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Linking Soil Microbial Activity to Water- and Air-Phase Contents and Diffusivities

^a Danish Institute of Agricultural Sciences, Department of Crop Physiology and Soil Science, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark
^b Aalborg University, Department of Environmental Engineering, Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark

View larger version (41K): [in a new window]   Fig. 1. Map of the sampling location at Lerbjerg with kriged contours of soil clay content. The area shown is located approximately 20 m from the border of a field of approximately 20 ha. The positions of the sampling sites L1, L3, and L5 are shown together with those of the L2, L4, and L6 sites included in previous studies (e.g., Schjønning et al., 1999; Thomsen et al., 1999).

View larger version (51K): [in a new window]   Fig. 2. Pore-size distribution derived from the soil water characteristics assuming d 3000/||, where d is the tube-equivalent pore diameter in micrometers, and is the matric potential in hectopascals. sampl indicates the water regime at sampling.

View larger version (20K): [in a new window]   Fig. 3. Net nitrification (upper figures) and CO2 evolution (lower figures) as averaged for each combination of soil and matric potential. Bars represent standard error. The values below the CO2 data points for the L3 and L5 soils indicate the numeric matric potentials assigned to the average results shown in the figure. For the L1 soil, the order of succession of data points along the water content axis was as expected from the matric potentials.

View larger version (24K): [in a new window]   Fig. 4. Carbon dioxide evolution for individual soil cores drained to the potentials to the dry side of the detected optima for net nitrification. The CO2 evolution is related to (a) volumetric water content or (b) the relative solute diffusivity.

View larger version (20K): [in a new window]   Fig. 5. Net nitrification for all individual soil cores as related to soil air content (upper figures) or relative gas diffusivity (lower figures). Note the different scales.

View larger version (17K): [in a new window]   Fig. 6. Relative substrate (broken line) and gas (solid and dash line) diffusivity simulated by selected models. The plots at the upper part show the results if assuming equal importance of solute and gas diffusivity ( = ß = 1). The plots at the lower part show the results if is calculated with a fixed ß = 1 and using the observed optima for soil water content. Data points are mean values of measured relative gas diffusivity for each water potential including all fifteen replicate cores. Bars represent standard error. The values at the data points indicate the numeric matric potentials.

View larger version (23K): [in a new window]   Fig. 7. Theoretical optimum water regime for aerobic microbial activity predicted from the Equation: = /ß = 2 [( - opt)/](2+3/b)/[1.1 opt (opt - 0.02 b)], and given as (a) the soil water content, , and (b) the water-filled pore space, WFPS. Solid lines indicate = 0.4 and broken lines = 0.6. Calculations were performed for three levels of the Skopp parameter, .