Diurnally variable δ18O signatures of soil CO2 fluxes indicate carbonic anhydrase activity in a forest soil

Oxygen isotopes are valuable tools for studying the gas exchange between terrestrial ecosystems and the atmosphere. We determined the δ18O signatures of soil CO2 fluxes from soil chamber measurements over the diurnal cycle in September 2000, May 2001 and July 2001 in a Sitka spruce plantation in Scotland. Concurrent estimates of the δ18O composition of soil water were obtained from soil samples collected in the vicinity of the chambers. The observed δ18O signatures of net soil CO2 fluxes were diurnally variable and strongly depleted compared to those expected from a simple evasion of respired CO2 at isotopic equilibrium with soil water. We then simulated the δ18O signatures of soil CO2 fluxes using a model of soil gas exchange that includes atmospheric invasion of CO2 with concurrent isotopic equilibration with soil water and evasion of the equilibrated CO2. This brought the modeled δ18O signatures closer to the observations, but complete agreement was only achieved when acceleration of isotopic exchange between CO2 and soil water by carbonic anhydrase activity was included. We hypothesize that carbonic anhydrase is present in the litter or surface soil layers. This introduces a feedback that can result in diurnally variable δ18O signatures of net soil CO2 fluxes. Such effects can only be captured in models that have an explicit description of the canopy air space with a variable δ18O signature of CO2

A mechanistic model of H 2

The concentration of 18O in atmospheric CO2 and H2O is a potentially powerful tracer of ecosystem carbon and water fluxes. In this paper we describe the development of an isotope model (ISOLSM) that simulates the 18O content of canopy water vapor, leaf water, and vertically resolved soil water; leaf photosynthetic 18OC16O (hereafter C18OO) fluxes; CO2 oxygen isotope exchanges with soil and leaf water; soil CO2 and C18OO diffusive fluxes (including abiotic soil exchange); and ecosystem exchange of H218O and C18OO with the atmosphere. The isotope model is integrated into the land surface model LSM, but coupling with other models should be straightforward. We describe ISOLSM and apply it to evaluate (a) simplified methods of predicting the C18OO soil-surface flux; (b) the impacts on the C18OO soil-surface flux of the soil-gas diffusion coefficient formulation, soil CO2 source distribution, and rooting distribution; (c) the impacts on the C18OO fluxes of carbonic anhydrase (CA) activity in soil and leaves; and (d) the sensitivity of model predictions to the d18O value of atmospheric water vapor and CO2. Previously published simplified models are unable to capture the seasonal and diurnal variations in the C18OO soil-surface fluxes simulated by ISOLSM. Differences in the assumed soil CO2 production and rooting depth profiles, carbonic anhydrase activity in soil and leaves, and the d18O value of atmospheric water vapor have substantial impacts on the ecosystem CO2 flux isotopic composition. We conclude that accurate prediction of C18OO ecosystem fluxes requires careful representation of H218O and C18OO exchanges and transport in soils and plants

The Indian Ocean experiment : widespread air pollution from South and Southeast Asia

The Indian Ocean Experiment (INDOEX) was an international, multiplatform field campaign to measure long-range transport of air pollution from South and Southeast Asia toward the Indian Ocean during the dry monsoon season in January to March 1999. Surprisingly high pollution levels were observed over the entire northern Indian Ocean toward the Intertropical Convergence Zone at about 6 degrees S. We show that agricultural burning and especially biofuel use enhance carbon monoxide concentrations. Fossil fuel combustion and biomass burning cause a high aerosol loading. The growing pollution in this region gives rise to extensive air quality degradation with local, regional, and global implications, including a reduction of the oxidizing power of the atmosphere