sparsely vegetated areas is a major source
of cations. Even in Hawaii, dust from Asia,
over 6000km away, is an important input of
phosphorus, especially during glacial times,
when vegetation cover was sparse and wind
speeds were high (Chadwick et al. 1999) (Box
9.2). In situ weathering of parent material is
Box 9.2. Geochemical Tracers to Identify Source of Inputs to Ecosystems
rus content of the dust, it is then possible to
calculate phosphorus inputs by this pathway.
Atmospheric inputs of phosphorus are much
lower than weathering for the first million
years or more of soil development. However,
by 4 million years, rock-derived phosphorus
has nearly disappeared, and Asian dust provides
most of the phosphorus input to the
soil. The biological availability of phosphorus
is low in old sites, but it would be much
lower were it not for inputs of Asian dust,
most of it transported more than 10,000
years ago (Chadwick et al. 1999).