DRAFT Inventory of U.S Greenhouse Gas Emissions and Sinks

2017_complete_report

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Carbon dioxide emissions and removals 34 due to changes in mineral soil C stocks are estimated using a Tier 3 method for the majority of annual crops (Ogle et al. 2010). A Tier 2 IPCC method is used for the remaining crops not included in the Tier 3 method (see Methodology section for a list of crops in the Tier 2 and 3 methods) (Ogle et al. 2003, 2006). In addition, a Tier 2 method is used for very gravelly, cobbly, or shaley soils (i.e., classified as soils that have greater than 35 percent of soil volume comprised of gravel, cobbles, or shale) regardless of crop, and for additional changes in mineral soil C stocks that are not addressed with the Tier 3 approach (i.e., change in C stocks after 2010 due to CRP enrollment). Emissions from organic soils are estimated using a Tier 2 IPCC method. Land-use and land management of mineral soils are the largest contributor to total net C stock change, especially in the early part of the time series (see Table 6-28 and Table 6-29). (Note: Estimates after 2012 are based on NRI data from 2012 and therefore do not fully reflect changes occurring in the latter part of the time series). In 2015, mineral soils are estimated to sequester 42.1 MMT CO 2 Eq. from the atmosphere (11.5 MMT C). 35 This rate of C storage in mineral soils represents about a 41 percent decrease in the rate since the initial reporting year of 1990. Carbon dioxide emissions from organic soils are 28.0 MMT CO 2 Eq. (7.6 MMT C) in 2015, which is a 7 percent decrease compared to 1990. In total, United States agricultural soils in Cropland Remaining Cropland sequestered approximately 14.0 MMT CO 2 Eq. (3.8 MMT C) in 2015. Table 6-28: Net CO2 Flux from Soil C Stock Changes in Cropland Remaining Cropland (MMT CO2 Eq.) Soil Type 1990 2005 2011 2012 2013 a 2014 a 2015 a Mineral Soils (71.2) (56.2) (47.1) (49.5) (43.7) (42.9) (42.1) Organic Soils 30.3 29.7 27.9 28.1 28.1 28.1 28.0 Total Net Flux (40.9) (26.5) (19.1) (21.4) (15.6) (14.8) (14.0) a Quality control uncovered errors in the estimates of mineral soils and the total net flux for 2013, 2014 and 2015, which will be updated following public review. The corrected mineral soil estimates are (47.6), (46.8), and (46.0) MMT CO2 Eq., respectively for 2013, 2014, 2015, and the total net flux is (19.6), (18.7) and (18.0) MMT CO2 Eq., respectively for the three years. Notes: Estimates after 2012 are based on NRI data from 2012 and therefore may not fully reflect changes occurring in the latter part of the time series. Totals may not sum due to independent rounding. Parentheses indicate net sequestration. 18 19 20 21 22 Table 6-29: Net CO2 Flux from Soil C Stock Changes in Cropland Remaining Cropland (MMT C) Soil Type 1990 2005 2011 2012 2013 a 2014 a 2015 a Mineral Soils (19.4) (15.3) (12.8) (13.5) (11.9) (11.7) (11.5) Organic Soils 8.3 8.1 7.6 7.7 7.7 7.7 7.6 Total Net Flux (11.2) (7.2) (5.2) (5.8) (4.3) (4.0) (3.8) a Quality control uncovered errors in the estimates of mineral soils and the total net flux for 2013, 2014 and 2015, which will be updated following public review. The corrected mineral soil estimates are (13.0), (12.8) and (12.6) MMT C, respectively for 2013, 2014 and 2015, and the total net flux is (5.3), (5.1) and (4.9) MMT C, respectively for the three years. Notes: Estimates after 2012 are based on NRI data from 2012 and therefore may not fully reflect changes occurring in the latter part of the time series. Totals may not sum due to independent rounding. Parentheses indicate net sequestration. Soil C stocks increase on Cropland Remaining Cropland across the entire time series, largely driven by the nearly 10 million hectares of land currently enrolled in CRP (i.e., set-aside program), as well as from increased hay production, adoption of conservation tillage (i.e., reduced- and no-till practices), and intensification of crop 34 Removals occur through uptake of CO2 into crop and forage biomass that is later incorporated into soil C pools. 35 Quality control uncovered errors in the mineral soil and total net flux estimates for 2015, which will be updated following public review. Based on the revision, soil C stocks increased by 46.0 MMT CO2 Eq. (12.6 MMT C) in 2015. The total net flux implies C sequestration of 18.0 MMT CO2 Eq. (4.9 MMT C). The corrected overall trend is a decrease in soil C stock change by 56 percent since the initial reporting year in 1990. 6-48 DRAFT Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2015

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 production by limiting the use of bare-summer fallow in semi-arid regions. However, there is a decline in the net amount of carbon sequestration (i.e., 2015 is 41 percent less than 1990), and this decline is due to less annual cropland enrolled in the CRP 36 that was initiated in 1985. For example, over 1.8 million hectares, which had been enrolled in the CRP, were returned to agricultural production during the last 3 years resulting in a loss of soil C. Soil C losses from drainage of organic soils are relatively stable across the time series with a small decline associated with the land base declining by 7 percent for Cropland Remaining Cropland on organic soils since 1990. The spatial variability in the 2015 annual C stock changes are displayed in Figure 6-4 and Figure 6-5 for mineral and organic soils, respectively. Isolated areas with high rates of C accumulation occur throughout the agricultural land base in the United States, but there are more concentrated areas with gains in the northern Great Plains, which has high rates of CRP enrollment. High rates of net C accumulation in mineral soils also occurred in the Corn Belt region, which is the region with the largest amounts of conservation tillage, along with moderate rates of CRP enrollment. The regions with the highest rates of emissions from drainage of organic soils occur in the Southeastern Coastal Region (particularly Florida), upper Midwest and Northeast surrounding the Great Lakes, and isolated areas along the Pacific Coast (particularly California), which coincides with the largest concentrations of organic soils in the United States that are used for agricultural production. Figure 6-4: Total Net Annual CO2 Flux for Mineral Soils under Agricultural Management within States, 2015, Cropland Remaining Cropland 18 19 36 The Conservation Reserve Program (CRP) is a land conservation program administered by the Farm Service Agency (FSA). In exchange for a yearly rental payment, farmers enrolled in the program agree to remove environmentally sensitive land from agricultural production and plant species that will improve environmental health and quality. Contracts for land enrolled in CRP are 10 to 15 years in length. The long-term goal of the program is to re-establish valuable land cover to help improve water quality, prevent soil erosion, and reduce loss of wildlife habitat. Land Use, Land-Use Change, and Forestry 6-49