The nitrogen economy of tropical pasture grasses under shade

Abstract

The immobilisation of nitrogen in soil organic matter and microbial biomass limits the productivity of tropical grass pastures (Graham et al, 1981; Robbins et al, 1989; Myers and Robbins, 1991; Robertson et al, 1993). Shade can stimulate the uptake of Nby tropical grasses when growth in full sun is limited by N deficiency, and this is associated with increased productivity in these pastures (Wong and Wilson, 1980; Wilson et al, 1986, 1990). The source of this response is not clear however, and research reported here forms part of an experimental program investigating the growth and nitrogen uptake of tropical grasses in shaded pastures. Green panic (Panicum maximum var. trichoglume) was grown in solution culture with/without nitrogen limitation, in full sun or 50% sunlight (artificial shades). Total plant DM and Nyields were lower under shade (-56% and -51%), and the decreases observed were proportional to the reduction in light (50% Sarlon cloth). Shoot/Root ratio responded strongly to N supply but was much less affected by shade. Shade reduced plant RGR, tillering, and delayed the onset of flowering. Shade did not stimulate N uptake by the plant when grown in solution culture. A pulse of15N labelled ammonium sulphate was introduced into an established green panic pasture with / without ~65% sunlight (artificial shades). Shaded pasture took up more 15 Nfrom the soil system, especially from the soil surface lay er (0-2.5 cm), but the results were inconclusive in terms of overall dry matter and nitrogen yield increases. Little change in S/R ratio of green panic was observed under shade. Soil microbial C was not different between treatments. There was no evidence of rhizosphere N2 fixation, or of reduced nitrogen losses under shaded pasture. The effects of tree (55-84% sunlight) and artificial shade (50% Sarlon cloth) upon the pasture: microclimate; dry matter, nitrogen and phosphorus yields; earthworm numbers; litter decomposition; and nitrogen mineralisation, were assessed for a bahia grass (Paspalum notatum) pasture established in and alongside an 8yr. old rose gum OEucalvptus grandis) plantation at Samford, Queensland. Shading by artificial shades or under trees had little effect upon the red/far red ratio, and hence upon light quality. Shade had little effect upon air temperature or relative humidity, but had a large effect upon peak and mean surface soil temperatures, with only minor differences occurring below 5cm. Soil moisture was improved by artificial shade reducing the rate at which the soil dried out. Soil under trees was consistently drier than soil in full sun or artificial shade.