Two ways of measuring substrate-induced respiration (SIR) following additio
n of glucose to soil, viz. solid glucose or glucose in solution, were teste
d on 13 soils sampled from arable, grassland and woodland sites ranging fro
m pH 3.2 to 7.5. Generally similar patterns of CO2 evolution were found bet
ween soils following addition of glucose as liquid or solid (r = 0.93) for
unamended, ryegrass-amended and fumigated soils. Glucose added in solution
to adjust the soils to 1.2-fold WHC was therefore preferred for analytical
convenience. The optimum time of CO2 measurement was between 0.5 and 2.5 h
for routine use. It was found to be unnecessary to make any correction for
CO2 dissolved in the soil solution for soils below pH 6.5. Maximum inhibiti
on of bacterial respiration was obtained at 4 to 8 mg added streptomycin an
d of fungal respiration at 8-12 mg added cyclohexamide g(-1) soil. In a gra
ssland soil (24% clay) the bacteria comprised 19 +/- 4.2% and the fungi 82
+/- 4.0% of the total biomass and in a grass ley (8% clay) the proportions
were 25 +/- 1.2% for bacteria and 76 +/- 4.5% for fungal biomass. (C) 1999
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