Organic Matter and Sustainability - Sustainable Management of Soil Organic Matter - page 8

soil aggregates. The capacity of a soil to store organic matter is related to
the association of organic matter with clay and clay plus silt (2-20 µ m

diameter) particles, soil microaggregates (20-250 µ m diameter) and

macroaggregates (> 250 µ m diameter), and the accumulation of sand-sized

(> 50 m) macroorganic matter (Christensen, 1996; Tisdall, 1996; Feller

and Beare, 1997; Hassink, 1997).

µ

Carbon inputs, macroorganic matter and organic matter functions
in soil

Climate and soil type can significantly influence the accumulation and
storage of soil organic matter due the interactions of temperature and
moisture on plant productivity and the ability of the soil's mineral compo-
nents to retain organic matter (Carter, 1996). Holding constant parent
material and topography, the potential for soil organic matter accumulation
increases with increasing precipitation and decreasing temperature (Cole
et al ., 1993). Moist, warm or hot climates favour rapid soil organic matter
decomposition relative to wet, cool climates.
Variable topography increases the potential for soil erosion which, in
turn, may also influence the accumulation and storage of soil organic
matter (Gregorich et al ., 1998). Although mineralization causes a direct loss
of soil organic matter, soil erosion can affect organic matter concentration
via removal, dilution and deposition processes.
Within any one soil type, increasing carbon inputs via agricultural man-
agement is the key to increasing soil organic matter quantity (Jenkinson,
1990). Where the initial concentration of organic matter is low, i.e. below
the capacity of a specific soil to store organic matter, it has been observed
that soil organic matter concentrations increase linearly with increasing
input levels, although the slope of the line depends on climate, soil type and
soil management (Parton et al ., 1996). The latter variable influences carbon
inputs mainly in the following three ways: increasing primary production
(e.g. perennial crops, plant nutrition and organic amendments, Angers and
Carter, 1996; Reeves, 1997; Janzen et al ., 1998); increasing the proportion
of primary production returned to, or retained by the soil (e.g. crop residue
retention and placement); and influencing both microbe- and plant-
induced changes in soil structure that can suppress the rate of decomposi-
tion through enhancing soil aggregation (Hassink et al ., 1997; Angers and
Caron, 1998). Under certain climates, placement of residues at depth can
allow a relative increase in stored organic matter (Carter et al ., 1998), while
surface accumulation of crop residue and plant material can occur under
conditions of desiccation or waterlogging. In addition to carbon inputs,
vegetative differences and quality of crop residue can also influence the
quantity of soil organic matter (Juma, 1993; Parton et al ., 1996).