Principles of terrestrial ecosystem ecology.pdf

The stimulation of microbial activity by warm
temperatures also initiates a series of feedback
loops that influence decomposition. The consumption
of oxygen by microbial and root respiration
constrains decomposition in wet soils
or wet microsites (e.g., the interior of soil aggregates).
On the other hand, the nutrients
released by decomposition at high temperatures
increase the quantity and quality of litter
produced by plants, altering the substrate available
for decomposition. High temperatures also
increase the rate of chemical weathering, which
in the short term enhances nutrient supply. In
cold climates, low temperature leads to a layer
of permanently frozen soils (permafrost) that
restricts drainage and therefore decomposition.
Most of the indirect temperature effects
enhance soil respiration at warm temperatures
and contribute to the more rapid decomposition
observed in warm climates.
Moisture
Moisture-mediated
effects
Evaporation
Water
availability
Oxygen
(+/-)
Direct effects
Carbon accumulation is greatest in wet soils
because decomposition is more restricted by
high soil moisture but is less restricted by low
soil moisture than is NPP. Decomposers, like
plants, are most productive under warm moist
conditions, provided sufficient oxygen is available.
This accounts for the high decomposition
Temperature
Microbial
activity
Soil
respiration
Factors Controlling Decomposition 161
Litter
quantity and
quality
Nutrient-mediated
effects
Weathering
Nutrient
availability
Plant
growth
Root
respiration
Figure 7.6. Direct and indirect effects of temperature on soil respiration.
rates in tropical forests (Gholz et al. 2000). The
decomposition rate of mineral soil generally
declines at soil moistures less than 30 to 50%
of dry mass (Haynes 1986), due to the reduction
in thickness of moisture films on soil
surfaces and therefore the rate of diffusion
of substrates to microbes (Stark and Firestone
1995). Osmotic effects further restrict the
activity of soil microbes under conditions of
extremely low soil moisture or salt accumulation.
Bacteria function at lower water availability
than do plant roots, so decomposition
continues in soils that are too dry to support
plant activity. The high concentrations of
osmotic metabolites synthesized by microbes in
dry or saline conditions create severe osmotic
gradients after soil wet-up, causing many microbial
cells to burst. This results in pulses of nutrient
availability after the first rains. Even
short-term drying–wetting cycles, such as rain
storms or the daily formation and evaporation
of dew, can strongly influence decomposition in
the litter layer and surface soils. The net effect
of drying–wetting cycles is the stimulation of
decomposition, if the cycles are infrequent (as
generally occurs in soils). Frequent moisture
fluctuations, as in the litter layer, can, however,
reduce microbial population numbers to an
extent that decomposition rates are reduced
(Clein and Schimel 1994). Drying–wetting