This article is sponsored by Indigo Ag.

Advancing sustainable farming means working to improve the way we measure both its financial and environmental benefits. This allows a stronger market to develop where farmers are compensated for transitioning to regenerative management. 

One way to achieve this is by using modeling and soil sampling to set a high standard for how soil carbon changes are quantified. That’s why in 2019, Indigo Ag launched Carbon by Indigo to break new ground in soil carbon quantification in close partnership with the scientific community. Our DayCent-CR biogeochemical model is calibrated, validated and reviewed by independent experts — and then approved for use by crediting programs such as the Climate Action Reserve (CAR). Importantly, our approach also accounts for the uncertainties inherent in carbon measurement. 

Buy-in from the scientific community is critical for supporting the advancement of models such as DayCent-CR. Recently, the Indigo team published a peer-reviewed paper on Indigo Ag’s carbon quantification model in the leading science journal Geoderma. In the paper, we provide deeper transparency for a key component of our overall MRV (monitoring, reporting and verification) processes. This expands on the documentation and review of the model validation for use under the CAR Soil Enrichment Protocol (SEP) that are already publicly available along with other documentation for each of our credit issuances. 

Indigo is the only agricultural offset producer to have both registry approval of our model validation as well as academic peer-review, demonstrating our commitment to the scientific integrity of our programs. Publication of Indigo's calibration and validation work in Geoderma will increase stakeholder (including buyers, verifiers, academics, government and others) confidence in the quality of Indigo's sustainability programs and carbon credits. 

Yet many sustainable farming stakeholders, including buyers, verifiers, academics and regulators have questions about soil carbon monitoring. Here are four common questions we receive: 

What is a soil carbon model, and why do we need to calibrate and validate it?

Soil carbon monitoring tools include biogeochemical process models, such as DayCent-CR, that simulate changes in soil organic carbon (SOC) at the soil-plant-atmosphere interface. Tools such as this allow us to quantify the number of soil carbon credits produced through regenerative agriculture. DayCent is the daily time-step version of the CENTURY model originating out of Colorado State University, and the "CR" signifies the proprietary "credit-ready" version used in Indigo’s carbon crediting program. We use the model as the core component of the comprehensive quantification approach required by the Soil Enrichment Protocol.

While DayCent-CR comes with a standard "factory setting," we need to calibrate it to the various crops, practices and geographies that are within Indigo’s project. We then need to measure how well the calibrated model performs through a validation exercise. If a model is used without being calibrated, or shows poor performance through validation, it will produce inaccurate estimations. As credits are purchased by companies to offset their own emissions, it’s of incredibly high importance to ensure that any carbon credit produced and sold is based on real carbon reductions and sequestration.

What data are used in the calibration and validation process?

Users of soil carbon models must be careful about the data used in the calibration and validation process. For DayCent-CR, the model is calibrated and validated with measurements of SOC from published field studies obtained through a literature search. The CAR Soil Enrichment Protocol (SEP) model validation guidance sets the criteria for what SOC data can and cannot be included. In total, we used 668 SOC stock measurements for calibration and validation of DayCent-CR.

Our model will also be improved over time through ingesting additional, high-quality data.

How do you address model uncertainty?

To produce credits, we use our calibrated DayCent-CR coupled with actual soil carbon measurements collected from project fields. The SOC measurements are then used to initialize our model. This process enables us to produce credits on a much larger acreage and on a shorter timescale than soil sampling alone. "Validating DayCent-CR for cropland soil carbon offset reporting at a national scale" also explains how we calculate model uncertainty and the implications of model uncertainty. We have also discussed uncertainty in this position paper: "Managing Quantification Uncertainty in Agricultural Carbon Markets." With model improvements from DayCent-CR 1.0 to DayCent-CR 1.0.2, we were able to reduce our project-level uncertainty deduction from 37 percent to 18 percent. No model will give estimates that are 100 percent certain, so we deduct credits from the overall total that we produce to account for uncertainty and ensure we are selling an appropriately conservative number of credits.

Why is it important to report model results?

In order for credits to undergo verification against the CAR SEP, we must explain our model validation process and showcase the results in a validation report. The reports must be reviewed by an independent third party modeling expert. Once approved, these reports appear on CAR’s website and are accessible for anyone to read and scrutinize. 

Since the inception of the carbon program, we have completed two model validation reports: DayCent-CR version 1.0 in 2021 and DayCent-CR version 1.0.2 in 2022. Every time we add a new crop or potential practice change to our model, we must re-calibrate it and create an updated model validation report.

"Validating DayCent-CR for cropland soil carbon offset reporting at a national scale" highlights the results from Indigo’s second validation report, where we added cotton as newly eligible crop in 2022.

What’s next for soil carbon measurement?

In 2019, we started a long-term, large-scale soil carbon experiment on cropland fields that have transitioned to regenerative management. We are beginning to analyze soil carbon stock change at early (less than five a year) time scales. We plan to publish the data, and then add them to our model calibration and validation dataset to improve DayCent-CR. The experiment is ongoing, and we will continue to monitor SOC and additional soil health properties.

Learn more from Indigo Ag about the progress being made to make agriculture more sustainable for people and the planet.