Greenhouse Gas Emissions from U.S. Agriculture and Forestry: A ...

combination with conventional tillage (T‐AGG, 2010). The C sequestration potential of eliminating
summer fallow in the U.S. has been estimated to range from 0.1 to 0.6 Mg C (0.4 to 2 Mg CO2 eq.) ha ‐ 1
yr ‐1 (Follett and McConkey, 2000; Follett, 2001; T‐AGG, 2010).
* Fertilization and organic amendments
Fertilization and organic amendments are common practices in agricultural systems to stimulate crop
biomass production. The resulting increase in residue C inputs to the soil can lead to soil C
sequestration. Some studies have found increased SOC levels as a result of N fertilization (see Follett,
2001 and Snyder et al., 2007, 2009), however, other studies have found little to no changes in SOC
stocks (see T‐AGG report, 2010). These different trends may be attributed to differences in soil
properties, in particular the initial C content of the soil (with C‐rich soils closer to their C saturation level,
Stewart et al., 2007), as well as differences in climate conditions, as decomposition tends to be
stimulated at higher temperatures (Snyder et al., 2007, 2009; Johnson et al., 2007). Most U.S. cropland
soils already receive N fertilizer additions to attain high crop yields and thus further increases in N
addition, in the absence of other management changes, are not likely to substantially increase crop
residue inputs to soil. In addition, the benefits of N fertilization in terms of C sequestration can be
largely offset by N2O emissions generated by nitrification and denitrification of applied fertilizer N, as
well as by the C‐cost of fertilizer manufacturing, distribution and application (T‐AGG, 2010). A review by
Franzluebbers (2005) took into account these C costs and found an optimum fertilization rate to
maximize net C offset between 24 to 37 kg N ha ‐1 yr ‐1 to achieve SOC sequestration of 0.07 to 0.11 Mg C
(0.3 to 0.40 Mg CO2 eq.) ha ‐1 yr ‐1 . Organic amendments, on the other hand, may displace N fertilizer use
and can, depending on how manure is treated and applied, reduce N2O emissions. Land applications of
manure can also decrease CH4 emissions when compared to alternative handling of manure, such as the
long‐term storage in anaerobic lagoons or stockpiles (Johnson et al., 2007).
The impacts on SOC from application of animal waste (e.g., farmyard manure, poultry litter), and to a
lesser degree compost, have been studied extensively in long‐term field experiments (see Paustian et al.
1997; Ogle et al. 2005; T‐AGG, 2010). Soil organic matter invariably increases with manure/compost
additions and typically at a higher rate per unit of organic matter added than for additions as crop
residues or green manure. This is not surprising given the fact that the original plant matter has already
been partially decomposed (i.e., in animal digestive tract and manure storage or composting system)
and therefore is stabilized to a greater degree than fresh plant material (Paustian et al., 1997). In a
global meta‐analysis, Ogle et al. (2005) found that the cropland management activity that increased SOC
levels the most was high inputs of organic amendments, usually manures, with an average increase of
around 35% in SOC in the top 30 cm compared to reference cropping systems with no manure addition.
One issue in considering organic amendments (manure, compost) as a soil carbon sequestration activity
is that the organic matter in the manure often originates from off‐site locations (i.e., as purchased
livestock feed), and thus a balanced accounting of the impact of manure additions requires an estimate
of effects of the removal of plant C where the feedstocks were produced (Smith and Powlson, 2000).
Animal waste is the most commonly applied organic amendment in agricultural systems. Recently,
biochar has received attention as a promising soil amendment with benefits both for soil fertility, plant
productivity and GHG mitigation (Lehmann et al., 2006; Lehmann, 2007; De Gryze et al., 2010). Biochar
Contract # GS‐23F‐8182H, Project Technical Report 20