Organic Matter and Sustainability - Sustainable Management of Soil Organic Matter - page 9

Addition of organic matter to the soil, in the form of crop residues or
organic amendments, increases the concentration of 'free, low-density'
macroorganic matter which can range from 10 to 40% of total soil organic
matter (Carter et al ., 1998; Kay, 1998). This form of organic matter func-
tions in improving the mechanical properties of soil, specifically in reducing
soil compactibility (Soane, 1990), and enhancing the range of soil
'available' water for plant growth (Kay, 1998) and soil friability (Watts
and Dexter, 1998). On the basis of this important role in organic matter
functioning, it would be advantageous to establish critical ranges for
macroorganic matter as it is sensitive to soil management, and easily
measured and quantified.

Soil aggregation, aggregated organic carbon and organic matter
functions in soil

Soil type characteristics (e.g. mineralogy and particle size distribution)
regulate the capacity of a soil to preserve organic matter and control
soil aggregation. The interrelationship between soil aggregation and soil
organic matter constrains both decomposition (e.g. separate carbon sub-
strate from decomposer organisms) and predation (e.g. separate microbes
from predators) processes and subsequently conserves and stabilizes soil
organic matter (Juma, 1993).
Arable soils contain less organic matter than adjacent grassland soils,
but the amount of organic carbon associated with the clay plus silt (i.e.
2-20

m diameter) can be similar (Hassink, 1997). More than 80% of the
organic carbon in temperate soils can be associated with < 20

µ

m diameter
organomineral particles (Christensen, 1996). Once the clay plus silt is
saturated with organic matter, additional organic matter would be found
mainly in the sand-sized macroorganic matter fraction. Thus, grassland and
forest soils that can contain relatively high concentrations of organic matter
generally have more sand-sized organic matter than arable soils (Carter
et al ., 1998). In soils with low concentrations of sand fraction carbon,
> 90% of the soil organic matter can be found in the clay plus silt particles
(Christensen, 1996). Thus, silt plus clay carbon, being an inherent measure
of the capacity of any one soil type to store organic matter (Hassink, 1997),
provides a basic measure of aggregated organic carbon. In addition, silt plus
clay carbon is easily estimated and quantified.
In most soils, the development of aggregation distributes organic
matter into different sized aggregates with an increasing susceptibility to
decomposition as follows: within clay plus silt particles, within micro-
aggregates (i.e. intra-microaggregate), within macroaggregates but external
to microaggregates (i.e. includes the light fraction, macroorganic and
microbial biomass carbon) and 'free' macroorganic matter (Carter, 1996;

µ