In the northern cereal belt of Australia, farmers are reluctant to apply nitrogen (N) fertilizers because of a perception that if N is added to the soil and no crop is subsequently planted due to lack of rain, the N is 'lost'. An experiment was conducted on a cracking clay soil over three seasons to compare the response of grain sorghum to N applied to the current crop v. N applied the previous season which was then either planted or left fallow (to simulate a missed planting opportunity). Recovery of 15~-labelled fertilizer by the crop and that remaining in the soil were simultaneously determined in microplots. The effect of tillage practice [zero (ZT) and conventional (CT)] was also examined. Sorghum grain yield responded to fresh applications of N in 1993 and 1993194 but not 1992, reflecting the importance of timing of rainfall rather than the total amount received within the season. Applications of N to the current crop always improved yield more than equivalent amounts of N applied to the previous crop. Grain yields of plots that were previously fallowed (fallow-sorghum rotation) were higher than the combined yields of sorghum-sorghum rotations, although fallowing was an inefficient means of accumulating both water and mineral N. Recovery of applied 1 5 ~ by sorghum varied from 48% in 1992 to 36% in 1993 but was not related to the overall N responsiveness of the crop. Sorghum recovered a similar proportion of 1 5 ~ from plots which had been fertilized and then fallowed the previous year compared to fresh applications to the current crop, despite the fallow plots having less 1 5 ~ in them due to losses from the previous season. Losses of 1 5 ~ from the soil/plant system varied markedly with year and appeared to be related to the pattern of rainfall occurring and its possible effect on denitrification. Tillage practice did not affect grain yields or PAWC, had minimal effect on the amount of mineral N present, and little influence on the fertilizer N requirements of sorghum per se. This study suggests that there is only a small residual value to subsequent sorghum crops of fertilizer N if added initially to a successful crop. However, if N is applied pre-plant and the crop is not planted, for example due to lack of planting rain, a large proportion of this N can remain available to the following crop depending on the nature of the subsequent rainfall.

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.

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.

The feasibility of using legume leys to redress declining levels of soil nitrogen (N) fertility on the heavy clay Vertisols of the northern Australian grain belt depends partly on the ability of plant residues to supply N directly to subsequent cereal crops. An alternative is the use of fertiliser N in continuous cereal cropping. Two experiments were conducted (one in the field, the other under polyhouse conditions) to compare the uptake of N from either plant residues or ammonium sulfate fertiliser that had been labelled with 15N. In a field trial, 15N-labelled shoots of grain sorghum and Desmanthus virgatus and ammonium sulfate were applied to micro-plots and the flux of the added N between different soil pools and a wheat crop was followed over 219 days. Only small amounts of residue-derived N (<5%) were recovered in the mineral N of the soil at a depth of 0-10 cm, whereas over 88% of the fertiliser N was present as mineral N soon after adding the fertiliser. Soil microbial biomass-N was increased following addition of residues. Recovery of added 15N in the wheat crop was much higher from the fertiliser (35%) than from the 2 residue sources (<5%). The pot trial compared a wider range of 15N-labelled residues (shoot and root residues of Desmanthus virgatus, Lablab purpureus, and sorghum) with several rates of ammonium sulfate, applied in the presence and absence of non-labelled grain sorghum residues, over 4 cropping cycles. Dry matter production and N uptake were increased by application of fertiliser N, although the response was reduced in the presence of non-labelled sorghum residues; responses to residue N were much smaller than those to fertiliser N. In the first crop following residue application <7% of residue N was recovered, increasing to 12-23% over the 4 crops. Recovery of fertiliser N by the crops increased with the rate of application, and also depended on whether it was applied together with residues. A feature of the results, in both the field and pot experiments, was the large proportion of applied 15N that could not be accounted for in either the soil or the crops, and these losses have been attributed to denitrification.