Biomass and Nutrient Dynamics in a C 4 Semi-Arid Australian Grassland Community

Abstract

SUMMARY (1) The seasonal pattern of above-ground live biomass production of a C4 Mitchell (Astrebla spp.) grassland community in subtropical, semi-arid Queensland, Australia, was studied over a 2.5 yr period. Actual evapotranspiration and the sequential uptake of nitrogen and phosphorus over continuous summer and winter growing periods were also measured. (2) There was a pronounced seasonal pattern of live biomass production, the total biomass produced over the winter growing period representing only about 7% of the peak total summer biomass production of 1960 kg ha-1. The yield production potential of the perennial grasses was severely depressed by low winter temperatures. (3) The maximum daily above-ground live biomass production rate for the perennial grass component during summer was 39 kg ha-1 day-1, compared with a maximum value of only 1 kg ha-1 day-' for the winter growing period. (4) The reduction in above-ground biomass production rate over the summer growing period was associated with a decline in the rate of nitrogen uptake. Phosphorus uptake was linear over almost the entire summer growing period. Nutrient uptake rate over the winter growing period was very low. (5) The ratio of actual evapotranspiration to precipitation for both the summer and winter growing seasons was around unity. (6) The results are compared with similar data for a major Australian C3 grassland in a climatic zone identical to that of the Mitchell grassland studied. The possible role of limiting soil nutrients and water as factors controlling the distribution and production of both grassland communities is discussed. (7) Some comments on animal management decision-making for Mitchell grasslands are also made.