Agriculture Reference
In-Depth Information
The complex, multistructured, multicomponent pool of organic materials consti-
tuting SOM includes: (1) decomposing plant residues, (2) associated microorgan-
isms and their products, and (3) the biochemically transformed (“humic”) fraction.
KBS LTER studies have shown that all these components can be associated with
Ca, minerals, and sesquioxides that greatly increase their MRTs. Long-term incu-
bations to allow microorganisms to metabolize labile components, together with
stable isotopic tracers to identify sources and acid hydrolysis to quantify the resis-
tant fraction, have provided a useful framework to interpret SOM dynamics at KBS.
Interpretation of the long-term incubation results distinguishes three pools for
modeling purposes—the active, slow, and resistant fractions—although SOM is
known to represent an oxidation continuum (Paul et al. 2001b). The active pool rep-
resenting the most labile components is small at ~5% of the SOM, and is composed
of some of the interaggregate plant residues measured by physical fractionation.
The active pool also contains a portion of the microbial biomass and has MRTs of
months to a few years. This pool is important in the management of nutrients on a
growing season basis. The slow pool, with MRTs ranging from months to decades,
accounts for ~40% of total SOM. This is the major source of soil nutrients that
change with management, often requiring 6 to 7 years to show measurable differ-
ences in yield and soil sustainability. The dynamics of the slow pool are controlled
by aggregation (intraaggregate SOM) and the association of microbial products
with Ca, as well as the silt and clay fractions. The intraaggregate fraction measured
by physical separation also has young and old constituents, but on average has
MRTs of decades. The silt and clay SOM fractions and the nonacid hydrolyzable
residues have the longest turnover times. They also contain the largest concentra-
tions of aluminosilicate minerals and sesquioxides. The actual mechanism of sta-
bilization of the oldest SOM in soils (the resistant pool) has yet to be determined.
Soil organic matter is a dynamic system with some components that can change
rapidly with changes in management and environmental and biotic controls, while
other components respond very slowly. The fractions we have identified, while
reflecting these controls, are not discrete. For example, in young fractions, such as
those protected by aggregates, there is a small amount of old C, possibly charcoal.
The older component, such as the SOM associated with clays, can also have a small
amount of young C, such as that coming from absorbed microbial biomass constitu-
ents. The MRTs and pool sizes we have measured have been useful for interpreting
the effects of management practices, as well as providing fundamental knowledge
of the basic controls affecting SOM dynamics.
The recent use of molecular-structure analysis to measure the biochemical
composition of SOM provides insights into SOM dynamics (Paul et al. 2008).
The major components derived from plant and microbial products such as car-
bohydrates, proteins, and related N compounds, lignins, sterols, aromatics, and
fatty acids are readily identified by pyrolysis-mass spectrometry. These constitu-
ents vary with ecosystem type and management. However, they also illustrate
the effect of microbial processing under unifying controls in that the molecular
structure of SOM also shows significant similarities among different soils. In
addition, there are some unidentified organic components that have century-to-
millennia turnover rates.
