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

The effects of forest fires on soil C have been found to be highly dependent on fire intensity and the
invasion of N‐fixing vegetation after fire (Johnson, 1992), as well as on the time since fire (Johnson and
Curtis, 2001). The meta‐analysis by Johnson and Curtis (2001) counter‐intuitively found a decrease in soil
C following prescribed fire whereas wildfires on average increased soil C, owing to increased abundance
of charcoal and recalcitrant, hydrophobic organic matter produced during wildfires and the effects of
naturally invading, post‐fire, N‐fixing vegetation. However, in contrast to the findings by Johnson and
Curtis (2001), the studies reviewed by Hoover (2003) generally found only small and short‐lived effects
of prescribed burning and limited to SOC in the surface soil, substantial losses (also at deeper depth) of
SOC (as high as 50%) as a result of wildfires, and no general effect of fire intensity. The authors
concluded that the response of forest soils to wildfires is likely a result of interactions between forest
type, climate, initial soil conditions, the presence/absence of vegetative recovery, elevation and other
factors. In addition, the role of black carbon (charcoal) in carbon stabilization following forest fires
remains to be better understood and needs to be elaborated in future studies looking at fire effects on
forest carbon pools.
Forest soils can also loose significant amounts of C by soil erosion (Page‐Dumroese et al., 2000). Soil
erosion occurs when forests are disturbed either naturally (fires, extreme rainfall events) or by forest
management (e.g., tillage, harvesting, thinning, burning, etc.) (Elliot, 2003). Typical soil carbon losses
due to soil erosion in Northwestern U.S. forests range between 0.12 and 5 kg C (between 0.44 and 18 kg
carbon dioxide equivalent (CO2 eq.)) ha ‐1 yr ‐1 , depending on type of disturbance (Elliot, 2003). These
rates can be minimized by proper forest management. For example, thinning and prescribed fire can
reduce the risk of wildfires and their associated large erosion rates.
II.a.2. Inorganic carbon
Inorganic carbon is present in soils as primary or secondary carbonate minerals which can sequester or
release CO2 depending on climate, vegetation, groundwater dynamics, soil properties and land‐
use/management practices. Due to its dominant presence at deeper depths, SIC is less susceptible to
physical disturbance from tillage and other cropping practices than soil organic carbon (SOC) (Lal, 2003).
On an ecosystem level, weathering of primary carbonate minerals (e.g., limestone) in humid
environments can result in a net loss in total soil C storage, although the amount of dissolved inorganic
carbon (DIC) that is eventually released to the atmosphere will depend on whether it is leached and re‐
precipitated lower in the profile, retained in groundwater or released in surface waters. In contrast, in
arid and semi‐arid systems, formation of secondary (pedogenic) carbonate minerals can result in the
sequestration of CO2 derived from the atmosphere, if base cations are supplied from weathering of non‐
carbonate primary minerals (e.g., base‐rich silicate minerals). Management practices such as irrigation
or the use of acidifying fertilizers (ammonia, sulfur) in these alkaline environments can impact both
dissolution and precipitation processes (Nordt et al., 2000). Irrigation can either result in a net storage
or release of CO2 from SIC depending on the amount and quality of irrigation water (i.e., concentration
of Ca or Mg and bicarbonate ions) and the solution flow path. Literature is limited on the impacts of
management practices on SIC dynamics in semi‐arid lands, and CO2 fluxes from the turnover of this pool
remain highly uncertain. Changes in soil inorganic carbon stocks (excluding added agricultural lime) are
not included in IPCC national greenhouse gas inventory methods.
Contract # GS‐23F‐8182H, Project Technical Report 8