Back in 2007, Congress passed the US Energy Independence and Security Act, which was intended to be a mechanism towards removing our country’s dependence on petroleum and promote renewable energies. The Senate opposed the original bill’s attempt to cut oil subsidies, so the final bill instead focused on a production mandate of cellulosic ethanol set at 61 billion liters per year by 2022 (more info can be found here). Researchers and companies have been pushing along towards this mandate, and one of the main methods of accumulating the biomass necessary is through crop residue. Imagine a corn field – crop residue is all the excess stalks and leaves from the plants that fall and cover the soil.
Crop residue seems, at first glance, to be a great source of biomass that’s not being used for anything else. However, a new study in Nature Climate Change has shown that removal of the crop residue increases CO2 emissions to the atmosphere due to increased soil erosion and oxidation when the crop waste is removed. A tremendous amount of organic carbon (about 2700 Gigatons) is stored in the soil, mainly from plant matter that has decomposed. Removing the crop residue exposes the soil to water and wind, both of which promote erosion and the subsequent escape of carbon, as well as oxidation, in which organic carbon is oxidized to become CO2.
To arrive at this conclusion, the authors used a massive supercomputer simulation to model CO2 emissions from corn fields across the Midwest. They had access to a data regarding the level of organic carbon in the soil and plugged this into a model that took into account the balance between oxidation rates (loss of carbon) and new carbon inputs from residue and roots (gain of carbon). Results from the model were tested against and agreed well with actual data regarding CO2 emissions from fields in Nebraska.Above is the first major result. The dashed line shows the predicted SOC levels in the soil with residue removal, whereas the solid line shows the same without residue removal (the gray shaded region showing the difference). Clearly, residue removal decreases the amount of organic carbon in the soil, attributed to oxidation and erosion as discussed above. The cool thing about this study is that they had access to soil organic carbon data across a lot of the Midwest, so they could then predict which areas have the greatest loss due to residue removal. Here, we can see that Wisconsin and Minnesota (dark blue areas) have the greatest organic carbon loss from the soil. The authors explain this due to the low temperatures of the North, which slow oxidation and keep a lot more organic carbon in the soil. This leaves the potential for much more extraction when the system is perturbed, for example, by residue removal, leading to the elevated values.
The authors directly compare these results with the mandates set by Congress. The model predicts that 50-70 g CO2 are emitted to the atmosphere for every megajoule of energy generated from the biofuels (created from the residue that was removed). This level of CO2 emissions will lead to an inability to meet the greenhouse gas emission reduction that Congress set to be met by 2020. One possible solution is to use compost or some other substitute to cover the soil once residue is removed. Interestingly, this issue has already been known to farmers for a long time, as residue has apparently been used as standard practice to prevent erosion and keep carbon in the soil. Now it’s official, research-wise, and we’ll see how our legislators respond.
Liska, A., Yang, H., Milner, M., Goddard, S., Blanco-Canqui, H., Pelton, M., Fang, X., Zhu, H., & Suyker, A. (2014). Biofuels from crop residue can reduce soil carbon and increase CO2 emissions Nature Climate Change DOI: 10.1038/nclimate2187