Studies on the Nitrate Nutrition of Two Indigenous Rhodesian Grasses

Abstract

This relationship between nitrogen fertility and secondary succession suggests that the physiological aspects of nitrogen metabolism of a grass might be expected to be adapted according to the successional position of the species. The object of our study was to investigate this. The behaviour of nitrate reductase (NR) in indigenous grasses is of interest since it is inducible by nitrate (Hageman & Hucklesby 1971) and hence the activity of the enzyme at any time should give an indication of the nitrate status of the soil. Since there are no reports of measurements of NR in Rhodesian grasses it was necessary to establish optimal conditions for assay procedures before comparisons between the species could be made. The grass species Hyparrheniafilipendula and Sporobolus pyramidalis were chosen for this study since they are similar in habit and distribution yet differ in their 'positions' in secondary succession. Hyparrhenia filipendula is a climax species while Sporobolus pyramidalis is seral. Leachable soil nitrate concentration and its seasonal variations occurring in 'disturbed' and 'undisturbed' veld types were also measured to enable an investigation of the relationship between soil nitrate and plant nitrate-metabolism and secondary succession. The relationship was further investigated by observing the effects of raising soil nitrogen fertility on grass nitrogen-metabolism.