Synthetic fertilizers are used throughout agriculture — and especially in the United States’ Corn Belt — to help plants grow. But the fertilizers also emit a greenhouse gas known as nitrous oxide (N2O) that is almost 300 times more potent, pound for pound, than carbon dioxide.Now, a recent study out of the University of Minnesota suggests that emissions from nitrous oxide have been severely underestimated, by as much as 40 percent in some places.
Nitrous oxide emissions have historically been calculated in two ways: either by adding up the amount of nitrogen used as fertilizer (known as the bottom-up method) or by taking measurements from the air (known as the top-down approach). But these two techniques haven’t always yielded compatible results, and regional measurements taken with a top-down approach showed more nitrous oxide emissions than in the bottom-up models used by the Intergovernmental Panel on Climate Change, leading researchers to speculate that the IPCC was likely underestimating global nitrous oxide emissions.
Researchers at University of Minnesota wondered where the discrepancy in the two models came from — what was the top-down model measuring that the bottom-up models were missing?
The answer, published Monday in the Proceedings of the National Academy of Sciences, came from looking at N2O emissions across Minnesota not just from the soil, but also from streams and rivers, where nitrogen fertilizers can often end up due to drainage and runoff.
The researchers found that when these river and stream systems are taken into account, estimates of nitrous oxide emissions tended to increase. The researchers also noticed a strong relationship between the size of the stream or river and its emissions, finding that small streams close to land had the highest emissions.
“Even very small amounts of N2O can be very harmful from a greenhouse gas balance perspective,” Peter Turner, a PhD candidate in the University of Minnesota’s Department of Soil, Water, and Climate, told BBC News. “We found that there was a nine fold underestimation with streams in the area, which translates to about a a 40 percent underestimation of the agricultural budget.”
According to the Environmental Protection Agency, about 74 percent of nitrous oxide emissions in the United States come from agricultural soil management. To mitigate emissions, scientists suggest improving a crops’ ability to absorb the nitrogen that is applied to the soil, as nitrogen that remains in the soil increases the potential for nitrous oxide emissions.
But this new study suggests that a better understanding of how nitrous oxide emissions interact with streams and rivers could also help researchers and farmers develop better strategies for nitrous oxide mitigation.
Press link for more: Natasha Geiling | think progress.com