between clay surfaces and organic anions. In acidic soils, on the other hand, Fe and
Al complexes are dominant in stabilizing SOM. Thus, Rasmussen et al. (2018) argue
that we need to look “beyond clay” as a predictive variable to explain organic matter
dynamics—we need to capture more than merely particle size but also actual
mineralogy and the mechanisms by which the different mineral phases interact
with organic molecules (Kleber et al. 2007). Despite that, it may still be easier to
predict the “vote” of the silent mineral majority than of microbes because minerals
are more geographically constrained and defined. The factors that control overall
clay mineralogy are understood well enough to map them on geographic scales that
readily feed into biogeochemical models.
4.4 Organic Molecules: Molecular Structure
The amount of organic matter in soil or subsoil has long been seen as an important
characteristic of a soil and of an ecosystem (e.g., Cameron and Breazeale 1904).
However, although we often discuss soil organic matter (SOM) as if it were a single
aggregate entity, this is of course not true. In fact, SOM is an enormously complex
mix of materials. The ways in which SOM behaves chemically, structures soil
physically, and alters the surfaces of soil minerals make SOM a dominant party in
the Democracy of Dirt. The chemical nature and availability of organic molecules
for extracellular reactions or microbial uptake often regulate the “voting” of the soil
microbes.
Older conceptual models had characterized SOM as “humic materials”; these
were thought to be large poly-condensed molecules whose stability was a function of
their chemistry. Individual humics were described as large (>5 kDa; Perminova
et al. 2003) and so complex that almost no two might be identical (Stevenson 1982),
analogous to snowflakes. In this concept, SOM molecules were viewed as being
chemically recalcitrant: too large for a cell to take up, too complex to be susceptible
to targeted enzymatic attack, and too aromatic to be easy to metabolize (Schnitzer
et al. 1991). Hence they were thought to decompose slowly and to be inefficiently
assimilated into biomass. However, the humic “snowflake” model is no longer
broadly accepted. Rather, humics are now thought to either result from condensation
reactions in the extraction process or to comprise conglomerates of small molecules
that produce artificially inflated estimates of molecular weight (Lehmann and Kleber
2015).
Problems with describing soil organic matter dynamics based on a humic/fulvic/
humin fractionation had long been recognized; thus, mathematical models such as
CENTURY and Roth-C (the Rothamsted Carbon Model, which is structurally
similar to CENTURY, describing C-turnover in non-waterlogged soil) that were
developed as far back as the 1980s did not describe SOM by its chemical composition, but rather assigned OM to discrete pools based on turnover times, for
example, Parton et al. (1987) assigned the turnover times of the “active,” “slow,”
and “passive” C as 1.5 y, 25 y, and 1000 y.
4 The Democracy of Dirt: Relating Micro-Scale Dynamics to Macro-Scale Ecosystem. . .
95
Al complexes are dominant in stabilizing SOM. Thus, Rasmussen et al. (2018) argue
that we need to look “beyond clay” as a predictive variable to explain organic matter
dynamics—we need to capture more than merely particle size but also actual
mineralogy and the mechanisms by which the different mineral phases interact
with organic molecules (Kleber et al. 2007). Despite that, it may still be easier to
predict the “vote” of the silent mineral majority than of microbes because minerals
are more geographically constrained and defined. The factors that control overall
clay mineralogy are understood well enough to map them on geographic scales that
readily feed into biogeochemical models.
4.4 Organic Molecules: Molecular Structure
The amount of organic matter in soil or subsoil has long been seen as an important
characteristic of a soil and of an ecosystem (e.g., Cameron and Breazeale 1904).
However, although we often discuss soil organic matter (SOM) as if it were a single
aggregate entity, this is of course not true. In fact, SOM is an enormously complex
mix of materials. The ways in which SOM behaves chemically, structures soil
physically, and alters the surfaces of soil minerals make SOM a dominant party in
the Democracy of Dirt. The chemical nature and availability of organic molecules
for extracellular reactions or microbial uptake often regulate the “voting” of the soil
microbes.
Older conceptual models had characterized SOM as “humic materials”; these
were thought to be large poly-condensed molecules whose stability was a function of
their chemistry. Individual humics were described as large (>5 kDa; Perminova
et al. 2003) and so complex that almost no two might be identical (Stevenson 1982),
analogous to snowflakes. In this concept, SOM molecules were viewed as being
chemically recalcitrant: too large for a cell to take up, too complex to be susceptible
to targeted enzymatic attack, and too aromatic to be easy to metabolize (Schnitzer
et al. 1991). Hence they were thought to decompose slowly and to be inefficiently
assimilated into biomass. However, the humic “snowflake” model is no longer
broadly accepted. Rather, humics are now thought to either result from condensation
reactions in the extraction process or to comprise conglomerates of small molecules
that produce artificially inflated estimates of molecular weight (Lehmann and Kleber
2015).
Problems with describing soil organic matter dynamics based on a humic/fulvic/
humin fractionation had long been recognized; thus, mathematical models such as
CENTURY and Roth-C (the Rothamsted Carbon Model, which is structurally
similar to CENTURY, describing C-turnover in non-waterlogged soil) that were
developed as far back as the 1980s did not describe SOM by its chemical composition, but rather assigned OM to discrete pools based on turnover times, for
example, Parton et al. (1987) assigned the turnover times of the “active,” “slow,”
and “passive” C as 1.5 y, 25 y, and 1000 y.
4 The Democracy of Dirt: Relating Micro-Scale Dynamics to Macro-Scale Ecosystem. . .
95