Microbial activity in response to water-®lled pore space of variably eroded southern Piedmont soils

Potential C and N mineralization and soil microbial biomass C (SMBC) are soil biological properties important in understanding nutrient and organic matter dynamics. Knowledge of soil water content at a matric potential near ®eld capacity is needed to determine these biological properties. The objective of this study was to examine whether adjustment of soil water content to a common level of water-®lled pore space (WFPS) may be an acceptable alternative that would require little prior analysis in comparison with adjustment based on matric potential. Potential C and N mineralization and SMBC were determined from 15 variably eroded soils of the Madison±Cecil±Pacolet association (clayey, kaolinitic, thermic Typic Kanhapludults) in response to WFPS. The levels of WFPS to achieve maximum activity and biomass under naturally settled conditions were unaffected by clay content and occurred at 0.42 0.03 m mÿ3 for net N mineralization during 24 days of incubation, 0.51 0.22 m mÿ3 for speci®c respiratory activity of SMBC, 0.60 0.07 m mÿ3 for cumulative C mineralization during 24 d of incubation, and 0.76 0.27 m mÿ3 for SMBC. Selecting a common WFPS level of 0.5 m mÿ3 resulted in 96 2%, 97 5%, 97 4%, and 88 10% of the maximum for these four properties, respectively, and was a reasonable compromise when attempting to estimate these properties during simultaneous incubations. Adjusting soil water content based on WFPS was simpler and nearly as reliable as based on matric potential, in which soil water content at ÿ33 kPa varied from 0.16 to 0.30 g gÿ1. # 1999 Elsevier Science B.V.