MEASURING BACTERIAL AND FUNGAL SUBSTRATE-INDUCED RESPIRATION IN DRY SOILS

The substrate-induced respiration inhibition (SIRIN) method of Anderso n and Domsch for partitioning bacterial and fungal contributions to so il respiration was modified for application to dry soils. This new met hod also provided a comparative basis when measuring SIRIN in soils of different moisture contents. Soil was incubated under optimum moistur e conditions (55% water-filled pore space) to maximize microbial activ ity and to ensure homogeneous incorporation of substrate and inhibitor s into soil. Soil samples were packed to a uniform bulk density prior to measurement of CO2 evolution by gas chromatography. Glucose (3 mg g (-1)) was added together with streptomycin (0.5 or 1.0 mg g(-1)) and/o r cycloheximide (15 mg g(-1)) for selective respiratory inhibition. Th e procedure included conditioning for 16 h at 4 degrees C, followed by 1.5-h equilibration and 2-h incubation. The method yielded consistent and reproducible CO2 respiration measurements for soils from a semi-a rid region having gravimetric moisture contents ranging between 7.5 an d 23.2%. Method sensitivity was not sufficient to detect variations in the fungal-to-bacterial ratio due to management practice for the soil under study. Measured fungal-to-bacterial ratios of 29:1 and 15:1, fo r conventionally and no-till managed soil, were not significantly diff erent at a probability level of 5%. Copyright (C) 1996 Elsevier Scienc e Ltd