Processes involving soil CO2 dynamic in a sector of Chaco-Pampean plain, Argentina: An isotope geochemical approach

Abstract

The magnitude and spatial variability of CO₂ surface emissions and processes involving CO₂ released to the atmosphere from the soils are relevant issues in the context of climate change. This work evaluated CO₂ fluxes and ¹³C/¹²C ratio of vegetation, organic matter, and soil gases from no disturbed soils of Chaco Pampean Plain (Argentina) with different soil properties and environmental conditions (PL and PA units). Soil organic decomposition from individual layers was accompanied by δ¹³C of total organic carbon (δ¹³C-TOC) values more enriched to depth. δ¹³C-TOC values in the upper soil profile ~ ca. 0–15 cm were like the plant community of this area (~−33 to −29 ‰) while δ¹³C-TOC varied stronger bellow horizon A, till ~ −24‰. Both δ¹³C-TOC and soil δ¹³C-CO₂ were similar (~ −24 to 26 ‰) at deeper horizons (~ 50–60 cm). Toward the superficial layers, δ¹³C-TOC and δ¹³C-CO₂ showed more differences (till ~ 4 ‰), due influence of the diffusion process. Horizon A layer (~ 0–20 cm) from both PL and PA units contained the most enriched δ¹³C-CO₂ values (~ −15–17 ‰) because atmospheric CO₂ permeated the soil air. A simple two-component mixing model between sources (atmospheric δ¹³C-CO₂ and soil CO₂) confirmed that process. Isotopically, CO₂ fluxes reflected the biodegradation of C3 plants (source), diffusive transport, and CO₂ exchange (atmosphere/soil). Soil moisture content appeared as a determining factor in the diffusion process and the magnitude of CO₂ surface emissions (12–60 g·m⁻²·d⁻¹). That condition was confirmed by CO₂ diffusion coefficients estimated by air-filled porosity parameters and soil radon gradient model.