Soil Teeming with Life: New Frontiers for Soil Science - Sustainable Management of Soil Organic Matter - page 387

rate of supply of nutrients and electron-accepting resources, is also critical
to the microbial community.
4. The soil microbial community is a product of more than 3.5 billion years of
evolution . The fossil record indicates that prokaryotes have been on Earth
for an extremely long period of time; 85% of their history occurred before
Pangea separated. This long period of evolution and natural selection
under a wide range of conditions is probably responsible for the enormous
microbial diversification. It has also probably selected for organisms that
survive stress conditions including starvation, desiccation and freezing. In
some sense, a gram of soil may contain a reasonable historical record of the
early evolutionary history of life.
The basic premise behind an attempt to understand the complex soil
community is that further knowledge will pay off in improved agriculture,
environmental decision making and management, and many of the major
practical issues listed above. In the past, soil biological processes have been
studied at the level of the 'Grand Mean', i.e. lumping all of the diversity and
complexity as an average value per gram, kilogram or hectare, for example.
This approach has been what was feasible and no doubt useful. The
basic question now is can, or in what cases will, a more detailed level of
understanding or a mechanistic level of insight be useful? Schimel (1995)
has suggested that in some cases it will be and in some cases it will not be.
An example of the former is when particular communities selected by one
environment has kinetic features or tolerance properties somewhat different
from those of communities selected under a different condition. In
this case, models of nutrient flux, for example based on Grand Mean
coefficients, will not be accurate for both cases. Other examples where
knowledge about particular organisms matters would be a PGPR (plant
growth-promoting rhizobacterium) that works in one soil type but not in
another, or that atmospheric methane is consumed by soils of one ecosys-
tem type but not by another. In other cases, the populations may not differ
in ways that affect function, but instead a new level of understanding can
be obtained which provides more insight into how or how fast a process is
controlled, e.g. the triggering of the molecular regulation of denitrification
or the response of quorum sensors that initiate root pathogenesis.
Operational Model for Understanding Soil Biocomplexity
A more in-depth understanding of the soil community and its activity
implies exploring biological processes at the organism and molecular levels
and understanding how those levels are controlled by soil physical, chemical
and climatic factors and by the overlying vegetation. Figure 6.1 shows the
continuum in biological organization in the soil community and the adap-
tive features important at each scale of organization. The adaptive features