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1. 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)

   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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2. Plant Uptake Offsets Silica Release From a Large Arctic Tundra Wildfire

   https://doi.org/10.1029/2019EF001149  

   Carey, Joanna C. ; Abbott, Benjamin W. ; Rocha, Adrian V. ( September 2019 , Earth's Future)

   Rapid climate change at high latitudes is projected to increase wildfire extent in tundra ecosystems by up to fivefold by the end of the century. Tundra wildfire could alter terrestrial silica (SiO₂) cycling by restructuring surface vegetation and by deepening the seasonally thawed active layer. These changes could influence the availability of silica in terrestrial permafrost ecosystems and alter lateral exports to downstream marine waters, where silica is often a limiting nutrient. In this context, we investigated the effects of the largest Arctic tundra fire in recent times on plant and peat amorphous silica content and dissolved silica concentration in streams. Ten years after the fire, vegetation in burned areas had 73% more silica in aboveground biomass compared to adjacent, unburned areas. This increase in plant silica was attributable to significantly higher plant silica concentration in bryophytes and increased prevalence of silica‐rich gramminoids in burned areas. Tundra fire redistributed peat silica, with burned areas containing significantly higher amorphous silica concentrations in the O‐layer, but 29% less silica in peat overall due to shallower peat depth post burn. Despite these dramatic differences in terrestrial silica dynamics, dissolved silica concentration in tributaries draining burned catchments did not differ from unburned catchments, potentially due to the increased uptake by terrestrial vegetation. Together, these results suggest that tundra wildfire enhances terrestrial availability of silica via permafrost degradation and associated weathering, but that changes in lateral silica export may depend on vegetation uptake during the first decade of postwildfire succession.

    

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3. A Case Study of Soil Moisture and Infiltration after an Urban Fire

   https://doi.org/10.3390/fire3020022  

   Alkin, Quinn ; Kinoshita, Alicia M. ( June 2020 , Fire)

   null (Ed.)

   There is an increased risk of future fire disturbances due to climate change and anthropogenic activity. These disturbances can impact soil moisture content and infiltration, which are important antecedent conditions for predicting rainfall–runoff processes in semi-arid regions. Yet these conditions are not well documented. This case study provides critical field measurements and information, which are needed to improve our understanding of mechanisms such as precipitation and temperature that lead to the variability of soil properties and processes in urban and burned landscapes. In June 2018, a fire burned a portion of the riparian zone in Alvarado Creek, an urban tributary of the San Diego River in California, United States. This fire provided an opportunity to observe soil moisture content and infiltration for one year after the fire. Three transects (one burned and two unburned) were monitored periodically to evaluate the complex spatial and temporal dynamics of soil moisture and infiltration patterns. Average dry season soil moisture content was less than five percent volume water content (%VWC) for all transects, and the burned transect exhibited the lowest %VWC during the wet season. Infiltration rates displayed a high degree of spatial and temporal variability. However, the location with the highest burn severity had the lowest average infiltration rate. The observed differences between the burned and unburned transects indicate that the fire altered hydrologic processes of the landscape and reduced the ability of the soil to retain water during the wet season. This research provides the first high-resolution soil moisture and infiltration field analysis of an urban fire-disturbed stream in southern California and a method to characterize post-fire hydrologic conditions for rainfall–runoff processes. 

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4. Less fuel for the next fire? Short‐interval fire delays forest recovery and interacting drivers amplify effects

   https://doi.org/10.1002/ecy.4042  

   Braziunas, Kristin H. ; Kiel, Nathan G. ; Turner, Monica G. ( April 2023 , Ecology)

   As 21st‐century climate and disturbance dynamics depart from historic baselines, ecosystem resilience is uncertain. Multiple drivers are changing simultaneously, and interactions among drivers could amplify ecosystem vulnerability to change. Subalpine forests in Greater Yellowstone (Northern Rocky Mountains, USA) were historically resilient to infrequent (100–300 year), severe fire. We sampled paired short‐interval (<30‐year) and long‐interval (>125‐year) post‐fire plots most recently burned between 1988 and 2018 to address two questions: (1) How do short‐interval fire, climate, topography, and distance to unburned live forest edge interact to affect post‐fire forest regeneration? (2) How do forest biomass and fuels vary following short‐interval versus long‐interval severe fires? Mean post‐fire live tree stem density was an order of magnitude lower following short‐interval versus long‐interval fires (3240 vs. 28,741 stems ha⁻¹, respectively). Differences between paired plots were amplified at longer distances to live forest edge. Surprisingly, warmer–drier climate was associated with higher seedling densities even after short‐interval fire, likely relating to regional variation in serotiny of lodgepole pine (Pinus contortavar.latifolia). Unlike conifers, density of aspen (Populus tremuloides), a deciduous resprouter, increased with short‐interval versus long‐interval fires (mean 384 vs. 62 stems ha⁻¹, respectively). Live biomass and canopy fuels remained low nearly 30 years after short‐interval fire, in contrast with rapid recovery after long‐interval fire, suggesting that future burn severity may be reduced for several decades following reburns. Short‐interval plots also had half as much dead woody biomass compared with long‐interval plots (60 vs. 121 Mg ha⁻¹), primarily due to the absence of large snags. Our results suggest differences in tree regeneration following short‐interval versus long‐interval fires will be especially pronounced where serotiny was high historically. Propagule limitation will also interact with short‐interval fires to diminish tree regeneration but lessen subsequent burn severity. Amplifying driver interactions are likely to threaten forest resilience under expected trajectories of a future fire.

    

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5. Fire modifies plant–soil feedbacks

   https://doi.org/10.1002/ecy.3994  

   Warneke, Christopher R. ; Yelenik, Stephanie G. ; Brudvig, Lars A. ( March 2023 , Ecology)

   Although plant–soil feedbacks (interactions between plants and soils, often mediated by soil microbes, abbreviated as PSFs) are widely known to influence patterns of plant diversity at local and landscape scales, these interactions are rarely examined in the context of important environmental factors. Resolving the roles of environmental factors is important because the environmental context may alter PSF patterns by modifying the strength or even direction of PSFs for certain species. One important environmental factor that is increasing in scale and frequency with climate change is fire, though the influence of fire on PSFs remains essentially unexamined. By changing microbial community composition, fire may alter the microbes available to colonize the roots of plants and thus seedling growth post‐fire. This has potential to change the strength and/or direction of PSFs, depending on how such changes in microbial community composition occur and the plant species with which the microbes interact. We examined how a recent fire altered PSFs of two leguminous, nitrogen‐fixing tree species in Hawaiʻi. For both species, growing in conspecific soil resulted in higher plant performance (as measured by biomass production) than growing in heterospecific soil. This pattern was mediated by nodule formation, an important process for growth for legume species. Fire weakened PSFs for these species and therefore pairwise PSFs, which were significant in unburned soils, but were nonsignificant in burned soils. Theory suggests that positive PSFs such as those found in unburned sites would reinforce the dominance of species where they are locally dominant. The change in pairwise PSFs with burn status shows PSF‐mediated dominance might diminish after fire. Our results demonstrate that fire can modify PSFs by weakening the legume‐rhizobia symbiosis, which may alter local competitive dynamics between two canopy dominant tree species. These findings illustrate the importance of considering environmental context when evaluating the role of PSFs for plants.

    

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