Biological studies of some tussock-grassland soils

Abstract

A quantitative and qualitative study of the bacteria of three tussock-grassland soils has been made to determine the relative influence of vegetation and soil type on the flora. Greatest numbers of bacteria tend to occur in the topsoil rather than in the subsoil and in the spring rather than the summer or autumn. Of the five mam taxonomic groups recognised, one (the Achromobacteriaceae) is considered to be chiefly derived from the leaf flora while the distribution of the aerogenic fermenters (Enterobacteriaceae and Aeromonas) is considered to be related to the relative organic content and the extent of the rooting systems in the three soils. The predominant flora consists chiefly of pseudomonads and is very similar in all three soils. Generally, the flora consists of strongly oxidative and proteolytic strains, psychrophilic or mesophilic rather than thermophilic, requiring readily available sources of energy but not requiring growth factors and capable of using ammonium as a sole source of nitrogen. There is a positive correlation between the saccharolytic and proteolytic activities of the strains and denitrification. Generally, it is considered that the activity of the flora is controlled primarily by the condition of the plant cover and particularly by root activity. Neither moisture nor temperature appear to be serious limiting factors.