Quantification of transient CO2 production in a sandy unsaturated zone

Temporal and spatial respiration rates were determined in a 5.7-m thick, sa ndy, unsaturated zone over a 550-day period using measured CO2 concentratio ns, CO2 fluxes to the atmosphere, moisture contents, and temperatures. Cycl ical patterns in CO2 concentrations were measured in duplicate nests of nin e gas samplers. Maximum CO2 gas concentrations occurred during the summer ( 0.85-1.22%), and minimum concentrations occurred during the winter (0.04-0. 24%). CO2 gas concentrations decreased with increasing depth during the sum mer and increased with depth during the winter. A one-dimensional finite el ement model was developed to quantify transient respiration rates through t he unsaturated zone. The model was calibrated to the measured CO2 concentra tions. Temperature and moisture content variations were represented with an analytical expression and linear interpolation of field-measured values, r espectively, in the model. Simulation results provided very good approximat ions to the field-measured CO2 concentrations, but predicted CO2 fluxes to the atmosphere were higher than measured. Respiration rates ranged from 5 m u g C g(-1) d(-1) in the soil horizon during the summer to about <10(-4) mu g C g(-1) d(-1) in unsaturated sections of the C horizon. A sensitivity an alysis showed that the respiration rates in the C horizon must be <10(-3) m u g C g(-1) d(-1) and that the majority of the elevated CO2 concentrations in this thick unsaturated zone are the result of respiration in the soil ho rizon. Overall, roots contribute about 75% of the CO2 in the summer months. O-2 gas, microbial analyses, and the distribution of root biomass supporte d this conclusion. These observations also imply that although microorganis ms are present in subsurface environments their in situ activity in this sa ndy unsaturated zone may be very low.