An evaluation of the substrate-induced respiration method

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 Elsevier Science Ltd. All rights reserved.