SUSTAINING PRODUCTIVITY OF A VERTISOL AT WARRA, QUEENSLAND, WITH FERTILIZERS, NO-TILLAGE, OR LEGUMES .1. ORGANIC-MATTER STATUS

Management practices involving legume leys, grain legumes, and no-till age and stubble retention, along with nitrogen (N) fertiliser applicat ion for wheat cropping, were examined for their effectiveness in incre asing soil organic matter (0-10 cm depth) from 1986 to 1993 in a field experiment on a Vertisol at Warra, Queensland. The treatments were (i ) grass + legume leys (purple pigeon grass, Setaria incrassata; Rhodes grass, Chloris gayana; lucerne, Medicago sativa; annual medics, M. sc utellata and M. truncatula) of 4 years duration followed by continuous wheat; (ii) 2-year rotation of annual medics and wheat (Triticum aest ivum cv. Hartog); (iii) 2-year rotation of lucerne and wheat; (iv) 2-y ear rotation of chickpea (Cicer arietinum cv. Barwon) and wheat; (v) n o-tillage (NT) wheat; and (vi) conventional tillage (CT) wheat. Fertil iser N as urea was applied to both NT wheat and CT wheat at 0, 25, and 75 kg N/ha.year. The CT wheat also received N at 12.5 and 25kg N/ha.y ear. After 4 years, soil organic carbon (C) concentration under grass + legume leys increased by 20% (650 kg C/ha.year) relative to that und er continuous CT wheat. Soil total N increased by 11, 18, and 22% afte r 2, 3, and 4 years, respectively, under grass + legume leys relative to continuous CT wheat. These increases in soil organic matter were mo stly confined to the 0-2.5 cm layer. After the start of wheat cropping , organic C and total N levels declined steadily but were still higher than under CT wheat and higher than initial values in December 1985. Although 2-year rotations of lucerne-wheat and medic-wheat had a small effect on soil organic C, soil total N concentrations were higher tha n in the chickpea-wheat rotation and continuous CT wheat from November 1990 to November 1992. Soil under chickpea-wheat rotation had organic C and total N concentrations similar to continuous CT wheat, although from the former, about 70 kg/ha.year of extra N was removed in the gr ain from 1989 to 1993. No-tillage practice had a small effect on soil organic C, although total N concentration was higher than under CT whe at in November 1993. These effects were mainly confined to the surface 0-2.5 cm depth. The C to N ratio was only affected in soil under gras s + legume leys, and no-tillage treatments. These data show that resto ration of soil organic matter in Vertisol requires grass + legume leys , primarily due to increased root biomass, although soil total N can b e enhanced by including legume leys for longer duration in cropping sy stems in the semi-arid and subtropical environment.