Carbon Dioxide Generation in the Soil-Vadose Continuum: Substrate Characteristics and Residence Times

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Fixation of atmospheric CO2 by plants and mineralization of the fixed C (CO2 production) by respiring biological communities link the soil and atmospheric C reservoirs in a dynamic equilibrium. Resent hydrogeological research has shown that CO2 production also occurs in subsoil vadose zones and that its short-term and long- term impacts on the terrestrial-atmospheric C balance can be substantial. Therefore the effort to predict the consequences of perturbations to the soil C reservoir must now be extended to subsoil vadose zones. This means that controls on CO2 production in the entire soil-vadose continuum must be identified the understood. One key control on subsurface CO2 production is the availability of organic C to support respiration. Here we will study characteristics and mean residence times (MRT) of this C, and how these parameters vary in two contrasting geologic settings beneath temperate grasslands. Determination of the 14C age of the CO2 produced at various depths will identify modern, pedogenic, and ancient C sources, provide estimates of depth variation of MRT of the C sources, and thereby also provide estimates of the time required production in the soil-vadose continuum. We hypothesize that it if possible to isolate and characterize a fraction of C in soil-vadose geologic material which can be identified as respiration substrate by its C isotope signature. Such identification would yield new insight into the types of organic C serving as respiration substrates and their evolution with depth. It may also provide methodological basis for construction of substrate MRT-vs-depth functions using a relatively simple isolation procedure.