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Jan Woźniak
Sr. Application Scientist at Picarro
Picarro Spotlight is a blog series showcasing important scientific work from our customers around the world. Each blog is selected and summarized by our team. Enjoy!
Nitrous oxide (N₂O) is a greenhouse gas roughly 300 times more potent than CO₂ over a 100-year horizon — and its atmospheric growth rate has been accelerating over the past decade at a pace that existing models cannot fully explain. What is driving that acceleration? A team of researchers in Austria may have found a critical piece of the answer, and a Picarro isotope analyzer was at the heart of their discovery.
A Reasonable Assumption — That Turned Out to Be Wrong
When a soil dries out, the textbook assumption is straightforward: dry soils are well-oxygenated, so N₂O production should shift toward nitrification — an aerobic pathway — while anaerobic denitrification recedes. When rain returns, denitrification rebounds, driving a short-lived pulse of emissions.
Dr. Eliza Harris and her colleagues at the University of Innsbruck, along with collaborators from the University of Natural Resources and Life Sciences Vienna, Helmholtz Zentrum München, Technical University of Munich, the University of Manchester, and Forschungszentrum Jülich, set out to test this assumption rigorously — and found it to be wrong in a way that matters for climate projections worldwide.
Their study, published in Science Advances as “Denitrifying pathways dominate nitrous oxide emissions from managed grassland during drought and rewetting”, followed 16 intact subalpine grassland monoliths through a full growing season of fertilization, severe drought, and rewetting — with isotopic measurements of N₂O taken continuously and automatically throughout.
The Instrument That Made It Possible
At the core of the experimental setup was a Picarro G5131-i N₂O isotope analyzer (currently succeeded by the newer model PI5131-i), coupled via automated chambers to a Picarro G2301 CO₂ and CH₄ analyzer. Together, these instruments provided fully automated, continuous, high frequency measurements — one isotopic reading for each chamber every ~17 minutes, for an entire growing season.
This measurement density was not a convenience — it was a scientific necessity. N₂O source pathways are highly dynamic, shifting on timescales of hours in response to rainfall, temperature, and substrate availability. Previous studies relying on manual or low frequency sampling had simply lacked the temporal resolution to track these changes. The Picarro G5131-i (now PI5131-i), based on Cavity Ring-Down Spectroscopy (CRDS), provided the sensitivity and stability needed to determine not just how much N₂O was being emitted, but where it was coming from — by measuring isotopic “site preference” (SP) and ¹⁵N values that act as fingerprints for specific production pathways.
The Surprising Results: Denitrification Doesn't Wait for Rain
The key finding upended the conventional view: even at very low soil moisture — well below 30% water filled pore space, when soils should be fully oxygenated — denitrifying pathways produced 70–90% of all N₂O emissions. Nitrification, the expected dominant under drought, played a surprisingly minor role throughout the entire growing season.
Why? Nanoscale imaging using NanoSIMS revealed that drought caused an enrichment of nitrogen bearing organic matter (N SOM) on the surface of soil microaggregates. These submicron scale structures appear to maintain oxygen depleted microsites even when the bulk soil is dry — providing local havens where anaerobic N₂O production, specifically through chemo and co-denitrification pathways, can continue uninterrupted.
Rewetting: A Larger Pulse Than Expected
When the drought was ended experimentally and monoliths were rewet, N₂O emissions surged — peaking as high as 11.6 nmol m⁻² s⁻¹, with 92% attributed to denitrification, occurring approximately 51 hours after rewetting. Crucially, the isotopic data showed that this initial pulse bore the signature of chemo or co-denitrification, as the N SOM accumulated during drought was rapidly consumed when water became available.
Perhaps most importantly for climate projections, the study found that the total seasonal N₂O budget from drought plots was not significantly lower than control plots — the large rewetting pulse effectively offset the reduced emissions during the dry period. For some monoliths, drought rewetting actually increased net seasonal emissions.
What This Means for a Warming World
These results carry direct relevance for climate science. The non linear sensitivity of N₂O emissions to soil moisture means that even modest increases in precipitation can substantially amplify global emissions — the study showed a 55% increase in N₂O flux for just +25% precipitation. The authors calculated that just two months of +25% precipitation globally could add 0.34 Tg N₂O N per year to the atmosphere.
Meanwhile, the accelerating shift in global rainfall patterns — more frequent droughts followed by more intense rewetting events — creates precisely the drought rewetting cycle dynamics documented here. The authors argue this feedback is sufficient to help explain the acceleration in atmospheric N₂O growth observed over the past decade, a trend that had previously lacked a clear mechanistic cause.
Incorporating these pathway dynamics — particularly the underappreciated role of chemo and co-denitrification — into biogeochemical models will be essential for accurate climate projections and effective policy design.
Read the Paper — Harris et al., Science Advances, 2021