Amazing what you can do with actual measurements.
Nitrogen fertilizers are incredibly efficient, but they make climate change a lot worse
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Conventional analysis of N2O emissions from human activities are estimated from various indirect sources. This include country-by-country reporting, global nitrogen fertilizer production, the areal extent of nitrogen-fixing crops and the use of manure fertilizers.
Our study instead used actual atmospheric concentrations of N2O from dozens of monitoring stations all over the world. We then used atmospheric modeling that explains how air masses move across and between continents to infer the expected emissions of specific regions.
We found global N2O emissions have increased over the past two decades and the fastest growth has been since 2009. China and Brazil are two countries that stand out. This is associated with a spectacular increase in the use of nitrogen fertilizers and the expansion of nitrogen-fixing crops such as soybean.
We also found the emissions reported for those two countries, based on a methodology developed by the Intergovernmental Panel on Climate Change, are significantly lower than those inferred from N2O levels in the atmosphere over those regions.
This mismatch seems to arise from the fact that emissions in those regions are proportionally higher than the use of nitrogen fertilizers and manure. This is a departure from the linear relationship used to report emissions by most countries.
There appears to be a level of nitrogen past which plants can no longer effectively use it. Once that threshold is passed in croplands, N2O emissions grow exponentially.
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all future emission scenarios consistent with the goals of the Paris Agreement require N2O emissions to stop growing and, in most cases, to decline—between 10% and 30% by mid-century.
Interestingly, emissions from the U.S. and Europe have not grown for over two decades, yet crop yields across these regions increased or remained steady.
https://phys.org/news/2019-11-nitrogen-fertilizers-incredibly-efficient-climate.html