Marginality Principle - Principles of Sustainable Soil Management in Agroecosystems - page 24

FIGURE 2.3 Degradation of soil across a landscape showing the variation in surface organic
matter. (From USDA-NRCS. Available at photogallery.nrcs.usda.gov, accessed July 13, 2012.)
Degradation of the soil is a complex process linking the physical, chemical, and
biological processes within the soil into a set of complex interactions, which are trig-
gered by natural and anthropogenic disturbances. The types of soil degradation are
shown in Table 2.2 . Scientific evidence suggests that the hierarchy of these factors
begins with degradation in the soil biological component followed by physical and
chemical changes. If we monitor these types of degradation, it is easy to see that
degradation will induce a vertical and horizontal component because the factors that
give rise to the spatial variation shown in Figures 2.1 and 2.2 will respond differently
as the degradation forces are applied. Changes in the soil are most apparent at the
surface more than with depth because the forces causing degradation manifest them-
selves in this part of the soil profile more quickly than with depth. These changes
are not uniform across a landscape because different soils respond differently to
degradation forces.
2.3.1 B iological D egraDation
Biological degradation is associated with the dynamics of the microbial systems
within the soil profile. Microbial activity and soil biodiversity are linked with the
soil organic C (SOC) pools in the soil and ultimately are associated with the deple-
tion of SOC and the turnover rates of the SOC pool. Assessments of soil quality must
include one or more indicators of microbial activity within the soil profile. Bastida et
al. (2008) developed a biological index to assess soil quality and compared different