Agriculture Reference
In-Depth Information
et al. 2000b). Grazing increased SOC sequestration relative to haying, likely due to
return via animal feces to land. The cool-season tall fescue increased SOC seques-
tration relative to the warm-warm season Bermuda grass, which may have been in
response to differential temporal distribution of moisture for plant growth and soil
microbial decomposition.
Management effects on the vertical depth distribution of SOC are a topic of keen
interest for scientists and good managers. SOC sequestration could be expected near
the soil surface where there are large stocks of plant and animal residues, and where
cold and/or dry conditions limit their decomposition. One might also expect carbon
stocks in the soil profile where soil remains undisturbed and low available nutrients
might limit decomposition. Sequestering SOC deep in the soil profile would likely
lead to longer-term storage; however, the time required to achieve this with small
downward flux of C is limiting. Data from land use comparisons in the Great Plains
of the United States suggest that SOC change with management will be limited to
the surface 50 cm of the profile (Figure 4.9). On eroded cropland converted to a pine
(
Pinus
spp.) plantation in South Carolina, organic carbon sequestration was 0.95 Mg
ha
−1
year
−1
in the forest-floor litter layer, 0.04 Mg ha
−1
year
−1
in the 0-15-cm soil
depth, and unchanged or tending to decline at 15-60-cm depths (Richter et al. 1999).
Detailed soil characterization with time would likely be necessary to detect signifi-
cant changes in SOC with depth in management systems with vigorous deep-rooting
capability. To provide some perspective, roots of Bermuda grass at the end of 3 years
of experimentation were 3.3 ± 1.7, 0.9 ± 0.5, and 0.9 ± 0.5 Mg ha
−1
at depths of 0-30,
30-60, and 60-90 cm, respectively (Adams et al. 1966). Assuming half of the root
biomass accumulated within a year, 40% of the biomass was organic carbon, and
Soil organic carbon (Mg ha
−1
)
0
10
20
30
40
50
0−50 cm
50−100 cm
Native pasture
CRP (5-year)
Cropland
100−200 cm
200−300 cm
Five locations in Kansas, Nebraska, and Te xas
FIGURE 4.9
Soil organic C content as affected by management (native pasture, reestablish-
ment of grassland for 5 years through conservation reserve program, and cultivated cropland) and
depth of sampling across five locations in the Great Plains of the United States. (From Gebhart,
D.L., H.B. Johnson, H.S. Mayeux, and H.W. Polley,
J. Soil Water Conserv.
, 49, 488, 1994.)
