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

application in agricultural and forest management systems is being considered as a potential technology
for generating C offset credits under several U.S. voluntary offset programs. Biochar is the product of
pyrolysis, which is the incomplete combustion of biomass into charred and highly recalcitrant organic
matter. Its GHG mitigation potential lays in its ability to sequester C by adding recalcitrant C to the soil,
stabilizing existing soil C and increasing crop production, as well as potential impacts on N2O and CH4
emissions (Johnson et al., 2007; Gaunt and Driver, 2010). Biochar recalcitrance and its potential to
stabilize C in soils appears to differ among biochar types and seems to depend on factors such as
biomass source (e.g., crop residues, urban waste, manure, wood, etc.) and the conditions of the
pyrolysis process (temperature, time) (Gaunt and Driver, 2010). In addition, the impacts of biochar on
soil C, N2O and CH4 emissions may vary across soil types. However, the contribution of different soil
conditions, such as hydrology, texture, microbial community structure, etc. to the impact of biochar on
soil C sequestration and GHG emissions needs further investigation (T‐AGG, 2010). Other research needs
are to evaluate the GHG mitigation potential of different types of biochar upon large‐scale application in
different agricultural systems in the U.S.
* Irrigation
In semi‐arid climates, irrigation can significantly increase crop production and, as a consequence, return
more residue C inputs to soils. Carbon sequestration rates through irrigation have been indirectly
estimated at 50 to 150 kg C (180 to 550 kg CO2 eq.) ha −1 yr −1 based on increased plant C inputs (Lal et al.,
1998; Follett, 2001). However, irrigation may also decrease SOC storage by stimulating organic matter
decomposition as a result of enhanced microbial activity and soil water content (Gillabel et al., 2007;
Sainju et al., 2008). Moreover, irrigated fields often undergo more intensive tillage, and crop residues
are not always returned to the soil, which could further reduce C storage (Follett, 2001). Finally,
irrigation could also lead to greater losses of C from soil erosion (Follett, 2001). Therefore, broad
estimates of soil C accumulation based solely on C input may overestimate the SOC accumulation via
irrigation. Yet, similar or even much higher C sequestration rates than those estimated by Lal et al.
(1998) have been documented in the literature, based on soil C measurements, with rates as high as
0.82 Mg C (3.0 Mg CO2 eq.) ha ‐1 yr ‐1 (T‐AGG, 2010), indicating a clear potential of irrigation for
sequestering atmospheric C. However, this C sequestration potential of irrigation needs to be weighed
against increased emissions of N2O and CH4 likely to occur due to the increased anaerobic conditions in
irrigated soils (T‐AGG, 2010).
Research on C sequestration under irrigation is scarce, is mostly limited to C changes in the soil surface
(< 30 cm) and frequently disregards the soil inorganic carbon (SIC) pool. The few studies that have
looked at soil inorganic carbon (SIC) dynamics under irrigation have shown contrasting results (e.g.,
Entry et al., 2004; Denef et al., 2008; Blanco‐Canqui et al., 2010). Therefore, more research is needed on
the factors controlling SIC dynamics in irrigated systems, to develop more accurate estimates of C fluxes
from this pool. Moreover, the majority of studies looking at irrigation effects on soil C have primarily
focused on the comparison of irrigated vs. non‐irrigated fields. More studies are needed to address the
impacts of variable levels of irrigation, in particular in semiarid regions facing reduced water supplies.
Contract # GS‐23F‐8182H, Project Technical Report 21