Geoscience Reference
In-Depth Information
of EC
a
using unsupervised classification (ERDAS, 1997). These twelve classes were then recoded
(combined) to form four productivity zones. Twelve geo-referenced sampling sites were selected in
each of the eight fields, three per EC
a
zone, for a total of ninety-six sites across the section.
Data collected spanned both spatial and biological scales (Figure 18.1). Soil samples were taken
(0 to 7.5 and 0 to 30 cm depths) using the EC
a
-based sampling scheme, and a portion of each sample
was separated into 1 to 2 mm aggregates (significant to soil structure). Microbial-scale measures of
the presence and activity of vesicular-arbuscular mycorrhizal (VAM) fungi were made on this frac-
tion. These organisms were selected for study because of their contributions to both crop produc-
tivity and soil conservation (aggregation). Measurements were made using C16:1(
cis
)11 fatty acid
methyl ester biomarker, immunoreactive total glomalin immunoassay, and wet aggregate stability
methods. Sampling-site scale
analyses of nineteen whole-soil physical and chemical characteristics
and surface residue were also performed, and field-scale
Global Positioning System (GPS)-refer-
enced yield maps were collected for winter wheat and corn in 1999 and 2000.
Using geographic information system (GIS) technology, microbial, soil, crop yield, and EC
a
data layers were superimposed to create a “grid stack” wherein data at identical geo-referenced
positions within the various layers could be exported in spreadsheet format for comparison and sta-
tistical evaluation. Microbial, soil, and crop yield data were appraised for significant relationships to
EC
a
and EC
a
zones using regression and analysis of variance (ANOVA) for a randomized complete
block strip-split plot design with crop and EC
a
zones as treatment factors. All statistical analyses
were conducted using SAS (SAS Inst., Cary, NC).
Geo-aligned data layers were analyzed in phases to address the various sustainability issues
outlined in the research objectives. Each phase built upon the findings of the previous phases to
expand linkages between EC
a
and EC
a
productivity zones, factors underlying soil production poten-
tial (edaphic, crop residue, and biological components), and crop yields. Experimental phases are
individually addressed in the five sections below; associated publications are cited in the section
headings for those interested in more detailed information.
18.3
ReSUltS And dISCUSSIon
18.3.1 P
h a s e
i: t
h e
ec
a
M
a P
(J
o h n s o n
e t
a l
., 2001)
Although soil heterogeneity arises from interactions among the soil-forming factors (Jenny, 1941),
it is confounded by management such as tillage and fertilization (Bouma and Finke, 1993). At the
FICS, it is interesting to note that mapped EC
a
reflects both historical and recent management. The
V-shaped patterns in the four corner fields of the section are consistent with the plow path followed
in the 1930s when the site was farmed as two half sections (Figure 18.3 and Figure 18.4). In addi-
tion, varying mean levels of conductivity, for the eight fields in the study, distinguish among differ-
ent crops grown the year before EC
a
mapping.
18.3.2 P
h a s e
ii: ec
a
v e R s u s
s
of i l
e
d a P h i c
P
R o P e R t i e s
(J
o h n s o n
e t
a l
., 2001)
Sampling-site scale
analyses of surface residue and nineteen soil physical and chemical parameters
(0 to 7.5 and 0 to 30 cm) were compared with EC
a
maps (0 to 30 cm) to “ground-truth” EC
a
and EC
a
zones. Except for NO
3
- and NH
4
-N, which exhibited a narrow range of variability across the site,
all residue and soil parameters were significantly different among EC
a
productivity zones at one or
both depths of analysis (0 to 7.5 and 0 to 30 cm). Surface residue mass and soil properties related
to yield potential were negatively correlated with EC
a
, and properties associated with soil erosion
were positively correlated (Table 18.1). These relationships are a function of the calcareous soils at
the FICS, where clay content and CaCO
3
salts dominate measured EC
a
(0 to 30 cm). The loss of top-
soil in less-productive eroded areas of each field has exposed underlying clay and CaCO
3
horizons,
