J.R. WILSON, V.R. CATCHPOOLE and K.L. WEIER

CSIRO, Division of Tropical Crops and Pastures, 306 Carmody Road, St Lucia, Qld 4067, Australia

Abstract

Shading a rundown green panic pasture stimulated greater growth and higher nitrogen (N) concentration of the green panic compared with adjacent plots in full sunlight. The pasture was 16 years-old and located on a grey, brown and red clay Brigalow soil in southern Queensland. The shades ("sarlon" cloth transmitting 37% light) were in place over two growing seasons.
Shade increased uptake of N by the rundown pasture by 5.3 g m^-2(≡ 53 kg N ha^-2) in the above-ground herbage over the two years; thus giving double the amount of N harvested from the full sun plots. Transfer of N from roots to tops did not appear to explain the result; at the end of the experiment shade roots had lost only 0.55 g N m^-2 compared to the full sun plots. Soil microbial biomass carbon, and the nitrogen fixing activity of roots measured at the end of the experiment did not differ between treatments. The shade stimulation of N uptake by the grass needs further investigation to understand the mechanisms involved.