Organic Matter and Sustainability - Sustainable Management of Soil Organic Matter - page 5

Soil organic matter and soil functions and processes
At the agroecosystem, or 'field' scale, organic matter influences many
readily measurable soil functions or processes (Schnitzer, 1991). Organic
matter is both a source and a sink for plant nutrients, and provides an
energy substrate for soil organisms. Soil macro- and microaggregation, that
aid the infiltration of air and water, are promoted and stabilized by soil
organic matter (Tisdall, 1996). Organic matter promotes water retention
and influences the efficacy and fate of applied pesticides (Gregorich et al .,
1994, 1997). It also influences certain soil physical processes such as
compactibility (Soane, 1990), friability (Watts and Dexter, 1998) and the
range of soil 'available' water for plant growth (Kay, 1998). Overall, the
positive interrelationship between soil organic matter and soil aggregation
has important benefits on both water and air infiltration, soil erodibility and
conservation of organic matter and nutrients (Feller and Beare, 1997).
Soil organic matter properties, attributes and constituents
Gregorich et al . (1994) indicated that soil organic matter should be
viewed as a set of fractions rather than a single entity. These fractions are
descriptive of the 'quality' of soil organic matter. Important fractions of
organic matter are the light fraction, macroorganic matter (i.e. particulate
carbon), microbial biomass carbon, mineralizable carbon, carbohydrates
and enzymes. These fractions have biological significance as they are
involved in several soil functions and processes such as aggregation and
formation of soil structure, and nutrient cycling and storage. Chemical
characterization of organic matter, that provides information on chemical
structure and functional groups, is also useful to evaluate the influence
of land use changes on organic matter (Monreal et al ., 1995; Mahieu et al .,
1999). However, the utility of such measurements in soil quality evaluation
is not so clear.
Soil biota are important soil constituents that are usually strongly asso-
ciated with or, in some cases (i.e. microbial biomass carbon), components
of soil organic matter. Measurements of soil biota abundance, diversity or
activity are considered potential indicators of soil quality (Gregorich et al .,
1997). The microbial biomass is the main agent that supports the soil
function and associated processes involved with the storing and cycling
of nutrients and energy (Carter et al ., 1999). Mycorrhizal fungi play an
important role in sustainable plant productivity and in the formation
and maintenance of soil structural stability (Tisdall, 1996; Gregorich et al .,
1997), while soil fauna are major determinants of soil processes influencing
nutrient cycling, aggregate formation and permeability of soil (Lavelle et al .,
1997).