More Like this

1. Looking back to look ahead: a vision for soil denitrification research

   https://doi.org/10.1002/ecy.2917  

   Almaraz, Maya; Wong, Michelle Y.; Yang, Wendy H. (December 2019, Ecology)

   Abstract Denitrification plays a critical role in regulating ecosystem nutrient availability and anthropogenic reactive nitrogen (N) production. Its importance has inspired an increasing number of studies, yet it remains the most poorly constrained term in terrestrial ecosystem N budgets. We censused the peer‐reviewed soil denitrification literature (1975–2015) to identify opportunities for future studies to advance our understanding despite the inherent challenges in studying the process. We found that only one‐third of studies reported estimates of both nitrous oxide (N₂O) and dinitrogen (N₂) production fluxes, often the dominant end products of denitrification, while the majority of studies reported only net N₂O fluxes or denitrification potential. Of the 236 studies that measured complete denitrification to N₂, 49% used the acetylene inhibition method, 84% were conducted in the laboratory, 81% were performed on surface soils (0–20 cm depth), 75% were located in North America and Europe, and 78% performed treatment manipulations, mostly of N, carbon, or water. To improve understanding of soil denitrification, we recommend broadening access to technologies for new methodologies to measure soil N₂production rates, conducting more studies in the tropics and on subsoils, performing standardized experiments on unmanipulated soils, and using more precise terminology to refer to measured process rates (e.g., net N₂O flux or denitrification potential). To overcome the greater challenges in studying soil denitrification, we envision coordinated research efforts based on standard reporting of metadata for all soil denitrification studies, standard protocols for studies contributing to a Global Denitrification Research Network, and a global consortium of denitrification researchers to facilitate sharing ideas, resources, and to provide mentorship for researchers new to the field. 

   more » « less
2. Elevated temperature and nutrients lead to increased N2O emissions from salt marsh soils from cold and warm climates

   https://doi.org/10.1007/s10533-023-01104-0  

   Comer-Warner, Sophie A.; Ullah, Sami; Dey, Arunabha; Stagg, Camille L.; Elsey-Quirk, Tracy; Swarzenski, Christopher M.; Sgouridis, Fotis; Krause, Stefan; Chmura, Gail L. (January 2024, Biogeochemistry)

   Abstract Salt marshes can attenuate nutrient pollution and store large amounts of ‘blue carbon’ in their soils, however, the value of sequestered carbon may be partially offset by nitrous oxide (N₂O) emissions. Global climate and land use changes result in higher temperatures and inputs of reactive nitrogen (Nr) into coastal zones. Here, we investigated the combined effects of elevated temperature (ambient + 5℃) and Nr (double ambient concentrations) on nitrogen processing in marsh soils from two climatic regions (Quebec, Canada and Louisiana, U.S.) with two vegetation types,Sporobolus alterniflorus(= Spartina alterniflora) andSporobolus pumilus(= Spartina patens), using 24-h laboratory incubation experiments. Potential N₂O fluxes increased from minor sinks to major sources following elevated treatments across all four marsh sites. One day of potential N₂O emissions under elevated treatments (representing either long-term sea surface warming or short-term ocean heatwaves effects on coastal marsh soil temperatures alongside pulses of N loading) offset 15–60% of the potential annual ambient N₂O sink, depending on marsh site and vegetation type. Rates of potential denitrification were generally higher in high latitude than in low latitude marsh soils under ambient treatments, with low ratios of N₂O:N₂indicating complete denitrification in high latitude marsh soils. Under elevated temperature and Nr treatments, potential denitrification was lower in high latitude soil but higher in low latitude soil as compared to ambient conditions, with incomplete denitrification observed except in LouisianaS. pumilus. Overall, our findings suggest that a combined increase in temperature and Nr has the potential to reduce salt marsh greenhouse gas (GHG) sinks under future global change scenarios. 

   more » « less
3. Isotopic signals in an agricultural watershed suggest denitrification is locally intensive in riparian areas but extensive in upland soils

   https://doi.org/10.1007/s10533-022-00898-9  

   Sigler, W. A.; Ewing, S. A.; Wankel, S. D.; Jones, C. A.; Leuthold, S.; Brookshire, E. N.; Payn, R. A. (March 2022, Biogeochemistry)

   Abstract Nitrogen loss from cultivated soils threatens the economic and environmental sustainability of agriculture. Nitrate (NO 3 − ) derived from nitrification of nitrogen fertilizer and ammonified soil organic nitrogen may be lost from soils via denitrification, producing dinitrogen gas (N 2 ) or the greenhouse gas nitrous oxide (N 2 O). Nitrate that accumulates in soils is also subject to leaching loss, which can degrade water quality and make NO 3 − available for downstream denitrification. Here we use patterns in the isotopic composition of NO 3 − observed from 2012 to 2017 to characterize N loss to denitrification within soils, groundwater, and stream riparian corridors of a non-irrigated agroecosystem in the northern Great Plains (Judith River Watershed, Montana, USA). We find evidence for denitrification across these domains, expressed as a positive linear relationship between δ 15 N and δ 18 O values of NO 3 − , as well as increasing δ 15 N values with decreasing NO 3 − concentration. In soils, isotopic evidence of denitrification was present during fallow periods (no crop growing), despite net accumulation of NO 3 − from the nitrification of ammonified soil organic nitrogen. We combine previous results for soil NO 3 − mass balance with δ 15 N mass balance to estimate denitrification rates in soil relative to groundwater and streams. Substantial denitrification from soils during fallow periods may be masked by nitrification of ammonified soil organic nitrogen, representing a hidden loss of soil organic nitrogen and an under-quantified flux of N to the atmosphere. Globally, cultivated land spends ca. 50% of time in a fallow condition; denitrification in fallow soils may be an overlooked but globally significant source of agricultural N 2 O emissions, which must be reduced along-side other emissions to meet Paris Agreement goals for slowing global temperature increase. 

   more » « less
4. Little Effect of Land Use on N ₂ O and NO Emission Pulses Following Rewetting of Dry Soils Across Seasonally Dry sub‐Saharan Africa

   https://doi.org/10.1029/2020JG005742  

   Hickman, Jonathan E.; Kaya, Bocary; Kebede, Abhraham; Kandji, Serigne; Fitch, Laura; Neill, Christopher; Nyadzi, Gerson; Diru, Willy; Palm, Cheryl A. (January 2021, Journal of Geophysical Research: Biogeosciences)

   Abstract In seasonally dry ecosystems, which are common in sub‐Saharan Africa, precipitation after dry periods can cause large pulses of nitrous oxide (N₂O), a greenhouse gas, and of nitric oxide (NO), a precursor to tropospheric ozone pollution. Agricultural practices can change soil characteristics, affecting trace N gas emissions. To evaluate the effects of land use on trace gas pulses at the start of the rainy season, we conducted laboratory measurements of N₂O and NO fluxes from soils collected from four pairs of agricultural and natural savannah sites across the Sudano‐Sahelian zone. We also conducted in situ wetting experiments, measuring NO fluxes from fallow sandy soils in Tanzania and NO and N₂O fluxes from clayey soils in Kenya with different histories of fertilizer use. In incubation studies, NO increased by a factor of 7 to 25 following wetting, and N₂O fluxes shifted from negative to positive; cumulative NO fluxes were an order of magnitude larger than cumulative N₂O fluxes. In Kenya and Tanzania, NO increased by 1 to 2 orders of magnitude after wetting, and N₂O increased by a factor of roughly 5 to 10. Cumulative NO fluxes ranged from 87 to 115 g NO‐N ha⁻¹across both countries—a substantial proportion of annual emissions—compared to roughly 1 g N₂O‐N in Kenya. There were no effects of land use or fertilization history on the magnitude of NO or N₂O pulses, though land use may have been confounded with differences in soil texture potentially limiting the ability to detect land use effects. 

   more » « less
5. 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. 

   more » « less