Agriculture Reference
In-Depth Information
In a survey of agricultural land uses in the Piedmont and Coastal Plain regions of
the southeastern United States, SOC under pastures was significantly greater in the
0-5-cm and 5-12.5-cm depths than under conventionally tilled cropland, but no dif-
ferent at 12.5-20 cm depth (Causarano et al. 2008). Although information on pasture
age and whether it was hayed or grazed was obtained in this study, more informa-
tion on specific management practices employed would have been helpful for more
insightful interpretation. The average SOC sequestration rate of 0.74 Mg ha
−1
year
−1
during 24 ± 11 years was lower than the value of 1.03 Mg ha
−1
year
−1
during 15 ± 17
years reported for 12 other pasture vs. crop comparisons in the southeastern United
States (Franzluebbers 2005). It is expected that effective SOC sequestration would
decrease with longer periods.
On Brazilian farms, Carvalho et al. (2010a) showed nondegraded pasture under
fertile soil conditions had greater SOC to 30-cm depth than native vegetation. At
another site with degraded pasture, SOC was lower than that of native vegeta-
tion. Converting pasture to annual cropping resulted in lower SOC than in nonde-
graded pasture; however, SOC was similar with cropping as with degraded pasture.
Converting sole cropping systems to an integrated crop-livestock system on these
sites resulted in an increase in SOC of 7.4 ± 4.6 Mg ha
−1
. In Chile, cropping systems
rotated with 2-5 years of pasture had greater total and macroaggregate-associated
organic C than continuous cropping systems (Sandoval et al. 2007).
How SOC sequestration changes with time is illustrated in Figure 4.8. These data
suggest that about 50% of the maximum SOC accumulation will have occurred dur-
ing the first 10 years of pasture establishment, while about 80% of maximum storage
could be expected with 25 years of management. The type of forage management
had a large effect on the rate of SOC sequestration within the first 25 years, i.e., 0.21
Mg ha
−1
year
−1
under hayed Bermuda grass (
Cynodon dactylon
) (Franzluebbers et al.
2000b), 0.33 Mg ha
−1
year
−1
under grazed Bermuda grass (Wright et al. 2004), and
0.55 Mg ha
−1
year
−1
under grazed tall fescue (
Festuca arundinacea
) (Franzluebbers
100
80
Maximum
soil organic C
accumulation
(%)
60
40
20
0
0
10
20
30
40
50
Ye ars of management
FIGURE 4.8
Soil organic C accumulation with time as a percentage of long-term maximum.
(Data derived from Wright, A.L., F.M. Hons, and F.M. Rouquette, Jr.,
Soil Biol. Biochem.
36,
1809, 2004 [in Texas] and Franzluebbers, A.J., J.A. Stuedemann, H.H. Schomberg, and S.R.
Wilkinson,
Soil Biol. Biochem.
32, 469, 2000 [in Georgia].)
