Organic Matter and Sustainability - Sustainable Management of Soil Organic Matter - page 10

Christensen, 1996). Table 1.2 illustrates the change in turnover time for
soil organic matter fractions and organic matter in aggregates based on
isotopic studies. The aggregation process itself is a means to both conserve
organic matter and allow the stored organic matter to function as a reservoir
of plant nutrients and energy. In addition, the role of organic matter in the
aggregation process has major implications for the functioning of soil in
regulating air and water infiltration. This interaction has allowed organic
matter to be identified as an indicator associated with the processes of soil
permeability and erodibility (Feller and Beare, 1997).

Conclusions

The tension between natural resource and economic sustainability in
agroecosystems, which has important consequences for conservation of soil
organic matter, underlines the need to develop quantitative estimates of the
value of the functions and services provided by organic matter. The soil
quality framework provides a logical procedure to evaluate soil organic
matter quality along the sequence of 'function' to 'methodology', although
at present the question of 'critical values' and 'standardization' are limiting
steps in this evaluation.
Organic matter inputs and the soil aggregation process are important
factors in the maintenance and regulation of organic matter functioning in
soil. Both the sand-sized macroorganic matter and silt plus clay carbon are
useful and easily measured indicators.

Table 1.2. Estimates of turnover time for soil organic matter in different fractions and in soil
aggregates (after Carter, 1996; Gregorich and Janzen, 1996; Collins et al., 1997; Monreal et al.,
1997).

Type of organic matter Estimated turnover time (years)

Organic matter in fractions
Litter, crop residue 0.5-2 .

Microbial biomass 0.1-0.4

Macroorganic matter 1-8

Light fraction 1-15

Organic matter in aggregates
Non-aggregated soil 1-7

Macroaggregates a (> 250 µ m diameter)

1-23

Microaggregates (20-250 µ m diameter) 3-80

Silt plus clay (< 20 m diameter) 5-1000

µ

a Organic matter in macroaggregates but external to microaggregates (i.e. inter-aggregate).