Organic Broadcaster

Researchers use 30-year cropping systems experiment to evaluate if farm fields can serve as carbon sinks

By Randy Jackson, Gregg Sanford, Matt Ruark, Anna Cates, Ashley Becker, Yichao Rui, David LeZaks

There’s a palpable buzz around soil carbon in the agricultural community. It’s exciting to see so much attention and energy directed toward addressing climate change and, in particular, so much exploration about how agriculture can be part of the solution rather than part of the problem. Investment organizations are clamoring to find the scheme where their capital will “do good.” Policymakers are scrambling to develop incentive mechanisms for practices believed to build soil carbon. Farmers are touting their management for soil health and carbon accumulation.

With this backdrop, a panel discussion at the Growing Stronger Conference explored the evidence that agricultural soils might help draw down atmospheric carbon dioxide levels to help slow climate change. Relying mostly on results from a long-term experiment at Arlington Agricultural Research Station in south-central Wisconsin—the Wisconsin Integrated Cropping Systems Trial —Dr. Randy Jackson relayed the group’s overall message: Tweaking our dominant grain-based annual crops with reduced tillage, cover crops, and manure addition is not likely to make those systems atmospheric carbon sinks (Fig. 1).

Fig. 1. Twenty-year differences in soil organic carbon (1989 to 2009) across entire soil depth (~3 ft.) at the Wisconsin Integrated Cropping Systems Trail (WICST). Crops on x-axis from L to R: (1) high-input continuous corn, (2) high-input corn-soybean rotation, (3) organic corn-soybean rotation with wheat cover crop, (4) high-input corn-alfalfa rotation, (5) organic corn-alfalfa rotation with oat cover crop, and (6) cool-season pasture with managed rotational grazing (modified from Sanford et al. 2012 Agriculture, Ecosystems & Environment).

Data shared by the group indicated that agriculture based on perennial grassland was our best and perhaps only approach to building soil carbon, although even pasture was struggling to maintain soil carbon stocks over the 20-year period (Fig. 1). Hypotheses for this lack of significant carbon accumulation in their grassland plots included the inherently shallow roots of cool-season pasture grasses and feedback from the already changing climate. During the discussion period following the panel presentations, attendees speculated that perhaps carbon would accumulate in deeper soils if warm-season grasses from the tallgrass prairie, known for their deeper roots, were grazed.

Dr. Gregg Sanford described why the soil carbon balance of an agroecosystem is so precarious. Data show that only 3 or 4 months of the year are periods when the input of carbon from the atmosphere via photosynthesis is greater than the output of carbon to the atmosphere from soil microbes. He also addressed the importance of deep soil sampling over time to accurately represent soil carbon change. Sanford showed how most of the “short-cut” methodologies for estimating change can result in questionable findings that tend to inflate estimates of soil carbon change.

Dr. Matt Ruark shared results of a soil carbon survey on organic grain farms concluding that the most important indicators of high soil carbon were whether the soils formed under prairie (that is, Mollisols) and whether recent management included pasture. Then, Dr. Anna Cates explained why establishing cover crops in the upper Midwest is so difficult. She described narrow windows each fall between main-crop harvest and temperatures that are too low for cover crop germination. Dr. Cates made clear how important cover crops are for soil and nutrient retention and reducing runoff, but showed how the amount of cover crop biomass is directly linked to whether it shifts the soil carbon balance of a cropping system.

Ashley Becker shared preliminary data from a survey of over 30 grazing farms in southern and central Wisconsin. Grazed pastures had almost 5 tons more carbon per acre in the surface 6 inches of soil than nearby sites on similar soil types under annual crop rotations of primarily corn and soybeans. Importantly, the same comparison at 6 to 12 inches depth showed no soil carbon difference.

To explore the mechanisms of soil carbon, Dr. Yichao Rui showed how more particulate organic matter (partially decomposed plant debris) was accumulating in the pasture soil and that the microbes in these pasture soils were more efficiently using this carbon source to build their own bodies. Moreover, when these microbes died, their dead cells were accumulating to a greater degree in the soil carbon pool known as the mineral-associated organic matter, which is thought to be how carbon actually gets “sequestered” from the atmosphere.

When woven together, the stories shared by these scientists raise concerns about emerging agricultural carbon markets. Dr. David LeZaks echoed these concerns from the perspective of his work in the investment capital world. He made clear what an exciting time we’re in, given the Biden Administration’s signals that climate-smart agriculture will be incentivized and rewarded moving forward. But Dr. LeZaks also described how devastating the failure of a new carbon market would be to sustainable agriculture efforts if investments, credits, and promises related to carbon sequestration do not materialize. He and other attendees emphasized the importance of emerging markets that rely on more than just carbon balance and include water quality, flood reduction, and wildlife habitat.

In the discussion that followed these presentations, attendees echoed concerns of the panelists that carbon markets will be co-opted by agricultural practices that are not likely to reduce atmospheric carbon dioxide. The sentiment expressed by several of these folks was that we need transformational change of our agricultural system rather than incremental tweaks or fixes to the existing one.

If you registered for the Growing Stronger Conference, you may watch the recording of this workshop: Can Regenerative Agriculture Stabilize Soil Carbon? If you missed conference week, you may still sign up to view more than 100 recordings and access resources in dozens of exhibit booths for the same admission price of $125.

 

Randy Jackson is Campbell-Bascom Professor of Grassland Ecology in the Department of Agronomy at University of Wisconsin-Madison. Gregg Sanford is Cropping Systems Agronomist with the UW-Madison Department of Agronomy. Matt Ruark is Professor of Soil Science in the Department of Soil Science at the University of Wisconsin-Madison. Anna Cates is State Soil Health Specialist with the University of Minnesota.Ashley Becker is a Graduate Student Research Assistant in the Nelson Institute for Environmental Studies at UW-Madison. David LeZaks is Senior Fellow at the Croatan Institute. Yichao Rui is Soil Scientist at the Rodale Institute.

 

From the March | April 2021 Issue

 

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