APSIM (Agricultural Production Systemsof both the total amounts in the whole projle and their distribution with depth. Since neither of these datasets included measurements of the runof component of the water balance, this aspect of model performance was evaluated, and shown to be generally good, using data from a third source where runoff had been measured from contour bay catchments. 0 1997

The decline in soil organic matter with cropping is a major factor affecting the sustainability of cropping systems. Changes in total C levels are relatively insensitive as a sustainability measure. Oxidation with different strength K M n Q has been shown to be a more sensitive indicator of change. The relative size of soil C fractions oxidised by 333mM KMnO4 declined with cropping, whilst the relative size of the unoxidised fraction increased. Changes in 813C ratio have been used to measure C turnover in systems which include C3 and C4 species.

Management practices involving legume leys, grain legumes, and no-tillage and stubble retention, along with nitrogen (N) fertiliser application for wheat cropping, were examined for their effectiveness in increasing 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. scutellata and M. truncatula) of 4 years duration followed by continuous wheat; (ii) 2-year rotation of annual medics and wheat (Triticum aestivum cv. Hartog); (iii) 2-year rotation of lucerne and wheat; (iv) 2-year rotation of chickpea (Cicer arietinum cv. Barwon) and wheat; (v) no-tillage (NT) wheat; and (vi) conventional tillage (CT) wheat. Fertiliser 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. year. After 4 years, soil organic carbon (C) concentration under grass + legume leys increased by 20% (650 kg C/ha. year) relative to that under continuous CT wheat. Soil total N increased by 11, 18, and 22% after 2, 3, and 4 years, respectively, under grass + legume leys relative to continuous CT wheat. These increases in soil organic matter were mostly 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 than 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 grain from 1989 to 1993. No-tillage practice had a small effect on soil organic C, although total N concentration was higher than under CT wheat 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 grass + legume leys, and no-tillage treatments. These data show that restoration of soil organic matter in Vertisol requires grass + legume leys, primarily due to increased root biomass, although soil total N can be enhanced by including legume leys for longer duration in cropping systems in the semi-arid and subtropical environment.

On the Darling Downs the growth and yield of a semi-dwarf wheat (cv. Oxley) under supplementary irrigation was increased by the application of up to 400 kg/ha of nitrogen (N) at planting. Nitrogen at 50 or 100 kg/ha applied at the boot stage to supplement 100 kg/ha applied at planting increased grain yield by 459 and 478 kg/ha, respectively. However, yields were still below those where all the N was applied at planting. In contrast, supplementary N (0, 25, 50 or 100 kg/ha) at flowering or after flowering generally did not increase grain yield. One exception to this was where only 50 kg/ha was applied at planting; an additional 100 kg/ha at flowering increased grain yield by 602 kg/ha. Applied at planting, more than 200 kg/ha of N was needed to produce premium grade wheat (i.e. protein content above 11.4%). To achieve this protein content where 100 kg/ha had been applied at planting an additional 100 kg/ha was needed at the boot stage or 50 kg/ha at flowering. Applied after flowering, up to 100 kg/ha of additional N produced wheat of a protein content too low to attract a premium payment. A similar quantity of N was assimilated whether the entire N application was applied at planting or where the application was split between planting and boot or flowering. Less N was assimilated when the application was split between planting and after flowering. More N was assimilated from soil than from foliar applications at the boot stage. Soil and foliar applications were equally effective at flowering in increasing the amount of N assimilated as well as the grain protein content. However, after flowering foliar application was the more effective method. The application of N at flowering to increase the protein content of this semi-dwarf cultivar is not an attractive commercial practice. The price ratio of premium to Australian Standard White wheat in recent years (<1.071 ) is less than that needed (1.0954-1.3013) to justify splitting the N application to lift grain protein content above 11.4% at the expense of yield.

Abstract. Continuous cultivation and cereal cropping of southern Queensland soils previously supporting native vegetation have resulted in reduced soil nitrogen supply, and consequently decreased cereal grain yields and low grain protein. To enhance yields and protein concentrations of wheat, management practices involving N fertiliser application, with no-tillage and stubble retention, grain legumes, and legume leys were evaluated from 1987 to 1998 on a fertility-depleted Vertosol at Warra, southern Queensland. The objective of this study was to examine the effect of lucerne in a 2-year lucerne-wheat rotation for its nitrogen and disease-break benefits to subsequent grain yield and protein content of wheat as compared with continuous wheat cropping.Dry matter production and nitrogen yields of lucerne were closely correlated with the total rainfall for October-September as well as March-September rainfall. Each 100 mm of total rainfall resulted in 0.97 t/ha of dry matter and 26 kg/ha of nitrogen yield. For the March-September rainfall, the corresponding values were 1.26 t/ha of dry matter and 36 kg/ha of nitrogen yield. The latter values were 10% lower than those produced by annual medics during a similar period. Compared with wheat-wheat cropping, significant increases in total soil nitrogen were observed only in 1990, 1992 and 1994 but increases in soil mineralisable nitrogen were observed in most years following lucerne. Similarly, pre-plant nitrate nitrogen in the soil profile following lucerne was higher by 74 kg/ha (9-167 kg N/ha) than that of wheat-wheat without N fertiliser in all years except 1996. Consequently, higher wheat grain protein (7 out of 9 seasons) and grain yield (4 out of 9 seasons) were produced compared with continuous wheat. There was significant depression in grain yield in 2 (1993 and 1995) out of 9 seasons attributed to soil moisture depletion and/or low growing season rainfall. Consequently, the overall responses in yield were lower than those of 50 kg/ha of fertiliser nitrogen applied to wheat-wheat crops, 2-year medic-wheat or chickpea-wheat rotation, although grain protein concentrations were higher following lucerne.The incidence and severity of the soilborne disease, common root rot of wheat caused by Bipolaris sorokiniana, was generally higher in lucerne-wheat than in continuous wheat with no nitrogen fertiliser applications, since its severity was significantly correlated with plant available water at sowing. No significant incidence of crown rot or root lesion nematode was observed. Thus, productivity, which was mainly due to nitrogen accretion in this experiment, can be maintained where short duration lucerne leys are grown in rotations with wheat.