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1. Soil CO ₂ and O ₂ Concentrations Illuminate the Relative Importance of Weathering and Respiration to Seasonal Soil Gas Fluctuations

   https://doi.org/10.2136/sssaj2019.02.0049  

   Hodges, Caitlin; Kim, Hyojin; Brantley, Susan L.; Kaye, Jason (July 2019, Soil Science Society of America Journal)
2. Thermal Adaptation of Enzyme‐Mediated Processes Reduces Simulated Soil CO ₂ Fluxes Upon Soil Warming

   https://doi.org/10.1029/2024JG008619  

   Van_de_Broek, Marijn; Riley, William J; Tang, Jinyun; Frey, Serita D; Schmidt, Michael_W I (December 2024, Journal of Geophysical Research: Biogeosciences)

   Abstract Understanding factors influencing carbon effluxes from soils to the atmosphere is important in a world experiencing climatic change. Two important uncertainties related to soil organic carbon (SOC) stock responses to a changing climate are (a) whether soil microbial communities acclimate or adapt to changes in soil temperature and (b) how to represent this process in SOC models. To further explore these issues, we included thermal adaptation of enzyme‐mediated processes in a mechanistic SOC model (ReSOM) using the macromolecular rate theory. Thermal adaptation is defined here to encompass all potential responses of soil microbes and microbial communities following a change in temperature. To assess the effects of thermal adaptation of enzyme‐mediated processes on simulated SOC losses, ReSOM was applied to data collected from a 13‐year soil warming experiment. Results show that a model omitting thermal adaptation of enzyme‐mediated processes substantially overestimates observed CO₂effluxes during the initial years of soil warming. The bias against observed CO₂effluxes was lower for models including thermal adaptation of enzyme‐mediated processes. In addition, for a simulated linear 3°C soil warming over 100 years, models including thermal adaptation of enzyme‐mediated processes simulated SOC losses of a factor of three smaller than models omitting this process. As thermal adaptation of microbial community characteristics is generally not included in models simulating feedback between the soil, biosphere and atmosphere, we encourage future studies to assess the potential impact that microbial adaptation has on soil carbon – climate feedback representations in models. 

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3. Daily Cropland Soil NO _x Emissions Identified by TROPOMI and SMAP

   https://doi.org/10.1029/2020GL089949  

   Huber, Daniel E.; Steiner, Allison L.; Kort, Eric A. (November 2020, Geophysical Research Letters)

   Abstract We use TROPOMI (TROPOspheric Monitoring Instrument) tropospheric nitrogen dioxide (NO₂) measurements to identify cropland soil nitrogen oxide (NO_x = NO + NO₂) emissions at daily to seasonal scales in the U.S. Southern Mississippi River Valley. Evaluating 1.5 years of TROPOMI observations with a box model, we observe seasonality in local NO_xenhancements and estimate maximum cropland soil NO_xemissions (15–34 ng N m⁻² s⁻¹) early in growing season (May–June). We observe soil NO_xpulsing in response to daily decreases in volumetric soil moisture (VSM) as measured by the Soil Moisture Active Passive (SMAP) satellite. Daily NO₂enhancements reach up to 0.8 × 10¹⁵ molecules cm⁻²4–8 days after precipitation when VSM decreases to ~30%, reflecting emissions behavior distinct from previously defined soil NO_xpulse events. This demonstrates that TROPOMI NO₂observations, combined with observations of underlying process controls (e.g., soil moisture), can constrain soil NO_xprocesses from space. 

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4. Impacts of Soil NO _x Emission on O ₃ Air Quality in Rural California

   https://doi.org/10.1021/acs.est.0c06834  

   Sha, Tong; Ma, Xiaoyan; Zhang, Huanxin; Janechek, Nathan; Wang, Yanyu; Wang, Yi; Castro García, Lorena; Jenerette, G. Darrel; Wang, Jun (February 2021, Environmental Science & Technology)
5. Multiple constraints cause positive and negative feedbacks limiting grassland soil CO ₂ efflux under CO ₂ enrichment

   https://doi.org/10.1073/pnas.2008284117  

   Fay, Philip A.; Hui, Dafeng; Jackson, Robert B.; Collins, Harold P.; Reichmann, Lara G.; Aspinwall, Michael J.; Jin, Virginia L.; Khasanova, Albina R.; Heckman, Robert W.; Polley, H. Wayne (January 2021, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Terrestrial ecosystems are increasingly enriched with resources such as atmospheric CO 2 that limit ecosystem processes. The consequences for ecosystem carbon cycling depend on the feedbacks from other limiting resources and plant community change, which remain poorly understood for soil CO 2 efflux, J CO2 , a primary carbon flux from the biosphere to the atmosphere. We applied a unique CO 2 enrichment gradient (250 to 500 µL L −1 ) for eight years to grassland plant communities on soils from different landscape positions. We identified the trajectory of J CO2 responses and feedbacks from other resources, plant diversity [effective species richness, exp(H)], and community change (plant species turnover). We found linear increases in J CO2 on an alluvial sandy loam and a lowland clay soil, and an asymptotic increase on an upland silty clay soil. Structural equation modeling identified CO 2 as the dominant limitation on J CO2 on the clay soil. In contrast with theory predicting limitation from a single limiting factor, the linear J CO2 response on the sandy loam was reinforced by positive feedbacks from aboveground net primary productivity and exp(H), while the asymptotic J CO2 response on the silty clay arose from a net negative feedback among exp(H), species turnover, and soil water potential. These findings support a multiple resource limitation view of the effects of global change drivers on grassland ecosystem carbon cycling and highlight a crucial role for positive or negative feedbacks between limiting resources and plant community structure. Incorporating these feedbacks will improve models of terrestrial carbon sequestration and ecosystem services. 

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