Principles of terrestrial ecosystem ecology.pdf

LONG-TERM
CONTROLS
STATE
FACTORS
BIOTA
TIME
PARENT
MATERIAL
CLIMATE
Interactive
controls
Plant
functional
types
Soil
resources
Figure 9.5. The major factors controlling nitrification
in soils (Robertson 1989). These controls range
from concentrations of reactants that directly control
nitrification to the interactive controls, such as
climate and disturbance regime, that are the ultimate
require oxygen for oxidation of NH 4. Oxygen
availability, in turn, is influenced by many
factors, including soil moisture, soil texture, soil
structure, and respiration by microbes and
roots (Fig. 9.5).
Nitrifier activity is sensitive to temperature.
It does, however, continue at low rates at low
temperatures, so over a long winter season,
substantial nitrification can occur, particularly
in nitrogen-rich agricultural soils. Nitrification
rates are slow in dry soils primarily because thin
water films restrict NH 4 + diffusion to nitrifiers
(Stark and Firestone 1995). Under extremely
dry conditions, low water potential further
restricts the activity of nitrifiers. The importance
of acidity in regulating nitrification rates
is uncertain. In laboratory cultures of agricultural
soils, maximum nitrification rates occur
between pH 6.6 and 8.0 and are negligible
below pH 4.5 (Paul and Clark 1996). Many
natural ecosystems with acidic soils, however,
have substantial nitrification rates, even at pH
4 (Stark and Hart 1997).
The fraction of mineralized nitrogen that is
oxidized to nitrate varies widely among ecosys-
Indirect
controls
Plant NH 4 +
uptake
Litter quantity
Carbon quality
Root / microbial
respiration
Temperature
H 2 O
Soil texture
Internal Cycling of Nitrogen 209
SHORT-TERM
CONTROLS
(-)
Ammonium
concentration
(-)
Direct
controls
(-)
Oxygen
concentration
NITRIFICATION
determinants of nitrification rate. Thickness of the
arrows indicates the strength of effects.The influence
of one factor on another is positive unless otherwise
indicated (-).
tems. In many unpolluted temperate coniferous
and deciduous forests, nitrification is only a
small proportion of net mineralization (e.g., 0
to 4%) but, as ecosystems receive increasing
nitrogen deposition, the fraction of nitrification
can increase to 25% (McNulty et al. 1990). In
tropical forests, in contrast, net nitrification is
typically nearly 100% of net mineralization,
even in sites with low rates of net mineralization
and without inputs of additional nitrogen
(Vitousek and Matson 1988) (Fig. 9.6). In
tropical ecosystems, plant and microbial
growth are frequently limited by nutrients
other than nitrogen, and their demand for
nitrogen is low, so nitrifiers have ready access
to NH4 + .
The potential fates of nitrate are absorption
by plants and microbes, exchange on anion
exchange sites, and loss from ecosystems via
denitrification or leaching. Because nitrate is
relatively mobile in soil solutions, it readily
moves to plant roots by mass flow or diffusion
(see Chapter 8) or can be leached from the soil.
Microbes also absorb nitrate and use it for
synthesis of amino acids through assimilatory