Introduction
Soil age is a major factor influencing ecosystem nitrogen dynamics.
Nitrogen (N) is virtually absent during the early stages of pedogenesis
because the N content of most primary minerals is exceedingly small,
if not lacking entirely. Over time, biological N-fixation and atmospheric
deposition lead to the accumulation of N in terrestrial ecosystems.
Because soil development takes place over time scales of millennia,
a chronosequence approach is often taken to infer changes in N storage
and N cycling rates during pedogenesis (Stevens & Walker 1970).
A chronosequence is a series of study sites which vary in age, but
climate, vegetation, topography, and parent material are kept constant
(Jenny 1941).
Chronosequence studies in humid ecosystems suggest that N storage
and cycling rates increase during early stages of soil formation,
but a shift from N- to P-limitation leads to a decline in N storage
late in ecosystem development (Walker & Syers 1976, Crews et
al. 1995). These patterns may not hold for semiarid climates, however,
as soil development should proceed much more slowly because of lower
weathering rates of parent material. In addition, the strong control
of water availability on ecosystem processes, as well as the patchy
vegetation distribution in arid and semiarid environments may alter
the pattern of ecosystem development with soil age.
As pedogenesis proceeds, water holding capacity and plant available
water storage should increase due to an increase in clay content.
Such increases in water storage may significantly alter N-cycling
rates during the development of semiarid ecosystems. Additionally,
changes in the spatial pattern of nutrient distribution may also
be an important aspect of development in dryland ecosystems. Several
studies have documented that nutrients such as N and P are more concentrated
under large shrub or tree canopies in arid and semiarid ecosystems
(Schlesinger et al. 1996). It remains unclear how this vegetation-soil
nutrient pattern might change over the course of long-term soil development.
We examined N pools and fluxes under tree canopies as well as in
intercanopy spaces along a three-million-year soil chronosequence
within a semiarid woodland ecosystem in northern Arizona. We hypothesized
that:
1. Total N, microbial biomass-N, and N-availability would increase
consistently with soil age because of slower weathering rates than
in more humid ecosystems, and because of the strong influence of
increased soil water availability with soil age.
2. Differences in soil N content among cover types would lessen with time because,
as soils age, leaf litter input and root turnover will eventually influence
the majority of soil microsites, leading to a more even distribution of N in
older soils.
3. The d15N signature of both soils and foliage would become more depleted
with soil age because, as clay content and soil water-holding capacity increases,
increased production should lead to increased plant uptake and a larger organic
C pool which would favor N immobilization, thus decreasing the relative N-loss
through fractionating pathways. |
