The people who have made this research possible are too many to list here. However, we would like to express particular gratitude to Joshua Posner for founding the Wisconsin Integrated Cropping Systems Trial (WICST); Mark Walsh, Mercedes Talvitie, and Lexi Schank for invaluable field work; Janet Hedtke, L. Gary Oates, Maggie Phillips, and Erin Pierce for lab analyses and coordination of people; and the many undergraduate workers, field technicians, and lab technicians, especially Carolyn Tull, whose combined hard work made it possible for us to better understand this piece of the tremendous challenge of climate change.
We have a problem – increasing levels of greenhouse gases in Earth’s atmosphere are wreaking havoc on climate patterns and, on average, slowly turning up the heat in the only place in the universe known to support life. To fight this, we must reduce greenhouse gases in the atmosphere, particularly CO2. The most effective approach would be to reduce the extraction and combustion of fossil fuels, the main driver of climate change. However, there is also great interest in removing CO2 directly from the atmosphere and storing it in the soil using longtime friends of humanity – plants.
When plants photosynthesize, they take CO2 from the atmosphere and use the carbon to build their stems, leaves, and roots. So why haven’t plants solved climate change for us? Well, when plants decompose, much of their carbon is consumed by microbes and exhaled as CO2, returning once more to the atmosphere. For plants to use photosynthesis to slow down climate change, their carbon needs to stay out of the atmosphere.
This brings us to the soil. As soil microbes decompose plants, a fraction of the plant carbon becomes part of the soil, prevented from returning to the atmosphere for decades or even centuries. This carbon-based soil material is called soil organic matter, and the carbon within is called soil organic carbon (SOC). In addition to trapping carbon where it won’t harm the climate, SOC provides a myriad of benefits. It helps soils stick together and absorb water, reducing erosion and regulating soil moisture for crops.
In short, more SOC is a good thing. However, many factors control how much SOC can accumulate. Some of these factors are beyond our immediate control such as rainfall, temperature, and innate soil qualities. However, some factors depend on how we choose to use the land. To start, different plants contribute different amounts of organic matter as they decompose. Also, if soils are disturbed, such as when they are tilled up, microbes tend to consume more SOC and then exhale it as CO2. Lastly, soils that have lost SOC in the past are thought to have a higher potential to gain SOC, compared to soils that are already near or at their maximum carbon-holding capacity.
In the past, the introduction of row-crop agriculture to forests and grasslands caused massive SOC losses as soils were tilled to grow crops that contribute relatively little organic matter to the soil [Sanderman et al. 2017]. Today, this makes row-crop soils prime candidates to regain and store SOC, but only if management can increase organic matter inputs or reduce the amount of organic matter returned to the atmosphere as CO2 by microbes. However, uncertainty abounds about the rate of SOC accrual and the total amount of SOC storage possible [Oldfield et al. 2024].
To address some of this uncertainty, we estimated changes in SOC over 30 years for 7 common agricultural land uses in the U.S. Upper Midwest: six cropping rotations and a periodically-burned restored prairie. These land uses exist side-by-side in an agricultural research station experiment called the Wisconsin Integrated Cropping Systems Trial or WICST for short.
WICST cropping rotations included continuous maize, a strip-tilled maize-soybean rotation, an organically-managed maize-soybean-wheat rotation, a rotation with maize followed by three years of alfalfa, an organically-managed rotation with maize followed by an oats/alfalfa mix followed by a second year of alfalfa, and a pasture grazed rotationally by dairy cattle. WICST also has plots of restored prairie grasses.
The soils at WICST are naturally high in carbon, which was added to the soil by centuries of prairie plants. However, prairie soils used for row-crop agriculture have lower SOC than prairie soils that were never tilled [Beniston et al. 2014], so we hypothesized that there was potential for some of the treatments to regain SOC with improved management. SOC at WICST was measured in 1989 when the experiment began, and again in 2009 and 2019. These measurements were compared to understand how SOC was changing due to land use.
WICST is particularly valuable for understanding SOC change for several reasons:
First, the plots at WICST are large (0.28 ha) relative to many agricultural experiments and we manage them with full-scale agricultural equipment, following university guidelines and regionally accepted best management practices. This provides a “real-world” aspect to our findings that small plot studies often lack, improving our understanding of SOC dynamics [Oldfield et al. 2024].
Second, with data spanning three decades, WICST has been going much longer than most experiments measuring SOC! This is important because SOC changes very slowly and varies a lot, even in the same field. To accurately measure SOC changes, we need enough time to pass that the changes over time are clear, even with all the “noise” from the natural variability.
Third, SOC at WICST is sampled deep into the soil. Many SOC studies only sample the top 15 or 30 centimeters, but at WICST, we measured 90 centimeters deep! (We would have measured even deeper, but below 90 cm, a layer of gravel left over from the last glacier would have damaged our equipment.) Since the entire soil profile contains SOC that can be increased by plants or decreased by microbes, it is important to measure SOC change as deeply as possible to get a full picture of how the soil is affecting atmospheric CO2 concentrations.
Finally, when we analyzed the WICST data, we adjusted the measurements to account for changes in soil compaction. If we hadn’t done this (and unfortunately many studies don’t), we might have estimated more SOC in soils that were compacted, just because there was more soil crammed into the top 90 cm [von Haden et al. 2020].
We were able to compare our results to what we would have found if we hadn’t used these more comprehensive methods. We found that less comprehensive methods overestimated SOC and would have led us to overly optimistic conclusions about these soils’ ability to store SOC and fight climate change. Using more comprehensive methods, we found that SOC was not increasing in any of our treatments. In fact, we were losing SOC in all of the row-crop systems and only maintaining SOC in the perennial grassland treatments: prairie and grazed pasture.
Treatment abbreviations are as follows: maize, cropping system of continuous maize; MS, minimum tillage cropping rotation of maize to soybean; org. MSW, organic cropping rotation of maize to soybean to winter wheat with cover crop; MaAA, cropping rotation of maize followed by 3 years of conventional alfalfa; org. Mo/aA, organic cropping rotation of maize followed by oats/alfalfa followed by alfalfa; MIRG, management intensive rotationally grazed pasture seeded to red clover, timothy grass, smooth bromegrass, and orchardgrass; prairie, cool-season grassy waterways established in 1990 planted to soy in 1998 and to warm-season grass mixes in 1999.
The center bar represents average change in SOC between 1989 and 2019 estimated by a linear mixed effects model based on data collected at WICST. Colored boxes represent the average plus and minus the standard error. Whiskers represent upper and lower 90% confidence limits. Letters represent results of pairwise comparisons within each depth at alpha = 0.1.
In other words, if the colored box or whiskers of a cropping rotation overlap with the dashed red line, that cropping rotation did not gain or lose a significant amount of SOC in 30 years. If the box and whiskers fall below the dashed line, that rotation lost SOC, and if they fall above the dashed line, that rotation gained SOC. Cropping rotations that share a letter are not significantly different in their SOC estimates.
This has substantial implications for the role of these soils in our battle with climate change. First, long-term perennial grasslands appear to be the most “climate-smart” option, at least where SOC is concerned. Although they did not trap additional atmospheric CO2, which would have lead to SOC increases, they also did not lose SOC to the atmosphere.
In contrast, the row crop treatments all lost SOC, meaning that they were sources of CO2 to the atmosphere and a driver of climate change, not a solution. While several of the row crop treatments had “improved” row crop management (for example, organic management, conservation tillage, inclusion of short-term perennial crops like alfalfa), which come with numerous benefits like reduced erosion and improved water quality, our study showed that they were indistinguishable from their conventional counterparts in terms of SOC.
This research joins others in highlighting the importance of perennial grasslands for maintaining SOC and avoiding the ongoing SOC losses found in row-crop systems. It also showed that increased SOC storage may be beyond the capabilities of these carbon-rich soils, and trying to use them to store excess atmospheric CO2 may prove impossible. Other solutions, such as replacing the extraction and combustion of fossil fuels with renewable energy, must be pursued with increased urgency.
Author Contributions:
Written by C. Dietz with edits by R. Jackson and G. Sanford.
References:
Beniston, Joshua W., et al. “Soil Organic Carbon Dynamics 75 years after land-use change in perennial grassland and annual wheat agricultural systems.” Biogeochemistry, vol. 120, no. 1–3, 16 Apr. 2014, pp. 37–49, https://doi.org/10.1007/s10533-014-9980-3.
Oldfield, Emily E., et al. “Greenhouse gas mitigation on croplands: Clarifying the debate on knowns, unknowns and risks to move forward with effective management interventions.” Carbon Management, vol. 15, no. 1, 25 June 2024, https://doi.org/10.1080/17583004.2024.2365896.
Sanderman, Jonathan, et al. “Soil Carbon Debt of 12,000 years of human land use.” Proceedings of the National Academy of Sciences, vol. 114, no. 36, 21 Aug. 2017, pp. 9575–9580, https://doi.org/10.1073/pnas.1706103114.
Von Haden, Adam C., et al. “Soils’ dirty little secret: Depth‐based comparisons can be inadequate for quantifying changes in soil organic carbon and other mineral soil properties.” Global Change Biology, vol. 26, no. 7, 18 May 2020, pp. 3759–3770, https://doi.org/10.1111/gcb.15124.