RELATIVE CARBON FLUXES FROM SOIL, DEEP VADOSE ZONE AND GROUNDWATER TO ATMOSPHERE AND RIVER OF A SEMI-ARID FLOODPLAIN IN COLORADO

Understanding of terrestrial carbon cycling relies primarily on studies of topsoils that are typically shallower than 0.3 m and seldom deeper than 1 m. Much less is known about carbon fluxes through deeper strata where about half of the Earth’s terrestrial carbon inventory resides. This is especially true in semi-arid and arid regions, which represent about 40% of the Earth’s land surface. Unique field instrumentation was installed within a semi-arid floodplain of the Colorado River to understand carbon inventories and carbon and water fluxes. Measurements were made over a 2 year period along a transect extending laterally for 250 m and vertically for 7 m through the unsaturated zone into the underlying aquifer. Year-round vadose zone pore-water sampling revealed the existence of a high dissolved organic carbon (DOC) influx from the rhizosphere infiltrating into the deeper vadose zone. The field and associated laboratory derived seasonal- and depth-resolved CO₂ fluxes and respiration rates combined with vadose zone and groundwater flow rates led to the important discoveries that 1) about 30% of the CO₂ emitted from the land surface is originating from depths below 1 m and above the water table, although the current global scale land models (CLM/ALM) show practically no CO₂ emission from below 1 m depth. 2) The CO₂ efflux to the atmosphere accounts for ~85% of the total floodplain carbon export, with only ~15% of the total carbon as DIC (the characteristics of the semi-arid climate) and < 1% of total DOC export to the aquifer and river, despite being along the river. To our knowledge, these analyses of a novel dataset are the first to quantify the relative contributions to carbon exports from different subsurface compartments into the atmosphere and river at a semi-arid floodplain. While studies of carbon cycling in other semi-arid region floodplains are needed, a similar hierarchy for magnitudes of carbon exports is expected in other semi-arid flood plains because of efficient OC utilization in soils and the deep vadose zone, and relatively low flow rates and DOC concentrations in groundwater.