Soil ecology and agroecosystem studies - Microbial Ecology in Sustainable Agroecosystems - page 2

Agricultural practices (row crop, orchard, forest management, and rangeland man-
agement) that disrupt the soil profile with aggressive tillage for crop production or weed
control or structural rehabilitation, remove crops and their stubble on a regular basis, or
rely on inorganic fertilizers tend to promote declines in natural SOM reserves (Six et al.,
2000; Coleman et al., 2004; Wang and Dalal, 2006). Ways to ameliorate the losses of SOM
pools in croplands and forested lands are being explored by scientists in several countries
(Elliott and Coleman, 1988; Grace et al., 1998; Six et al., 2000, 2004). These efforts were ini-
tially promoted to sustain croplands for food production but more recently are seen as
mechanisms for the sequestration of C in soil due to the concern over global warming and
rising levels of atmospheric CO 2 and the fact that soils represent the largest terrestrial pool
of C, containing 1500 pg, twice that of the atmosphere (Schlesinger, 1996).
The importance of SOM and the processes that contribute to its formation and to soil
formation has been understood for some time (Jenny, 1941). The different forms of SOM
arise from the transformation of plant, microbial, and animal-derived detritus through
chemical and physical processes that are mediated by weathering and biotic activity. The
processes behind the transformations of detritus into SOM are central in controlling and
regulating key nutrient cycles in natural, agricultural cropland, rangeland, and agrofor-
estry systems. In this chapter, we set forth the principal mechanisms and the biotic interac-
tions involved in the transformation of detritus into SOM. Organic detritus is produced by
photosynthesizing and heterotrophic organisms and enters the soil system both above- and
belowground. The physical localization of the detritus and its chemical makeup (e.g., C/N
ratio and lignocellulose content) have strong influences on subsequent system behavior. In
fact, soils, with their strong roles in governing biogeochemical cycles of carbon, nitrogen,
and phosphorus, should be viewed as central organizing entities within terrestrial ecosys-
tems (Coleman et al., 1998). A thermodynamic approach to ordering and dissipative pro-
cesses in soil-plant systems provides a comparison of system-level anabolic and catabolic
processes to achieve this synthesis ( Table 1.1 ; Addiscott, 1995). This approach provides a

Table 1.1 Ordering and Dissipative Processes in Soil-Plant Systems Categorized
as Biological or Physical

Ordering processes Dissipative processes

Entropy decreases Entropy increases

Biological

Photosynthesis Respiration

Growth Senescence

Formation of humus Decomposition of humus

Physical

Water flow (profile development) Water flow (erosion, leaching)

Flocculation Dispersion

Aggregation Disaggregation

Development of structure Breakdown of structure

Larger units Smaller units

Fewer of them More of them

More ordered Less ordered

Sources: Coleman et al., 1998, from Addiscott, T.M. 1995. Entropy and sustainability. European
Journal of Soil Science 46:161-168. With permission from John Wiley and Sons.
Note: The pairs are not exact opposites.