Principles of terrestrial ecosystem ecology.pdf

alization is the net accumulation of inorganic
nitrogen in the soil solution over a given time
interval. Net mineralization occurs when microbial
growth is limited more strongly by carbon
than by nitrogen, whereas net immobilization
occurs when microbial communities are nitrogen
limited.
Net nitrogen mineralization is an excellent
measure of the nitrogen supply to plants in
ecosystems with rapid nitrogen turnover, where
there is little competition for nitrogen between
plants and microbes. The annual net mineralization
in the deciduous forests of eastern
North America, for example, approximately
equals nitrogen uptake by vegetation
(Nadelhoffer et al. 1992). In less fertile ecosystems,
such as arctic tundra, net nitrogen mineralization
rate substantially underestimates the
amount of nitrogen that is annually acquired by
plants (Nadelhoffer et al. 1992). There are at
least two explanations for this apparent discrepancy.
(1) Plant roots and their mycorrhizal
fungi, which are excluded in net mineralization
assays, may be good competitors with sapro-
LONG-TERM
CONTROLS
STATE
FACTORS
BIOTA
TIME
PARENT
MATERIAL
Climate
Interactive
controls
Plant
functional
types
Soil
resources
Indirect
controls
Figure 9.3. The major factors controlling ammonification
(net nitrogen mineralization) in soils. These
controls range from the proximate control over
nitrogen mineralization (the concentration of DON,
physical environment, and microbial carbon to
nitrogen ratio) to the state factors and interactive
Plant
uptake of
DON
Litter
quantity
Carbon quality
Internal Cycling of Nitrogen 205
phytic microbes for mineralized nitrogen in the
real world, so this assay may underestimate the
quantity of nitrogen that would be mineralized
in the presence of roots (Stark 2000). (2) Plants
and their mycorrhizal fungi that absorb amino
acids may also not depend heavily on nitrogen
mineralization to meet their nitrogen requirements
in infertile ecosystems due to absorption
of DON, so the low rates of net nitrogen mineralization
in these ecosystems may be an
accurate reflection of small fluxes that naturally
occur through this pathway.
Nitrogen mineralization rate is controlled by
the availability of DON and inorganic nitrogen,
the activity of soil microbes, and their relative
demands for carbon and nitrogen. The quantity
and quality of organic matter, including its
relative amounts and forms of carbon and
nitrogen, that enter the soil are the major
determinants of the substrate available for
decomposition (see Chapter 7) and therefore
the substrate available for nitrogen mineralization
(Fig. 9.3). Thus the state factors and interactive
controls that promote productivity and
SHORT-TERM
CONTROLS
(-)
Direct
controls
Dissolved
organic
nitrogen
Microbial
C:N
Temperature
H 2 O
(-)
NET NITROGEN
MINERALIZATION
(AMMONIFICATION)
controls that ultimately determine the differences
among ecosystems in mineralization rates. Thickness
of the arrows indicates the strength of effects. The
influence of one factor on another is positive unless
otherwise indicated (-).