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

Abstract

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.

Plain Language Summary Soil CO2 emissions are a key part of the Earth's carbon cycle and could accelerate climate change more than human activities. Tropical soils, with their fast carbon cycling, vary widely in CO2 emissions, making it critical to gather more data across different soil types, land use changes, vegetation, and climate conditions for better future predictions. In this study, we measured soil CO2 emissions and identified the carbon pools contributing to these emissions. Using stable carbon isotopes and radiocarbon in soil organic carbon (SOC), soil pore space CO2, and respired CO2, we found that SOC contributes minimally to emissions, which are mostly driven by root respiration and fresh organic matter decay. This indicates that surface CO2 fluxes do not fully reflect the dynamics of older carbon in the soil, complicating estimates of how long carbon remains in the soil.