Deep Vadose Zone Respiration Contributions to Carbon Dioxide Fluxes from a Semiarid Floodplain

Abstract

Core Ideas A significant fraction of CO2 fluxes from a semiarid floodplain originates from below the root zone. Measured and calculated field CO2 fluxes are consistent. Laboratory‐measured respiration rates are consistent with field results. Although CO2 fluxes from soils are often assumed to originate within shallow soil horizons (<1‐m depth), relatively little is known about respiration rates at greater depths. We compared measured and calculated CO2 fluxes at the Rifle floodplain along the Colorado River and measured CO2 production rates of floodplain sediments to determine the relative importance of deeper vadose zone respiration. Calculations based on measured CO2 gradients and estimated effective diffusion coefficients yielded fluxes that are generally consistent with measurements obtained at the soil surface (326 g C m−2 yr−1). Carbon dioxide production from the 2.0‐ to 3.5‐m depth interval was calculated to contribute 17% of the total floodplain respiration, with rates that were larger than some parts of the shallower vadose zone and underlying aquifer. Microbial respiration rates determined from laboratory incubation tests of the sediments support this conclusion. The deeper unsaturated zone typically maintains intermediate water and air saturations, lacks extreme temperatures and salinities, and is annually resupplied with organic carbon from snowmelt‐driven recharge and by water table decline. This combination of favorable conditions supports deeper unsaturated zone microbial respiration throughout the year.

Hot Spots and Hot Moments of Nitrogen in a Riparian Corridor

“…The primary monitoring well network used for the current study was installed in 2013. A detailed description of the borehole instrumentation and field sampling protocol is provided in Tokunaga et al () and Williams et al (). Briefly, instrumentation including tensiometers and soil water and gas samplers were installed at various depths in the vadose zone for sampling.…”

“…Separate multilevel boreholes were installed for groundwater monitoring and sampling. The groundwater sampling wells accessed three distinct depths near the capillary fringe zone, 1 m below the water table, and the deep aquifer region (Tokunaga et al, ). Apart from gas samplers, similar instrumentation sets were used for groundwater sampling wells.…”

Evidence for microbial mediated nitrate cycling within floodplain sediments during groundwater fluctuations

“…Installation and well sampling have recently been described in full (Tokunaga et al, 2016). Briefly, lysimeters and gas sampler ports were installed as part of a drilling operation from the ground surface to the water table (∼ 3.25m at the time of drilling).…”

“…This is unsurprising given the low NH 4 + concentrations in the Rifle subsurface, as the AOA have previously been characterized as having half-saturation constants in the nM range (Martens-Habbena et al, et al, 2006) and biological nitrogen fixation (Swanner & Templeton, 2011;Lau et al, 2014) might be a source of subsurface nitrogen it is more likely that OM mineralization contributes the bulk of NH 4 + . Rifle sediments can be carbon rich due to the advective downward transport of DOM (Tokunaga et al, 2016), and heterogeneously distributed fine-grained sediment lenses 10 (Janot et al, 2016). These lenses are enriched in organic carbon that likely represent the deep burial of soil horizons (Janot et al, 2016), common within floodplain sediments (Blazejewski et al, 2009;Hill, 2010).…”

“…The model outputs simulated trends in microbial functional group composition and contribution to biogeochemical processes of interest, including the production of N 2 O and N 2 . donor based on measurements of total organic matter at depth (Tokunaga et al, 2016), assuming 10 % of the total was available for microbial mineralization. Organic matter concentrations decreased with depth from 2 to 3 m, consistent with the structure of spatially discrete hotspots identified at the Rifle site (Wainwright et al, 2016).…”