Sorry about that; it causes Adobe Acrobat to start up to read the .pdf.
Here's some highlights [with comments]
TgC (TeraGrammesCarbon) = MtC (MegaTonnesCarbon) ...or 1/3CO2 -(I think). ...and 1000 MtC = 1 GtC (GigaTonnesCarbon).
...numbers in parentheses (xx.x) refer to sequestration of carbon or CO2.
[bracketed] [comments are mine] [observations]
INVENTORY OF U.S. GREENHOUSE GAS EMISSIONS AND SINKS:
1990 - 2005
APRIL 15, 2007
U.S. Environmental Protection Agency
1200 Pennsylvania Ave., N.W.
Washington, DC 20460
U.S.A.
Released for printing: April 15, 2007
Soils are the largest pool of C in agricultural land, and also have the greatest potential for storage or release of C because biomass and dead organic matter C pools are relatively small and ephemeral compared with soils.
...changes in soil organic C stocks due to: (1) agricultural land-use and management activities on mineral soils, (2) agricultural land-use and management activities on organic soils, and (3) CO2 emissions that result from liming of soils with dolomite and limestone. Mineral soil C stock changes and C emissions from organic soils are reported here for Land Converted to Grassland, but emissions from liming are reported in Cropland Remaining Cropland....
Drainage of organic soils for grazing management led to annual losses of 0.9 Tg CO2 Eq. in 2005.
The net increase in soil C stocks over the period from 1990 through 2005 was largely due to an increase in annual cropland enrolled in the Conservation Reserve Program, intensification of crop production by limiting the use of bare-summer fallow in semi-arid regions, increased hay production, and adoption of conservation tillage (i.e., reduced- and no-till practices). At present (2005), cropland enrolled in the Conservation Reserve Program accounts for 32 percent of the increase of C stocks for Cropland Remaining Cropland on mineral soils.
The highest rates of sequestration in mineral soils occurred in the Midwest, where there were the largest amounts of cropland managed with conservation tillage adoption. Rates were also high in the Great Plains due to enrollment in the Conservation Reserve Program. Emission rates from drained organic soils were highest along the southeastern coastal region, in the northeast central United States surrounding the Great Lakes, and along the central and northern portions of the west coast.
Net Soil C Stock Changes in Grassland Remaining Grassland (Tg CO2 Eq.)
1990 1995 2000 2001 2002 2003 2004 2005
(3.7) 12.7 12.6 12.6 12.5 12.5 12.5 12.4
[Notice the sequestration in 1990; but 1995 and on?]
Land Converted to Grassland
1990 1995 2000 2001 2002 2003 2004 2005 (TgCO2)
(14.6) (16.3) (16.3) (16.3) (16.3) (16.3) (16.3) (16.3)
[Notice how sequestration increases from 1990-1995 and then remains constant. If we'd kept increasing at that rate, we'd be sequestering almost 20 Megatonnes of CO2/yr (5+ MtCO2/yr. over 1990 levels).]
[...and this is just for some (mineral soils) land converted to grassland (how often is that happening vs. the reverse?)]
[...converting other (organic soils) lands to grasslands actually increases CO2 emissions (<1MtCO2/yr.).]
Overall, the recalculations resulted in an average annual increase in emissions of 7.4 Tg CO2 Eq. (46.2 percent) for soil C stock changes in Grassland Remaining Grassland over the period from 1990 through 2004.
[...due to warming and drying?]
A Tier 3 model-based approach was used to estimate C stock changes for mineral soils in Grassland Remaining Grassland, except for lands with very gravelly, cobbly or shaley soils (greater than 35 percent by volume). An IPCC Tier 2 method was used to estimate stock changes for the gravelly, cobbly or shaley soils and additional changes in C stocks in mineral soils. A Tier 2 method was also used to estimate additional stock changes associated with sewage sludge amendments.... Liming emitted another 4.0 Tg CO2 Eq.
Agricultural soils are also important sources of other greenhouse gases, particularly N2O from application of fertilizers, manure, and crop residues and from cultivation of legumes, as well as CH4 from flooded rice cultivation. These emissions are accounted for in the Agriculture chapter, along with non-CO2 greenhouse gas emissions from field burning of crop residues and CH4 and N2O emissions from livestock digestion and manure management.
[One thing that I notice is that in general growing stuff on "mineral soils" sequesters lots (50-60-70 MtCO2/yr); whereas growth on "organic soils" tends to be a net emitter of CO2 (10-20-30 MtCO2/yr).]
~SA
