Soil gas dynamics and carbon cycling in the vadose zone of a large, regulated river near Austin, Texas

Globally, over half of all rivers are regulated in some way. In some cases, river regulation is the dominant factor governing fluid exchange and soil gas dynamics in the hyporheic and unsaturated zones of the river banks. Where this is the case, it is important to understand the effects that an artificially-induced change in river stage can have on the chemical, plant, and microbial components of the unsaturated zone. Daily releases from an upstream dam cause rapid stage fluctuations in the Lower Colorado River east of Austin, Texas. Previous findings indicate that meter-scale changes in river stage propagate through the bank and are dampened to centimeter-scale pulses in the water table. We propose that these conditions create an area in the subsurface that is subjected to frequent cycles of saturation and drainage (the ‘active zone’), supporting enhanced microbial oxidation of organic carbon and acting as a biogeochemical filter. CO2 production through the active zone appears to be decoupled from patterns of diurnal respiration but closely linked to increased soil-water content following a water table rise. Our data indicate that neither surface water nor groundwater is the source of carbon for this zone. Instead, two sources of carbon are possible: 1) litter-derived carbon is leaching down through the soil column into the active zone, or 2) at low water contents particulate organic carbon accumulates in macropores where it is enzymatically decomposed to labile carbon. Additionally, our results suggest that the active zone may sustain denitrifying activity and a region of methanogenesis.Geological Science