| Year | Annual SOC Stock (kg C / ha) | Annual SOC Stock (kg CO2 / ha) | Annual Change (kg CO2 / ha) | Annual Emissions (kg CO2 / ha) |
|---|---|---|---|---|
| 2008 | 7580 | 27793.3 | NA | NA |
| 2009 | 7520 | 27573.3 | -220.0 | 220.0 |
| 2010 | 7486 | 27448.7 | -124.7 | 124.7 |
| 2011 | 7431 | 27247.0 | -201.7 | 201.7 |
| 2012 | 7376 | 27045.3 | -201.7 | 201.7 |
| 2013 | 7345 | 26931.7 | -113.7 | 113.7 |
| 2014 | 7294 | 26744.7 | -187.0 | 187.0 |
| 2015 | 7268 | 26649.3 | -95.3 | 95.3 |
| 2016 | 7208 | 26429.3 | -220.0 | 220.0 |
| 2017 | 7165 | 26271.7 | -157.7 | 157.7 |
| 2018 | 7108 | 26062.7 | -209.0 | 209.0 |
| 2019 | 7076 | 25945.3 | -117.3 | 117.3 |
| 2020 | 7022 | 25747.3 | -198.0 | 198.0 |
| 2021 | 6991 | 25633.7 | -113.7 | 113.7 |
| 2022 | 6947 | 25472.3 | -161.3 | 161.3 |
| 2023 | 6905 | 25318.3 | -154.0 | 154.0 |
| 2024 | 6860 | 25153.3 | -165.0 | 165.0 |
Soil Carbon Stock Changes
Methods 5.0
on-farm
non-mechanical
CO2 emissions or sequestrations based on estimated changes in soil carbon stocks.
Introduction
The advent of conservation agriculture initiatives has driven demand for models and tools to quantify GHG flux from croplands, including the flux of different pools of carbon in the soil. This demand is due to evidence that cultivating crops under certain conditions can lead to increased soil carbon stocks (i.e. carbon removal from the atmosphere). Functions from SWAT-C (Zhang et al. 2013) have been incorporated into the SWAT+ model (Bieger et al. 2017) to support agricultural field-scale simulations of soil carbon stock changes. Version 5 of the Fieldprint Calculator has integrated the SWAT+ model with support from collaborators at Colorado State University and members of Field to Market.
Tip
Read the Soil Carbon section to learn more about the development and advantages of SWAT+ for soil carbon modeling.
Methods
This method is more fully described in the Soil Carbon section. Below we present a reduced example demonstrating the output from SWAT+ and how it is used in version 5 of the Fieldprint Calculator.
SWAT+ produces estimates of soil carbon stock changes on a calendar-year basis. Table 1 is an example of the annual outputs from SWAT+ and the conversions to use the model simulations for a given field.
Table 1 shows:
- Year: sequence of years of the simulation, from 2008 to the most recent year.
- Annual SOC Stock (kg C / ha): Annual output from SWAT+, in units of kg of carbon.
- Annual SOC Stock (kg CO2 / ha): The output from SWAT+ is multiplied by 44/12 (ratio of molecular weights, CO2 to carbon) to obtain units of kg CO2 / ha.
- Annual Change (kg CO2 / ha): Difference in SOC stocks in consecutive calendar years.
- Annual Emissions (kg CO2 / ha): The variable
Annual Change (kg CO2 / ha)is multiplied to reverse its sign to represent GHG emissions.
In the case above, the field is slowly depleting its SOC stock every year. For FPv5, GHG emissions from decreasing SOC stocks are represented with positive numbers (e.g., 259 kg CO2 / ha / year) so that they add to the total GHG emissions, while carbon sequestrations (i.e. removals) due to increasing SOC stocks are represented with negative numbers (e.g., -450 kg CO2 / ha / year) so they subtract from the total GHG emissions.
Attribution of CO2 emissions or sequestration by crop interval
The Fieldprint Platform will assign CO2 sequestration or emissions to a crop interval according to the number of days the crop interval covers within a calendar year. The following scenario illustrates the approach:
- Calendar year: 2023
- Soil carbon stock change estimate: 425 kg CO2 / ha / year or 1.16 kg CO2 / ha / day.
- Two crop intervals in 2023:
- 2023 soybeans (2022-10-31 to 2023-10-10), covering 283 days in 2023.
- This crop interval gets assigned 1.16 kg CO2 / ha / day * 283 days = 329.5 kg CO2 / ha.
- 2024 corn grain, (2023-10-11 to 2024-10-20), covering 82 days in 2023.
- This crop interval gets assigned 1.16 kg CO2 / ha / day * 82 days = 95.5 kg CO2 / ha.
- 2023 soybeans (2022-10-31 to 2023-10-10), covering 283 days in 2023.
- The soybean crop interval would also include emissions from 2022, and the corn crop interval would include emissions for 2024, using the same approach.
References
Bieger, Katrin, Jeffrey G. Arnold, Hendrik Rathjens, Michael J. White, David D. Bosch, Peter M. Allen, Martin Volk, and Raghavan Srinivasan. 2017. “Introduction to SWAT+, a Completely Restructured Version of the Soil and Water Assessment Tool.” JAWRA Journal of the American Water Resources Association 53 (1): 115–30. https://doi.org/10.1111/1752-1688.12482.
Zhang, Xuesong, R. César Izaurralde, Jeffrey G. Arnold, Jimmy R. Williams, and Raghavan Srinivasan. 2013. “Modifying the Soil and Water Assessment Tool to Simulate Cropland Carbon Flux: Model Development and Initial Evaluation.” Science of The Total Environment 463-464 (October): 810822. https://doi.org/10.1016/j.scitotenv.2013.06.056.