CO2 flux and carbon dynamics in soil and respired CO2 in a semi-arid region of western India

Soil CO₂ emissions surpass anthropogenic fluxes by an order of magnitude, with tropical soils exhibiting significant flux variability. We measured soil CO₂ flux, stable carbon isotope ratio (δ¹³C), and radiocarbon (¹⁴C) in soil-respired CO₂ as well as in soil pore space CO₂ (soil CO₂) and soil organic carbon (SOC). The objectives were to estimate CO₂ flux, identify influencing factors, and trace the sources of CO₂ in soil pore space and surface emissions. Soil CO₂ concentrations ([CO₂]) ranged from 13,780 to 26,300 ppmv. The surface CO₂ flux varied between 4.6 and 8.6 µmolCO₂/m²/s. It was strongly influenced by soil moisture content. Under relatively dry summer day with soil moisture content in the range of 7.7%–9.5% by weight, the flux varied between 8.0 and 8.6 µmolCO₂/m²/s. Under increased soil moisture conditions (14.3%–17.9%), CO₂ flux decreased to 4.6–6.6 µmolCO₂/m²/s, with larger fluctuations attributed to moisture variability. The ¹⁴C in soil and respired CO₂ is predominantly modern, while SOC exhibited much older radiocarbon ages, ranging from 2700 before present (BP) at 10 cm to 12,900 BP at 150-cm depth. Therefore, the SOC contributes minimally (at most 5%) to both soil and respired CO₂. Instead, root respiration and the decomposition of fresh organic matter are the dominant sources, even at deeper soil layers. As a result, the SOC pool and soil CO₂ appear to function as largely decoupled systems, suggesting that estimating the mean residence time of SOC based solely on surface CO₂ flux may be misleading.