118
SOM pool vary as a function of depth-at any
depth the SOM pool may undergo additions, losses,
and transformations, as well as the exchange of
material with the soil above and below. Insights
into SOM cycling can be gained from either simple
or complex models of the SOM pool, and we
will review, at the end of each section, applications
of these approaches to problems of ecological
significance.
SOM Additions
Carbon (and related elements in organic matter) is
added to soil via a variety of processes: (1) surficial
litter (i.e., leaves, branches, crop residue, etc.),
(2) root inputs (i.e., root death, root exudation), and
(3) downward (i.e., leaching via dissolved or suspended modes in water, bioturbation) or upward
(bioturbation) transport (Fig. 8.1). A quantitative
understanding of these processes in most soil systems is often poor due to the difficulty of monitoring them in the field.
It is likely easiest to quantify surficial litter inputs
(via litter traps, standing biomass measurements,
etc.) (see Chapter 2). However, these inputs only
Simplified Model of
Carbon Cycling Processes in Soil
root
detrius
-FIGURE 8.1. Carbon processes in the soil environment.
Ronald Amundson and W. Troy Baisden
contribute to the SOM pool if they are incorporated
via transport processes. These are discussed below.
It is far more difficult to measure rates of root inputs
into the soil (see Chapter 4), and a good understanding of these additions is not available for all
ecosystems.
With respect to N, there are several important
processes in addition to those discussed above for
C. First, biological N fixation is an important N
addition in many ecosystems. An additional input
is atmospheric deposition of nitrate (NO;) and ammonia (NH3) via wet and dry pathways (Schlesinger 1991). The deposition rates appear to vary
greatly from one location to another (Galloway et
al. 1994; see also Chapter 17), and in some regions
have been greatly modified by industrial and agricultural activity (Kinzig and Soclow 1994). Finally,
fertilizer additions have, in some areas, exceeded
natural N additions (Kinzig and Soclow 1994).
SOM Losses
With respect to C, most losses from soils are via
(1) CO 2 produced through heterotrophic decomposition of SOM, (2) dissolved organic C (DOC)
removal with downward percolating water, and
(3) erosion. In most soils, respiration dominates the
losses. Soil respiration (C0 2 flux across the soilatmosphere interface) can be relatively easy to
quantify (e.g., via sodium hydroxide [NaOH] traps,
infrared [IR] gas analyzers, etc.). However, total
soil respiration consists of CO 2 released via decomposition of SOM as well as CO2 produced from the
respiration of living plant roots. The proportion of
total respiration due to decomposition of humic
substances versus living root respiration is poorly
known, although estimates of the humus contribution to respiration have been made based on comparisons of the net primary productivity (NPP) of
a site to the total soil respiration (Schlesinger 1977)
and natural abundance carbon-14 e 4 C) (Dorr and
Miinnich 1986) and carbon-13 (l3C) (Robinson and
Scrimgeour 1995; Rochette and Flanagan 1997)
mass balance approaches.
In established ecosystems, soil N loss pathways
include (1) plant uptake, with lesser losses via
(2) gaseous losses (nitrous oxide [N 2 0], nitric oxide
[NO], N 2 , NH 3 ) (see Chapter 15), (3) dissolved
losses (NO;, ammonium [NHt], dissolved or-
SOM pool vary as a function of depth-at any
depth the SOM pool may undergo additions, losses,
and transformations, as well as the exchange of
material with the soil above and below. Insights
into SOM cycling can be gained from either simple
or complex models of the SOM pool, and we
will review, at the end of each section, applications
of these approaches to problems of ecological
significance.
SOM Additions
Carbon (and related elements in organic matter) is
added to soil via a variety of processes: (1) surficial
litter (i.e., leaves, branches, crop residue, etc.),
(2) root inputs (i.e., root death, root exudation), and
(3) downward (i.e., leaching via dissolved or suspended modes in water, bioturbation) or upward
(bioturbation) transport (Fig. 8.1). A quantitative
understanding of these processes in most soil systems is often poor due to the difficulty of monitoring them in the field.
It is likely easiest to quantify surficial litter inputs
(via litter traps, standing biomass measurements,
etc.) (see Chapter 2). However, these inputs only
Simplified Model of
Carbon Cycling Processes in Soil
root
detrius
-FIGURE 8.1. Carbon processes in the soil environment.
Ronald Amundson and W. Troy Baisden
contribute to the SOM pool if they are incorporated
via transport processes. These are discussed below.
It is far more difficult to measure rates of root inputs
into the soil (see Chapter 4), and a good understanding of these additions is not available for all
ecosystems.
With respect to N, there are several important
processes in addition to those discussed above for
C. First, biological N fixation is an important N
addition in many ecosystems. An additional input
is atmospheric deposition of nitrate (NO;) and ammonia (NH3) via wet and dry pathways (Schlesinger 1991). The deposition rates appear to vary
greatly from one location to another (Galloway et
al. 1994; see also Chapter 17), and in some regions
have been greatly modified by industrial and agricultural activity (Kinzig and Soclow 1994). Finally,
fertilizer additions have, in some areas, exceeded
natural N additions (Kinzig and Soclow 1994).
SOM Losses
With respect to C, most losses from soils are via
(1) CO 2 produced through heterotrophic decomposition of SOM, (2) dissolved organic C (DOC)
removal with downward percolating water, and
(3) erosion. In most soils, respiration dominates the
losses. Soil respiration (C0 2 flux across the soilatmosphere interface) can be relatively easy to
quantify (e.g., via sodium hydroxide [NaOH] traps,
infrared [IR] gas analyzers, etc.). However, total
soil respiration consists of CO 2 released via decomposition of SOM as well as CO2 produced from the
respiration of living plant roots. The proportion of
total respiration due to decomposition of humic
substances versus living root respiration is poorly
known, although estimates of the humus contribution to respiration have been made based on comparisons of the net primary productivity (NPP) of
a site to the total soil respiration (Schlesinger 1977)
and natural abundance carbon-14 e 4 C) (Dorr and
Miinnich 1986) and carbon-13 (l3C) (Robinson and
Scrimgeour 1995; Rochette and Flanagan 1997)
mass balance approaches.
In established ecosystems, soil N loss pathways
include (1) plant uptake, with lesser losses via
(2) gaseous losses (nitrous oxide [N 2 0], nitric oxide
[NO], N 2 , NH 3 ) (see Chapter 15), (3) dissolved
losses (NO;, ammonium [NHt], dissolved or-