7.3 Soil & Manure Management

    Category ID Description EIC
    1688 Agricultural Soil Management (GHG only) - N2O 53091858000000
    1746 Soil Liming (GHG only) - CO2 53091858000000

    Introduction

    This document describes the methodology used to estimate greenhouse gas (GHG) emissions from agricultural soil and manure management in the San Francisco Bay Area (SFBA), as reported in categories 1688 and 1746. Category 1688 covers nitrous oxide (N2O) emissions from direct and indirect emissions from soil and manure management, while 1746 covers carbon dioxide (CO2) emissions from liming (treating with lime) of soil using limestone (CaCO3) or dolomite (CaMg(CO3)2). Emissions of N2O include both direct and indirect emissions from managed and unmanaged soils and indirect emissions from manure management. Direct emissions result from volatilization of the fertilizer applied to soil, manure, decomposition of crop residues, and mineralization of nitrogen in soil organic matter following drainage of the soils (histosols). Indirect emissions result after the applied fertilizer volatilizes or is otherwise mobilized and deposits in waters where it undergoes nitrification and denitrification processes that release N2O. Liming is used to reduce soil acidity, thus improving plant growth in agricultural fields (CARB, 2016).

    The emissions reported here are derived from the California Air Resource Board’s (CARB’s) Greenhouse Gas (GHG) Inventory (CARB, 2024). The emissions aggregated and reported in category 1688 are the sum of thirty-four different sub-categories reported by CARB, and those in category 1746 are the sum of two CARB sub-categories.

    A list of specific descriptions and activity details contained in each CARB sub-category, which is based on the Intergovernmental Panel for Climate Change (IPCC) Level 3 categorization is given in the table below:

    Air District Category

    IPCC Level 3 description

    Activity Details

    1688

    3C4 - Direct N2O Emissions from Managed Soils

    Commercial use of nitrogen fertilizer on turf - Synthetic fertilizers

    3C4 - Direct N2O Emissions from Managed Soils

    Drained histosols (organic soils from Sacramento River Delta)

    3C4 - Direct N2O Emissions from Managed Soils

    Nitrogen applied in fertilizer - Organic fertilizers

    3C4 - Direct N2O Emissions from Managed Soils

    Nitrogen applied in fertilizer - Synthetic fertilizers

    3C4 - Direct N2O Emissions from Managed Soils

    Nitrogen in crop residues

    3C4 - Direct N2O Emissions from Managed Soils

    Nitrogen in managed manure - Beef cattle

    3C4 - Direct N2O Emissions from Managed Soils

    Nitrogen in managed manure - Dairy cows

    3C4 - Direct N2O Emissions from Managed Soils

    Nitrogen in managed manure - Dairy heifers

    3C4 - Direct N2O Emissions from Managed Soils

    Nitrogen in managed manure - Poultry

    3C4 - Direct N2O Emissions from Managed Soils

    Nitrogen in managed manure - Sheep, goat, horse

    3C4 - Direct N2O Emissions from Managed Soils

    Nitrogen in managed manure - Swine

    3C4 - Direct N2O Emissions from Managed Soils

    Nitrogen in unmanaged manure - Beef cattle

    3C4 - Direct N2O Emissions from Managed Soils

    Nitrogen in unmanaged manure - Dairy cows

    3C4 - Direct N2O Emissions from Managed Soils

    Nitrogen in unmanaged manure - Dairy heifers

    3C4 - Direct N2O Emissions from Managed Soils

    Nitrogen in unmanaged manure - Poultry

    3C4 - Direct N2O Emissions from Managed Soils

    Nitrogen in unmanaged manure - Sheep, goat, horse

    3C4 - Direct N2O Emissions from Managed Soils

    Nitrogen in unmanaged manure – Swine

    3C4 - Direct N2O Emissions from Managed Soils

    Residential use of nitrogen fertilizer on turf - Synthetic fertilizers

    3C5 - Indirect N2O Emissions from Managed Soils

    Commercial use of nitrogen fertilizer on turf - Synthetic fertilizers

    3C5 - Indirect N2O Emissions from Managed Soils

    Nitrogen applied in fertilizer - Organic fertilizers

    3C5 - Indirect N2O Emissions from Managed Soils

    Nitrogen applied in fertilizer - Synthetic fertilizers

    3C5 - Indirect N2O Emissions from Managed Soils

    Residential use of nitrogen fertilizer on turf - Synthetic fertilizer

    3C6 - Indirect N2O Emissions from Manure Management

    Nitrogen in managed manure - Beef cattle

    3C6 - Indirect N2O Emissions from Manure Management

    Nitrogen in managed manure - Dairy cows

    3C6 - Indirect N2O Emissions from Manure Management

    Nitrogen in managed manure - Dairy heifers

    3C6 - Indirect N2O Emissions from Manure Management

    Nitrogen in managed manure - Poultry

    3C6 - Indirect N2O Emissions from Manure Management

    Nitrogen in managed manure - Sheep, goat, horse

    3C6 - Indirect N2O Emissions from Manure Management

    Nitrogen in managed manure - Swine

    3C6 - Indirect N2O Emissions from Manure Management

    Nitrogen in unmanaged manure - Beef cattle

    3C6 - Indirect N2O Emissions from Manure Management

    Nitrogen in unmanaged manure - Dairy cows

    3C6 - Indirect N2O Emissions from Manure Management

    Nitrogen in unmanaged manure - Dairy heifers

    3C6 - Indirect N2O Emissions from Manure Management

    Nitrogen in unmanaged manure - Poultry

    3C6 - Indirect N2O Emissions from Manure Management

    Nitrogen in unmanaged manure - Sheep, goat, horse

    3C6 - Indirect N2O Emissions from Manure Management

    Nitrogen in unmanaged manure – Swine

    1746

    3C2 – Liming

    Dolomite

    3C2 – Liming

    Limestone

    Based on the activity details in the table above, there are five major classification groups of emissions within these two categories that will be discussed specifically with regard to how the statewide emissions are downscaled to the SFBA and county scales:

    • turf and lawn,
    • drained histosols,
    • fertilizer and crop residues,
    • livestock, and,
    • soil liming.

    Methodology

    These soil and manure management source categories are considered area sources as they account for process emissions from distributed activities that are not permitted by the Air District and hence are not systematically or annually catalogued. The inventory development for these categories uses an area source calculation approach, since the process-level emissions are widely distributed across thousands of emission release points and not necessarily measured directly.

    The emissions data for these categories come directly from CARB’s Greenhouse Gas Inventory (CARB, 2024; latest data for year 2022). GHG emission inventory data for the state of California was directly downloaded from CARB archives for the years 2000 to 2022. The general methodology used by CARB (and modified as needed by the Air District) to calculate emissions for the base year(s) of the agricultural soil and manure management area source categories is as follows:

    Emissionsstate;national,pollutant = Activity Data × Emission Factorpollutant

    Base Year(s) Emissions county,pollutant =

    Emissionsstate;national,pollutant × Control Factorpollutant × Fractioncounty × GWPpollutant

    • Base Year: is a year for which emissions data is directly reported by CARB and available.
    • Activity Data: is the total statewide (or regional) throughput or activity data for applicable base year(s).
    • Emissionsstate;national,pollutant: is the amount of emissions from a larger area (e.g. state or national level) to be allocated to a smaller regional area based on a proportional measure, such as allocating based on the ratio of county to state population.
    • Emission Factorpollutant: is a factor that allocates an amount of emissions, in mass, of a particular pollutant by unit of activity data.
    • Control Factorpollutant: is a fractional ratio (between 0 and 1) that captures the estimated reduction in emissions as a result of rules and regulations.
    • Fractioncounty: is the fraction of total regional emissions (between 0 and 1) estimated to be allocated to a particular county.
    • Fractionin District: The BAAQMD jurisdiction covers only a portion of Solano and Sonoma County. For this reason, additional allocation must be made for these counties to determine the proportion of the county’s emissions occurring within BAAQMD’s jurisdiction.
    • GWPpollutant is the Global Warming Potential of a particular GHG pollutant. The current version of the GHG emissions inventory incorporates the global warming potential (GWP) reported in the Fifth Assessment report of the Intergovernmental Panel for Climate Change (IPCC, 2014). The GWPs for the three principal GHGs are 1 for carbon dioxide (CO2), 34 for methane (CH4), and 298 for nitrous oxide (N2O), when calculated on a 100-year basis with climate-carbon feedback included. The CARB emissions are given in MMTCO2eq using IPCC Assessment Report 4 (AR4; IPCC, 2007) and the Air District uses the values in IPCC Assessment Report 5 (AR5; IPCC, 2014). Since the GWPs for both species reported in this chapter, CO2 and N2O (including climate feedback), for both Assessment Reports, are the same, no correction is applied.

    Once base year emissions are determined, historical backcasting and forecasting of emissions relative to the base year emissions are estimated using growth profiles as follows:

    Current Year Emissionscounty = Base Year(s) Emissioncounty x Growth Factor

    Where:

    • Growth Factor: is a scaling factor that is used to derive historical emissions estimates for years for which activity data and/or emissions are not available, and to forecast emissions for future years, using surrogates that are assumed to be representative of activity and/or emissions trends.

    More details on the inputs and variables used above are provided in the following subsections.

    Emissions

    As stated above, the GHG emissions for these two categories are derived directly from statewide emissions inventory of thirty-six sub-categories (CARB, 2024) produced and updated annually by CARB, as listed in the table above.

    County Fractions

    Turf and Lawn

    The county fractions for residential use of synthetic nitrogen fertilizer on turf and lawns are determined using the fractions of residential acreage determined by Association of Bay Area Government’s (ABAG) Plan Bay Area 2050 linearly interpolated to the year 2022 (ABAG, 2021). Similarly, those for commercial use are based on the fractions of commercial and industrial acres, again from Plan Bay Area 2050 for the year 2022 (ABAG, 2021).

    Drained Histosols

    The county fractions were determined by examining available public resources estimating the area of the Sacramento – San Joaquin River Delta that lies within the Air District’s jurisdiction and within each SFBA county and comparing this to the State’s total for delta wetlands and peatlands, in order to downscale and derive a regional fraction and thereby county fractions for the SFBA. Only Alameda, Contra Costa, and Solano Counties contain relevant acreage.

    Fertilizer and Crop Residues

    The county fractions for organic fertilizer application are determined by ratioing the individual counties’ acreage using organic fertilizer to the total for the SFBA (USDA, 2022). Similarly, the county fractions for synthetic fertilizer application are from the relative proportions of the total cropland fertilized in each county to the total for the Air District’s jurisdiction (USDA, 2022).

    The county fractions for crop residues are determined by ratioing the individual counties’ acreage of fertilized cropland to the total for the SFBA(USDA, 2022).

    Livestock

    The county fractions for emissions from manure from the different types of livestock are determined by ratioing the individual counties’ numbers of animals of each type to the total for the SFBA (USDA, 2022).

    Soil Liming

    The county fractions are determined by ratioing the individual counties’ acreage of cropland fertilized to the total for the SFBA (USDA, 2022).

    ID Description ALA CC MAR NAP SF SM SNC SOL SON
    1688 Agricultural Soil Management (GHG only) - N2O 0.15 0.17 0.06 0.08 0.01 0.07 0.16 0.16 0.14
    1746 Soil Liming (GHG only) - CO2 0.04 0.07 0.01 0.31 0.00 0.01 0.11 0.29 0.16

    BAAQMD Jurisdiction Fraction

    The BAAQMD jurisdiction only accounts for a part of Solano and Sonoma counties. The remaining area for each is covered by other Air Districts.

    Turf and Lawn

    The Solano and Sonoma County fractions within the Air District jurisdiction for this sub-group are taken to be the fractions of total number of households in each county within BAAQMD (ABAG, 2021), which are 69.8% and 86.7%, respectively.

    Drained Histosols

    The entire Solano County region in the Sacramento River Delta, that contributes emissions from drained histosols, lies within the Air District’s jurisdiction. Sonoma County contains no land for histosols.

    Fertilizer and Crop Residues

    The fractions of Solano and Sonoma Counties within the Air District’s jurisdiction for this sub-group is assumed to be 48.0% and 37.4%, respectively, as determined by the relative amount of land area for each county within the Air District (ABAG, 2021).

    Livestock

    Estimates for the dairy, range, and feedlot cattle populations within the Air District’s jurisdiction are based on an inventory methodology put together by CARB for Animal Husbandry subsector (CARB, 2004) for the year 2000, that utilizes animal population data from California Department of Food and Agriculture (CADFA, 2001). Based on this report, the livestock fractions of Solano and Sonoma Counties within the Air District’s jurisdiction are assumed to be 48% and 39%, respectively.

    Soil Liming

    The fractions of Solano and Sonoma Counties within the Air District’s jurisdiction for this sub-group is assumed to be 48.0% and 37.4%, respectively, as determined by the relative amount of land area for each county within the Air District (ABAG, 2021).

    Emission Factors and Local Controls

    Because emissions are derived directly from CARB’s GHG inventory, no emission factors or controls are used. The Air District does not currently regulate emissions from these sources.

    Historical Emissions / Backcast

    Historical emissions for the years 1990-1999 are backcast using averages of the sum of the individual CARB sub-category emissions (as laid out in the table in the Introduction section) for years 2000-2004. These historical statewide emissions are then scaled down to the SFBA and to the county-level using the same sub-group fractional apportionment approach as that applied to the emissions data for years 2000-2022.

    Future Projections / Growth Factor

    Future projection forecasts are similarly determined as the approach described in the Historical Emissions subsection by holding year 2023-2050 emissions constant and to be equal to the averages of the sum of the individual CARB subcategory emissions for years 2018-2022.

    Sample Calculations

    An example calculation for year 2022 N2O emissions (in million metric tons of CO2-eqivalent) for category 1688 for Santa Clara County is shown below:

    Step 1

    Obtain 2022 California emissions for 34 subcategories that comprise category 1688 for IPCC Level 3 - Agriculture, Forestry and Other Land Use

    (CARB, 2024; see table in the Introduction section for the subcategories)

    7.225 MMTCO2eq

    Step 2

    Apply ratio for downscaling emissions from California to SFBA (weighted average ratio = 0.0712)

    = 7.225 MMTCO2eq

    × 0.07117

    = 0.514 MMTCO2eq

    Step 3

    Apply fraction for downscaling to Santa Clara County (varies for different subcategories, from 0 for drained histosols to 0.221 for synthetic fertilizers; weighted average = 0.155)

    = 0.514 MMTCO2eq

    × 0.155

    = 0.080 MMTCO2eq

    Assessment of Methodology

    The general methodology for determining emissions for these categories has not changed, although all of the data inputs have been updated.

    Base Year

    Revision

    Reference

    2022

    1. Use base years of 2000-2022 from CARB’s GHG inventory released in 2024.
    2. Update county proportions using fractions from United States Department of Agriculture Census of Agriculture. Volume 1, Chapter 2: County Level Data, various chapters, as described above in the County Distribution/Fraction section
    3. Update BAAQMD portions of Solano and Sonoma counties using Plan Bay Area 2050
    4. Backcast is constant, using the average of 2000-2004 for each subcategory
    5. Forecast is constant, using the average of 2018-2022.
    6. No local controls are implemented.
    1. CARB, 2024
    2. USDA, 2022;

    ABAG, 2021

    1. ABAG, 2021;

    CARB, 2004

    1. Direct calculation
    2. Direct calculation

    2015

    1. Used CARB’s 2000-2012 base year data from then current versions of the GHG inventory for category 1688 and CARB’s 1990-2004 data for category 1746. There were far fewer subcategories in the CARB GHG inventory in these earlier data sets (twelve for soils management in the 2000-2012 data set versus 34 in the 2000-2022 set; two for soil liming for both 1990-2004 and 2000-2022 data sets). No data for IPCC Level 3 subsector 3C6 emissions (manure management) were used.
    2. Downscaling to the BAAQMD region was done using the fraction of cropland in the Bay Area relative to total in the State (0.0296) for category 1746 and category 1688 crop residue; using the fractions of different fertilizers applied for organic and synthetic fertilizer application; using fractions of population of different types of livestock for livestock manure management; basis for apportionment of histosol emissions to different counties is unknown.
    3. Apportioning emissions to individual counties was done by downscaling from regional emissions using derived fractions, as described above in the County Distribution/Fraction section.
    4. Forecasting and backcasting was done using Agriculture and Natural Resources Employment statistics (ABAG, 2014).
    5. No controls were implemented
    1. CARB, 2014; CARB, 2007
    2. BAAQMD, 2017
    3. BAAQMD, 2017

    Emissions

    The table below summarizes greenhouse gas emissions for the base year 2022 in metric tons of CO2 equivalents (MTCO2eq).

    ID Description CO2 N2O Total
    1688 Agricultural Soil Management (GHG only) - N2O 0.0 514221.0 514221.0
    1746 Soil Liming (GHG only) - CO2 3611.4 0.0 3611.4

    Summary of Base Year 2022 Emissions

    The soil and manure management (including soil liming) categories contribute a little less than 1% (0.52 MMTCO2eq) to the 2022 base year SFBA regional GHG emissions inventory (65.8 MMTCO2eq). Although the CO2 emissions from processes described in this chapter are insignificant relative to the total anthropogenic CO2 emissions in the Air District, CO2 emissions from soil liming do contribute 2.7% of Agricultural emissions for this climate pollutant (shown below). Nitrous oxide emissions from category 1688 (shown below) are the dominant source of N2O, comprising 97.1% of Agricultural N2O emissions and 41.7% of total N2O within the Air District’s jurisdiction.

    Contribution of Soil & Manure Management Emissions by Sector
    Subsector Sector Subsector GHG Emissions (MMTCO2eq) Sector GHG Emissions (MMTCO2eq) % of Sector
    Soil & Manure Management Agriculture 0.52 1.26 41.04%

    Contribution of Soil & Manure Management Emissions to Regional Total
    Subsector Subsector GHG Emissions (MMTCO2eq) Regional Total GHG Emissions (MMTCO2eq) % of Regional Total
    Soil & Manure Management 0.52 65.68 0.79%

    Trends

    The time series chart below shows the emission trends for all soil and manure management source categories.

    Summary of Trends

    Because of the small magnitude of emissions and the Air District’s lack of information regarding earlier emissions, the emissions backcasted from 2000 to 1990 are assumed to have been constant, as are those for the future. Within the period of the base year emissions (2000-2022), there are peaks for both categories in 2005 and 2012-2013, as shown in the time series plots below.

    Uncertainties

    The main sources of uncertainties in this calculation approach arise from the accuracy of the emissions data collected across the state and the factors assumed in the development of CARB’s GHG inventory (CARB, 2022). CARB publishes a new GHG inventory every year, adding data for another year and sometimes modifying previous emissions. Therefore, the base year inventory data may change yearly. There is also uncertainty involved in scaling a state-level inventory to produce Bay Area specific estimates, since different statistics (e.g., land area, herd sizes, and the area of Sacramento River Delta land within the Bay Area) are used for this scaling.

    Contact

    Author: Sally Newman

    Reviewer: Abhinav Guha

    Last Update: 07/09/2025

    References

    ABAG. 2014. Plan Bay Area 2040 projections, Association of Bay Area Governments. https://abag.ca.gov/our-work/land-use/forecasts-projections

    ABAG. 2021. Plan Bay Area 2050, Association of Bay Area Governments. https://planbayarea.org/finalplan2050

    BAAQMD. 2017. Greenhouse Gas Emission Estimates and Draft Forecasts, Draft v2017-Q1, Bay Area Air Quality Management District. https://www.baaqmd.gov/~/media/files/planning-and-research/plans/2017-clean-air-plan/ghg_emissions_and_forecasts_draft.pdf

    CADFA. 2001. 2001 Agricultural Resource Directory, California Department of Food and Agriculture. http://www.cdfa.ca.gov/card/card_new02.htm

    CARB. 2004. Livestock Husbandry Inventory Methodology, Chapter 7.6, California Air Resources Board. https://ww3.arb.ca.gov/ei/areasrc/fullpdf/full7-6.pdf

    CARB. 2016. California’s 2000-2014 Greenhouse Gas Emission Inventory Technical Support Document, California Air Resources Board. https://ww2.arb.ca.gov/sites/default/files/classic/cc/inventory/ghg_inventory_tsd_00-14.pdf

    CARB. 2024. Greenhouse Gas Inventory 2024 Edition: Years 2000-2022 (AR4 GWPs), California Air Resources Board. https://ww2.arb.ca.gov/ghg-inventory-archive

    IPCC. 2007. Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland: Changes in Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Intergovernmental Panel for Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. 106 pp. Available here: https://www.ipcc.ch/site/assets/uploads/2018/02/ar4-wg1-chapter2-1.pdf

    IPCC. 2014. Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II, and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyers (eds.)]. Intergovernmental Panel for Climate Change, Geneva, Switzerland, 151 pp. Available here: https://www.ipcc.ch/site/assets/uploads/2018/02/SYR_AR5_FINAL_full.pdf

    USDA. 2022. Census of Agriculture, Volume 1, Chapter 2: County Level Data. Table 40 (Fertilizers and Chemicals Applied: 2022 and 2017). Table 11 (Cattle and Calves – Inventory and Sales: 2022 and 2017). Table 12 (Hogs and Pigs – Inventory and Sales: 2022 and 2017). Table 13 (Sheep and Lambs – Inventory and Sales: 2022 and 2017). Table 14 (Milk Goats – Inventory and Sales: 2022 and 2017). Table 18 (Equine – Inventory and Sales: 2022 and 2017). Table 19 (Poultry – Inventory and Sales: 2022 and 2017), United States Department of Agriculture. https://www.nass.usda.gov/Publications/AgCensus/2022/Full_Report/Volume_1,_Chapter_2_County_Level/California/