- A variety of soil properties have been proposed and tested as potential
indicators of soil quality. This study was conducted in an effort to produce a
standardized soil quality index for systems in Oregon. Soils in both forested and
agricultural systems were sampled in a wide variety of locations, climates, and
management regimes in an attempt to represent the diversity across the state. Sites
were chosen such that managed and unmanaged treatments were paired at each
location. Soil properties were measured and the results were analyzed to produce
indices capable of differentiating between managed and unmanaged systems. Soil
physical properties, such as particle size distribution, aggregate stability, bulk
density, and water content were measured to characterize the soils in question.
Chemical properties extractable nitrogen (N) and phosphorous (P), total carbon (C),
and pH were measured to further describe the nature of the soil environment.
Finally, biological properties such as enzyme activities (β-glucosidase,
arylsulfatase, and fluorescein diacetate hydrolysis, a broad measure of hydrolytic
activity in the soil), total biomass C, total bacterial and fungal biomass, respiration,
and soil ergosterol (a sterol found almost exclusively in viable fungi) concentration
were measured to examine the activity of the microbial community. Canonical
Discriminant Analysis, a multivariate statistical technique for differentiating
between groups of measures, was used to suggest which parameters might be most
useful as an index differentiate between managed and unmanaged systems. The
index chosen for forested sites, soil ergosterol concentration normalized to total C,
indicates the pervasiveness of fungal influence in forest systems. In the agricultural
systems, a linear combination of enzyme activities, normalized to clay content was
the best discriminator between treatments. The index for agricultural sites correctly
assigned samples to the managed or unmanaged treatment 90% of the time. The
forest index was successful with 95% of sites sampled.
One of the enzyme activities included in the soil quality index for the
agricultural sites was 3-glucosidase. Previous research has shown that β-glucosidase activity can detect soil management effects and has potential as a soil
quality indicator. However, much remains unknown about the ability of this
enzyme to discriminate between treatments. Consequently, a secondary study was
conducted to determine how management affects the kinetics of this enzyme and its
stabilization on soil colloids using samples from three of the agricultural sites.
Microwave radiation was used to heat soil samples to denature the fraction of β-
glucosidase associated with viable microorganisms in these soils and estimate the
extracellular (abiontic) activity of this enzyme stabilized in the soil.