7
Decomposition and Soil Organic
Matter Dynamics
G. Philip Robertson and Eldor A. Paul
Introduction
Decomposition is the process whereby dead organic material is broken down to its constituent
parts, ultimately to carbon dioxide (C02) and inorganic ions such as ammonium (NHt), calcium
(Ca2+), potassium (K+), and other elements originally assimilated by the organisms now decomposing. At various points along this path the
decomposing organic matter is consumed and metabolized by other organisms, which themselves
will die and decompose. Decomposition is thus a
dynamic recursive process that involves a complex
array of physical, chemical, and biological interactions that complete the biogeochemical nutrient
cycles.
Rates of decomposition vary tremendously as a
function of the structure and chemical composition
of the organic matter being decomposed (substrate
quality), the abiotic environment in which decomposition is occurring (temperature, moisture, and
aeration status in particular), and the degree to
which the decomposing substrate is exposed to heterotrophs, both microbial and faunal (Kurcheva
1960; Bunnell et al. 1977; Jenny et al. 1949; Swift
et al. 1979; Paustian et al. 1997). One might thus
expect differences in rates of decomposition both
within and between ecosystems. Within-ecosystem
variability will occur as different materials are deposited and decompose in microhabitats that range
from plant canopies to different positions along topographic gradients, different positions within soil
profiles, and even different positions within soil aggregates. Differences among ecosystems arise from
expressions of environmental and biotic gradients
104
at larger scales, especially with respect to climate,
ecosystem fertility, and successional status (Olson
1963; Meentemeyer 1978; Melillo et al. 1982; Wallace and Freedman 1986; Aber et al. 1990).
Operationally, in terrestrial ecosystems, decomposition typically defines the breakdown of organic
matter into either inorganic ions or into an organic
matter size class that permits its entry into the soil
organic matter (SOM) pool. The SOM pool receives the breakdown products of decomposition,
and the further decomposition and humification of
these products falls under the general rubric of
SOM dynamics-which also includes the turnover
of microbes and soil invertebrates that further oxidize and assimilate SOM.
Typically, decomposition is measured as mass
and nutrient loss from dead organic material placed
in that part of the ecosystem in which it normally
occurs-usually the soil surface for senescent plant
leaves, branches, stems, and other aboveground litter, and various soil horizons for roots, fungi, and
other belowground organisms or parts thereof.
More often than not, the usual strategy for assessing
mass loss is to entrain a known quantity of material
at a specific location and to then periodically evaluate it or a subsample for mass and nutrient loss.
Methods for assessing organic matter dynamics
(as opposed to decomposition per se) are varied and
often more difficult to interpret than decomposition
assays owing to the heterogeneous nature of SOM.
The most comprehensive and least controversial
measure is that of the total SOM pool itself. Combustion techniques provide an adequate representation of total organic carbon (C) stores in soil and
can be used to document changes in these stores
Decomposition and Soil Organic
Matter Dynamics
G. Philip Robertson and Eldor A. Paul
Introduction
Decomposition is the process whereby dead organic material is broken down to its constituent
parts, ultimately to carbon dioxide (C02) and inorganic ions such as ammonium (NHt), calcium
(Ca2+), potassium (K+), and other elements originally assimilated by the organisms now decomposing. At various points along this path the
decomposing organic matter is consumed and metabolized by other organisms, which themselves
will die and decompose. Decomposition is thus a
dynamic recursive process that involves a complex
array of physical, chemical, and biological interactions that complete the biogeochemical nutrient
cycles.
Rates of decomposition vary tremendously as a
function of the structure and chemical composition
of the organic matter being decomposed (substrate
quality), the abiotic environment in which decomposition is occurring (temperature, moisture, and
aeration status in particular), and the degree to
which the decomposing substrate is exposed to heterotrophs, both microbial and faunal (Kurcheva
1960; Bunnell et al. 1977; Jenny et al. 1949; Swift
et al. 1979; Paustian et al. 1997). One might thus
expect differences in rates of decomposition both
within and between ecosystems. Within-ecosystem
variability will occur as different materials are deposited and decompose in microhabitats that range
from plant canopies to different positions along topographic gradients, different positions within soil
profiles, and even different positions within soil aggregates. Differences among ecosystems arise from
expressions of environmental and biotic gradients
104
at larger scales, especially with respect to climate,
ecosystem fertility, and successional status (Olson
1963; Meentemeyer 1978; Melillo et al. 1982; Wallace and Freedman 1986; Aber et al. 1990).
Operationally, in terrestrial ecosystems, decomposition typically defines the breakdown of organic
matter into either inorganic ions or into an organic
matter size class that permits its entry into the soil
organic matter (SOM) pool. The SOM pool receives the breakdown products of decomposition,
and the further decomposition and humification of
these products falls under the general rubric of
SOM dynamics-which also includes the turnover
of microbes and soil invertebrates that further oxidize and assimilate SOM.
Typically, decomposition is measured as mass
and nutrient loss from dead organic material placed
in that part of the ecosystem in which it normally
occurs-usually the soil surface for senescent plant
leaves, branches, stems, and other aboveground litter, and various soil horizons for roots, fungi, and
other belowground organisms or parts thereof.
More often than not, the usual strategy for assessing
mass loss is to entrain a known quantity of material
at a specific location and to then periodically evaluate it or a subsample for mass and nutrient loss.
Methods for assessing organic matter dynamics
(as opposed to decomposition per se) are varied and
often more difficult to interpret than decomposition
assays owing to the heterogeneous nature of SOM.
The most comprehensive and least controversial
measure is that of the total SOM pool itself. Combustion techniques provide an adequate representation of total organic carbon (C) stores in soil and
can be used to document changes in these stores