A study has been made of the rhizosphere effect of four subantarctic plant associations at Macquarie Island. A stimulation occurred in each case in the rhizosphere, even in the more primitive Azorella selago and Dicranoweisia antarctica associations.No qualitative differences were found between rhizosphere and non-rhizosphere isolates. The temperature range of most isolates was 1 0 ' to 25°C. with a high proportion giving maximum growth at 25" C.Respiration measurements were made on I I soils using the Warburg respirometer and a marked correlation was found between total carbon content of the soil and respiration. The variations in respiratory quotient which occurred were not related to carbon content and were probably due to differences in chemical constitution of the organic-matter fractions and soil aeration.

Laboratory tillage of soils causes an increase in the oxygen uptake of soil microorganisms. The effect is closely related to the extent of aggregate disruption caused by the tillage treatment and is attributed largely to exposure of organic matter that was previously inaccessible. In some soils drying and rewetting after tillage causes a further increase in microbial activity, but this is not accounted for by disaggregation. As drying and rewetting commonly follow tillage in the field the resultant higher microbial activity could contribute to the faster mineralization of organic matter in arable land.

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