Measuring bacterial and fungal substrate-induced respiration in dry soils

The substrate-induced respiration inhibition (SIRIN) method of Anderson and Domsch for partitioning bacterial and fungal contributions to soil respiration was modified for application to dry soils. This new method also provided a comparative basis when measuring SIRIN in soils of different moisture contents. Soil was incubated under optimum moisture conditions (55% water-filled pore space) to maximize microbial activity and to ensure homogeneous incorporation of substrate and inhibitors 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/or cycloheximide (15 mg g−1) for selective respiratory inhibition. The procedure included conditioning for 16 h at 4°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-arid region having gravimetric moisture contents ranging between 7.5 and 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, for conventionally and no-till managed soil, were not significantly different at a probability level of 5%.