Agriculture Reference
In-Depth Information
20% of the organic carbon could be retained as SOC following a year of decomposi-
tion, then it would have taken 9, 35, and 32 years to achieve an SOC accumulation of
1.5 Mg ha
−1
(a minimum detectable limit) at depths of 0-30, 30-60, and 60-90 cm,
respectively. However, much larger detectable differences can be expected; for
example, values of 10.3 ± 3.0 Mg ha
−1
were reported for three studies in Ontario and
Illinois at a depth of 40-50 cm (Yang et al. 2008).
Fertilization of pasture with an organic amendment should increase SOC, given
the high carbon concentration of the amendment relative to its nitrogen concentration.
However, the evidence available to support this effect is not overwhelmingly strong.
Two on-farm surveys of pastures in Alabama and Oklahoma found 5.7 Mg ha
−1
greater SOC after one to three decades with broiler litter application than without
(Sharpley et al. 1993; Kingery et al. 1994). In a 12-year pasture study in Georgia,
SOC was statistically greater with broiler litter than without in only one of four
management scenarios (Franzluebbers and Stuedemann 2010). The calculated rate
of SOC sequestration with broiler litter was 0.21 ± 0.43 Mg ha
−1
year
−1
among the
four regimes, which was an average retention of 9% of carbon applied as broiler
litter. In a review of literature, retention of carbon from manure application was
estimated as 7 ± 5% in thermic regions and 23 ± 15% in temperate or frigid regions
(Franzluebbers and Doraiswamy 2007), suggesting that manure application could
have a more positive impact on SOC accumulation in the northern half of the eastern
United States owing to temperature limitation on decomposition.
When animals graze pastures, there is a balance between carbon removal by
grazing and deposition via manure that becomes available for storage as SOC. As
theorized by Odum et al. (1979), pasture productivity could increase with a moder-
ate level of grazing pressure and decline with time under excessive grazing pres-
sure compared with no grazing (Figure 4.10). In a 5-year evaluation of coastal
Bermuda grass in Georgia, the mean annual forage production was 8.6 Mg ha
−1
under unharvested management, 9.2 Mg ha
−1
under low grazing pressure, and
7.5 Mg ha
−1
under high grazing pressure (Franzluebbers et al. 2004). Similar to the
response in forage productivity, SOC stock and various other soil biochemical prop-
erties at the end of 5 years of management were greatest at a moderate stocking rate
(Sollenberger et al. 2012). At the end of 12 years of Bermuda grass/tall fescue man-
agement in Georgia, SOC sequestration to a depth of 90 cm followed the order low
grazing pressure (1.17 Mg ha
−1
year
−1
) > unharvested (0.64 Mg ha
−1
year
−1
) = high
grazing pressure (0.51 Mg ha
−1
year
−1
) > hayed management (−0.22 Mg ha
−1
year
−1
)
(Franzluebbers and Stuedemann 2009).
From a long-term pasture survey in Georgia, SOC was greater when Bermuda
grass was grazed than when hayed (Franzluebbers et al. 2000b). The surface residue
carbon was 1.8 Mg C ha
−1
when grazed and 1.2 Mg C ha
−1
when hayed. SOC to a
depth of 20 cm was 38.0 Mg ha
−1
when grazed and 31.1 Mg C ha
−1
when hayed. The
difference in soil and residue carbon was 7.5 Mg C ha
−1
, suggesting an SOC seques-
tration rate in response to grazing vs. haying of 0.46 Mg ha
−1
year
−1
. In these temper-
ate, humid conditions, moderate grazing levels had beneficial impacts on SOC.
Establishment of perennial grass pastures in the southeastern United States can
sequester SOC at rates of 0.25-1.0 Mg ha
−1
year
−1
. SOC sequestration rate can be
affected by forage type, fertilization, forage utilization, animal behavior, and soil
