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- Title
Controls on the rate of oxygen isotope exchange between soil waters and carbon dioxide.
- Authors
Jones, Sam; Kaisermann, Aurore; Ogée, Jérôme; Wohl, Steven; Cheesman, Alexander; Cernusak, Lucas; Lloyd, Jon; Wingate, Lisa
- Abstract
The oxygen isotope composition (δ18O) of CO2 can be used to separate net land-atmosphereCO2 exchanges into photosynthesis and respiration. This requires a good understanding ofthe contributions of these two large and opposite fluxes to variations in the δ18O ofatmospheric CO2. Global scale photosynthetic estimates are particularly sensitive tothe δ18O of CO2 exchanged between the soil and atmosphere. Soils influence theδ18O of CO2 in the atmosphere as respired CO2 or atmospheric CO2 invading thesoil profile dissolves and undergoes hydration in soil water. Owing to the relativeabundance of molecules, the exchange of oxygen atoms during hydration imparts theδ18O of water to CO2. The degree to which CO2 exchanged between the soil andatmosphere is labelled by the δ18O signature of soil water depends on the residencetime of CO2 in the soil profile and the rate of the hydration reaction. It has beenshown that soil microbes, like plants, express carbonic anhydrases that catalyse thisprocess. However, the variability and environmental controls on this enzymaticactivity are poorly understood in soils. Here we investigate variations in the rateof hydration through controlled laboratory gas exchange measurements on soilmicrocosms. To understand the drivers of these variations we measured soils, with differentchemical and physical properties, sampled from 44 sites across western Europe andnortheastern Australia. Observed hydration rates exceeded theoretical uncatalysed ratesfor the incubation conditions by up to three orders of magnitude highlighting theimportance of the role played by catalysts such as carbonic anhydrase. Variations inthe rate of hydration were best explained by positive relationships with soil pHand microbial biomass, and a negative relationship with nitrate availability underacidic conditions. These results reinforce the emergent view of pH as the principaldriver of carbonic anhydrase expression by soil microbial communities and forthe first time we show the sensitivity of this activity to nitrate availability in soils.This work highlights the need for a better understanding of interactions betweenthe rate of hydration and nutrient availability in order to refine modelling effortsaimed at accounting for the influence of soils on the δ18O of atmospheric CO2.
- Subjects
AUSTRALIA; ISOTOPE exchange reactions; SOIL moisture; CARBON dioxide; OXYGEN isotopes; SOIL profiles; CARBONIC anhydrase; HETEROTROPHIC respiration
- Publication
Geophysical Research Abstracts, 2019, Vol 21, p1
- ISSN
1029-7006
- Publication type
Article