Hydroxyl radical (•OH) is produced in soils from oxidation of reduced iron (Fe(II)) by dissolved oxygen (O2) and can oxidize dissolved organic carbon (DOC) to carbon dioxide (CO2). Understanding the role of •OH on CO2 production in soils requires knowing whether Fe(II) production or O2 supply to soils limits •OH production. To test the relative importance of Fe(II) production versus O2 supply, we measured changes in Fe(II) and O2 and in situ •OH production during simulated precipitation events and during common, waterlogged conditions in mesocosms from two landscape ages and the two dominant vegetation types of the Arctic. The balance of Fe(II) production and consumption controlled •OH production during precipitation events that supplied O2 to the soils. During static, waterlogged conditions, •OH production was controlled by O2 supply because Fe(II) production was higher than its consumption (oxidation) by O2. An average precipitation event (4 mm) resulted in 200 µmol •OH m−2 per day produced compared to 60 µmol •OH m−2 per day produced during waterlogged conditions. These findings suggest that the oxidation of DOC to CO2 by •OH in arctic soils, a process potentially as important as microbial respiration of DOC in arctic surface waters, will depend on the patterns and amounts of rainfall that oxygenate the soil.
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Revised microbial and photochemical triple-oxygen isotope effects improve marine gross oxygen production estimates
The biogeochemical fluxes that cycle oxygen (O2) play a critical role in regulating Earth’s climate and habitability. Triple-oxygen isotope (TOI) compositions of marine dissolved O2 are considered a robust tool for tracing oxygen cycling and quantifying gross photosynthetic O2 production. This method assumes that photosynthesis, microbial respiration, and gas exchange with the atmosphere are the primary influences on dissolved O2 content, and that they have predictable, fixed isotope effects. Despite its widespread use, there are major elements of this approach that remain uncharacterized, including the TOI dynamics of respiration by marine heterotrophic bacteria and abiotic O2 sinks such as the photochemical oxidation of dissolved organic carbon (DOC). Here, we report the TOI fractionation for O2 utilization by two model marine heterotrophs and by abiotic photo-oxidation of representative terrestrial and coastal marine DOC. We demonstrate that TOI slopes associated with these processes span a significant range of the mass-dependent domain (λ = 0.499 to 0.521). A sensitivity analysis reveals that even under moderate productivity and photo-oxidation scenarios, true gross oxygen production may deviate from previous estimates by more than 20% in either direction. By considering a broader suite of oxygen cycle reactions, our findings challenge current gross oxygen production estimates and highlight several paths forward to better understanding the marine oxygen and carbon cycles.
more » « less- PAR ID:
- 10381284
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- PNAS Nexus
- Volume:
- 1
- Issue:
- 5
- ISSN:
- 2752-6542
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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