How much nitrogen is fixed by pulse crops and what factors affect fixation?

Take home messages

• The amount of nitrogen (N) fixed by pulses varies widely (from 0 to 400 kg N/ha) and is impacted by crop species, soil nitrate at planting, effective nodulation and agronomic factors such as time of sowing, row spacing, plant population and variety.
• Narrower row spacing in pulses not only improves crop biomass and yield but also the proportion of N in that biomass that is fixed from the atmosphere and hence free for crop use.  This allows crops to be produced on lower levels of soil nitrate and gives more opportunity for crop residues to be higher in N that can mineralise for the following crop.
• Time of sowing should be optimised for maximum biomass production and longer time to accumulate fixed N.  The proportion of N in plants that is derived from the atmosphere (%Ndfa), i.e. fixed, is significantly greater when crops are sown earlier in the planting window rather than late, particularly in soybean and fababean crops.  If growers are planting late, more N will be fixed if plant populations are significantly increased.
• Some minor varietal differences in N fixing potential do exist and growers can aim for higher biomass varieties to fix more N.

Background

Average amounts of nitrogen (N) fixed annually by crop and pasture legumes are around 110 kg N/ha (ranging from close to zero to more than 400 kg N/ha).  The actual amount fixed depends on the species of legume grown, the site and the seasonal conditions as well as agronomic management of the crop or pasture.  The legume crop uses this N for its own growth and may fix significantly more than needed, leaving a positive N balance in the soil for proceeding crops.

Average estimates of N fixation for the major crop legumes grown in Australia (derived from many research trial studies) are given in Drew et al (2012) (Table 1), however, huge variations around these figures exist in practice.  Actual percent N fixed and amounts of N fixed by individual crops are influenced by environment and management effects, including soil nitrate levels at planting.  Importantly, both root and shoot N must be considered when calculating the total amount of N that was fixed and used by the plant for growth. Root N is substantial for all crops but does vary with species, for example chickpea have equal portions of N in their roots as they do in their shoots wheareas faba bean and mungbean have approximately half as much N in their roots as their shoots (Unkovich et al. 2010). N remaining in residues of shoots and roots of the pulse crop after harvest is a slow release form of N for the subsequent crop.  In this form, less is likely to be moved through the loss pathways that lead to loss of inorganic N fertiliser in the short term.

Table 1. Estimates of amounts of N fixed annually by crop legumes in Australia from Drew et al. (2012)

¹ Total crop N = shoot + root N
²Total N fixed = %N fixed x total crop N; Data sourced primarily from Unkovich et al. 2010

Results

Improving the amount of N fixed in a farming system by changing agronomic practices has been a focus in a recent northern region project.  Our results show that altering management practices such as row spacing, time of sowing and variety used can have large implications for the amount of N fixed by that crop.  This means better N nutrition for the pulse crop and also potentially for the crop following that pulse

Row spacing

Field trials with chickpea, mungbean, soybean and fababean have shown that significant increases in %Ndfa (percentage of nitrogen derived from the atmosphere) occurred when plants were grown on a narrower row spacing (i.e. 25 or 50 cm rows compared to 75 or 100 cm rows), keeping plant population the same.  This then translated into higher amounts of N (kg/ha) fixed by the plants as biomass was also greater and ultimately more N was left behind post-harvest for the following crop.  Figure 1 below demonstrates this higher amount of N fixed with narrower row spacing for two chickpea trials; one at Billa Billa near Goondiwindi and one near Dalby. After accounting for the N removed in the grain at harvest, an estimated 59 kg N/ha was added to the soil by the chickpeas at the Dalby site when grown on 0.25m rows, while only 23 kg N/ha was added at the 1.0 m row spacing.  In the trial at Goondiwindi, N fixation and biomass were much lower overall. Just 6 kg N/ha was added through N fixation at 0.25 m row spacing however if grown at on 1.0m rows, the crop actually depleted soil N by 6 kg/ha

Reducing rows from 90 down to 30 cm in mungbean also significantly increased both %Ndfa and total amount of N fixed.  Differences in varieties in their potential to fix N also was evident (Figure 2).

Figure 1. Total N fixed in chickpeas (shoots and roots) when grown at 3 different row spacings but keeping plant population the same at 30 plant/m².

Figure 2. Differences in total shoot and root nitrogen for 3 mungbean varieties, Crystal , Jade-AU and Satin II (LSD 5% = 7.65) and for two row spacings of 30 and 90cm (LSD 5% = 6.24).

Time of sowing

Mungbean, soybean and faba bean have all shown significant impacts of time of sowing on N fixation.  Not only is biomass of the crop reduced in a late planting for all three crops, so too is the proportion of the N in the plants that is fixed by the rhizobia (%Ndfa). Higher plant populations are therefore required to try to compensate for this loss in production and reduced amount of free N.  Faba bean varieties PBA Nasma and PBA Warda both showed that sowing late decreased %Ndfa by more than half and this combined with the reduced amount of biomass produced by the plants from this late May sowing date, meant much less N was fixed by the plants (Figure 3).  Increasing plant population partially compensated but did not completely overcome this loss.

Soybean planted in late January rather than December also was negatively impacted, with much lower %Ndfa and N fixed. One variety from the Australian Soybean Breeding Program, ‘Richmond’ fixed half the amount of N (81 kg N/ha less) in shoots when planted later (15 January 2014 compared 20 December 2013).  The variety PR443 fixed only a third as much N (163 kg/ha less) when sown at the later planting time.

Figure 3. Total amount of N fixed by fababean (mean of two varieties PBA Nasma and PBA Warda) was much lower when the crop was planted late. (N.B. Figures are for total N in shoots and roots assuming 40% of N in roots).

Inoculation

Trials focussing on the best form of inoculum for soybean and peanut in particular have shown little differences between peat, freeze-dried and granular inoculum forms.  Growers should be able to use either form with confidence depending on available equipment.  The use of liquid Zn fertilisers at recommended application rates and mixed with the chickpea inoculum strain CC1192 did not significantly impact the rhizobia or nodulation.  Mixing of inoculum with concentrated forms of any fertiliser however is not recommended and extreme caution must be taken at all times to protect the live bacteria in the inoculum which are extremely sensitive to heavy metals and low or high pH levels. Further research with rhizobia strain compatibility for soybean, mungbean and fababean strains is required.

Establishment of good nodulation is vital for N fixation and hence good inoculation practices are crucial for survival of the rhizobia on the seed or in the soil at planting.  Manufacturer guidelines as given on the packets should be followed and the correct rhizobia strain must be used.

Conclusions

Improving the amount of N fixed in a farming system by changing agronomic practices has been a focus in this project.  Our results show that altering management practices such as row spacing, time of sowing and variety used can have large implications for the amount of N fixed by that crop.  This can mean better N nutrition for the pulse but also for the crop following that pulse.  Field trials have compared the impact of different row spacing, plant population, time of sowing and variety on effective nodulation and N fixation in pulse crops.  This work has shown that narrower row spacing (for example 25 and 50cm rather than 75 or 100cm) in pulses can lead to higher levels of N fixed by the crop.  This has correlated well with growth of the tops (biomass) and in some cases yield.  Also, importantly, it has translated to greater amounts of N left in the soil.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

References

‘Inoculating legumes: A practical guide.’ Ground Cover Direct

Unkovich M, Baldock J and Peoples M (2010) Prospects and problems of simple linear models for estimating symbiotic N2 fixation by crop and pasture legumes. Plant and Soil 329:75-89.

Contact details

Nikki Seymour
Department of Agriculture and Fisheries
Leslie Research Facility, 13 Holberton St
Toowoomba 4350
Ph: 07 45291237
Fx: 07 46398800
Email: nikki.seymour@daf.qld.gov.au

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