After water, nitrogen (N) is the principal driver of agriculture in Australia. Of the 6 million tonnes N required each year for the growth of pasture and crop plants, about 3 million tonnes is derived from legume nitrogen (N2) fixation, 1 million tonnes is supplied as fertiliser N with the remaining 2 million tonnes derived from mineralisation of plant residues and soil humus.

N requirements of wheat and the other winter cereals

Australian farmers grow about 18 million ha of winter cereals each year, with wheat accounting for 70% of the area. The N requirements of wheat have been well researched. Higher grain proteins need extra N. A 3 t/ha crop at 8% protein requires 65 kg N/ha. At 10% protein, it is 86 kg N/ha and at 12% protein it is 125 kg N/ha. As the grain proteins increase, there is a parallel reduction in the efficiency with which N is taken up from the soil and partitioned within the plant into the grain.

Option for meeting the N demand

For established wheat farming soils, there are only three potential sources of N to supplement the N mineralised from the organic matter (humus) bank:
• Legume pasture leys
• Pulses
• Fertiliser and manures
Pasture and pulse legumes fix N. They absorb N2 from the soil atmosphere into small nodules on their roots and the bacteria (rhizobia) in the nodules convert the atmospheric N2 into ammonia (NH3). The ammonia is then converted into organic compounds by the plant and used for growth. The N-rich residues and exudates from the legumes add to the N of the soil to be used by other non-leguminous crops, such as cereals. The amount of N2 fixed by legumes is strongly linked to productivity.

Optimising N2 fixation (and rotational benefits) through agronomic management

Within the constraints of the climate and season, good legume management to maximise productivity will benefit N2 fixation. Examples of legume management include optimising nutrient inputs (e.g. P), reducing acidity with lime, managing weeds, disease and insects. Optimising the basic agronomy is critical in terms of legume productivity and N2 fixation. This means maintaining a good cover of stubble on the soil surface in the pre-crop fallow, sowing on time and establishing the appropriate plant density.

A management option for cropping that has gained popularity in recent years is no-tillage. Data from the NSW Department of Primary Industries (DPI) long-term rotation experiments in northern NSW showed a positive effect of no-tillage on productivity and N2 fixation of chickpea (Felton et al. 1998; Marcellos et al. 1998; Herridge et al. 1998). The effect was a result of increased soil water and decreased soil nitrate accumulation during the summer (pre-crop) fallow. Water availability and use have an overriding influence on crop productivity in Australian agriculture. Farmers have no control over seasonal weather but they have some control over the efficiency with which water is infiltrated into and stored in the soil (fallowing efficiency), coupled with the efficiency with which the water is used by crops (water-use efficiency). In the rotation experiments, the no-tilled plots had an average of 35 mm additional soil water at sowing (Table 1).

Table 1. Effects of tillage on soil water and nitrate at sowing, chickpea growth and grain yield and N2 fixationA

A means of 21 site/years of experiments (unpublished data of W. Felton, H. Marcellos, D. Herridge, G. Schwenke and M. Peoples)
B Crop N calculated as shoot N x 2

The no-tilled soils also had, on average, 15 kg nitrate-N /ha less than the cultivated soils. For cereals under no-tillage, additional fertiliser N may be required to supplement the reduced soil nitrate. For legumes, however, the lower nitrate levels lead to greater N2 fixation activity. As a result of the extra soil water and reduced soil nitrate, chickpea shoot dry matter (DM) and N were 16% higher under no-tillage, as were % crop N from N2 fixation (25% higher) and total crop N fixed (43% higher) (Table 1).

Optimising N2 fixation (and rotational benefits) through inoculation

Pulses must be well nodulated for maximum N2 fixation and rotational benefits. For the majority of situations, farmers will need to inoculate the seed or soil with the appropriate strain of rhizobia at sowing in order to ensure good levels of nodulation. In other situations, however, there will be adequate numbers of effective rhizobia already in the soil and inoculation will have no effect on either nodulation or crop growth. The NSW Department of Primary Industries and other state Departments of Agriculture take the conservative approach and recommend that all legumes are inoculated at sowing. There are far less problems with unnecessary inoculation than not using inoculants when they are needed. Unnecessary inoculation represents a small cost of production; N-deficient crops can mean substantial reductions in yield and income.

Until recently, the commonly-used method for inoculation was to apply a peat-based inoculant, produced and marketed by just one or two manufacturers, as a slurry to the seed just before sowing. Now, there are five manufacturers selling a more diverse range of inoculant products with different modes of application.

A farmer’s choice of inoculant will depend to a large extent on personal experience and product availability, relative cost and perceived efficacy. The commercial inoculant manufactures, Rhizobium scientists with the state Departments of Agriculture, the universities and CSIRO, as well as GRDC, work together to ensure than Australian farmers continue to have access to very high-quality legume inoculants. Peat inoculants (NODULAID™, Nodule N™, N-Prove™), applied to the legume seed as a slurry, remains the most widely-used of the formulations and the benchmark for efficacy. Under the right conditions, the freeze-dried formulation (EasyRhiz™) is highly efficacious. The clay and peat granular inoculants (NODULATOR™, ALOSCA®, N-Prove™), applied directly to the soil, are appealing to farmers because of ease-of-use and convenience and in the future may well supplant peat as the inoculant formulation of choice. New co-inoculant products, such as TagTeam® and BioStacked®, are exciting new products that promise yield increases and improved gross margins under certain conditions. Whilst the potential benefits of all formulations and products may be real and appealing, farmers should look for evidence of efficacy in their particular environment. They should also use the products strictly according to the label.

N2 fixation by the different pulses

There are now sufficient data from Australian and overseas studies to be able to categorise the pulses either as strong N2 fixers (lupin and fababean), medium N2 fixers (chickpea, field pea, mung bean and lentil) or weak N2 fixers (navy bean). The difference between N2 fixation of chickpea and fababean is highlighted in Table 2, using data from the NSW DPI long-term rotation experiments and commercial crops in northern NSW. With both data sets, fababean had a higher reliance on N2 fixation and fixed about 30% more N.

Table 2. Comparisons of N2 fixation and yields of chickpea and fababean in crop rotation experiments and on-farm surveys in northern NSW

A Means of 18 site/years/tillage treatments; soil water and nitrate to depth of 1.2 m (unpublished data of W. Felton, H. Marcellos, D. Herridge, G. Schwenke and M. Peoples)
B Means of 15 farmer crops; soil water and nitrate to depth of 0.9 m (Schwenke et al., 1998)
C Crop N calculated as shoot N x 2 for chickpea and shoot N x 1.4 for fababean

It is likely that field pea in the northern grains belt would have similar levels of N2 fixation to chickpea (Rochester et al. 1998).

Nitrogen and rotational benefits of pulses

Cereals grown after pulses commonly yield 0.5–1.5 t/ha more than cereals grown after cereals without fertiliser N. To generate equivalent yield in the cereal-cereal sequence, 40–100 kg fertiliser N/ha needs to be applied. The rotational benefit is made up of an N benefit and a biological benefit, the latter largely relating to the break effect of the pulse on soil- and stubble-borne diseases of cereals. Major diseases in the northern grains belt are crown rot, common root rot, root-lesion nematode and yellow leaf spot. A reasonable range for the disease-break effect is 0.2–0.5 t/ha.

Mike Lucy and colleagues summarised results from more than a decade (60 sites x years) of chickpea-wheat rotation experiments in the northern grains belt (Lucy et al. 2005) (Table 3). Major observations are:
• Wheat following chickpea outyielded wheat after wheat by an average of 0.7 t/ha in the NSW trials and by 0.6 t/ha in the Qld trials. Proteins were increased by an average of 1% (NSW) and 1.4% (Qld).
• Where water was not limiting, the yield benefit was greater than 1.5 t/ha.
• The major factor in the increased wheat yields was nitrate supply. In NSW, there was, on average, an additional 35 kg nitrate-N/ha in the 1.2 m profile after chickpea than in the continuous wheat. Increased soil nitrate following a pulse is attributed to both mineralisation of the N-rich residues and nitrate sparing by the legume, i.e. using less of the soil nitrate than a cereal crop.
• Chickpea yields were, on average, about 85% of the unfertilised wheat and about 70% of the N-fertilised wheat.

Table 3. Summary of a decade of experimental results from the northern grains belt showing the rotation benefits of chickpea on yield and grain protein levels of the following wheat crop (source: Lucy et al. 2005)

The rotational benefit of pulses on wheat yields tends to last for one season only. Marcellos et al. (1993) published results from 6 sites in northern NSW showing an average yield benefit of chickpea of +46% (3.2 t/ha for wheat after chickpea versus 2.2 t/ha for wheat after wheat) for the residual first wheat. For five of the six sites, there were no effects of the chickpea on yields of a second residual wheat crop. Soil nitrate levels following chickpea were, on average, 89% greater than after wheat (equivalent to about 40 kg nitrate-N/ha).

Clearly, most of the data on rotational benefits of pulses in the northern grains belt relates to chickpea. One of the objectives of the long-term no-tillage experiments of NSW DPI (Felton et al. 1998) was to compare chickpea and fababean in terms of N2 fixation and rotational benefits. The former (N2 fixation) was achieved to some extent whilst the latter (rotational benefits) was compromised by a combination of drought (1994), frost (1995) and a very wet season causing disease (1998). The limited data that are available shows:
• no difference between chickpea and fababean in terms of soil nitrate benefits when the legumes are sown into low nitrate soils
• slight superiority of fababean in higher nitrate soils in terms of soil nitrate and grain yield benefits

Comparison of the rotational benefits of pulses and short-term pasture leys

Published data from the Warra experiments in southern Qld were aggregated to compare the rotational benefits of chickpea and 1-year lucerne and annual medic leys (Table 4).

Table 4. Comparison of rotational benefits of 1-year pasture leys and chickpea at Warra in southern Qld (source: Dalal et al. 1998; Weston et al. 2002)

The data show clear benefits of the legumes, with increases in soil nitrate levels of 74–88 kg N/ha (pasture leys) and 35 kg N/ha (chickpea). The reduced nitrate benefit of chickpea is to be expected because of the removal of a substantial amount of chickpea N in the harvested grain. The higher soil-water use by the leys meant that yield benefits were less than for chickpea, but the extra nitrate meant far greater grain protein responses.

Knowledge package on legumes and N in farming systems

Evidence for the rotational benefits of pulses in the cereal-production systems of the northern grains belt of NSW and Qld is overwhelming. We now have a large enough data set from both published and unpublished experiments during the 1980s and 1990s to be able to predict N2 fixation and the N benefits of chickpea and fababean in rotation sequences, and to build those algorithms into decision support packages such as the NSW DPI ‘Crop Mate’. The ‘Crop Mate’ package is designed to provide users (farmers and advisers) with up-to-the-minute weather and climate forecasting data to be used together with relevant information on soil N supply and crop N demand, disease and commodity prices to assist in decision making. ‘Crop Mate’ and the algorithms that drive it will be further developed during the next 1–2 years and will be rigorously tested against data sets. If they prove to be accurate, they should become useful management tools for farmers and advisers alike.

References

Dalal RC, Strong WM, Doughton JA, Weston EJ, Cooper JE, Wildermuth GB, Lehane KJ, King AJ, Holmes CJ (1998) Sustaining productivity of a Vertisol at Warra, Queensland, with fertilisers, no-tillage or legumes 5. Wheat yields, nitrogen benefits and water-use efficiency of chickpea-wheat rotation. Australian Journal of Experimental Agriculture 38, 489-501.
Felton WL, Marcellos H, Alston C, Martin RJ, Backhouse D, Burgess LW, Herridge DF (1998) Chickpea in wheat-based cropping systems of northern New South Wales. II. Influence on biomass, grain yield, and crown rot in the following wheat crop. Australian Journal of Agricultural Research 49, 401-407.
Herridge DF, Marcellos H, Felton WL, Turner GL, Peoples MB (1998) Chickpea in wheat-based cropping systems of northern New South Wales. III. Prediction of N2 fixation and N balance using soil nitrate at sowing and chickpea yield. Australian Journal of Agricultural Research 49, 409-418.
Lucy M, McCaffery D, Slatter J (2005) Northern Grain Production – a farming systems approach.
Marcellos H, Felton WL, Herridge DF (1993) Crop productivity in a chickpea-wheat rotation. Proc. 7th Australian Agronomy Conference, Australian Society of Agronomy. pp 276-278.
Marcellos H, Felton WL, Herridge DF (1998) Chickpea in wheat-based cropping systems of northern New South Wales. I. N2 fixation and influence on soil nitrate and water. Australian Journal of Agricultural Research 49, 391-400.
Schwenke GD, Peoples MB, Turner GL, Herridge DF (1998) Does nitrogen fixation of commercial, dryland chickpea and faba bean crops in north-west New South Wales maintain or enhance soil nitrogen? Australian Journal of Experimental Agriculture 38, 61-70.
Weston EJ, Dalal RC, Strong WM, Lehane KJ, Cooper JE, King AJ, Holmes CJ (2002) Sustaining productivity of a Vertisol at Warra, Queensland, with fertilisers, no-tillage or legumes. 6. Production and nitrogen benefits from annual medic in rotation with wheat. Australian Journal of Experimental Agriculture 42, 961-969.

Contact details

David Herridge
NSW Department of Primary Industries
Tamworth Agricultural Institute, 4 Marsden Park Rd, Calala, NSW 2340
Ph: 02 6763 1143
Fx: 02 6763 1222
Email: david.herrige@dpi.nsw.gov.au

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