The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 303

CO 2 -C captured in sample traps and in blanks (glass jars with no soil). The
CO 2 emissions divided by total soil organic C were used to calculate C
turnover. Soil inorganic N (NO 3 -N and NH 4 -N) was extracted with 2 M
KCl and measured by standard colorimetric procedures using a Technicon
Autoanalyzer II (Technicon Corp., Tarrytown, New York). The net N
mineralization was the difference between the final and initial inorganic
N contents for the incubation.
Statistical analyses of data were performed using the mixed procedure
of the Statistical Analysis System at an established a priori level of P 0.10

(SAS, 1996). The term 'trend' is used to designate appreciable, but
non-significant, treatment effects which differed at the 0.10 < P < 0.20
level.

≤

Results and Discussion

For both crop species, elevated CO 2 resulted in a significant increase in
the residue mass returned to plots (Torbert et al ., 1997), resulting in a
significant increase in total C and total N (0-5 cm soil depth) after 5 years
(Table 4.11.1). At the 5-10 cm depth, a similar trend for an increase in
total N was observed under elevated CO 2 , but not with total C. The
increased retention of N in the soil system in the elevated CO 2 treatment
resulted in a significant reduction in the C : N ratio for soil under soybeans
at the 0-5 cm depth, and for both the sorghum and the soybean at the
5-10 cm depth (Table 4.11.1). The high C : N ratio at both depths and
the significant difference between treatments indicated that the soil C was
in an unsteady state (Table 4.11.1).

Table 4.11.1. Effect of plant species and atmospheric CO 2 level on soil total C (g kg − 1 ), total N
(g kg − 1 ) and C : N ratio 1 .

Total C Total N C : N ratio

CO 2 level Sorghum Soybean Sorghum Soybean Sorghum Soybean

0-5 cm
Ambient

3.7 a 4.1 a 0.21 a 0.25 a 17.3 a 16.1 a

Elevated 4.2 b 5.4 b 0.24 b 0.36 b 17.3 a 14.9 b

5-10 cm
Ambient 2.3 a 2.4 a 0.13 a 0.13 a 18.7 a 18.2 a

2.4 a 2.2 a 0.14 a 0.14 a 17.9 b 15.7 b

Elevated

1 Values represent means of three replicates. Means within a column followed by the same letter do
not differ significantly (0.10 level).