Principles of terrestrial ecosystem ecology.pdf

212 9. Terrestrial Nutrient Cycling
chemodenitrification converts NO2 - (nitrite)
abiotically to nitric oxide gas (NO) where NO2 -
accumulates in the soil at low pH. Chemodenitrification
is typically much less important than
biological denitrification.
The sequence of NO3 - reduction is NO3 - Æ
NO2 - Æ NO Æ N2O Æ N2. The last three products,
particularly N2O and N2, are released as
gases to the atmosphere (Fig. 9.4). Most denitrifiers
have the enzymatic potential to carry
out the entire reductive sequence but produce
variable proportions of N2O and N2, depending
in part on the relative availability of oxidant
(NO3 - ) versus reductant (organic carbon).
When NO3 - is relatively more abundant than
labile organic carbon, more N2O than N2 is produced.
Other factors that favor N2O over N2
production include low pH, low temperature,
and high oxygen. Although NO is often
released during denitrification in laboratory
incubations, this seldom occurs in nature
because its diffusion to the air is impeded
by water-filled pore spaces. Some of the NO
that is produced serves as a substrate for
LONG-TERM
CONTROLS
STATE
FACTORS
BIOTA
TIME
PARENT
MATERIAL
CLIMATE
Interactive
controls
Plant
functional
types
Soil
resources
Indirect
controls
Plant NO 3 -
uptake
Litter quantity
Carbon quality
Root / microbial
respiration
Temperature
H 2 O
Soil texture
Figure 9.7. The major factors controlling denitrification
in soils. These controls range from concentrations
of substrates that directly control nitrification
to the interactive controls such as climate and disturbance
regime that are the ultimate determinants
further reduction to N2O or N2 by denitrifying
bacteria.
The three conditions required for significant
denitrification are low oxygen, high nitrate concentration,
and a supply of organic carbon (Fig.
9.7) (Del Grosso et al. 2000). In most nonflooded
soils, oxygen availability exerts the
strongest control over denitrification. Oxygen
supply is reduced by high soil water content,
which impedes the diffusion of oxygen through
soil pores. Soil moisture, in turn, is controlled
by other environmental factors such as slope
position, soil texture, and the balance between
precipitation and evapotranspiration. Soil
oxygen concentration is also sensitive to its
rate of consumption by soil microbes and roots.
It is consumed most quickly in warm, moist
environments.
The second major control over denitrification
is an adequate supply of the substrate
NO3 - . Because nitrification is a primarily an
aerobic process, the low-oxygen conditions that
are optimal for denitrification frequently limit
NO3 - supply. Some wetlands, for example, have
SHORT-TERM
CONTROLS
(-)
DIRECT
CONTROLS
(-)
Nitrate
concentration
(-)
Labile
carbon
Oxygen
concentration
DENITRIFICATION
of denitrification rate. Thickness of the arrows
indicates the strength of effects. The influence of
one factor on another is positive unless otherwise
indicated (-).
(-)