Evaluation of the acetylene-reduction assay of nitrogen fixation in pastures using small soil-core samples

Abstract

Acetylene reduction (AR) assays of N2 fixation in clover-ryegrass pasture were made on bulked samples of 2.54 cm diameter by 7.5 cm deep soil cores. The assays gave a reasonable reflection of N2 fixation as indicated by their relationship to assays on pasture turfs and by their responses to seasonal conditions and imposed treatments. Variability in the assay was large (25% coefficient of variation on a 12-core sample) apparently because of irregular spatial distribution of N2 fixation in pasture, and AR activity was subject to sharp short-term fluctuations presumably reflecting climatic conditions before the assay. Consequently, quantitative interpretation of the assay in terms of N2 fixed was considered liable to substantial error. There was also some indication that the AR potential of pastures was not fully expressed by the assay method used. The experiments suggest that the AR assay using small soil cores could be a useful index of N2 fixation in making between-treatment comparisons under the same climatic conditions, but it is of limited value for absolute determination of N2 fixation.