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The easiest way to tackle fertilizer pollution is to lower the amount of nitrogen applied to crops, thereby reducing nutrient losses into the air and water. The problem is, reducing fertilizer rates can also shrink crop yields, which means less income for farmers and less food on our plates.
So here’s the question: how can we slash nitrous oxide emissions from agriculture without sacrificing productivity?
To meet this challenge, scientists need to understand the relationship between “nitrogen surplus” (the amount of applied nitrogen fertilizer not taken up by the plant), “nutrient use efficiency” (the ratio of how much yield you get from each pound of fertilizer applied) and nitrous oxide emissions that contribute to climate change. The more nitrogen a plant absorbs, the less it releases into the air in the form of nitrous oxide and into the water where it can contribute to harmful algal blooms.
Until now, the only option for farmers wanting to reduce nitrous oxide emissions has been to reduce their fertilizer rate and risk impacting yield. But new research by EDF’s Trevor Anderson suggests that nitrous oxide emissions can be reduced by shifting the balance between fertilizer rate and crop yield in ways that can actually increase farm income.
Emissions tipping points
Trevor examined interactions between these measures of fertilizer use and found that reductions in nitrogen surplus levels and associated increases in nutrient use efficiency rates can reduce greenhouse gases – but only up until a certain threshold of emissions is reached:
- When the nitrogen surplus amount is less than 50 kilograms per hectare, greenhouse gas emissions stay stable. But if more than 50 kilograms of nitrogen per hectare is left in the soil after application, nitrous oxide emissions start to skyrocket.
- When it comes to nutrient use efficiency, farmers who produced at least 60 kilograms of grain per one kilogram of fertilizer applied saw emissions reductions. But beyond this tipping point, increases in efficiency rate no longer have a significant effect on reducing emissions.
These findings allow farmers to set specific targets for nutrient applications and emissions reductions – and to measure avoided GHG emissions. These data can help facilitate their participation in new carbon markets for agriculture that will soon be on the horizon.
Farmers looking to gather data on their operations can do two things to help determine nutrient optimization: use a soil test to measure the amount of nitrogen leftover in the soil at harvest time, and/or they can calculate the nitrogen surplus amount by using their nitrogen application rate and their yield.
EDF’s Trevor Anderson examined the links between nitrogen use efficiency measures and nitrous oxide emissions.
More data needed
Trevor examined 13 peer-reviewed studies from seven locations across the U.S., and conducted a meta-analysis of nearly 300 emissions data points.
“We tried to focus our research in the Midwest, but there weren’t enough data,” noted Trevor. “So we examined a broader swath of the U.S. and focused on continuous corn and corn-soybean crop rotations.”
What we do know is that reducing nitrogen surplus can improve or increase nutrient use efficiency. And we now know that there are tipping points beyond which efficiencies don’t have a significant effect on nitrous oxide emissions.
Trevor’s research also pointed out that the best way to reduce emissions while maintaining yields is to maximize “nitrogen uptake,” the amount of nitrogen that is absorbed by plants. But we don’t know enough about how to do this. That’s our next research challenge.