Agriculture Reference
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variation (CV) of only 20.2 (Table 2). A three-fold range of soil organic C
(6.7-20.0 g C kg
−1
soil) and N (0.8-2.1 g N kg
−1
soil) occurred across the
set of fields, and soil C:N ranged from 8.1 to 9.8 (Table 2). Soil C and N
were highly correlated with one another (p < 0.0001, r
2
= 0.95). Soil pH
was near neutral (6.3-7.2) and varied the least of all measured soil proper-
ties with a CV of 3.7. Soil NH
4
+
, NO
3
−
, and Olsen P had the most variation
of all measured variables with CVs of 70.8, 127.5, and 76.7, respectively,
and were positively skewed, especially NO
3
−
( Table 3). Field 4 had the
highest level of NO
3
−
(44.9 μg-N g
−1
soil) while field 1 had the lowest (0.2
μg-N g
−1
soil).
MBC ranged from 67.7 μg-C g
−1
soil in fi eld 1, to 165.8 μg-C g
−1
in
fi eld 13, but it did not consistently increase across the C gradient (Table 3).
Overall, MBC had a positive relationship with soil C (p = 0.03, R
2
= 0.36),
soil N (p = 0.02, R
2
= 0.41), and silt (p = 0.05, R
2
= 0.31), although the low
R
2
values indicate that none of these variables was highly associated with
the variation in MBC. MBN was not signifi cantly related to any soil vari-
able other than MBC (p = 0.003, R
2
= 0.56) and showed more variability
than MBC with a CV of 39.8 vs. 21.9 for MBC. Total FAMEs, which can
be considered an alternative measure of microbial population size, had a
positive but weak relationship with MBC (p = 0.05, R
2
= 0.31) and similar
associations with soil C and N as MBC. Total FAMEs also had positive re-
lationships with EOC (p = 0.02, R
2
= 0.41) and EON (p = 0.02, R
2
= 0.40).
13.3.2 PATTERNS OF SOIL ENZYME ACTIVITIES
The activities of nine soil enzymes showed different trends across the 13
fields (Fig. 1). Phosphodiesterase activity showed the strongest field effect
(F-statistic = 27.3); higher F-statistic values indicate the relative magnitude
of the “field effect”, i.e., a higher ratio of variance among fields to vari-
ance within a field (see methods section). Six out of nine enzyme activities
had an F-statistic greater than 20. β-glucosaminidase activity was the least
variable of the measured enzymes (F-statistic = 8.2), followed by the ac-
tivities of l-asparaginase (F-statistic = 14.1) and urease (F-statistic = 19.1).
The geometric mean of enzyme activities ( Table 2), an indicator of the
overall metabolic potential, showed a strong positive relationship with
