Relationships between carbon age and CO ₂ efflux in agricultural and drainage ditch soils using the thermonuclear bomb pulse

Agricultural soils contribute significantly to global CO2 emissions. Soils can be both a CO2 sink and source depending on vegetation cover and the physical-chemical characteristics of the soil environment. This study uses a new C-14 dating approach to elucidate soil physical and geochemical properties that drive the production and transport of CO2 in cultivated and noncultivated drainage ditch soil environments. As expected, CO2 production and fluxes decrease with soil depth. Relating depth-specific subsurface radiocarbon of CO2 ((CO2)-C-14) to the atmospheric scenario generated from the testing of thermonuclear bombs in the 1950s and 1960s provides a means to date the labile organic C pools responsible for soil respiration that are otherwise generalized as having a modern age (<= 1950 AD). Depth-specific (CO2)-C-14 measurements showed that CO2 in shallow soils (<= 22.5-cm depth) have a younger age than CO2 in deeper soils (>22.5-cm depth). The (CO2)-C-14 of surface emissions were comparable to those of shallow soils (<= 22.5-cm depth), showing that respiration in the deeper soils (>22.5-cm depth) do not contribute significantly to surface emissions, and the higher CO2 concentrations at depth in soil result from processes limiting CO2 transport to surface. Coupling the C-14 data with CO2 flux profiles confirms that CO2 effluxes are derived from modern soil organic C (<10 yr since sequestered) from the shallowest soil depths in all soil environments studied, but changes in physical soil processes governing gas transport at depth could release these deeper CO2 stores.