Abstract. The effect of 2 tillage practices (zero v. conventional), fertiliser application (nitrogen, phosphorus and zinc), and pulse-cereal rotation on changes in soil mineral nitrogen, plant-available water in the soil, grain yield and protein, and key soil fertility parameters (total nitrogen, organic carbon) in the Central Highlands of Queensland were examined between 1991 and 1998. Four pasture treatments (perennial legume, perennial grass, annual legume and legume-grass mixes) were included in January 1995, following previously unsuccessful attempts to grow lucerne and annual medics. The experiment was conducted as an opportunity cropping system on an open downs soil at Gindie that is representative of a large proportion (70%) of soils in the Central Highlands. Tillage practice did not affect the amount of mineral nitrate or the plant-available water content of the soil at planting, except in 1991 and 1998 when plant-available water content was higher under conventional tillage than zero tillage. However, zero tillage improved grain yield in 2 of 4 years (wheat in 1992; sorghum in 1996), increased uptake of nitrogen in every crop and produced greater grain protein levels in both wheat crops grown than conventional tillage. There were grain responses to nitrogen + phosphorus fertilisers (wheat in 1991 and sorghum in 1997). Grain protein was increased with applications of nitrogen regardless of whether phosphorus was added in 3 of the 4 crops planted. Sowing a pulse did not significantly increase grain yields in the following crop although it did increase soil mineral nitrogen at planting. Soil nitrate remained low in control (P 0 N 0 ) plots (<39 kg N/ha) when crops were planted each year but increased significantly (average 84 kg N/ha) following a long fallow of 3.5 years resulting from drought. Plant-available water content of the soil at sowing was lower where chickpeas had been grown the previous season than with wheat.

An experiment, established on a cracking clay (Vertisol) at Emerald, central Queensland, studied the dry matter (DM) production, nitrogen (N) fixation, and water use of several potential ley-legume species over 4 seasons (1994–1997). Four ley legumes (siratro, Macroptilium atropurpureum cv. Siratro; lucerne, Medicago sativa cv. Trifecta; lablab, Lablab purpureus cv. Highworth; and desmanthus, Desmanthus virgatus cv. Marc) were compared with a pulse (mungbean, Vigna radiata cv. Satin), and grain sorghum (Sorghum bicolor) was included as a non-legume control. Overall, the annual legumes lablab (17.5 t/ha) and mungbean (13.4 t/ha) and the perennial siratro (16.2 t/ha) accumulated more DM than the perennials lucerne (9.6 t/ha) and desmanthus (7.1 t/ha). Lucerne produced little DM in its first year, but in later years had similar production to siratro and lablab. Desmanthus produced >4 t/ha of DM in the first year but barely survived during later seasons. Annual legumes grew faster and exhausted soil water more rapidly than the perennials. The perennials were able to extract more water from the soil than the annual legumes and sorghum, but were inefficient at converting small to moderate rainfall events (25–50 mm) into DM production. During the fallow following the growth of lablab and mungbean, nitrate-N in soil increased and was always greater at the time of re-sowing than for the perennial legumes and sorghum. Initially, the 2 annual legumes derived a high proportion (50% to >70%) of their above-ground N from fixation (%Ndfa) but this declined as the experiment progressed to low values (<13%) in the third and fourth years, reflecting increased supply of nitrate from the soil. In contrast, %Ndfa peaked at 72% for siratro and >90% for lucerne, and remained high (25–50%) throughout the experiment. N fixation rates were strongly negatively correlated with soil nitrate. Over the 4 years, siratro fixed 161 kg N/ha, lucerne 120, lablab 119, mungbean 78, and desmanthus 19 based on above-ground biomass. Mungbean had a net negative N balance (–80 kg N/ha) due to N exported in grain.

Incorporating legumes into the cropping system has been shown to significantly improve the nitrogen nutrition of cereal crops in Central Queensland. However, little is known about the effect of these legumes on the chemical and physical properties of soil. We examined changes in soil chemical (total nitrogen, organic carbon and pH) and physical (bulk density, cone penetrometer resistance and saturated hydraulic conductivity) properties following either continuous cropping (sorghum or mungbean) or pasture legumes (siratro, lucerne, lablab and desmanthus) over 4 years. Soil carbon was also fractionated using a KMnO4 oxidation procedure which classifies the soil carbon into either labile or non-labile pools. All pasture legumes except desmanthus increased soil total nitrogen in the topsoil (0–10 cm) after only 2 years compared with sorghum. Total nitrogen in the soil did not significantly change under mungbean. Soil organic carbon progressively increased under siratro, desmanthus and sorghum but remained unchanged under the other legumes. Before the experiment, the percentage of total soil carbon classified as labile (oxidised by 333 mmol KMnO4/L) ranged from 14 to 17%. The amount of labile carbon increased by 17% after 3 years of siratro, remained unchanged under desmanthus and sorghum, and decreased under the annual legumes and lucerne. Non-labile carbon remained either unchanged or increased under all legumes, whereas it tended to decrease after 3 consecutive sorghum crops. Soil pH was generally highest under sorghum and lowest under lablab. Soil after sorghum had higher bulk density and penetrometer resistance compared with the effect of legumes but these differences were comparatively small. Saturated hydraulic conductivity of the soil was much higher on the soil surface than at 10 cm. On the surface, soil hydraulic conductivity (saturated) values were generally lower following siratro and higher after sorghum than the other species. At 10 cm depth, soil hydraulic conductivity (saturated) was generally lower in sorghum and, to a lesser extent, in mungbean plots reflecting the significantly lower density of macropores under these crops. It was concluded that although all legumes generally enhanced the chemical and physical properties of the cracking clay, perennial legumes such as siratro would have a greater beneficial effect in the longer term than annual legumes.

Summary. The growth and ability of 12 summer-growing annual and perennial legumes to fix nitrogen and the response of a subsequent wheat crop was examined in a field trial on a deep cracking clay soil in the Central Highlands of Queensland. Twelve legumes [Lablab purpureus cv. Highworth, Vigna radiata cv. Satin, Macroptilium atropurpureum cv. Siratro, Medicago sativa cv. Trifecta, Vigna trilobata (CPI 13671), Macroptilium bracteatum (CPI 27404), Glycine latifolia (CQ 3368), Desmanthus virgatus cv. Marc, Desmanthus virgatus cv. Bayamo, Stylosanthes sp. aff scabra (104710), Clitoria ternatea cv. Milgarra, Cajanus cajan cv. Quest)] and grain sorghum (Sorghum bicolor cv. Tulloch) as a non-legume control were established in November 1994 and their growth monitored until March 1995. The legumes averaged greater than 5 t/ha dry matter production and 77 kg N/ha (above-ground only). Dry matter production ranged from less than 2 t/ha for G. latifolia and M. sativa to greater than 9 t/ha for D. virgatus cv. Bayamo and C. cajan. Annual legumes initially had much higher relative growth rates than the perennial legumes but they rapidily exhausted all the plant available water content of the soil thus allowing the well-established perennials to eventually match this production. The proportion of plant nitrogen (above ground) derived from N2 fixation was generally low, reflecting high soil NO3, but varied widely between species ranging from less than 20% for D. virgatus cv. Marc and G. latifolia to over 45% for C. ternatea, S. scabra and V. trilobata. The quantity of nitrogen derived from fixation was correlated with above-ground dry matter and nitrogen content. There was a significant (P<0.05) growth response by wheat following legumes compared with that following sorghum in the increasing order V. radiata = M. atropurpureum = L. purpureus > C. cajan = M. sativa = V. trilobata = M. bracteatum = G. latifolia > S. scabra = D. virgatus = C. ternatea. Previous legume growth had no significant (P>0.05) effect on yield or nitrogen concentration in a second ‘plant-back’ crop (sorghum). It was concluded that a wide range of pasture-ley legumes have the potential to improve cereal crop production in this region.

Poor yields and low grain protein in cereal crops resulting from declining soil fertility, especially nitrogen (N), are major threats to the grains industry in central Queensland. The effect of 4 different pasture-ley legumes [siratro (Macroptilium atropurpureum cv. Siratro), lucerne (Medicago sativa cv. Trifecta), lablab (Lablab purpureus cv. Highworth), and desmanthus (Desmanthus virgatus cv. Marc)] on grain yield and quality of sorghum crops was compared with that of a pulse (mungbean; Vigna radiata cv. Satin) or continuous cropping with grain sorghum (Sorghum bicolor). Legume leys consistently resulted in large increases in grain yield (188–272%), N uptake by sorghum (145–345%), and grain protein (0.21–7.0% increase in grain protein) in sorghum test-crops compared with continuous sorghum crops to which no fertiliser N had been added. The positive effect of legumes persisted up to 3 sorghum test-crops after only 1 year of legumes, although by the third year the effect was comparatively small. Mungbean and lablab generally produced the largest benefit in sorghum test-crops in the first year after legumes, whereas desmanthus and lucerne produced the least benefit. Adding fertiliser N (up to 75 kg N/ha) significantly improved grain yields and N uptake of sorghum test-crops in 3 of 4 years. However, responses to fertilisers were less than those resulting from legumes, which was ascribed to increased availability of legume N to sorghum. Legumes progressively increased soil nitrate in all subsequent sorghum test-crops (compared with continuous sorghum crops), rising from 6.8–18.9 kg NO3-N/ha after 1 year of legumes to 24.2–59.6 kg NO3-N/ha after 3 years of legumes. There was little difference between the legumes in their ability to increase soil nitrate, with the exception of desmanthus, which consistently resulted in the lowest amount of soil nitrate for subsequent test-crops and lowest uptake of N by these crops. Plant-available water content (PAWC) at planting of the sorghum test-crop was only significantly (P<0.05) affected by previous species in 1997, when it was lowest in plots previously sown to siratro and lucerne and highest in sorghum and mungbean plots. In both 1996 and 1997, plots sown to sorghum had significantly higher PAWC at anthesis and grain maturity when previous plots were sorghum rather than legumes.