FACTORS REGULATING ACETYLENE REDUCTION ASSAY FOR MEASURING HETEROTROPHIC NITROGEN FIXATION IN WATER-LOGGED SOILS

概要

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In the C_2H_2-C_2H_4 assay for measurement of heterotrophic N_2 fixation in water-logged soils, the diffusion of C_2H_2 into the soil and the recovery of C_2H_4 from it are critical factors regulating the assay result. To establish an C_2H_2-C_2H_4 assay technique suitable for water-logged soils, the C_2H_2-reducing activities (ARA), assayed by varying the method of assay gas filling, the pC_2H_2 of the assay gas, the duration of assay incubation and of soil vibration before the gas sampling, were compared. A maximum ARA was measured when the following set of procedures were applied to the soil sample in assay flasks : 1) a 4-fold repetition of 1-min evacuation under 0.01 atmospheric pressure and the subsequent 1-min filling under 1 atmospheric pressure with assay gas at pC_2H_2 of 0.1 atm, 2) an assay incubation for 3 hr, and 3) a sampling of an aliquot of the headspace gas after strongly vibrating the flask for 1 min. The ARA measured by this technique was several times larger than those measured by the techniques hitherto applied, and corresponded to an almost 80% of the V_<max> of the sample. This technique was, therefore, proposed for the assay of heterotrophic N_2 fixation in water-logged soils. A striking depression of ARA in the soil sample prepared with agitation indicated that a microbial ecosystem established in the soil should be kept as undisturbed as possible throughout the C_2H_2-C_2H_4 assay.