Inorganic carbon fluxes across the vadose zone of planted and unplanted soil mesocosms

Abstract. The efflux of carbon dioxide (CO₂) from soils influences atmospheric CO₂ concentrations and thereby climate change. The partitioning of inorganic carbon (C) fluxes in the vadose zone between emission to the atmosphere and to the groundwater was investigated to reveal controlling underlying mechanisms. Carbon dioxide partial pressure in the soil gas (pCO₂), alkalinity, soil moisture and temperature were measured over depth and time in unplanted and planted (barley) mesocosms. The dissolved inorganic carbon (DIC) percolation flux was calculated from the pCO₂, alkalinity and the water flux at the mesocosm bottom. Carbon dioxide exchange between the soil surface and the atmosphere was measured at regular intervals. The soil diffusivity was determined from soil radon-222 (²²²Rn) emanation rates and soil air Rn concentration profiles and was used in conjunction with measured pCO₂ gradients to calculate the soil CO₂ production. Carbon dioxide fluxes were modeled using the HP1 module of the Hydrus 1-D software.

The average CO₂ effluxes to the atmosphere from unplanted and planted mesocosm ecosystems during 78 days of experiment were 0.1 ± 0.07 and 4.9 ± 0.07 μmol C m⁻² s⁻¹, respectively, and grossly exceeded the corresponding DIC percolation fluxes of 0.01 ± 0.004 and 0.06 ± 0.03 μmol C m⁻² s⁻¹. Plant biomass was high in the mesocosms as compared to a standard field situation. Post-harvest soil respiration (R_s) was only 10% of the R_s during plant growth, while the post-harvest DIC percolation flux was more than one-third of the flux during growth. The R_s was controlled by production and diffusivity of CO₂ in the soil. The DIC percolation flux was largely controlled by the pCO₂ and the drainage flux due to low solution pH. Modeling suggested that increasing soil alkalinity during plant growth was due to nutrient buffering during root nitrate uptake.