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useful theoretical framework linking biological and physical factors and mediated pro-
cesses in which to consider the biogeochemical phenomena described in this chapter. The
capacity for self-organization resides in the soil population. This population, made up
of soil organisms, provides the entropy-maximizing characteristics that characterize soil
thermodynamically (Addiscott, 2010).
Our approach incorporates the contributions of invertebrates and microbes to this
process by melding and combining the earlier food web analyses of Hunt et al. (1987) and
Moore et al. (2003) to provide a more mechanistic and realistic synthesis of the conver-
gence of aboveground and belowground aspects, that is, “green and brown worlds.” In the
following sections, we provide an overview of both empirical and theoretical approaches
to a successful integration of the green and brown worlds via the all-important detritus
pathway in terrestrial ecosystems.
The production, translocation, and decomposition of SOM in managed ecosystems, includ-
ing forests and rangelands, have been studied empirically and modeled conceptually and
mathematically (Jenkinson et al., 1991; Moorhead et al., 1999; Parton et al., 1987; Paustian et
al., 1990, 1997). These approaches include a conjoint consideration of SOM with key biotic
and abiotic processes, including C/N ratio, lignin/C ratio, and 1/percentage sand, moving
SOM fractions from litter sources into different pools depending on their recalcitrance and
turnover times. Employing models backed up with empirical studies has been useful for
land managers who compare management treatments across large land areas (Barrett and
Burke, 2000; Epstein et al., 1997; Grace et al., 2006). Models of the dynamics of SOM have been
used to predict trends in SOM with reasonable accuracy. Of these, the CENTURY model
(Parton et al., 1987) has proven useful. There is a specific agroecosystem version (Version
4.0), which is being incorporated into several other cropping systems models (Gijsman et al.,
2002) and can effectively simulate C accumulation during soil formation (Parton et al., 1987).
The CENTURY model also offers a heuristic tool to study the key components and
processes important to detritus and the formation and dynamics of SOM. CENTURY
includes three important features (
Figure 1.1
)
. First, the origins, rates of production, and
quantity of production of organic carbon are taken into consideration. Second, the forms of
organic carbon produced and its structural quality in relation to its rate of degradation are
examined. Third, the biotic and abiotic factors and processes that mediate the degradation
and transformation of organic matter are included. The biotic components are based on
microbes whose activities are governed in large part by soil moisture, temperature, and
substrate quality. These components will require a more explicit consideration of the full
suite of soil biota given that the approach lacks implicit consideration of the detrital food
web interactions and habitat heterogeneity effects.
CENTURY is comprised of submodels, one that represents the inputs of organic mate-
rial from plant residues and animal excreta and another that represents SOM; these are
structured and parameterized for different types of ecosystems (e.g., grassland/crop, forest,
and savanna). For each ecosystem type, the submodels possess different rates of primary
production and partition organic matter into aboveground and belowground residues.
Residue is divided into above- and belowground metabolic and structural pools that differ
in quality based on lignin and N contents. The former decompose rapidly and cycle soluble
and labile inputs, while the latter pools decompose more slowly and contain metabolically
resistant compounds, including plant lignin. The SOM submodel simulates the flow of C
and N through the above- and belowground components into multiple SOM pools with
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