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Depleted soil nitrogen supplies in long-term continuously cultivated soil for cereal grain cropping have resulted in reduced cereal yields, low grain proteins and hence low economic returns. This has necessitated the development of alternative management practices to sustain crop yields, as well as to restore and maintain soil fertility. In the present study we examined the comparative performance of several management options over a 12-year period, including: a 4-year rotation of grass + legume pasture followed by wheat (GL–wheat); 2-year rotations of lucerne–wheat, annual medic–wheat and chickpea–wheat; and continuous conventional tillage (CT) or no-tillage (NT), without or with fertiliser N application (0, 25 and 75 kg N ha–1 for each crop). Average wheat grain yields were highest in the chickpea–wheat rotation, followed by the NT wheat with 75 kg N ha–1; the lowest grain yields were in the CT or NT wheat treatment without fertiliser N application. Crop water use and gross margin were strongly correlated. However, there was an increasing potential for the deep leaching of nitrate-N at 75 kg N ha–1 application, as well as from the GL pasture initiated in 1987, but not from that initiated in 1986, emphasising the effect of variability in growing seasons. Soil organic C stocks increased under the 4-year GL pasture in the 0–0.1 m depth only, then decreased steadily following the cropping phase. The rotation of 4-year GL pasture followed by wheat cropping for 4–6 years may maintain initial soil organic C stock, but a shorter cropping phase is required to increase soil organic C and N stocks and soil fertility in the long term. Partial economic analysis of the treatments suggested that restoring or maintaining soil N fertility, either through legume-based pastures, grain legume and/or N fertiliser, provides long-term positive economic return.

Wheat yields are highly variable in northern Australia because rainfall is variable. Economic benefits from applying nitrogen fertilisers are uncertain because yield responses depend on a good supply of soil moisture during the growing of the crop to provide a high potential yield. While an experiment in 1996 indicated that up to 90 kg N/ha of N fertiliser is profitable, it was not known whether this response is reliable or typical. A crop model and historical climate records (1960–1993) were used in this study to produce a long-term record of yield and grain protein responses to N fertilisers. Responses in 1996 are shown to be atypical due to the favourable conditions in that year. Under typical growing conditions, it is not economic to use N fertiliser. The simulations quantified the relationship between responses to fertiliser and the amount of soil moisture available at sowing. Applications of N fertilisers will be most profitable if used when measurements indicate that the plant-available soil moisture content before sowing is above average.

The supply of adequate amount of nitrogen is essential to produce sustainable cereal grain yields and good grain quality while maintaining or enhancing soils’ natural resource base. To achieve these objectives, a long-term field experiment was established in 1986 to include forage legumes and grain legume in rotation with wheat, and fertilizer N application (0, 25 and 75 kg N ha-1 y-1) to no-till compared with conventional till on a fertility-depleted vertisol.

Rainfed grain production, based on winter cereals, is marginal in south-west Queensland, Australia, because of low and variable rainfall and high evapotranspiration. Also, grain yield and grain quality have decreased as soil fertility, particularly soil nitrogen supply, has declined on older cropping lands. An option for improving soil N supply is to include legume-based pastures in rotation with winter cereals. The objective of this study was to determine the effects of short-term (18 months) legume pastures (annual medics and lucerne + annual medics), and longer term (3 years) mixed grass (Bambatsi panic) and legume (lucerne + annual medics) pasture phases on sheep production and on soil water and N supply and production of subsequent wheat crops on a grey Vertosol soil. Two separate phases of annual medics and lucerne + annual medics pastures produced mean total aboveground dry matter yield of 7.10 t/ha of annual medics and 5.80 t/ha of lucerne + annual medics over the 18-month periods. For two phases of the grass + legume pastures, mean total aboveground dry matter yield was 3.95 t/ha for grass and 8.19 t/ha for legume over 3 years. Over an 18-month period, sheep bodyweight gains and fleece weights were similar for the annual medics, lucerne + annual medics and grass + legume pastures and were approximately five times greater than those from native pasture as a result of the greater stocking rate possible on the sown pastures. Greater drying of the soil profile occurred following lucerne + annual medics and grass + legume pasture phases than continuous wheat, resulting in lower soil water content at sowing of wheat crops following these pasture phases on several occasions. Mean soil nitrate-N benefits before wheat sowing in the first year following termination of the 18-month annual medics, lucerne + annual medics, and the 3-year grass + legume pasture phases were 45, 44 and 42 kg N/ha, respectively. Grain N yields and gross margins of the first wheat crops following the 18-month annual medics, lucerne + annual medics, and the 3-year grass + legume pasture phases were similar in value to continuous wheat with ~60, 80, and 40–60 kg N/ha fertiliser applied at sowing, respectively. Improvements in grain N yield and gross margin were still evident in the fifth wheat crop following annual medics and lucerne + annual medics pastures and in the third wheat crop following grass + legume pasture, compared with continuous wheat without N fertiliser addition. Total gross margins from 1996 to 2005 were 1.6–2.5 times greater for the pasture–crop rotations than continuous wheat where no N fertiliser was applied to wheat. However, gross margins were greater in continuous wheat than in pasture–crop rotations where N fertiliser was applied to target prime hard grade grain protein in wheat. The 3-year grass + legume pasture phase showed potential to improve surface soil structure and water infiltration and to reduce decline in soil organic carbon concentration at 0–0.1 m depth, compared with continuous wheat cropping and shorter-term legume pasture phases.

Winter cereal cropping is marginal in south-west Queensland because of low and variable rainfall and declining soil fertility. Increasing the soil water storage and the efficiency of water and nitrogen (N) use is essential for sustainable cereal production. The effect of zero tillage and N fertiliser application on these factors was evaluated in wheat and barley from 1996 to 2001 on a grey Vertosol. Annual rainfall was above average in 1996, 1997, 1998 and 1999 and below average in 2000 and 2001. Due to drought, no crop was grown in the 2000 winter cropping season. Zero tillage improved fallow soil water storage by a mean value of 20 mm over 4 years, compared with conventional tillage. However, mean grain yield and gross margin of wheat were similar under conventional and zero tillage. Wheat grain yield and/or grain protein increased with N fertiliser application in all years, resulting in an increase in mean gross margin over 5 years from $86/ha, with no N fertiliser applied, to $250/ha, with N applied to target ≥13% grain protein. A similar increase in gross margin occurred in barley where N fertiliser was applied to target malting grade. The highest N fertiliser application rate in wheat resulted in a residual benefit to soil N supply for the following crop. This study has shown that profitable responses to N fertiliser addition in wheat and barley can be obtained on long-term cultivated Vertosols in south-west Queensland when soil water reserves at sowing are at least 60% of plant available water capacity, or rainfall during the growing season is above average. An integrative benchmark for improved N fertiliser management appears to be the gross margin/water use of ~$1/ha.mm. Greater fallow soil water storage or crop water use efficiency under zero tillage has the potential to improve winter cereal production in drier growing seasons than experienced during the period of this study.

Reduced supplies of nitrogen (N) in many soils of southern Queensland that were cropped exhaustively with cereals over many decades have been the focus of much research to avoid declines in profitability and sustainability of farming systems. A 45-month period of mixed grass (purple pigeon grass, Setaria incrassata Stapf; Rhodes grass, Chloris gayana Kunth.) and legume (lucerne, Medicago sativa L.; annual medics, M. scutellata L. Mill. and M. truncatula Gaertn.) pasture was one of several options that were compared at a fertility-depleted Vertosol at Warra, southern Queensland, to improve grain yields or increase grain protein concentration of subsequent wheat crops. Objectives of the study were to measure the productivity of a mixed grass and legume pasture grown over 45 months (cut and removed over 36 months) and its effects on yield and protein concentrations of the following wheat crops. Pasture production (DM t/ha) and aboveground plant N yield (kg/ha) for grass, legume (including a small amount of weeds) and total components of pasture responded linearly to total rainfall over the duration of each of 3 pastures sown in 1986, 1987 and 1988. Averaged over the 3 pastures, each 100 mm of rainfall resulted in 0.52 t/ha of grass, 0.44 t/ha of legume and 0.97 t/ha of total pasture DM, there being little variation between the 3 pastures. Aboveground plant N yield of the 3 pastures ranged from 17.2 to 20.5 kg/ha per 100 mm rainfall. Aboveground legume N in response to total rainfall was similar (10.6–13.2 kg/ha per 100 mm rainfall) across the 3 pastures in spite of very different populations of legumes and grasses at establishment. Aboveground grass N yield was 5.2–7.0 kg/ha per 100 mm rainfall. In most wheat crops following pasture, wheat yields were similar to that of unfertilised wheat except in 1990 and 1994, when grain yields were significantly higher but similar to that for continuous wheat fertilised with 75 kg N/ha. In contrast, grain protein concentrations of most wheat crops following pasture responded positively, being substantially higher than unfertilised wheat but similar to that of wheat fertilised with 75 kg N/ha. Grain protein averaged over all years of assay was increased by 25–40% compared with that of unfertilised wheat. Stored water supplies after pasture were

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.

Continuous cereal production in the summer-dominant rainfall region of north-eastern Australia has depleted native soil nitrogen fertility to a level where corrective strategies are required to sustain wheat grain yields and high protein levels for domestic and export markets. Annual medic pastures, along with other strategies, were evaluated from 1986 to 1998 on a Vertisol at Warra, southern Queensland, for their benefit to subsequent grain yield and protein content of wheat.Annual medic production and nitrogen yields were closely correlated with the growing season rainfall (March–September). Each 100 mm of growing season rainfall resulted in 1.39 t/ha of dry matter and 40 kg/ha of N yield. There were significant increases in soil mineralisable nitrogen following annual medic leys compared with continuous wheat in most years, but increases in total soil nitrogen were only observed in 1990, 1991 and 1992. However, pre-plant nitrate-nitrogen following an annual medic ley was always higher than that of continuous wheat without nitrogen fertiliser. This resulted in significant increases in wheat yield (6 of 9 seasons) and grain protein (in all seasons) compared with continuous wheat. The overall responses in yield and protein were similar to those of 50� kg/ha of fertiliser nitrogen applied to continuous wheat crops. A nitrogen harvest budget for the annual medic–wheat rotation over 11 years showed that it contributed 131 kg N/ha more than continuous wheat. Thus, using the seasonal conditions experienced from 1987 to 1998 as a reasonable representation of the rainfall range in the region, sustainable productivity can be maintained where annual medics are grown in short rotations with wheat.

Summary. In this study, the benefits of chickpea–wheat rotation compared with continuous wheat cropping (wheat–wheat rotation) were evaluated for their effects on soil nitrate nitrogen, wheat grain yields and grain protein concentrations, and water-use efficiency at Warra, southern Queensland from 1988 to 1996. Benefits in terms of wheat grain yields varied, from 17% in 1993 to 61% in 1990, with a mean increase in grain yield of 40% (825 kg/ha). Wheat grain protein concentration increased from 9.4% in a wheat–wheat rotation to 10.7% in a chickpea–wheat rotation, almost a 14% increase in grain protein. There was a mean increase in soil nitrate nitrogen of 35 kg N/ha.1.2 m after 6 months of fallow following chickpea (85 kg N/ha) compared with continuous wheat cropping (50 kg N/ha). This was reflected in additional nitrogen in the wheat grain (20 kg N/ha) and above-ground plant biomass (25 kg N/ha) following chickpea. Water-use efficiency by wheat increased from a mean value of 9.2 kg grain/ha. mm in a wheat–wheat rotation to 11.7 kg grain/ha.mm in a chickpea–wheat rotation. The water-use efficiency values were closely correlated with presowing nitrate nitrogen, and showed no marked distinction between the 2 cropping sequences. Although presowing available water in soil in May was similar in both the chickpea–wheat rotation and the wheat–wheat rotation in all years except 1996, wheat in the former used about 20 mm additional water and enhanced water-use efficiency. Thus, by improving soil fertility through restorative practices such as incorporating chickpea in rotation, water-use efficiency can be enhanced and consequently water runoff losses reduced. Furthermore, beneficial effects of chickpea in rotation with cereals could be enhanced by early to mid sowing (May–mid June) of chickpea, accompanied by zero tillage practice. Wheat of ‘Prime Hard’ grade protein (≥13%) could be obtained in chickpea–wheat rotation by supplementary application of fertiliser N to wheat. In this study, incidence of crown rot of wheat caused by Fusarium graminearum was negligible, and incidence and severity of common root rot of wheat caused by Bipolaris sorokiniana were essentially similar in both cropping sequences and inversely related to the available water in soil at sowing. No other soil-borne disease was observed. Therefore, beneficial effects of chickpea on wheat yields and grain protein were primarily due to additional nitrate nitrogen following the legume crop and consequently better water-use efficiency.

Summary. Continuous cereal cropping in southern Queensland and northern New South Wales has depleted native soil nitrogen fertility to a level where corrective strategies are required to sustain grain yields and high protein content. The objective of this study was to examine the performance of chickpea in chickpea–wheat rotations in terms of yields, water use and N2 fixation. The effects of sowing time and tillage practice have been studied. Chickpea grain yields varied from 356 kg/ha in 1995 to 2361 kg/ha in 1988; these were significantly correlated with the total rainfall received during the preceding fallow period and crop growth. Almost 48% of total plant production and 30% of total plant nitrogen were below-ground as root biomass. Mean values of water-use efficiency for grain, above-ground dry matter, and total dry matter were 5.9, 14.2 and 29.2 kg/ha.mm, respectively. The water-use efficiency for grain was positively correlated with the total rainfall for the preceding fallow and crop growth period although cultural practices modified water-use efficiency. The potential N2 fixation was estimated to be 0.6 kg nitrogen/ha.mm from 1992 total dry matter nitrogen yields assuming all of the nitrogen contained in chickpea was derived from the atmosphere. Sowing time had a much larger effect on grain yield and N2 fixation by chickpea than tillage practice (conventional tillage and zero tillage) although zero tillage generally increased grain yields. The late May–early June sowing time was found to be the best for chickpea grain yield and N2 fixation since it optimised solar energy use and water use, and minimised frost damage. Nitrogen fixation by chickpea was low, less than 40% nitrogen was derived from atmosphere, representing less than 20 kg nitrogen/ha.year. The potential for N2 fixation was not attained during this period due to below-average rainfall and high soil NO3-N accumulation because of poor utilisation by the preceding wheat crop. Increased soil NO3-N due to residual from fertiliser N applied to the preceding wheat crop further reduced N2 fixation. A simple soil nitrogen balance indicated that at least 60% of crop nitrogen must be obtained from N2 fixation to avoid continued soil nitrogen loss. This did not occur in most years. The generally negative soil nitrogen balance needs to be reversed if chickpea is to be useful in sustainable cropping systems although it is an attractive cash crop. Sowing time and zero tillage practice, possibly combined with more appropriate cultivars, to enhance chickpea biomass, along with low initial soil NO3-N levels, would provide maximum N2 fixation.

Summary. In many subtropical environments, cereal crops develop and mature largely on residual water in the soil. This research involves evaluation of the impact of plant available nitrogen (N) and water in soil at sowing on grain protein in wheat and barley in such a subtropical environment. Estimates of grain protein concentration of wheat (cv. Hartog) were made using plant available water and available N (soil nitrate-N and fertiliser N, where applied) at sowing using data obtained from an experiment conducted at Warra, Queensland, from 1987 to 1995. Treatments included: grass + legume leys of 4-year duration followed by continuous wheat with 0 or 50 kg N/ha.year applied as urea at sowing; 2-year rotation of lucerne and wheat; 2-year rotation of annual medics and wheat; 2-year rotation of chickpea and wheat, no-tillage wheat; and conventional tillage wheat. Fertiliser N as urea was applied to both no-tillage wheat and conventional tillage wheat at 0, 25 and 75 kg N/ha.year. The conventional tillage wheat also received N at 12.5 and 50 kg N/ha.year. Estimates of wheat grain yield required both rainfall during the fallow period or plant available water in the soil profile at sowing and rainfall from sowing to anthesis and, therefore, it could not be predicted precisely at sowing. Increasing plant available water (mm) in soil at sowing linearly reduced grain protein. In comparison, available N at sowing increased grain protein curvilinearly from 10.0% at 50 kg N/ha to 14.5% at 200 kg N/ha (0–120 cm depth). Variation in grain protein concentration was best accounted for by the available water : available N ratio at 0–90, 0–120 or 0–150 cm depths. The protein concentrations of wheat (cv. Hartog) grown in 1996 at Warra and Nindigully, and wheat (cv. Cunningham) grown from 1991 to 1995 at Billa Billa, and barley (cv. Tallon) grown in 1996 at Nindigully and Formartin, Queensland, were successfully predicted using the relationship between the available water : available N ratio and wheat grain protein concentration developed using data from Warra during 1987–95. Thus, available water should be matched by N supply at sowing to ensure the production of Prime Hard grade wheat and malting grade barley in the subtropical environment. As a ‘rule of thumb’, for 0–120 cm depth of soil sampling, each millilitre of available water matched with each kilogram of N per hectare of available N, at sowing, would produce about 13% protein wheat in this semi-arid region. It requires only 0.5 kg of N/ha for each millilitre of available water in 0–120 cm depth of soil to produce malting grade barley of about 10.5% protein concentration. Available water in soil at sowing can be approximated with rainfall during the fallow period, with rainfall (mm) : available N (kg/ha for 0–120 cm depth) ratios of 3.7 and 7.4 for respective 13 and 10.5% grain protein concentrations for both wheat and barley.

Mineral nitrogen release following legume-based cropping systems for restoring the fertility of a Vertisol and the yield response and N uptake of subsequent wheat crops was studied. Legume phases of pastures, including a 4 year grass+legume ley, and lucerne and medic leys (~1 year) were terminated in October 1988 or 1989 and rotated with wheat. Chickpea-wheat rotations matched those of lucerne and medic leys. Mineral N accumulations during a subsequent fallow period were determined by core sampling to 1.5 m in October, February and May. Grain yield and N uptake of wheat enabled comparisons of the fertility restorative effects of the various systems relative to continuous wheat cropping. Averaged for two fallow periods, increases in mineral N down to 1.2 m depth were 93, 91, 68, and 37 kg/ha following grass+legume, lucerne and medic leys, and chickpea, respectively, compared with the continuous wheat treatment. Wheat yields were generally lower in 1989 (1.85–2.88 t/ha) than in 1990 (2.08–3.59 t/ha) following all leys and crops due to seasonal conditions. There was a grain yield increase of 0.11 and 0.52 t/ha in 1989 and 1.23 and 1.26 t/ha in 1990 following lucerne and medic leys, respectively and 0.85 t/ha in 1990 following a 4 year grass+legume ley. Following chickpea there was a yield increase of 0.81 and 1.36 t/ha in 1989 and 1990 respectively. Nitrogen uptake by wheat was increased by 40 and 49 kg/ha in 1989 and 48 and 58 kg/ha in 1990 following lucerne and medic leys respectively and 63 kg/ha in 1990 following a 4 year grass+legume ley. Following chickpea N uptake by wheat was increased by 27 and 32 kg/ha in 1989 and 1990 respectively. Grain protein concentration of wheat was substantially higher following all pasture leys (11.7–15.8%) than following wheat (8.0–9.4%) or chickpea (9.4–10.1%). Therefore, there was substantial evidence of the effectiveness of pasture leys in soil fertility restoration, as reflected in mineral N, yield response and N uptake by subsequent wheat crops.

Unreliable rainfall during the crop growing season leads to a variable use of applied fertiliser nitrogen (N) by the crop, which may leave substantial fertiliser N residue in the soil. Residual effects of fertiliser N (0-150 kg/ha) applied to a succession of wheat crops over the period 1987-94 were studied in terms of increased crop returns ($A/ha) from fertiliser application and increased soil mineral N for the subsequent crop. In spite of the unreliability of wheat responses to applied N in this region, increases in financial returns over this sequence of crops suggest that a strategy of routine N application to wheat was highly profitable on this fertility-depleted soil. When increases in returns from 1 fertiliser application were summed over successive crops, financial returns generally increased with increasing rate of N applied up to the highest N rate (100 or 150 kg/ha). When N was applied to each successive crop, financial returns were similarly increased but applications >50 kg/ha were less profitable than rates 2t/ha) in 1989 and 1990. Consequently, low levels (46-63 kg/ha) of soil mineral N were apparently carried over for crops in 1989 and 1990 even where 75 kg N/ha was applied to the preceding crop. Subsequent recovery of financial losses, incurred in years of water deficit, made the routine application of 75 kg N/ha to fertility-depleted soils of this region profitable.

Effects on soil nitrogen accretion and potentially mineralisable nitrogen were studied as part of a long-term field experiment established in 1986 to test alternative legume-based systems for restoring fertility in a Vertisol. Organic C accretion was also measured to ascertain the changes in organic matter content. The systems, which were studied only during 1989 and 1990, were a grass+legume ley (purple pigeon grass, Rhodes grass, lucerne, annual medics) of 4 years duration followed by wheat; a 2-year rotation of wheat (lucerne undersown) and lucerne; a 2-year rotation of wheat (medic undersown) and medic; a 2-year rotation of chickpea and wheat; and continuous wheat as control. Soil total N and organic C significantly increased in the 0–10 cm soil layer only under the grass+legume ley. There was no significant change in the soil C/N ratio. Plant residues contained from 52 to 104 kg N/ha in 1990 at the end of the legume phase, with high values for root N in the grass+legume ley. A comparison of N accretion versus fixation at the end of the legume-based systems in 1990 showed that net accumulation of N exceeded fixation in soil under lucerne and grass+legume leys; in the latter, net accumulation of 779 kg N/ha over 3.75 years was measured compared with 384 kg N/ha for N2 fixation. Part of the accumulation of N may have been due to uptake of NH4-N from the deep subsoil. Although values for soil mineral N (0–120 cm) were low at the end of all the legume-based systems, a deep subsoil (120–300 cm) accumulation of NH4-N was found in all treatments. The nitrogen mineralisation potentials (No) for 0–10 cm depth samples taken at the end of the legume phase in 1989 were higher in all the legume-based systems (105–182 mg N/kg) than the wheat control (57 mg N/kg). The rapid biological tests of N availability, both waterlogged and aerobic incubation, were more sensitive to treatment differences than No, in the surface and subsoil (range 12–78 mg N/kg for 0–10 cm soil for the waterlogged procedure). The rapid chemical tests, hot KCl extraction and the autoclave index, showed small treatment effects and did not appear to be useful availability indices. The pasture management (graced v. mown and removed) had no significant effect on total N, organic C and N availability indices in this alkaline Vertisol during the study period.

Cereal production in the summer-dominant rainfall region of Australia, especially the north-east, has relied heavily on natural soil fertility. Continued cereal production has so depleted the fertility of some soils that corrective strategies are required to restore the production of high protein wheat needed for domestic and export markets. Application of nitrogen (N) fertilisers, along with other strategies to improve soil N status, was evaluated between 1987 and 1994 on a Vertisol located in an area of unreliable winter rainfall. Responses of wheat grain yield and protein content to applied N (0-150 kg/ha) under zero tillage (ZT) and conventional tillage (CT) were determined each year, except 1991 when severe drought prevented wheat sowing. The ZT practices increased grain yields, particularly in 1988 and 1992-93 when antecedent soil water supplies were moderate (about 1 m wet soil in 1988 and 1992) or low (about 0.6 m wet soil in 1993), apparently due to increased antecedent soil water. Tillage practice had little effect on available nitrate-N (kg/ha) to 1.5 m, but the greater water supply in ZT soil usually benefited the wheat crop when N was applied. Applying N increased returns from 5 of the 7 crops because of grain yield and/or grain protein responses. Grain yield responses were inconsistent in the year of fertiliser application where no N fertiliser had been applied to preceding crops. Nevertheless, grain protein usually increased with increasing N application at sowing, except in 1994, when drought after sowing prevented secondary root development and fertiliser uptake. Where N was applied with each successive crop, the crops receiving small N applications (0, 12.5 or 25 kg/ha.crop) produced grain of a low protein concentration (11.5%, due to the low market value of low protein wheat, or because of lower grain yields. Routine N application to crops over the period 1987-94, which included the longest drought (1990-94) in the lifetime of most producers, caused similarly inconsistent grain yield increases but increased grain protein concentrations (>11.5%) in all except the first crop (1987). Increased frequency of high protein wheat and a high anticipated market value of the higher protein grain should encourage greater producer confidence with routine application of N throughout this region.

Applied nitrogen (N) recovered by fertilised wheat and by successive wheat crops in a 4-crop sequence (1987-90) was studied by applying 15N-depleted ammonium nitrate (0, 2.5, and 7.5 g/m2) to a Vertisol in the summer-dominant rainfall region of northern Australia. Recoveries of applied N by each of the 4 crops in order of cropping sequence were 60.3� 4.2, 4.4 � 2.3, 1 . 3 � 0.49, and 0- 8 � 0.56%, there being no effect of 2 tillage treatments, conventional tillage (CT) and no till (NT), on uptake of applied N. There was very low recovery of residual fertiliser N after the first wheat crop was harvested; usually

Nitrogen fixation was measured using two isotopic techniques over 2 years as part of a long-term field experiment established to test alternative management strategies for restoring fertility in a vertisol at Warra, Southern Queensland. Treatments containing legumes were: grass- legume ley (purple pigeon grass and Rhodes grass, lucerne and annual medics) for 4 years followed by 4 years of wheat; a 2-year rotation of lucerne and wheat; a 2-year rotation of medic and wheat; and a 2-year rotation of chickpea and wheat. For the enriched-15N procedure, the proportion of N derived from air (% Ndfa) for the grass-legume and lucerne and medic leys ranged from 67 to 97%, and averaged 85%, with little evidence for effects of season, pasture establishment, time or species. The % Ndfa for chickpea was significantly lower (62%). Values for the natural abundance 15N procedure were mostly lower and more variable than for the enriched method, ranging from 62 to 91% for the grass-legume, lucerne and medic leys, and averaged 76%. It was concluded that the enriched procedure provided more reliable estimates of N2 fixed by the legumes. N2 fixation measured by the enriched-15N dilution method in the grass-legume ley averaged 80 kg N ha-1 during 2 years. A similar amount of N was fixed by the lucerne ley during 1 year (83 kg N ha-1 but medic ley fixed less (56 kg N ha-1 The amount of N2 fixed by chickpea was 72 kg N ha-lyear-1. The dry matter yield of the legumes in leys, mainly lucerne, was closely associated with the amount of N2 fixed, with a value of 28 kg of N2 fixed for each tonne of dry matter produced.

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.

Our paper examines recent developments in clim- utilization, animal production and financial analysis are be- atology, systems analysis and decision support which are ing developed for each savanna community in northern relevant to the management of northern Australian savan- Australia. The key processes in these models are plant nas. The structure, function and use of these communities growth as a function of climate inputs and the effect of have been well described in previous reviews which show grazing on plant survival and production. We describe a the importance of pastoralism as the major economic general experimental methodology to apply existing models activity. to specific grassland/soil combinations. Annual variability of rainfall is high, resulting in un- Examples of the application of these models show that: certainty in management decisions. Systems analysis (1) periods of overgrazing can be identified when model models of pastoral enterprises are being constructed output is combined with regional animal number statistics; which predict the response of savannas to management and (2) management decisions such as burning can be im- alternatives against a background of annual climatic vari- proved when ENSO based forecasts are used. ation as well as expected long-term global climate The challenge of future savanna studies is to influence in- change. dividual managers to make better management decisions In northern Australia, El Ninio/Southern Oscillation based on reliable models of savanna processes. The uncer- events account for over half the major ecologically sig- tainty of future climate change suggests more flexible stra- nificant droughts. The seasonal persistence of the Southern tegies will be required for the evolution of sustainable and Oscillation phase allows forecasts to be made before the economic savanna use. onset of summer rains. The potential of such forecasts is