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1. Rapid nitrate reduction produces pulsed NO and N2O emissions following wetting of dryland soils

   https://doi.org/10.1007/s10533-022-00896-x  

   Krichels, Alexander H.; Homyak, Peter M.; Aronson, Emma L.; Sickman, James O.; Botthoff, Jon; Shulman, Hannah; Piper, Stephanie; Andrews, Holly M.; Jenerette, G. Darrel (March 2022, Biogeochemistry)

   Abstract Soil drying and wetting cycles can produce pulses of nitric oxide (NO) and nitrous oxide (N 2 O) emissions with substantial effects on both regional air quality and Earth’s climate. While pulsed production of N emissions is ubiquitous across ecosystems, the processes governing pulse magnitude and timing remain unclear. We studied the processes producing pulsed NO and N 2 O emissions at two contrasting drylands, desert and chaparral, where despite the hot and dry conditions known to limit biological processes, some of the highest NO and N 2 O flux rates have been measured. We measured N 2 O and NO emissions every 30 min for 24 h after wetting soils with isotopically-enriched nitrate and ammonium solutions to determine production pathways and their timing. Nitrate was reduced to N 2 O within 15 min of wetting, with emissions exceeding 1000 ng N–N 2 O m −2  s −1 and returning to background levels within four hours, but the pulse magnitude did not increase in proportion to the amount of ammonium or nitrate added. In contrast to N 2 O, NO was emitted over 24 h and increased in proportion to ammonium addition, exceeding 600 ng N–NO m −2  s −1 in desert and chaparral soils. Isotope tracers suggest that both ammonia oxidation and nitrate reduction produced NO. Taken together, our measurements demonstrate that nitrate can be reduced within minutes of wetting summer-dry desert soils to produce large N 2 O emission pulses and that multiple processes contribute to long-lasting NO emissions. These mechanisms represent substantial pathways of ecosystem N loss that also contribute to regional air quality and global climate dynamics. 

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2. Nitrogen Availability and Changes in Precipitation Alter Microbially Mediated NO and N₂O Emissions From a Pinyon–Juniper Dryland

   https://doi.org/10.1111/gcb.70159  

   Zhao, Sharon; Krichels, Alexander H; Stephens, Elizah Z; Calma, Anthony D; Aronson, Emma L; Jenerette, G Darrel; Spasojevic, Marko J; Schimel, Joshua P; Hanan, Erin J; Homyak, Peter M (March 2025, Global Change Biology)

   ABSTRACT Climate change is altering precipitation regimes that control nitrogen (N) cycling in terrestrial ecosystems. In ecosystems exposed to frequent drought, N can accumulate in soils as they dry, stimulating the emission of both nitric oxide (NO; an air pollutant at high concentrations) and nitrous oxide (N₂O; a powerful greenhouse gas) when the dry soils wet up. Because changes in both N availability and soil moisture can alter the capacity of nitrifying organisms such as ammonia‐oxidizing bacteria (AOB) and archaea (AOA) to process N and emit N gases, predicting whether shifts in precipitation may alter NO and N₂O emissions requires understanding how both AOA and AOB may respond. Thus, we ask: How does altering summer and winter precipitation affect nitrifier‐derived N trace gas emissions in a dryland ecosystem? To answer this question, we manipulated summer and winter precipitation and measured AOA‐ and AOB‐derived N trace gas emissions, AOA and AOB abundance, and soil N concentrations. We found that excluding summer precipitation increased AOB‐derived NO emissions, consistent with the increase in soil N availability, and that increasing summer precipitation amount promoted AOB activity. Excluding precipitation in the winter (the most extreme water limitation we imposed) did not alter nitrifier‐derived NO emissions despite N accumulating in soils. Instead, nitrate that accumulated under drought correlated with high N₂O emission via denitrification upon wetting dry soils. Increases in the timing and intensity of precipitation that are forecasted under climate change may, therefore, influence the emission of N gases according to the magnitude and season during which the changes occur. 

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3. Evidence of both molecular cloud and fluid chemistry in Ryugu regolith

   https://doi.org/10.1126/sciadv.adp3037  

   Bose, Maitrayee; Root, Robert A; Guan, Yunbin; Eaton, Jacob; Wittmann, Axel; Skrmetti, Thomas; Desch, Steven J (July 2024, Science Advances)

   The sulfur chemistry of (162173) Ryugu particles can be a powerful tracer of molecular cloud chemistry and small body processes, but it has not been well explored. We report identification of organosulfurs and a sulfate grain in two Ryugu particles, A0070 and A0093. The sulfate grain shows oxygen isotope ratios (δ¹⁷O = −11.0 ± 4.3 per mil, δ¹⁸O = −7.8 ± 2.3 per mil) that are akin to silicates in Ryugu but exhibit mass-independent sulfur isotopic fractionation (Δ³³S = +5 ± 2 per mil). A methionine-like coating on the sulfate grain is isotopically anomalous (δ¹⁵N = +62 ± 2 per mil). Both the sulfate and organosulfurs can simultaneously form and survive during aqueous alteration within Ryugu’s parent body, under reduced conditions, low temperature, and a pH >7 in the presence of N-rich organic molecules. This work extends the heliocentric zone where anomalous sulfur, formed by selective photodissociation of H₂S gas in the molecular cloud, is found. 

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4. Post-fire soil emissions of nitric oxide (NO) and nitrous oxide (N2O) across global ecosystems: a review

   https://doi.org/10.1007/s10533-023-01072-5  

   Stephens, Elizah Z; Homyak, Peter M (September 2023, Biogeochemistry)

   Abstract Wildfires may increase soil emissions of trace nitrogen (N) gases like nitric oxide (NO) and nitrous oxide (N₂O) by changing soil physicochemical conditions and altering microbial processes like nitrification and denitrification. When 34 studies were synthesized, we found a significant increase in both NO and N₂O emissions up to 1 year post-fire across studies spanning ecosystems globally. However, when fluxes were separated by ecosystem type, we found that individual ecosystem types responded uniquely to fire. Forest soils tended to emit more N₂O after fire, but there was no significant effect on NO. Shrubland soils showed significant increases in both NO and N₂O emissions after fires; often with extremely large but short-lived NO pulses occurring immediately after fire. Grassland NO emissions increased after fire, but the size of this effect was small relative to shrublands. N₂O emissions from burned grasslands were highly variable with no significant effect. To better understand the variation in responses to fire across global ecosystems, more consistent measurements of variables recognized as important controls on soil fluxes of NO and N₂O (e.g., N cycling rates, soil water content, pH, and substrate availability) are needed across studies. We also suggest that fire-specific elements like burn severity, microbial community succession, and the presence of char be considered by future studies. Our synthesis suggests that fires can exacerbate ecosystem N loss long after they burn, increasing soil emissions of NO and N₂O with implications for ecosystem N loss, climate, and regional air quality as wildfires increase globally. 

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5. Underestimation of Future Agricultural Soil N ₂ O Emissions and Abatement Needs

   https://doi.org/10.1111/gcb.70919  

   Lu, Chaoqun; Li, Linchao; Winiwarter, Wilfried; Canadell, Josep G; Liu, Weihang; Tian, Hanqin (May 2026, Global Change Biology)

   ABSTRACT Agricultural soils are the largest human‐induced source of nitrous oxide (N₂O) due to the extensive fertilizer use in crop production. Despite progress made in global N₂O budget accounting, using the IPCC‐derived static emission factors (EF) limits our capability to project future changes in agricultural N₂O emissions and identify cost‐effective mitigation strategies. Here, we use a physics‐informed AI‐driven dynamic EF modeling framework to project direct agricultural soil N₂O emissions induced by mineral fertilizer additions under various climate and nitrogen (N) regulation policy scenarios. Compared to projections based on static EF, our study yields higher future N₂O emissions by 0.3–1.1 Tg N year⁻¹across scenarios by 2050, implying greater abatement needs to combat climate change. The gap between static and dynamic EF approaches is projected to widen to 29%–34% by mid‐century. Under moderate‐ to high‐ambition N regulation policies, we project that a 25% emission reduction can be reached before 2050, while a 45% reduction is only attainable under high‐ambition N policies and strong climate action. The potential for N₂O mitigation due to a policy shift varies substantially among regions, with seven top source regions contributing 76%–87% of global N₂O reduction. Adopting N₂O reduction technologies in hotspot areas would significantly accelerate the timeline for achieving the 25% reduction goal. Improving fertilizer management on croplands can provide climate benefits comparable to, or even exceeding, those of enhancing soil carbon sequestration, particularly in regions with low nitrogen use efficiency. Our findings highlight the higher mitigation potential of targeting N₂O emission hotpots and the urgency of implementing policy shifts. 

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