Critical Research Addresses Knowledge Gaps in Fertilizer Impact on Sustainability

Factors associated with nitrogen loss are less common on the High Plains than regions like the Midwest, making it the perfect location for sustainable cropping systems. It’s the location where sorghum thrives, and National Sorghum Producers is studying these differences and working to promote sorghum’s benefits in this area.

Article By Patrick Wade

No matter what field you work in, anyone who ever competed in an elementary school science fair can recall the scientific method. Before any budding scientists can begin dropping Mentos in a Coca-Cola bottle decorated like a volcano, they must first establish a hypothesis – a type of testable, educated guess.

Although the scientific community studies issues more important than toy volcanoes, scientific consensus is often still developed by testing one educated guess after another. Some of our most universally agreed-upon scientific truths began as one simple, testable educated guess. With the findings of each iterative study, these hypotheses grow increasingly precise and our collective understanding of the natural world sharpens. In this way, scientific discoveries are less like gold – naturally found within the earth, just waiting to be stumbled upon – than they are sedimentary rocks – crookedly but methodically layered on top of one another, one test at a time.

Policymakers, in turn, rely on those iterative and increasingly focused tests to craft programs that are both grounded in the real world and encourage scientifically-sound, positive behaviors. But what happens when scientists suspect that one of those initial assumptions at the base of a scientific consensus was never properly tested? What happens when they suspect that assumption informed over a decade’s worth of policymaking? They test it, of course.

Researchers at Kansas State University and Oklahoma State University went through this exact process when they speculated that a major assumption undergirding U.S. renewable fuel policy had not been properly tested in one of the country’s key agricultural producing regions. Peter Tomlinson, Ph.D., Lucas Haag, Ph.D., and Alan Schlegel, Ph.D., from Kansas State University and Jason Warren, Ph.D., from Oklahoma State University sought to determine if the United Nations’ Intergovernmental Panel on Climate Change’s (IPCC) estimates for nitrous oxide (N₂O) emissions associated with the production of grain sorghum applied to the semi-arid High Plains in the heart of the Sorghum Belt.

Presently, the application of nitrogen fertilizer is the largest source of greenhouse gas emissions associated with agricultural production. Different crops, production practices and geographies, however, can mitigate the amount of nitrogen that is emitted as nitrous oxide. Originally, the IPCC estimated that approximately 2 percent of nitrogen applied in the production of grain sorghum is emitted in the form of nitrous oxide – approximately 1 percent as direct emissions from fertilizer application and approximately 1 percent as indirect emissions from the breakdown of residue.

These emissions estimates provide part of the bases for carbon intensity scores assigned to ethanol feedstocks like grain sorghum, which in turn determine the market value assigned to these commodities in emerging carbon economies like renewable fuels.

Preliminary findings from these teams at K-State and OSU show that total observed nitrous oxide emissions associated with grain sorghum is in fact 0.3 percent, well below the IPCC estimates of 2 percent. These tests used a standard wheat-fallow-grain sorghum-fallow rotation in western Kansas, a region largely representative of the Sorghum Belt.

We sat down, virtually, with Peter Tomlinson, Ph.D., and Jason Warren, Ph.D., to discuss their preliminary findings and how this might affect the ever-evolving realm of carbon policymaking. This interview has been edited for length and clarity:

Patrick: Right off the bat, what has been your defining takeaway from this project?

Jason: I really like this project because we know it will inform an industry with region-specific data. There is a lot of information out there about how agriculture impacts greenhouse gases, and politics can often impact what we see on TV. But at the end of the day, if the goal is to have policies that reduce greenhouse gas emissions, we need sound science, which is why I work on projects like this.

Peter: For me, this project has been a great opportunity to develop and conduct research that can be directly applied to support producers and inform the ethanol industry and regulatory entities with regionally-appropriate data.

Patrick: What informed the IPCC’s default cumulative cropping system emission factor of 2 percent? Is the difference in your findings simply explained by the presence of better data, or something else?

Jason: The IPCC default value was developed as a result of a literature review of all the nitrous oxide emissions measurements collected throughout the world. It is my understanding that an emission of 2 percent of the nitrogen applied was found to be the average emission.

Peter The values presented in the IPCC report are based on all of the available data worldwide which captures many different environments, management systems and efficiencies. The 2 percent represents two components: the direct emissions from nitrogen fertilizer application estimated to be 1 percent and indirect emissions from residue estimated to be 1 percent. Our working hypothesis was that the semi-arid (dry) environment of the High Plains would result in N₂O emissions less than the default values.

Jason: This is why the research was so important, because there was no emissions data for dryland cropping systems in the High Plains.

Patrick: In your report’s results, you note that cumulative nitrous oxide flux was low across all treatments. Are there any treatment variables that weren’t present in your experiments that could dramatically change these findings? On a larger scale, what levels of flux might you expect from plots with different crop rotations, such as with cotton in Texas, or in other regions of the Sorghum Belt?

Peter: One key limitation of our study is that we have not looked at different nitrogen fertilizer sources. That being said, given the semi-arid environment I would not expect the nitrogen source to result in different emission results.

Jason: I don’t expect to see a rotation effect that would dramatically change the findings. As mentioned, the primary reason the values are so low is because of the semi-arid climate. What is very interesting is that 2018 and 2019 were certainly not below average rainfall years. With this, we are not simply seeing the effects of a “dry” year. I make this point because, again, N₂O is emitted when the soil is very moist, and the data makes it apparent that this condition simply does not occur that often in dryland systems in the High Plains, even in years with near or above average rainfall.

Peter: Another key consideration to minimize N₂O emissions is to make sure nitrogen fertilizer is not being over applied. Thus, it is important that soil samples are being collected to determine the available nitrogen in the profile so that the nitrogen fertilizer application rate is adjusted to account for the soil nitrogen levels.

Patrick: In your report’s introduction, you mention that recent changes to the lifecycle assessment of corn have hurt the favorability of grain sorghum for biofuel production. Do you believe your findings here may help to counter that?

Peter: Sorghum is not being discounted compared to corn, but without this research, the potential for premiums is nonexistent. Accordingly, I do feel this work is key to supporting High Plains sorghum production. The production systems in this region are distinct from the Midwest, which necessitates conducting regionally-appropriate research… Soil moisture is one of the key components needed for denitrification, which results in N₂O production. The dry environment is likely a key reason for the lower emissions that we have observed.

Jason: I think that the collection of data from the High Plains sorghum producing region of the U.S. will provide accurate data on the N₂O emissions from grain sorghum, and that the findings that the emissions are much lower than the IPCC default is good… I think this, along with the improved management over the past 30 years which allows for increased productivity and resilience to limited rainfall, should make sorghum more favorable to those who want to minimize greenhouse gas emissions by utilizing biofuel.


This story originally appeared in the Winter 2021 Issue of Sorghum Grower magazine.