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

data set of field experiments (126 articles) addressing the change in SOC storage for a variety of
agricultural activities including (1) long‐term cultivation, (2) changing tillage management, (3) modifying
C input to the soil by varying cropping practices, and (4) setting‐aside land from crop production. Their
study provided a number of ‘response factors’ for the change in SOC storage due to changing land use
and management practices, which can be applied in the development of national GHG inventories using
the 2006 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC, 2006). Long‐term cultivation
reduced SOC storage in almost every study in their analysis, with the exception of a few arid land sites.
These trends were relatively consistent with other syntheses that estimate losses of SOC due to long‐
term cultivation at 20–30% (e.g., Murty et al., 2002). Stimulation of organic matter decomposition
occurs during cultivation due to frequent tillage, which releases organic matter protected in aggregates
and redistributes organic matter in the soil profile where environmental conditions are more favorable
for decomposition (Six et al., 2002, 2004; Conant et al., 2007). Soil organic C has been shown to decline
significantly following even one tillage event (1–11% of soil C lost) (Conant et al., 2007). Soil carbon
losses are typically lower under less frequent and less intensive tillage management. Model analyses
indicate that cultivating and ripping are less disruptive than moldboard plowing (Conant et al., 2007).
No‐tillage (NT) in particular has been shown in a majority of studies to increase soil carbon stocks in
surface layers relative to plow tillage systems (Paustian et al., 1997; Smith et al., 1998; West and Post,
2002; Ogle et al., 2005) and as a consequence is being promoted as an alternative management practice
in mitigating GHG emissions from agricultural systems (CAST, 2004; T‐AGG, 2010) (see further details in
Section III on mitigation practices).
Cropping system and crop management can also influence CO2 emissions by affecting the quantity of C
inputs to the soil and the quality of crop residue returned to the soil. Management practices such as
irrigation and fertilization are typically implemented with the purpose of increasing crop productivity,
which directly controls the amount of C input to the soil. Other management practices increasing C
inputs to the soil are the use of winter cover crops, green manures, organic amendments (manure), high
residue crop varieties, or annual rotations that include one or more years of hay or pasture. On the
other hand, inclusion of bare fallow or rest years in a rotation, planting low residue crops such as
vegetables, or burning or removal of residues typically reduce C inputs to the soil. The meta‐analysis by
Ogle et al. (2005) produced input factors for moist and dry climates to estimate the relative effect of
increasing or decreasing C input to the soil on SOC storage compared to ‘medium’ input cropping
systems, defined as continuous cereal or row crop rotations with residues returned to the field after
harvest. Low input rotations were estimated to decrease SOC storage in the top 30 cm after 20 years to
91‐92% of the amount of SOC under medium input rotations in moist and dry climates, respectively.
SOC changes due to soil and crop management practices can differ widely across field studies, due to
spatially varying factors such as climate and soil properties as well as the impacts of previous land use
and management. Ogle et al. (2005) found that the impact on SOC storage of several management‐
related factors (e.g., tillage, C input levels, land set aside) tended to vary as a function of climate, with
largest to smallest changes in SOC as follows: tropical moist>tropical dry>temperate moist>temperate
dry.
Contract # GS‐23F‐8182H, Project Technical Report 6