What is the N legacy following pulses for subsequent crops and what management options are important to optimise N fixation?

Table 10. Concentrations of total residue N from legume crops in 2011, soil mineral N (0-1.2m) measured in autumn 2012 following either wheat, canola, lupins or field peas from brown manure (BM), and lupins, chickpeas or lentils for grain at Junee Reefs, NSW in 2011, and calculations of the apparent net mineralisation of N (soil mineral N net benefit) from legume residues.

Source: Peoples et al., 2015 GRDC update, Peoples et al., 2017 and un-published results.

mineralisation of the crop residues from the legume BM and lupin grain crop in the 2011 year represented around 10 % of the soils mineral N prior to sowing the second cereal crop (Table 11). It is evident that in those crops (chickpea and lentil 2011) that mineralised more N from their residues prior to sowing the first cereal crop, they provided no detectable N benefit for the second cereal crop in 2013 (Table 11). Peoples et al., (2017) also calculated that the soil mineral N benefit from the legume crops was 0.13 kg N/ha per millimetre of summer rainfall.

Table 11. Concentrations of total residue N from legume crops in 2011, soil mineral N (0-1.6m) measured in autumn 2013 following either wheat, canola, lupins or field peas from brown manure (BM), lupins, chickpeas or lentils for grain at Junee reefs, NSW in 2011, and calculations of the apparent net mineralisation of N from legume residues.  (Chickpea and lentil were not included as they did not provide benefits through to the second cereal crop)

Source: Peoples et al., 2015 GRDC update, Peoples et al., 2017 and un-published results.

^a measures of soil mineral N in 2013 following the 2011 chickpea and lentil treatments were not significantly different from the soil mineral N detected after the 2011 wheat treatment so were not included in the analysis.

How to optimise N fixation

Where a legume species is well suited and doesn’t have any obvious constraints to N fixation, it is likely to derive more than half of its N requirements for growth from N₂ fixation. To achieve the desired outcome of increased inputs of fixed N by legumes, the interaction between the best legume and rhizobial genotypes tailored to the local environment and grown with the best agronomic management is required. As outlined in equation 1, to maximise the amounts of N₂ fixed by legumes for the subsequent crop, the grower needs to produce the highest amount of legume N by growing the maximum quantity of legume DM with the highest %N content and ensure that there is a very high proportion of the legume N derived from atmospheric N₂ (%Ndfa).

Given the close relationships that have frequently been observed between legume productivity and the amounts of N₂ fixed by many different crop and forage legumes growing across a diverse range of locations in Australia (e.g., see Peoples et al., 2009;Unkovich et al., 2010; Peoples et al., 2012), management options specifically aimed at supporting greater legume growth will generally have the desired effect of improving inputs of fixed N.

Constraints to N₂ fixation and pulse growth

A. Restricted legume growth:

• Drought
• Poor in-crop weed control
• Carry-over of herbicide residues or in-crop residues
• Nutritional constraints associated with acid soils and P or Mo deficiency.

B. Low % Ndfa resulting from:

• Failure of legume to nodulate due low rhizobia numbers in the soil or poor inoculation
• Acidic subsurface layers
• High soil mineral N (60kgN/ha in Chickpeas, >100 kg N/ha in faba beans and other pulses).

Sub-surface acidity

Many growers are trying to diversify their cropping programs to include higher value pulse legumes to increase the profitability and sustainability of their properties.  Most growers have been implementing a liming program since the late 1980’s, however in a recent survey of paddocks sown to pulse crops across SE Australia between 2015-17, 83 % of these sites had acid sub-surface layers between 5-15 cm or 5-20 cm (Burns and Norton 2018) (Figure 2). Of the 55 sites, only 9 (17 %) of those soils were in the low-risk category and had a soil type suitable for growing acid-sensitive pulse crops.

The authors point out that the mean soil pH_Ca in the moderate and high-risk category soils at depths between 5 and 15 cm were (4.8-5.2, and 4.6-4.8) respectively, indicating that root development, nodulation and therefore production could be compromised. The severity and depth of the acid layer in the extreme risk category soils make these unsuitable for acid-sensitive pulse crops. To obtain maximum growth and maximum nitrogen fixation, correct paddock selection for each species with optimal soil pH are critical factors.

The optimal soil pH_ca for a range of pulse legumes is outlined in Table 12. Burns indicates that any potential paddocks where pulse crops are to be sown should be identified and checked for acidic

Figure 2. Mean soil pH_Ca in surface and subsurface layers of the 55 acidic sites surveyed, categorised (Low, Moderate, High or Excessive) for potential risk of poor nodulation and reduced seedling vigour of acid sensitive pulse species (Burns and Norton page 16).
pulses (Burns and Norton 2018).

sub-surface layers well in advance of sowing acid-sensitive pulses.  A liming program to rectify the surface and sub-surface layers then needs be implemented which may require more specialised machines to ensure the lime is moved into the sub-surface layer and enough time allowed for the pH to sufficiently increase to sow acid-sensitive pulses.  Depending on the environment, rainfall, soil type, mixing and quality of lime used, this may require up to 24 months in low rainfall zones.

Figure 3. The tolerance of legume species and their associated rhizobia to a range of soil pH_ca and the likelihood of successful nodulation (poor, sub-optimal or optimal). (Extracted from Burns and Norton 2018).

The GRDC/NSW DPI publication ‘Legumes in acidic soils’ (Burns and Norton 2018) and GRDC Update paper (Burns and Norton 2020) offer some practical information to assist growers to better understand the agronomic management required to grow pulses and ensure maximum biomass potential and N fixation is achieved.  There are a range of publications that can assist growers better understand the requirements for paddock selection, constraints, crop and variety selection, time of sowing, fertiliser/herbicide and fungicide applications.  A few of these publications include:

- Pulses: putting life into the farming system (2015). Armstrong E and Holding Di;

- GRDC Inoculating legumes: A practical guide (2011) Drew et al.

- GRDC Legumes in acidic soils – maximising production potential. (2018) Burns and Norton;

- GRDC Grow Notes for Lentil, Chickpea, Fababean, Lupins (all available at https://grdc.com.au/resources-and-publications/grownotes/crop-agronomy/lentil-southern-region-grownotes).

Sodicity and salinity

Unfavourable and hostile soils that limit legume root exploration (e.g. soil compaction, sodicity, salinity), inhibit nodulation or restrict shoot growth (e.g. soil acidity, nutrient deficiencies) should also be ameliorated (Peoples et al. 2009; Santachiara et al., 2019; Vanlauwe et al., 2019). Lentils and chickpeas are also very sensitive to saline soils.  Where the electrical conductivity (EC_se) of the saturated soil extract is 2 dS/m and 3 dS/m, a yield reduction of 20 % and 90 % has been found.

Soil mineral N

To achieve high %Ndfa, concentrations of available soil mineral N would also need to be low at sowing (<55-85 kg N ha^-1; Voisin et al. 2002; Salvagiotti et al. 2008), and > 60 kgN/a in the soil at depths of 0-1.2m prior to sowing chickpeas (Doughtan et al., 1993; Drew et al., 2012).  Higher concentrations of soil N would inhibit nodule initiation and the N fixation process (Peoples et al., 2009; Guinet et al., 2018). High N and ensuing suppression of N fixation is less likely to occur under reduced tillage practices where the retention of stubble from a previous cereal crop is more likely to immobilize soil mineral N resulting in higher rates of N fixation (Torabian et al., 2019).

Effective inoculation

Prospective agronomic practices to achieve this would include the use of high quality rhizobial inoculants at sowing, efficient inoculation practices, and the ameliorating of any soil conditions that are either hostile to rhizobia’s survival and persistence or results in erratic nodulation (e.g. soil pH or nutrient deficiencies).

Crop species

In terms of genetic factors, the choice of legume species (and maturity group) most adapted for the local soil type, season or climate is likely to play a crucial role (Peoples et al., 2009; Tagliapietra et al., 2021), as will plant improvement for enhanced disease resistance (Peoples et al. 2019).

Greenethorpe farming system trial results in 2021

In January 2020, 3.3/ha of lime was applied and incorporated using a Horsch-Tiger to a depth of 26 cm at the Greenethorpe Farming system site.  The 2021 year was extremely wet (952 mm) which resulted in some significant challenges such as higher disease levels in the pulse crops than experienced in 2020 (767 mm rainfall year).  The ameliorated lime improved the %Ndfa and shoot N fixation (12-24 kg N/t DM) in all pulse crops at Greenethorpe compared to 2020 even in such a wet year with high disease pressure (Table 12). A new northern type of faba beans was grown in 2021 that produced excellent grain yields (7.7 t/ha), but potentially produced less biomass compared to the longer maturing Samira faba bean that was sown in 2020. The high grain yield and reduced faba bean biomass DM has resulted in lower net inputs of fixed N remaining in the crop residues compared to what may have been remaining if a southern later maturing variety such as Samira had been sown (Table 13).

Table 12.Soil mineral N at sowing, legume shoot biomass, %N content, and estimates of the proportion (%Ndfa), and amounts of shoot and total plant (shoot+root) N fixed at Greenethorpe in 2021 for a range of legume crops.

# Total plant biomass is indicated in brackets if it is different than the total legume biomass.

The cool September/October resulted in delayed flowering of the chickpea variety Captain when compared to previous years.  The site did not reach the average daily temperature of 15 degrees Celsius until late October, with the daily temperature between mid-August and the end of October generally staying below 15 degrees Celsius (Figure 4).   The longer growing season assisted chickpea to produce more biomass and reasonable grain yields despite the continued impact of fungal diseases that included Ascochyta blight, sclerotinia and botrytis grey mould. This higher biomass resulted in high net inputs of fixed N (Table 13). The same chickpea population (35 plants/m²) was sown and established in the chickpea/linseed intercrop treatment, but the chickpeas were sown in alternate rows, 50 cm wide.  The linseed did not emerge in high numbers and this treatment became a predominately chickpea crop sown on 50 cm wide rows. Interestingly, there was considerably less biomass and grain yield compared to the chickpea monoculture sown on 25 cm row spacing.

Table 13. Grain and hay yields, grain %N, N removed in grain or hay and the estimated residual fixed N remaining after grain or hay removal from a range of legume crops at the Greenethorpe site in 2021.

# Total plant biomass is indicated in brackets if it is different than the total legume biomass.
* Hay calculated as 70% of the total plant dry matter

Figure 4. The daily and average daily temperature at the Greenethorpe trial site in 2018-2021 from interpreted data (Silo). Data courtesy of Dr Jeremy Whish.

Section 2: Legume crop legacy

Soil N

The main route for biologically fixed N to enter the soil N pool is through the decomposition of legume crop residues. The magnitude and timing of the release of legume N as plant-available forms represents a balance been the microbial-mediated mineralisation and immobilisation processes in the soil, which in turn are affected by the efficiency of use of the legume organic C by the decomposer population, and the microbial demand for C and N for growth (Kumar and Goh, 2000; Fillery, 2001). Inorganic N tends to be released from plant residues once excess C has been consumed by microbial growth. As compared to cereal crop residue, legume crop residue contains both a higher N content as well as a lower C to N ratio. These characteristics favour net N mineralisation and therefore lead to higher soil mineral N concentrations as legume crop residue breaks down. While legume crop residue breakdown is the primary source of soil N availability improvements after legume crops, this is not the only source. Other sources include: the carry-over of un-utilised mineral N after the legume crops and reduced N immobilisation by the soil biology compared to cereal stubbles.

Junee Reefs experiment 2011-2013

The large differences in soil mineral N observed following pulses grown for grain or BM in 2011 at the Junee Reefs experiment compared to wheat or canola top-dressed with fertiliser N at stem elongation, resulted in increases in wheat N uptake and higher wheat grain protein percentage in 2012 (Table 14).  However, the impact of the additional N supply was not fully reflected in grain yields, with only a 0.6-0.7 t/ha increase in wheat grain yield. The drier growing season of 2012 reduced the maximum grain yield to 4.1 t/ha.  The subsequent calculations indicate that the 2012 wheat crop recovered the equivalent of 29-39% (mean 32%) of pulse residue N (Table 14).  This compared to 49-61 % (average 55 %) of the top-dressed fertiliser N. When Peoples et al. (2017) examined a range of crops between 1990 and 2016 across New South Wales and South Australia,

Table 14. Grain yield and crop N uptake by wheat in 2012 following either wheat, canola, and lupin or field pea grown for brown manure (BM) or lupin, chickpea or lentil grown for grain at Junee, NSW in 2011, and calculations of the apparent recoveries by wheat of either N from pulse crop residues, or top-dressed fertiliser N.

Source: Peoples et al., 2015 GRDC update, Peoples et al., 2017 and un-published results. They found that the average apparent recovery of legume N was 30% from grain legume crops and 29% from BM crops.

The CSIRO/NSW DPI farming system teams will examine the current farming systems and determine if the apparent recovery legume N by the following crop is within the range that Peoples et al. (2017) reported.

Southern Farming Systems project results (2018-2021)

The inclusion of fully phased crop sequences with and without legumes across a range of locations (Wagga Wagga, Greenethorpe, Urana & Condobolin) and seasons (2018, 2019, 2020 & 2021) in this project has allowed the investigation of a number of key questions:

1. To what degree do legume crops boost the soil mineral N available to subsequent crops?
2. To what degree do legume crops boost the grain yield of subsequent crops, and
3. What is the approximate dollar value of these legume legacy benefits?

The legume crops at all four sites often resulted in more mineral N being available at sowing of the subsequent crops (Figure 5 and Table 15). Averaged across legume crop types, seasons and sites, an extra 50 kg/ha of extra mineral N was available at sowing in the subsequent season as compared to a cereal crop in the same season. Much of this N wasn’t available directly after the legume harvest, but became available over the summer fallow period.

Figure 5. Extra soil mineral N (0-2 m) available at sowing following a legume crop compared to a cereal crop; averaged across four legume crops (lentil, lupin, faba bean & vetch), four sites (Wagga Wagga, Greenethorpe, Urana & Condobolin) and three seasons (2018, 2019 & 2020). Comparisons made between equivalent timely sown, decile 2 N strategy crop sequences. n=33, average=50 kg N/ha. The blue area represents the middle 50 % of data points, the two outside lines represents the maximum and minimum data point and the dot represents an outlying data point.

As evident in Figure 5, a significant amount of variability exists in the amount of extra soil mineral N that was available to the subsequent crops following a legume crop. Some trends exist between field site and season, however few clear trends are evident between preceding crop type (Table 15). This highlights that legume crop choice is better governed by performance and profitability potential for a given farm enterprise rather than potential soil mineral N benefits, which is a secondary consideration.

Table 15. Extra soil mineral N (0-2 m) available at sowing following a range of legume crops compared to a cereal crop at each site; averaged across three seasons (2018, 2019 & 2020). Comparisons made between equivalent timely sown, decile 2 N strategy sequences.

With synthetic fertiliser prices at current all-time highs, more people are looking to legumes as a potential N source. One way to compare synthetic N sources to legume N sources is to value the short-term N benefit that legume crops can provide at the equivalent cost of urea. This comparison is presented in Table 16. At high urea prices as are currently being experienced ($1,200/t in early 2022), the value of legume N benefits can be significant at over $200/ha. It is important to note that this valuing of the soil N legacy left by legume crop only considers the extra mineral N accumulation over the summer period and does not consider any further in-crop mineralisation that can occur during the following growing season.

Table 16. Average extra soil mineral N (0-2 m) available at sowing following a legume crop compared to a cereal crop at each field site, with the value of this extra mineral N displayed at a range of urea prices. An assumption of 30 % N loss from applied urea has been applied.

Using the crop sequences implemented in the southern farming systems project, not only are we able to examine the soil N legacy effects following legume crops, but we are also able to examine the urea savings and grain yield benefits provided to subsequent crops.

The N management strategies compared across some crop sequences in this project were based on either a conservative seasonal outlook (decile 2), or a more optimistic (decile 7) seasonal outlook. For each non-legume crop in each year of the sequences, soil mineral N was measured pre-sowing and a potential yield estimate was made based on starting soil water, N level and seasonal conditions up to that time. N was then applied as urea assuming either a decile 2 or a decile 7 finish to the season. Assuming an average season is decile 5, this means that often the decile 2 N strategy would be too low, and the decile 7 treatment too high to maximise yield potential in any year. Using this approach, the legacy benefits of carry-over N from either legumes or unused fertiliser N would be accounted for in the pre-sowing tests and less N applied accordingly. This approach (compared to set N rates) better mimics farmer practice.

For a given N management strategy, the extra soil mineral N often available following legume crops results in a reduction in the rate of top-dressed urea needing to be applied to the subsequent canola crops. This saving is urea cost combined with any grain yield benefit can be used to provide an indication of the legume legacy benefit in $/ha. Averaged across the three field sites, four legume crop types and three seasons, the average urea saving and grain yield benefit to the following canola crop was 78 kg/ha and 0.22 t/ha respectively (Table 17). When these benefits are valued at $1,200/t for urea and $650/t for canola, the total value of the legume value ranges from $171 to $330/ha depending on the field site, with an average of $237/ha (Table 17).

The above comparisons are made under a decile 2 N strategy. However, at the Wagga Wagga field site we can also make comparisons with decile 7 N strategy cereal sequences. This allows the comparison of legume legacy benefits to non-legume sequences where N is less limiting due to higher rates of urea applied.

Table 17. Urea saving, extra canola grain yield and the dollar value of these benefits following a legume crop compared to a cereal crop at each field site; averaged across a range of legume crops (lentil, lupin, faba bean & vetch) and three seasons (2018, 2019 & 2020). Comparisons made between equivalent timely sown, decile 2 N strategy crop sequences.

*Only legacy effects from the 2019 legume crops included for the Greenethorpe site.

The implementation of the higher decile 7 nitrogen strategy instead of the decile 2 strategy on the non-legume sequence resulted in an increased canola grain yield. However, this increase in grain yield was not high enough to offset the significant extra urea cost. As a result, the $/ha value of the legume legacy benefits are even higher when compared to the decile 7 non-legume sequence (Table 18).

Table 18. Urea saving, extra canola grain yield and the dollar value of these benefits following a legume crop compared to a cereal crop across two N management strategies (decile 2 & decile 7) at the Wagga Wagga field site; averaged across a range of legume crops (Lentil, lupin, faba bean & vetch) and three seasons (2018, 2019 & 2020). Comparisons made between equivalent timely sown, decile 2 & 7 N strategy crop sequences.

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 especially thank Mr Rod Kershaw at “Iandra” Greenethorpe and Warakirri Cropping “Karoola Park” Urana for the use of land for experimental purpose and for management advice at the sites. We also thank Peter Watt (Elders), Tim Condon (DeltaAg), Greg Condon (Grassroots Agronomy), Heidi Gooden (DeltaAg) and Chris Baker (BakerAg) for the many useful discussions in their role on the Project Advisory Committee.

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Contact details

Tony Swan
CSIRO Agriculture and Food
Clunies Ross St, Acton, Canberra ACT 2601
Ph: 0428 145 085
Email: tony.swan@csiro.au
Twitter: @tony_swan64

Mathew Dunn
NSW Department of Primary industries
Wagga Wagga Agricultural Institute
Ph: 0447 164 776
Email: mathew.dunn@dpi.nsw.gov.au

Dr Mark Peoples
CSIRO Agriculture and Food
Clunies Ross St, Acton, Canberra ACT 2601
Email: mark.peoples@csiro.au

GRDC Project Code: CSP1703-007RTX,