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A Process-Based Model for Predicting Soil Carbon Dioxide Efflux and Concentration

Dep. of Forest Ecology, P.O. Box 27, FIN-00014 University of Helsinki, Finland

View larger version (27K): [in a new window]   Fig. 1. A schematic presentation of the simulation model. The CO2 production in each soil layer consists of microbial respiration (rm) and root respiration (rr), which are controlled by temperature (T) and by soil-water content (v). Carbon dioxide moves between the layers by diffusion and the CO2 flux (J) depends on the total porosity of soil (Eo), the soil-water content and the thickness of the layers (l) as well as the concentration gradient between the layers. The CO2 fluxes are denoted by thick arrows, and thin arrows represent information between parameters and processes. The amount of CO2 in a soil horizon is denoted by C and soil layers are denoted with capital letters O, A, B, and C. In the figure, processes are presented only for O- and A-horizons.

View larger version (21K): [in a new window]   Fig. 2. The relation between soil respiration and soil-water content in O-, A-, B-, and C-horizons. In the model, the maximum value of the moisture factor f(v) is limited to 1.

View larger version (22K): [in a new window]   Fig. 3. Flow-chart of the program. The initial values of soil parameters are given to the program. Then values of measured ambient air CO2 concentration, soil temperature and soil moisture of each soil layer are imported from the data file. First, the CO2 flux between the layers is calculated. Next the respiration of each soil layer is calculated. Then new values for CO2 concentrations in each layer are obtained using calculated respiration and flux values. New ambient air CO2 concentration, temperature, and moisture values are imported from the data file until the end of the file has been reached.

View larger version (19K): [in a new window]   Fig. 4. Measured and predicted daily average CO2 effluxes from the soil surface over 19 mo at the reference site from May 1998 to November 1999 excluding the winter months from December 1998 to April 1999. The CO2 efflux was measured continuously by two automatically operating chambers, and the efflux presented is an average of the two chambers.

View larger version (28K): [in a new window]   Fig. 5. (a) Soil temperature (°C) was measured by temperature sensors installed permanently in the O-, A-, B-, and C-horizons. (b) Soil-water content (m3 m-3) was measured by TDR probes installed permanently in corresponding soil horizons and denoted with similar symbols. Values presented in the figure are daily averages of five temperature sensors and TDR probes installed in each soil horizon. When the soil freezes in O-horizon in late November, the TDR does not measure the volumetric water content correctly.

View larger version (22K): [in a new window]   Fig. 6. The relation between measured and predicted CO2 efflux calculated with the model (a) without moisture factor and (b) with moisture factor. Daily average values are shown in the figure. Simulation without the moisture factor shows a clear over estimation of the efflux.

View larger version (18K): [in a new window]   Fig. 7. Simulated respiration in the O-, A-, B-, and C-horizons over 19 mo at the reference site from May 1998 to November 1999 excluding the winter months from December 1998 to April 1999.

View larger version (34K): [in a new window]   Fig. 8. (a) Measured and predicted daily average values for soil CO2 concentration in the O-, A-, B-, and C-horizons over 19 mo at the reference site from May 1998 to November 1999 excluding the winter months from December 1998 to April 1999. Measured values for soil CO2 concentration are an average of air samples taken from five permanently installed gas samplers in each horizon. The coefficient of variation of the samples ranged, on average, from 0.37 in the humus layer to 0.51 in the B-horizon. (b), (c), (d), and (e) measured and predicted daily average CO2 concentrations plotted on xy–plot for O-, A-, B-, and C-horizons, respectively.