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

and Vitousek, 2009). For example, in a review by Snyder et al. (2007, 2009), N2O emissions from
pastures were reported to be reduced by 40‐92% when using nitrification inhibitors.
Conversions from cropland to natural grassland (e.g., setting aside under the CRP program) will typically
result in lower N2O emissions due to the elimination of fertilizer N use. A recent review by Swan et al. (in
prep) reported decreased N2O emissions with conversion of cropland to grassland, by as much as 4.5 kg
N (2.2 t CO2 eq.) ha ‐1 yr ‐1 if the amount of N fertilizer application was also reduced. In the case of no
change in N fertilizer, N2O emission changes were estimated to be near zero, while N2O emissions were
estimated to increase by as much as 6.9 kg N (3.4 t CO2 eq.) ha ‐1 yr ‐1 if fertilization rates were higher on
the grassland. Their analysis suggested that conversion of cropland to grassland can change soil N2O
emissions, but the amount of change depends greatly on N fertilizer management. Planting herbaceous
buffers within a field or along a riparian area can further reduce N2O emissions by capturing NO3 ‐ runoff
from agricultural land before it becomes denitrified off‐site (T‐AGG, 2010).
Orchards, Agroforestry, Woody Crops
N2O emissions tend to be reduced in agroforestry systems or in woody crop cultivations as these
systems tend to rely less on N fertilization. However, results vary widely across studies. Machefert et al.
(2002) found much lower N2O emissions in forested versus agricultural land in Europe. Others have
found little differences in N2O emissions between annual crops and woody crop plantations (e.g.,
poplar) but much lower emissions from natural forests (Scheer et al., 2008). On the other hand,
conventional horticulture systems tend to be heavily fertilized to maximize crop yields. Much higher
fertilization rates were reported for example for lettuce compared to corn (T‐AGG, 2010) while N
removal by lettuce is much smaller than for corn. Combined with irrigation practices, horticulture land
may be a significant source of N2O emissions. N2O emissions from horticulture land may be reduced by
using the same mitigation strategies as promoted for annual cropland (e.g., timing and placement of N
fertilizer, enhanced‐efficiency fertilizers, etc.). Management practices such as cover crops or manure
applications might reduce the need for N fertilizer and decrease N2O emissions (e.g., Wyland et al.,
1995). Further research needs include evaluating N2O mitigation strategies for the different types of
horticulture crops, and quantifying N2O emissions from different management practices. In particular,
research is lacking in orchard systems regarding both soil C sequestration and N2O and CH4 emissions.
Contract # GS‐23F‐8182H, Project Technical Report 31