Geoscience Reference
In-Depth Information
NO
3
-N contributed 98, 98, 97, and 66 percent to
the variability between water-filled pore space
(WFPS) and NO
3
-N for 2000, 2001, 2002, and
2003, respectively. The +CC treatment indicates
NO
3
-N accounts for 81, 23, 53, and 29 percent of
the variability between WFPS and NO
3
-N. Stan-
dardized estimate differences for -CC and +CC
treatments can be attributed to the dry years, in
which the surveys occurred. The average annual
precipitation for the South-Central Nebraska
region is 690 mm; the rainfall for the 4-year
study was 541, 524, 374, and 492 mm. Below-
normal precipitation characterized the 4-year
period and is reflected in the standard estimate
values for the +CC treatment where both water
and nitrate dynamics reflect crop uptake. Ana-
lyzing all data from 2000 through 2003 shows
that NO
3
-N dynamics accounted for 79 percent of
the -CC variability between WFPS and NO
3
-N.
Additionally, comparisons were made between
NO
3
-N and WFPS differences as contributors to
the EMI differences; nitrate contributed 79, 98,
93, and 98 percent of the variability for years
2000, 2001, 2002, and 2003, respectively, and 86 percent for all 4 years combined. The primary
contributor to the differences between EMI -CC and +CC over the growing seasons was nitrate
level differences. The methodology demonstrated nitrate as a dynamic player in the crop production
cycle, and EMI as a viable tool for observing NO
3
-N dynamics in a crop production system.
tAble 19.1
Relative Contribution of no
3
-n and WfpS
in explaining variability of Apparent
electrical Conductivity (eC
a
)*
no
3
-n
WfpS
year
%
p<
%
p<
treatment
2000
81.2
0.004
18.8
0.12
+CC
2001
22.9
0.10
77.1
0.004
+CC
2002
53.0
0.005
47.0
0.007
+CC
2003
29.3
0.006
70.7
0.0001
+CC
2000
97.7
0.023
2.3
0.707
−CC
2001
98.4
0.0001
1.6
0.42
−CC
2002
97.0
0.0003
3.0
0.46
−CC
2003
66.3
0.0001
33.7
0.0001
−CC
*
Standard regression estimates shown for 2000-2003
comparing the relative contribution of NO
3
-N and wfps
(water-filled pore space) in explaining the variation in
EC
a
(all data from Rep 2). Comparisons are made to
temperature-corrected EC
a
values.
19.3.2.2
no-Cover, All treatments
Table 19.1 indicates that NO
3
-N is significant and the primary contributor to the -CC profile
weighted soil electrical conductivity plot dynamics for all 4 years. Plot treatment means distinc-
tive shapes (Figure 19.4) can be interpreted from the perspective of NO
3
-N as responsible for key
features. Every plot begins with manure or compost being applied early, followed by an EC
a
value
that gradually increases as the season progresses. Planting dates varied from DOY 114 to 137 and
did not have an immediate impact on EC
a
. The crop produces a visible change in EC
a
approximately
50 days after planting, as the crop achieves about 30 cm height (30 cm is approximately the v6-v9
stage, a time of increasing N uptake) (Ritchie et al., 1986); the apparent conductivity makes a notice-
able downturn that lasts until about the time the corn silks. The downturn in EC
a
corresponds to the
time of maximum nutrient uptake; a time when NO
3
-N was rapidly being removed from the soil.
Silk stage occurs at physiological maturity; it is also the point at which the EC
a
curve levels out until
harvest, when EC
a
begins a gradual increase, until the end of the season. Once maturity is achieved,
very little nutrient uptake is occurring and EC
a
is relatively stable. After harvest, mineralization
increases NO
3
-N concentration and EC
a
increases in proportion.
19.4 ConClUSIonS
A 4-year study, which included field measurement of EC
a
, identified the effects of manure, com-
post, fertilizer, and cover crop on EC
a
values. Compost and manure applied at the N rate resulted in
consistently higher conductivity and available N, when compared to the commercial fertilizer and
