Quantification of transient CO₂ production in a sandy unsaturated zone

Temporal and spatial respiration rates were determined in a 5.7‐m thick, sandy, unsaturated zone over a 550‐day period using measured CO 2 concentrations, CO 2 fluxes to the atmosphere, moisture contents, and temperatures. Cyclical patterns in CO 2 concentrations were measured in duplicate nests of nine gas samplers. Maximum CO 2 gas concentrations occurred during the summer (0.85–1.22%), and minimum concentrations occurred during the winter (0.04–0.24%). CO 2 gas concentrations decreased with increasing depth during the summer and increased with depth during the winter. A one‐dimensional finite element model was developed to quantify transient respiration rates through the unsaturated zone. The model was calibrated to the measured CO 2 concentrations. Temperature and moisture content variations were represented with an analytical expression and linear interpolation of field‐measured values, respectively, in the model. Simulation results provided very good approximations to the field‐measured CO 2 concentrations, but predicted CO 2 fluxes to the atmosphere were higher than measured. Respiration rates ranged from 5 µ g C g −1 d −1 in the soil horizon during the summer to about <10 −4 µ g C g −1 d −1 in unsaturated sections of the C horizon. A sensitivity analysis showed that the respiration rates in the C horizon must be <10 −3 µ g C g −1 d −1 and that the majority of the elevated CO 2 concentrations in this thick unsaturated zone are the result of respiration in the soil horizon. Overall, roots contribute about 75% of the CO 2 in the summer months. O 2 gas, microbial analyses, and the distribution of root biomass supported this conclusion. These observations also imply that although microorganisms are present in subsurface environments their in situ activity in this sandy unsaturated zone may be very low.