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Deep Vadose Zone Respiration Contributions to CO2 Fluxes from a Semi-arid Floodplain

Most of the carbon dioxide (CO2) fluxes leaving the soil surface originate from the shallow subsurface soil layer. These fluxes come from root and microbial respiration. However, recent work has shown that there exist contributions to this CO2 flux from the deeper subsurface. It has been estimated that there is as much organic carbon at depths greater than 1 m compared with the surface to 1-m depth. However there have been only a few measurements of CO2 flux measurements from deeper locations. In the case of floodplains, there have been numerous studies that have demonstrated the carbon fluxes to river systems. Groundwater recharge is often a slow process, and this provides ample opportunity for microbes to break down organic carbon and hence release CO2.

Authors of a recent article in Vadose Zone Journal set up their observational study in semiarid western Colorado—at the Rifle floodplain along the Colorado River. The surface soil extends to about 1.5 m deep and is underlain by floodplain alluvium with coarse alluvium extending to 6 to 7 m, and the water table is at around 3 to 4 m deep. The major annual infiltration from snowmelt and late spring rainfall occurs in the months of April through June. In later months (July to early fall), evapotranspiration dominates the hydrological cycle with little or no recharge through the vadose zone.

View of Colorado river

In this field site, boreholes were instrumented at five locations along the groundwater flow direction to investigate vadose zone to a depth of around 3.5 m. The hydraulic head measurements were obtained using tensiometers and piezometers, and the diffusion coefficients for CO2 were also obtained. In addition to the site monitoring, the soil water retention curves and gas diffusion coefficients were determined for soil and sediments in the laboratory. The surface CO2 flux was measured for a period of one year during the morning and the late afternoon–evening time period. Soil gas samples were also collected. In order to independently determine the effect of the deeper subsurface on CO2 fluxes at the surface, the respiration rates were measured on the deeper sediments.

It was observed that the CO2 flux measurements in the field vary from a low during winter to a high during summer. This is related to the warming of the shallow layer of the subsurface. In addition, the daytime CO2 fluxes are higher than the daily averaged fluxes. In the deep unsaturated zone, the CO2 concentration was higher, indicating that this is a source of CO2 for the flux to the atmosphere as well as diffusion to the groundwater with the larger gradient being towards the soil surface. As there are few or no roots deeper than 1.5 m, most of the deeper CO2 production comes from the native organic carbon in the deeper layer and seasonally by advection from the overlying soil or groundwater. The higher respiration rates at certain depths are consistent with the profiles of CO2 and the diffusion coefficients. Finally, the respiration contribution from the soil deeper than 2 m is about 17% of the total soil surface CO2 fluxes.

Read the full article in Vadose Zone Journal. Free preview available Oct 7 - Oct 14