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

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

Take home messages

  • Pulse legumes can improve the profitability and sustainability of your farming system. We found average legume legacy benefits to subsequent canola crops worth $237/ha from both higher grain yields and savings in urea costs
  • ’Grow what you can and grow it well‘ to maximise N input.  Select the best legume crop, variety and sowing time for your soil and get the agronomy right - ensure effective nodulation, maximise pulse dry matter, remove subsoil constraints, and avoid high soil mineral N and damaging herbicides
  • Crop end use (grain, silage/hay or brown manure) affects N legacies in subsequent crops – understand and account for these benefits
  • Pulse crops with high grain yield or cut for hay production may not always provide a net input of mineral N, but other benefits include the role as a double break, emergence in heavy stubble and high N residues that assist conversion of cereal stubble to humus to improve soil fertility.

Legume crops - introduction

The benefits of crop rotation are widely recognised in modern farming systems. In Southern NSW, cereal-dominated sequences (wheat and barley) often include canola as a break crop, but rarely include a legume break crop. The uptake of more diverse cropping sequences can provide a range of benefits that may outweigh the challenges and risk associated with growing and marketing legume crops, especially if viewed from a whole-of-system perspective.

System benefits from growing legumes can include soil chemical, structural and biological changes as well as impacts on pests, disease and weed levels that can influence the performance of subsequent crops in the sequence. However, much of the legacy benefit derived from legume crops relates to N supply (Angus et al., 2015; Peoples et al., 2017).

In a recent paper on sustainable intensification of cropping systems, Reeves (2020), highlighted several changes to farming systems to ensure our farms remain productive, profitable and sustainable.  He concluded that a “new revolution of diversified farming based on the effective integration of crops, pastures, livestock, shrubs and trees together with diverse practices are required to make farms more resilient financially and to the increasing challenges of climate change and climate extremes.”  To build this resilience, he notes that it is imperative to build soil C and N content and soil health generally (Reeves 2020). Unfortunately, our current intensive cropping systems are reducing both total soil C and N (Sanderman and Baldock 2010), soil organic N is declining over time (Figure 1; Lake 2012) and despite widespread use of lime, current acid soil management programs are not preventing acidification of layers within the 5-15 cm depth layers (Burns and Norton 2018).

Line graph showing accumulated deficits expressed as elemental N fertiliser equivalent in Australian temperate crop soils as estimated by two scenarios: Scenario 1 being the best possible case of N fertiliser usage on those crops and Scenario 2 being a more realistic assessment of likely N usage levels.  (Lake 2012).Figure 1. Accumulated deficits expressed as elemental N fertiliser equivalent in Australian temperate crop soils as estimated by two scenarios: Scenario 1 being the best possible case of N fertiliser usage on those crops and Scenario 2 being a more realistic assessment of likely N usage levels.  (Lake 2012).

Angus and Peoples (2012) calculated that a fallow typically reduced total soil N by 4.4 % annually and crops by 2.5 % and determined that more frequent inclusion of legumes would be required to offset this decline in soil organic N, or otherwise increased rates of fertiliser N application would be required to maintain yields. If this was to occur it has been predicted that fertiliser N costs would rise as a percentage of gross margin from 9-10 % to around 14.8% by 2037 and 17.5 % by 2067 (Table 1).

Table 1. The increase in fertiliser N calculated to maintain a 4 t/ha and a 2 t/ha wheat crop on a red Mallee soil between 2017 and 2067 (Angus and Peoples 2012).

Year

 

Soil N       (kg N/ha)

Fertiliser N required
(kg N/ha)

N cost
(% of GM)

2017

Red Soil
GSR = 300mm
4t/ha @ 10.5% protein

108

80

9.1

2037

54

134

14.6

2067

27

161

17.7

2017

Mallee Soil
GSR = 200mm
2t/ha @ 10.5% protein

45

53

10.5

2037

23

75

15.0

2067

10

88

17.5

In this paper we utilise the findings from recent systems experiments undertaken in southern and central NSW to quantify the contributions of N fixation to legume growth and soil N fertility and to examine the N legacy for following crops. Management options will also be described that can assist in optimizing both the performance of the legume and the flow-on N benefits for subsequent crops.

The GRDC Farming Systems experiments 2018-2021

Experiment outline

Four contrasting locations were selected in 2017 that represented a range of soil types and environmental factors and which encompassed a diverse range of grower and consultant groups.  The main core experiment site is located at the Wagga Wagga Agricultural Institute with three regional node sites located at Condobolin Research and Advisory Station, Greenethorpe and Urana.  There are six treatment sequences that are common to all sites, with the Wagga Wagga site encompassing all treatments.  The crop sequence treatments applied are provided in Table 2. All sites were sown to wheat in 2017 with the treatment sequences starting in 2018. Data from the Wagga Wagga, Greenethorpe and Urana sites are presented in this paper.

Table 2. Farming systems sites with sowing timing, N management and winter grazing strategies applied to different crop sequences.

Crop sequences

Condobolin & Urana

Wagga Wagga

Greenethorpe

Sowing

Nitrogen

Sowing + grazing

Nitrogen

Sowing + grazing

Nitrogen

Canola-wheat

E, T

Low, High

E+G, T

Low, High

E+G, T

Low, High

Canola-wheat-barley

T

Low

T

Low, High

  

Canola-wheat-wheat

    

T, L

High

Lentil-canola-wheat

E

Low, High

E, T

Low, High

E

Low, High

Lupin-canola-wheat

  

T

Low

  

Faba bean-canola-wheat

T

Low

  

T

Low

Chickpea-wheat

  

T

Low

T

Low

*Legume-canola-wheat

T

Low

E+G, T

Low, High

E+G, T

Low

Faba bean/canola-wheat

  

T

Low

T

Low

Wheat-wheat-wheat

  

T

Low, High

T

Low

Fallow-canola-wheat

E

High

E, T

High

  

Flexible one

Flexible

Flexible

Flexible

Flexible

Flexible

Flexible

Flexible two

Flexible

Flexible

  

Flexible

Flexible

E = Sown early from mid-March to mid-April period
T = Timely sown crops from 3rd week April to mid-May
G = Grazing (always winter grazed and sometimes a 2nd grazing or stubble graze)
Nitrogen = Low (top-dressed nitrogen in June-July for a decile 2-year (N2) grain yield, High (top-dressed nitrogen in June-July for a decile 7-year (N7) grain yield)

Prior to sowing the cereal crop at all sites in 2017, soil samples were taken and analysed for chemical characteristics. It was determined that at Condobolin, Greenethorpe and Wagga lime would need to be applied to ameliorate the soil and increase the soil pH (CaCl2) to > 5.5 in the surface 0-10 cm and > 5.2 between 10-20 cm.  A rate of 3 t/ha, 3.5 t/ha and 1 t/ha of lime was applied at the Condobolin, Greenethorpe and Wagga sites, respectively and incorporated to a depth of around 10 cm. The aim was to incorporate the lime deeper (> 15cm) at the Greenethorpe site, however due to the dry conditions, the offset discs were not able to penetrate deeper.  To ensure that the early March sown treatments were able to be sown on time with sufficient surface soil moisture to ensure germination and plant emergence at the start of 2018, the Greenethorpe site was not ploughed following a rainfall event in January 2018. We envisaged that the alkalinity from the lime would move lower in the profile to 10-15 cm over the next few years with sufficient rainfall.

Section 1: Nitrogen fixation and legume impacts on soil N dynamics - Results from previous and current farming systems experiments

Many experiments have demonstrated a close relationship between soil mineral N and wheat yield across a range of environments in eastern Australia (Angus et al., 2015). Both soil mineral N and wheat yields are generally lower following wheat crops and highest following legumes. The amount of N mineralised from legume residues that becomes available for a subsequent crop can be influenced by legume species and its end use (i.e., whether it is grown for grain, green or brown manured, grazed or cut for hay), and the amount of rainfall over the summer fallow between crops.

Legume inputs of fixed N

Cost-effective supply of legume N depends on productive and efficient N fixation. Matching species choice to the environment is an important factor that impacts on the total input of N fixed (kg N/ha). Specifically, the amounts of N fixed by legumes are regulated by two factors:

  1. The amount of legume N accumulated over the growing season (as determined by shoot dry matter (DM) production and %N content); and
  2. The proportion of the legume N derived from atmospheric N2 (often abbreviated as %Ndfa).

Equation 1: Amount of legume shoot N fixed = (legume shoot DM x %N/100) x (%Ndfa/100)

The greater the amount of biomass that a legume can produce, the higher the potential for more N fixation to occur (Peoples et al., 2009). Where a species is well suited and doesn’t have any obvious constraints to N fixation (see section on subsoil constraints), it is likely legumes will derive more than half of their N requirements for growth from atmospheric N2 via N fixation. Under these conditions it is common for around 15-20 kg of shoot N to be fixed on average per hectare for every tonne of legume shoot DM accumulated during the growing season (Table 3).  However, there can be a wide range in %Ndfa and the amounts of N fixed by different legumes across different environments. Analyses of on-farm samples of legumes collected from 61 commercial grower paddocks, indicated an average %Ndfa of 65%, but the range was 8 to 89%.  Similarly, the average shoot N fixed per tonne of shoot biomass was 16 kg N/t DM, with a range of between 2 to 25 kg N/t DM (Table 3).

Table 3. Summary of on-farm estimates of N fixation by 61 commercial pulse crops sampled between 2001-2017 (Peoples et al. un-published data).

Legume

Number paddocks

%Ndfa

Shoot N fixed
(kg N/ha)

Mean shoot N fixed & (range)
(kg N/t DM)

Chickpea

8

67%

47

14 (7-25)

Fababean

23

68%

126

17 (10-25)

Fieldpea

8

56%

46

14 (2-20)

Lentil

5

65%

83

18 (4-25)

Lupin

14

63%

83

16 (9-21)

Vetch

3

69%

89

17 (13-22)

Mean

 

65%

90

16

The estimate of amounts of N fixed per t of DM accumulated can be used to compare N fixation efficiency: 20+ indicating excellent fixation; > 15 is considered OK; but < 10 kg/t DM generally indicates that there is some constraint to root nodulation, the N fixation process or crop growth which will need to be identified and addressed to maximise future inputs of fixed N (see section on constraints to N fixation). In the case of the 61 commercial pulse crops summarized in Table 3, 20 % of the crops sampled (i.e., 12 crops) were deemed to have had sub-optimal N fixation.

Net inputs of fixed N2

The amount of shoot N fixed by legumes are informative, but what is more important is how much fixed N might be contributed to the soil at the end of the growing season. Since the root systems of legumes can contain between 25 % to 50 % of the total plant N, this below-ground contribution of fixed N can be a substantial component of the potential carry-over N benefit for following crops and should not be ignored (Peoples et al., 2009). Since it is extremely difficult to fully recover root systems of legumes in the field, total N fixed is usually calculated by adjusting the shoot measures of N fixation to include an estimate of how much fixed N might also be associated with the nodulated roots using a ‘root factor’ (Unkovich et al., 2008; Unkovich et al., 2010, Peoples et al., 2012). For many pulse legumes around one-third of the plant N is commonly below-ground in roots and nodules; in this case a ‘root factor’ of 1.5 would be used (Table 4).

Equation 2: Total N fixed = (shoot N fixed) x root factor.

Table 4. A ROUGH RULE OF THUMB for estimating the total amount of N fixed by different legume species to include shoot and root fixed N.

Species

Estimated shoot N fixed
(kg N/t DM)

Estimated below ground N
(% of total N)

Root factor

Estimated total plant N fixed
(kg N/t DM)

Fieldpeas, lupins, fababeans, vetch

20

33%

1.5

30

Chickpeas

20

52%

2.06

41

Lucerne

20

50%

2.0

40

Subclover

20

42%

1.72

34

The net inputs of fixed N (Equation 3) are derived by comparing the total amounts of N fixed to the amounts of N removed in harvested grain, hay, and/or animal products, or lost from the system via ammonia volatilisation from urine patches where the legume-based pastures or legume stubbles are grazed (Peoples et al., 2012).

Equation 3: Net input of fixed N = (total amount of N fixed) – (N removed + N lost)

The total amounts of N remaining in the crop vegetative residues and roots at the end of the 2011 growing season (Table 5) were calculated for pulse crops using Equation 4.

Equation 4: Total residue N = (total crop N) – (grain N removed)

Junee Reefs experiment 2011-2013

Data generated by experimentation at Junee Reefs indicated that brown manured legumes (BM: legume crops killed with knock-down herbicide before weed seed-set as a weed management tool) provided greater net returns of fixed N to soils than grain crops, as large amounts of N were removed in the high-protein legume grain (Table 5). However, it is clear from this dataset and others, that different legume species have very different potential for growth and N fixation, regardless of their eventual end-use (Table 5). In this experiment, legume DM ranged between 5.7 and 9.9 t/ha, with the lupin BM and lupin grain crops having the highest %Ndfa, lentils lower at 59 % and field peas and chickpeas lowest at 50 %.  When we examined the net N balance after grain removal compared to brown manuring, there was a range in net N balance between -1kg N/ha in the lentils to an additional 241 kg N/ha following the lupins BM (Table 5).

Table 5. Shoot and grain dry matter (DM) production, N accumulation, grain yield, inputs of N fixed by legume grain or brown manure (BM) crops and estimates of the amount of residual N remaining at the end of the growing season that was derived by fixation and total residual N at Junee Reefs in 2011.

Crop 2011

Biomass
(t DM/ha)

Grain yield (t/ha)

Total plant NA (kg N/ha)

Ndfa
(%)

Inputs of fixed NA
(kg N/ha)

Grain N
(kgN/ha)

Net N balance of fixed N
(kgN/ha)

Total residue N
(kgN/ha)

Lupin BM

8.4

-

290

83

241

-

+241

290

Field Pea BM

6.3

-

215

52

112

-

+112

215

Lupin

9.9

3.5

398

85

338

210

+128

188

Chickpea

6.4

1.8

247

50

141

77

+64

170

Lentil

5.7

3.2

248

59

137

138

-1

110

Wheat +Nb

11.1

4.8

  

49

87

 

64

Canola +Nb

10.6

3.2

  

49

94

 

111

LSD P<0.05)

1.3

0.5

-

9

-

11

-

22

Source: Legume data from Peoples et al., 2015 GRDC update and Peoples et al., 2017.
A The amount of total plant N and shoot N fixed were adjusted to include an estimate of N contributed by the nodulated roots as described by Unkovich et al., (2008), Unkovich et al., (2010).
b Urea fertiliser was applied to wheat at 49 kg N/ha and canola at 66 kg N/ha.

The GRDC farming systems experiments 2018-2020

A summary of the average N dynamics from the pulse legume crops for phase 1 (2018-2020) of the current GRDC farming systems experiments located at Greenethorpe, Wagga and Urana are outlined in Table 6. Generally, the high-density legume pastures (HDL) have produced on average, the highest quantities of shoot N fixation with estimates of shoot N fixed ranging between 16-20 kg N/t DM (Table 6). The faba bean at Urana, faba bean-canola inter-crop treatments at both Wagga and Greenethorpe in 2018 and 2019, lupins at Wagga and lentils (N2) at Urana also all had reasonable fixation rates that were > 17 kg N/t DM. Generally, the chickpeas and lentils at both the Wagga and Greenethorpe sites and the chickpeas at Urana had the lowest rates of N fixed with < 12 kg N/t DM (Table 6).

In the GRDC project experimental sites, no legume crop was managed as a brown manure (BM) crop. Rather the early sown HDL legume crops were grazed in June before cutting for hay in October, whilst the mid-April to early-May sown HDL crops were cut for hay in October of each year, with the aim to increase gross margin from the sale of the hay and the grazing if applicable. When we calculated the average net inputs of fixed N remaining in crop residues following grain or hay removal, we found that across the two decile 1 and one decile 9 year treatments at each site, the faba beans at Urana had the highest net return of fixed N of 116 kg N/ha, the HDL averaged across all sites was 75 kg N/ha, and generally all other crops produced less than 40 kg fixed N/ha in remaining residues (Table 6). In the cropping sequences where the wheat and canola preceding the pulse crop were fertilised at a higher nitrogen level (Decile 7 strategy), the fixation rate and the quantity of fixed N remaining after grain harvest was generally reduced (lentils at Urana - 9 cf 33, lentils at Wagga = 6 cf 40).  However, at the Greenethorpe site, less than 25 kg N/ha remained following the harvest of the faba bean or lentil crops.

Table 6. Average N dynamics of the legume crops at each field site in the ‘Southern Farming Systems’ project. Values presented are averages across three seasons (2018, 2019 & 2020).

Field Site

Crops
2018, 2019, 2020

Legume biomass
(t/ha)

Shoot
N fixed
(kg N/t DM)

Total fixed N from root & shootA  (kg N/ha)

N removed from grain or hay
(kg N/ha)

Fixed N remaining in crop resides
(kg N/ha)

Total Residue N in crop (kg N/ha)

Greenethorpe

HDLC un-grazed, T

4.7

20

166

78

89

167

HDL grazed, E

4.5

19

136

63

73

153

Chickpea

5.5

10

133

88

45

169

Fababean/canolaB

4.4

17

112

88

24

94

FababeanD

5.9

14

128

144

24

91

Lentil (N7) E

4.5

12

84

82

1

65

Lentil (N2) E

4.2

10

66

81

-15

66

Wagga

HDL grazed, E (N2)

4.6

21

148

69

79

116

HDL grazed, E (N7)

4.9

18

135

63

72

120

HDL un-grazed, T (N7)

4.4

18

116

54

62

117

HDL un-grazed, T (N2)

4.8

16

115

54

61

137

Lupin

4.4

25

144

131

47

85

Lentil (N2)

4.9

16

114

74

40

105

Chickpea

4.1

11

101

63

38

126

Lentil (N7)

5.0

11

83

77

6

111

Urana

Fababean

9.6

17

235

119

116

218

HDL un-grazed, T

6.1

17

168

79

38

182

Chickpea

4.7

12

118

79

38

107

Lentil (N2)

4.6

18

130

97

33

109

Lentil (N7)

3.7

16

91

82

9

86

A The amounts of shoot N fixed were adjusted to include an estimate of N contributed by the nodulated roots as describe by Unkovich et al. (2010)
B Sown mixture of fababean and canola – Intercrop in 2018 and 2019 only
C HDL – Pasture mix consisting of vetch, Arrowleaf and Balansa clover
D Average results from fababean at Greenthorpe in 2018 and 2019 only
E The N7 and N2 relate to the nitrogen requirement in the crop sequence, not the legume crop.

To better examine the year-to-year interaction across the three sites, a complete dataset for each year is provided in Tables 7 to 9.

2018

In 2018, the %Ndfa of the chickpea crops at Greenethorpe and Wagga were very low (26-31%) and shoot N fixed were 5-7 kg N/t DM. The %Ndfa of the lentil crop at Greenethorpe was also low (30-40%), with shoot N fixed representing 6-7 kg N/t DM (Table 7). By comparison, the lentil and faba bean crops performed very well on the alkaline soils at Urana with high shoot N fixed values (17-23 kg N/t DM). The HDL crops across all sites performed the best with high %Ndfa (58-79%), and high shoot N fixed (16-27 kg N/t DM).  However, more N was removed in grain and hay than was estimated to be fixed for the chickpeas and lentils at Urana and Greenethorpe (Table 7).

2019

In the extremely dry 2019 year, the total amount of total legume biomass produced was low and this ultimately reduced the quantity of fixed N remaining in the crop residues. Nonetheless, the faba bean/canola intercrop, faba bean, lupin and HDL treatments had good %Ndfa (67-81 %) and generally had the highest amounts of fixed N in the crop residues following harvest or hay cut (Table 8). The higher soil mineral N concentration at the start of 2019 at both Wagga and Urana probably resulted in the poorer N fixation and lower net inputs of fixed N (Table 8).

2020

In 2020, all sites received substantial rainfall and this impacted different pulse crops in different ways. The Greenethorpe site received 767 mm of rainfall and the combination of the high rainfall, the persistent subsoil acidity layer (7-15 cm) with a high aluminium concentration resulted in the death of the rhizobia in the faba bean crops.  To ensure a successful faba bean harvest and to not damage the long-term treatment, 170 kg/ha of urea was applied to ensure a 4-5 t/ha faba bean grain yield. As such no analysis of N fixation could occur.  The HDL and chickpea crops at Greenethorpe had a high legume biomass, high Ndfa% (70-92 %) and high rates of shoot N fixed (17-34 kg N/t DM), which resulted in significant net inputs of fixed N remaining in the residues after grain or hay was harvested (Table 9). By comparison, there was little or no fixed N remaining in the lentil residues.

At the Wagga site in 2020, all legume crops produced between 5 and 8t/ha of legume biomass and all crops except the lentil (N7) had > 50 % Ndfa. The HDL and chickpea crops generated the highest net inputs of fixed N following harvest (74-106 kgN/ha).  The lupin crop had a high %Ndfa (75 %) and high rates of shoot N fixed (24 kg N/t DM), but after removing the 4.7 t/ha of grain, only 32 kg fixed N/ha was calculated to remain in that treatment’s residues (Table 9).

The Samira faba beans at the Urana site in 2020 produced a massive 18.2 t/ha of legume biomass with a high %Ndfa and good shoot N fixed (17 kg N/t DM). So, after subtracting the N removed from the 5.3 t/ha of grain yield, there was potentially a net input of 256 kg fixed N/ha in the crop residues (Table 9). All crops performed very well in the alkaline soils of Urana in 2020, with high grain yields; however, the lower legume biomass from the lentil (N7 treatment) and the chickpea resulted in considerably less fixed N remaining in crop residues following harvest (Table 9).

Apparent mineralisation (calculated soil mineral N benefit)

Even though elevated concentrations of soil mineral N are frequently observed after legume crops (Angus et al. ,2015), only a fraction of the N in legume residues remaining at the end of the growing season becomes available immediately for the benefit of subsequent cereal crops (Peoples et al. 2009). The microbial-mediated decomposition and mineralisation of the N in legumes organic residues into plant-available inorganic forms, is influenced by three main factors: (i) rainfall to stimulate microbial activity, (ii) the amount of legume residues present, and (iii) the N content and quality of the residues (Peoples et al., 2015: Peoples et al., 2017).

We calculated the apparent mineralisation at Junee Reefs (Tables 10 and 11) in the year following the pulse crops (2012) using three different equations (Equations 5 to 7).

Equation 5: Apparent mineralisation of legume residues (kgN/ha per tonne of grain yield)

= 100 x [(mineral N after legume) – (mineral N after wheat)] / (grain yield 2011).

Equation 6: Apparent mineralisation of legume residues (kgN/ha per tonne of shoot residue N)

= 100 x [(mineral N after legume) – (mineral N after wheat)] / (legume shoot residue N). Where shoot residue = (peak biomass DM) – grain yield.

Equation 7: Apparent mineralisation of legume N (as a % 2011 total residue N) = 100 x [(mineral N after legume) – (mineral N after wheat)] / (total legume residue N).

Results suggest that the net mineralisation over the wet 2011/12 summer fallow period represented the equivalent of 11- 46 kg N/ha per tonne of grain yield, 16 -18 kg N/ha per tonne of shoot residue DM, and 22-56 % of the pulse crop residues (Table 10). Interestingly, the apparent net

Table 7: Soil mineral N at sowing, legume biomass (DM), shoot N content (%N), reliance upon N fixation for growth (%Ndfa), shoot N accumulation and estimated quantity of shoot N and total plant N (shoot+root) fixed, grain and hay DM yields, N removed in grain or hay at harvest and the calculated net inputs of fixed N at Greenethorpe, Wagga and Urana in 2018 for a range of legume crops and treatments.

Field site

Crops

Starting soil mineral N* 0-2m (kgN/ha)

Total biomass () & legume biomass# (t/ha)

Legume (%N)

Ndfa (%)

Shoot N (kgN/ha)

Shoot N fixed (kgN/ha)

Shoot N fixed (kgN/tDM)

Total N fixed by shoot & roots A (kgN/tDM)

Total hay () & legume hay yield B (t/ha)

Grain yield (t/ha)

GrainEF (%N)

Total fixed N from root & shoot (kgN/ha)

N removed from grain or hay (kgN/ha)

Fixed N remaining in crop resides (kgN/ha)

Greenethorpe

HDLC Grazed, E

145

(4.2) 4.0

3.5

78

140

108

27

41

(3.0) 2.8

  

163

76

87

HDL Un-Grazed,T

144

(5.0) 3.7

3.3

68

124

82

22

33

(3.5) 2.6

  

123

58

66

Fababean

139

6.1

2.3

58

141

83

13

20

 

2.1

4.4

124

94

30

Fababean/CanolaD

133

4.4

2.2

78

96

75

17

26

 

2.1

4.4

113

91

21

Chickpea

153

5.0

2.1

26

106

27

5

11

 

1.9

3.9

55

74

-19

Lentil (N2)

141

5.4

2.0

30

108

31

6

9

 

1.7

4.2

47

72

-25

Lentil (N7)

158

5.4

1.8

40

98

39

7

11

 

1.7

4.3

58

73

-15

Wagga

HDL Grazed, E

64

3.8

2.7

65

102

68

18

27

2.6

  

102

47

54

HDL Un-Grazed, T

64

4.1

2.2

79

87

67

17

25

2.8

  

100

47

54

Lentil (N2)

64

3.3

2.6

70

85

59

18

27

 

1.4

4.0

88

57

31

Lentil (N7)

69

3.1

2.3

74

72

52

17

26

 

1.3

4.0

79

54

24

Chickpea

64

2.5

2.4

31

59

18

7

15

 

1.3

3.7

38

47

-9

Urana

HDL Un-Grazed, T

73

3.0

2.8

58

82

47

16

24

2.1

  

71

33

38

Fababean

73

3.0

2.9

78

88

68

23

35

 

1.8

3.9

103

72

31

Lentil (N2)

73

2.3

2.6

64

59

38

17

25

 

2.6

4.0

57

104

-47

Lentil (N7)

73

2.2

2.7

68

58

39

18

27

 

1.9

4.0

58

76

-18

*Soil mineral nitrogen determined from 0-2m at Greenethorpe and Urana.
# Total plant biomass is indicated in brackets if it is different than the total legume biomass.
A The amounts of shoot N fixed were adjusted to include an estimate of N contributed by the nodulated roots as described by Unkovich et al. (2010)
B Hay calculated as 70% of the total plant dry matter
C HDL – Pasture mix consisting of Vetch, Arrowleaf and Balansa clover
D Sown mixture of fababean and canola – Intercrop
E Lentil grain %N at Urana and Wagga was derived from the Greenethorpe analysed chickpeas (2018-2021)
F The chickpea grain %N for Wagga was derived from the Greenethorpe analysed chickpeas (2018-2021)
Note: Legume crops had <8kgN/ha of added fertiliser at sowing. N2 or N7 refer to the other crops in the sequence fertilised at a low (N2) or high (N7) rate. If no rate indicated, other crops fertilised at low rate.

Table 8: Soil mineral N at sowing, legume biomass (DM), shoot N content (%N), reliance upon N fixation for growth (%Ndfa), shoot N accumulation and estimated quantity of shoot N and total plant N (shoot+root) fixed, grain and hay DM yields, N removed in grain or hay at harvest and the calculated net inputs of fixed N at Greenethorpe, Wagga and Urana in 2019 for a range of legume crops and treatments.

Field site

Crops

Starting soil mineral N* 0-2m (kgN/ha)

Total biomass () & legume biomass# (t/ha)

Legume (%N)

NdfaE (%)

Shoot N (kgN/ha)

Shoot N fixed (kgN/ha)

Shoot N fixed (kgN/tDM)

Total N fixed by shoot & roots A (kgN/tDM)

Total hay () & legume hay yield B (t/ha)

Grain yield (t/ha)

GrainFG (%N)

Total fixed N from root & shoot (kgN/ha)

N removed from grain or hay (kgN/ha)

Fixed N remaining in crop resides (kgN/ha)

Greenethorpe

Fababean/Canola

177

4.5

2.5

67

109

75

16

25

 

1.8

4.7

112

85

27

Fababean

220

5.7

2.3

69

128

88

16

23

 

2.4

4.7

132

114

18

HDL Grazed, E

193

(3.8) 3.7

3.3

13

119

15

4

6

(2.6)3.7

  

22

10

12

HDL Un-Grazed, T

229

(5.2) 3.4

3.5

12

117

14

4

6

(3.7) 2.4

  

21

10

11

Lentil (N7)

236

2.7

2.5

49

68

33

12

19

 

0.8

4.6

50

39

11

Chickpea

217

2.9

2.3

39

68

27

9

19

 

1.2

4.1

55

52

4

Lentil (N2)

252

2.4

2.5

27

61

17

7

10

 

0.9

4.7

25

40

-15

Wagga

HDL Grazed, E (N2)

93

4.4

2.7

80

120

96

22

33

3.1

  

144

67

77

HDL Grazed, E (N7)

112

4.4

2.4

72

106

76

17

26

3.0

  

114

53

61

HDL Un-Grazed, T (N7)

113

4.0

3.1

75

97

73

18

27

2.8

  

109

51

58

Lupin

82

3.9

2.8

70

111

78

20

30

 

1.3

4.5

117

59

58

Lentil (N2)

82

4.0

2.1

62

86

54

13

20

 

0.6

4.0

81

26

55

Chickpea

82

3.2

1.9

50

61

31

10

20

 

0.5

3.7

63

20

44

HDL Un-Grazed, T (N2)

82

3.7

2.9

46

108

50

14

20

2.6

  

75

35

40

Lentil (N7)

113

4.2

2.2

22

90

17

4

7

 

0.7

4.0

26

30

-5

Urana

Fababean

73

7.7

2.4

51

183

92

12

18

 

2.0

3.9

138

78

60

Lentil (N2)

73

4.6

2.3

63

105

67

14

22

 

1.1

4.0

101

46

55

HDL Un-Grazed, T

73

4.6

2.5

57

114

64

14

21

3.2

  

96

45

51

Lentil (N7)

159

4.2

2.5

43

103

45

11

16

 

1.3

4.0

68

51

16

*Soil mineral nitrogen determined from 0-2m at Greenethorpe and Urana.
# Total plant biomass is indicated in brackets if it is different than the total legume biomass.
A The amounts of shoot N fixed were adjusted to include an estimate of N contributed by the nodulated roots as described by Unkovich et al. (2010)
B Hay calculated as 70% of the total plant dry matter
C HDL – Pasture mix consisting of Vetch, Arrowleaf and Balansa clover
D Sown mixture of fababean and canola – Intercrop
E The non-reference plant delta’s that were used to dermine the percentage of nitrogen fixed by the legume at Urana was from 2018 and 2020 non-legume weed species. As such, all of the Nitrogen fixation values and estimates of nitrogen remaining after grain or hay removal are to be used as a guide only and not to be used for journal publishable data.
F Lentil grain %N for 2019 were derived from average grain nitrogen concentrations at Greenethorpe, Urana and Wagga (2018-2020)
G The chickpea grain %N for Wagga was derived from the Greenethorpe analysed chickpeas (2018-2021)
Note: Legume crops had <8kgN/ha of added fertiliser at sowing. N2 or N7 refer to the other crops in the sequence fertilised at a low (N2) or high (N7) rate. If no rate indicated, other crops fertilised at low rate.

Table 9: Soil mineral N at sowing, legume biomass (DM), shoot N content (%N), reliance upon N fixation for growth (%Ndfa), shoot N accumulation and estimated quantity of shoot N and total plant N (shoot+root) fixed, grain and hay DM yields, N removed in grain or hay at harvest and the calculated net inputs of fixed N at Greenethorpe, Wagga and Urana in 2020 for a range of legume crops and treatments.

Field site

Crops

Starting soil Mineral N* 0-2m (kgN/ha)

Total Biomass () & Legume Biomass# (t/ha)

Legume (%N)

Ndfa (%)

ShootN (kgN/ha)

Shoot NFixed (kgN/ha)

Shoot NFixed (kgN/tDM)

Total N Fixed by shoot & roots A (kgN/tDM)

Total Hay () & Legume Hay Yield B (t/ha)

Grain Yield (t/ha)

GrainE (%N)

Total Fixed N from root & shoot (kgN/ha)

N removed from grain or hay (kgN/ha)

Fixed N remaining in crop resides (kgN/ha)

Greenethorpe

HDL un-grazed, T

139

(7.0)6.9

3.3

92

247

236

34

52

(4.9)4.9

  

355

165

189

Chickpea

184

8.4

2.4

70

200

140

17

35

 

4.1

3.4

289

139

149

HDL grazed, E

167

(6.2) 5.8

3.7

86

174

149

26

39

(4.3) 4.1

  

224

104

119

Chickpea/linseedD

225

5.0

2.2

85

109

93

19

39

 

2.3

3.1

192

73

119

Lentil (N7)

230

5.4

2.4

74

129

96

18

27

 

3.1

4.4

143

135

8

Lentil (N2)

174

4.8

2.6

68

125

84

17

26

 

3.0

4.3

127

131

-4

FababeanF

235

7.9

2.6

NA

201

NA

NA

NA

 

5.2

4.3

NA

225

NA

Wagga

HDL grazed, E (N2)

106

5.5

2.7

83

148

132

23

35

3.9

  

199

93

106

HDL grazed, E (N7)

81

6.7

2.4

79

158

127

19

29

4.7

  

191

89

102

HDL un-grazed, T (N2)

107

6.7

2.9

59

186

113

17

25

4.7

  

169

79

90

Chickpea

101

6.6

2.4

64

155

99

15

31

 

3.4

3.7

203

124

79

HDL Un-Grazed, T (N7)

121

5.0

3.1

60

157

92

18

28

3.5

  

148

65

74

Chickpea/linseedD

81

5.6

2.3

53

130

68

12

26

 

2.7

3.7

140

100

40

Lentil (N2)

79

7.4

2.5

62

186

116

16

23

 

4.0

3.5

174

138

35

Lupin

87

6.7

3.3

75

219

163

24

37

 

4.7

4.5

245

213

32

Lentil (N7)

148

7.8

2.7

45

214

96

12

19

 

4.2

3.5

144

147

-3

Urana

Fababean

102

18.2

2.2

77

403

309

17

25

 

5.3

4.0

464

208

256

HDL Un-Grazed, T

101

10.6

3.1

70

326

226

21

32

7.4

  

339

158

181

Lentil (N2)

120

6.8

2.6

87

178

154

23

34

 

4.0

3.5

232

140

91

Lentil (N7)

137

4.7

2.6

79

124

98

21

31

 

3.3

3.6

146

117

29

Chickpea

121

6.2

2.1

66

129

87

14

27

 

4.3

3.7

169

158

11

*Soil mineral nitrogen determined from 0-2m at Greenethorpe and Urana.
# Total plant biomass is indicated in brackets if it is different than the total legume biomass.
A The amounts of shoot N fixed were adjusted to include an estimate of N contributed by the nodulated roots as described by Unkovich et al. (2010)
B Hay calculated as 70% of the total plant dry matter
C HDL – Pasture mix consisting of Vetch, Arrowleaf and Balansa clover
D Sown mixture of fababean and canola – Intercrop
E The chickpea grain %N for Wagga was derived from the Greenethorpe analysed chickpeas (2018-2021)
F Fababean: The interaction between subsoil acidity and a wet season resulted in the rhizobia being killed. An additional 170kg/ha of urea was applied to ensure sufficient N for fababean grain yield.
Note: Legume crops had <8kgN/ha of added fertiliser at sowing. N2 or N7 refer to the other crops in the sequence fertilised at a low (N2) or high (N7) rate. If no rate indicated, other crops fertilised at low rate.

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.

 

Total residue N from legume & non-legume crops by end 2011

Peak Biomass 2011 minus grain/hay yield

Additional soil mineral N from legumes

Calculated soil mineral N benefits

(kg N/ha)

Per tonne of grain yield

Apparent mineralisation of 2011 legume N (kg N/t DM)

Apparent mineralisation of 2011 legume N (% residue N)

Crop 2011

(kg N/ha)

(t/ha)

(kg N/ha)

   

Lupins BM

290

8.4

86

-

10

30%

Field Pea BM

215

6.3

43

-

7

20%

Lupin

188

6.4

40

11

6

21%

Chickpea

170

4.6

82

46

18

48%

Lentil

110

2.5

41

13

16

37%

Wheat

64

 

-

  

-

Canola

111

 

-

  

-

Average after BM crops

 

65

-

8.5

25%

Average after grain crops

 

54

23

13

35%

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)

Crop 2011a

Total residue N from legume & non-legume crops by end 2011
(kg N/ha)

Soil mineral N autumn 2013
(kg N/ha)

Additional soil mineral N from legumes in autumn 2013
(kg N/ha)

Apparent mineralisation of legume N
(% 2011 residues)

Lupins BM

290

167

34

12%

Field Pea BM

215

151

18

9%

Lupin

188

151

18

10%

Wheat

64

133

-

-

Canola

111

115

-

-

Average BM crops

 

26

11%

Average of grain crops

 

18

10%

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 pHCa 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 pHca 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

 Line graph showing mean soil pHCa 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). Mean soil pHCa 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).Figure 2. Mean soil pHCa 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.

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

Crop 2021

Starting soil mineral N* 0-2m (kgN/ha)

Legume biomass (t/ha)

Legume (%N)

Ndfa (%)

Shoot N (kgN/ha)

Shoot N fixed (kgN/tDM)

ShootDM (kgN/ha)

Total N fixed root & shoot (kgN/ha)

Vetch Un-Grazed (T)

110

7.1

2.9

82

207

171

24

36

Fababean

146

11.2

2.8

79

310

244

22

33

Chickpea (N2) (ChP-W)

106

9.4

1.9

63

176

111

12

24

Chickpea (N7) (C-W-ChP)

92

9.5

2.0

67

188

127

13

28

Chickpea (N2) (C-W-ChP)

91

8.9

2.5

57

226

128

14

29

Chickpea Intercrop

150

6.2

2.1

78

127

99

16

33

# 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.

Crop 2021

Total hay () & legume hay yield (t/ha)

Grain Yield (t/ha)

Grain (%N)

Total fixed N from root & shoot (kgN/ha)

N removed from grain or hay (kgN/ha)

Ffxed N remaining in crop resides (kgN/ha)

Vetch Un-Grased (T)

(5.0) 5.0

  

257

120

137

Fababean

 

7.7

4.3

366

331

36

Chickpea (N2) (ChP-W)

 

3.1

3.2

229

101

128

Chickpea (N7) (C-W-ChP)

 

2.6

3.2

229

101

128

Chickpea (N2) (C-W-ChP)

 

2.9

3.2

263

94

169

Chickpea Intercrop

 

2.0

3.2

203

64

139

# 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

Four line graphs showing the daily and average daily temperature at the Greenethorpe trial site in 2018-2021 from interpreted data (Silo). Data courtesy of Dr Jeremy Whish.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.

 

Soil mineral N autumn 2012
(kg N/ha)

N fertiliser applied 2012
(kg N/ha)

Wheat grain yield
(t/ha)

Wheat grain protein
%)

Wheat total N uptake
(kg N/ha)

Apparent recovery of legume or fertiliser N
(%)

Lupins BM

152

49

4.0

13.6

198

29%

Field Pea BM

113

49

4.1

12.3

177

29%

Lupin

110

49

3.9

12.4

170

30%

Chickpea

152

49

4.0

12.4

181

39%

Lentil

111

49

4.0

11.2

152

35%

Wheat

70

49

3.4

9.9

114

-

Wheat

70

100

3.8

11.7

145

61%

Canola

72

49

3.4

9.8

118

-

Canola

72

100

3.8

11.8

143

49%

Mean legume

    

32%

Mean fertiliser

    

55%

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.

Box plot showing 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 pointFigure 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.

Preceding crop type

Field site

Wagga Wagga

Greenethorpe

Urana

Condobolin

Extra mineral N (kg N/ha)

Lentil

34

-

70

42

Lupin

15

-

-

60

Fababean

-

67

50

-

Vetch

37

63

77

37

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.

 

Field site

 

Wagga Wagga

Greenethorpe

Urana

Condobolin

 

Average extra mineral N (kg N/ha)

 

29

64

66

47

Urea price ($/t)

Value of extra mineral N as Urea ($/ha)

600

54

119

123

88

800

72

159

164

117

1,000

90

199

205

146

1,200

108

239

246

175

1,400

126

278

287

204

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.

Field site

Wagga Wagga

Greenethorpe*

Urana

Condobolin

Average

Average urea saving (kg/ha)

29

120

69

94

78

Average extra canola yield (t/ha)

0.21

0.18

0.38

0.11

0.22

Value of urea saving:
Urea=$1,200/t ($/ha)

35

144

83

113

94

Value of extra canola yield: Canola=$650/t ($/ha)

137

117

247

72

143

Total value of legume legacy ($/ha)

171

261

330

184

237

*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.

Nitrogen strategy of non-legume crop sequence

Decile 2

Decile 7

Average urea saving (kg/ha)

29

204

Average extra canola grain yield (t/ha)

0.21

0.02

Value of urea saving: Urea=$1,200/t ($/ha)

35

245

Value of extra canola yield: Canola=$650/t ($/ha)

137

13

Total value of legume legacy ($/ha)

171

258

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.

References

Angus JF and Peoples MB (2012).  Nitrogen from Australian dryland pastures.  Crop and Pasture Science, 2012, 63, 746-758.  http://dx.doi.org/10.1071/CP12161

Angus JF, Kirkegaard JA, Hunt JR, Ryan MH, Ohlander L and Peoples MB (2015).  Break crops and rotations for wheat.  Crop and Pasture Science, 2015, 66, 523-552.  http://dx.doi.org/10.1071/CP14252

Subsurface acidity threatens central and southern NSW cropping areas. In: National Soil Science Conference, Canberra 2018. Available at: https://www.researchgate.net/publication/330305076_Subsurface_acidity_threatens_central_and_southern_NSW_cropping_areas

Burns H and Norton M (2018). Legumes in acidic soils.  Maximising production potential in south eastern Australia. Grains Research and Development Corporation.

Burns H, Norton M and Condon J (2020).  It’s not all about lime – management strategies to improve nodulation and N2 fixation on acidic soils.  2020 Wagga Wagga GRDC update paper.

Drew E, Herridge D, Ballard R, O’Hara G, Deaker R, Denton M, Yates R, Gemell G, Hartley E, Phillips L, Seymour N, Howieson J and Ballard N (2012).  Inoculating legumes: A practical guide. Grains Research and Development Corporation.

Doughtan JA, Vallis I and Saffigna PG (1993).  Nitrogen fixation in chickpea.  Influence of prior cropping or fallow, nitrogen fertiliser and tillage.  Aust. J.Agric.Res., 1993, 44, 1403-13.

Fillery IRP (2001). The fate of biologically fixed nitrogen in legume-based dryland farming systems: a review. Aust. J. Exp. Agric. 41, 277-463.

Guinet M, Nicolardot B., Revellin C, Durey V, Carlsson G, Voisin AS (2018). Comparative effects of inorganic N on plant growth and N2 fixation of ten legume crops: towards a better understanding of the differential response among species. Plant Soil 432, 207–227.

Kirkegaard JA, Peoples MB, Angus JF and Unkovich MJ (2011).  Diversity and evolution of rainfed farming systems in southern Australia. In “rainfed farming systems”. (Eds P Tow, I Cooper, I Partridge, C Birch) pp.715-754. (Springer: Dordrecht, The Netherlands).

Kirkegaard J, Hunt J, Peoples M, Llewellyn R, Angus J, Swan T, Kirkby C, Pratt T and Jones K (2017a). Opportunities and challenges for continuous cropping systems. GRDC update (2017) Adelaide, Wagga Wagga, Ballarat.

Kirkegaard J (2017). A review of factors constraining farming systems efficiency in southern NSW (Milestone 106 – Project CFF00011.

Kumar K, Goh KM (2000). Crop residues and management practices: effects of soil quality, soil nitrogen dynamics, crop yield, and nitrogen recovery. Adv. Agron. 68, 197-319.

Lake AHW (2012). Australia’s declining crop yield trends II: The role of nitrogen nutrition.  The Regional Institute Australia.  http://www.regional.org.au/au/asa/2012/nutrition/8166_lakea.htm

People MB, Brockwell J, Herridge DF, Rochester IJ, Alves BJR, Urquiaga S, Boddey RM, Dakora FD, Bhattarai S, Maskey SL, Sampet C, Rerkasem B, Khan DF, Hauggaard-Nielsen H, Jensen ES (2009). The contributions of nitrogen-fixing crop legumes to the productivity of agricultural systems.  Symbiosis 48, 759-786. https://doi.10.1071/CP12123

Peoples MB, Brockwell J, Hunt JR, Swan AD, Watson L, Hayes RC, Li GD, Hackney B, Nuttall JG, Davies SL (2012). Factors affecting the potential contributions of N2 fixation by legumes in Australian pasture systems. Crop Pasture Sci. 63, 759-786.

Peoples M, Swan T, Goward L, Hunt J, Li G, Harris R, Ferrier D, Browne C, Craig S, Van Rees H, Mwendwa J, Pratt T, Turner F, Potter T, Glover A and Midwood J (2015). Legume effects on soil N dynamics. Comparison of crop responses to legume and fertiliser N. In: GRDC Grower Update 2015, Corowa and Finely, NSW.

Peoples M, Swan T, Goward L, Hunt J, Hart R, Hart B (2015). Legume effects on available soil nitrogen and comparisons of estimates of the apparent mineralisation of legume nitrogen. Proceedings of the 17th ASA Conference, 20-24 September 2015, Hobart, Australia. Web site: www.agronomy2015.com.au

Peoples et al. (2016).  Profitable Break Crop Management Guide.

People M, Swan T, Goward L, Hunt J, Li G, Schwenke G, Herridge D, Moodie M, Wilhelm N, Potter T, Denton M, Browne C, Phillips L, Khan D (2017). Soil mineral nitrogen benefits derived from legumes and comparisons of the apparent recovery of legume or fertiliser nitrogen by wheat.  Soil Research. 2017; 55:600-615. https://doi.org/10.1071/SR16330

Peoples MB, Hauggaard-Nielsen H, Huguenin-Elie O, Jensen ES, Justes E, Williams M (2019). The contributions of legumes to reducing the environmental risks of agricultural production, in: Lemaire, G., De Faccio Carvalho, P.C., Kronberg, S., Recous, S. (Eds.), Agroecosystem diversity- Reconciling contemporary agriculture and environmental quality. Academic Press, Elsevier, London, U.K., pp. 123-143.

Reeves TG (2020).  Is sustainable intensification of cropping systems achievable? 2020 Wagga Wagga GRDC Update.

Sanderman J and Baldock JA (2010).  Accounting for soil carbon sequestration in national inventories: a soil scientist’s perspective. IOP Publishing Ltd. Environmental Research Letters, Volumn 5, Number 3.

Santachiara G, Salvagiotti F, Rotundo JL (2019). Nutritional and environmental effects on biological nitrogen fixation in soybean: A meta-analysis. Field Crops Res. 240, 106–115

Salvagiotti F, Cassman KG, Specht JE, Walters DT, Weiss A, Dobermann A (2008). Nitrogen uptake, fixation and response to fertilizer N in soybeans: A review. Field Crops Res. 108, 1-13.

Tagliapietra EL, Zanon AJ, Streck NA, Balest DS, da Rosa SL, Bexaira KP, Richter GL, Ribas GG, da Silva MR (2021). Biophysical and management factors causing yield gap in soybean in the subtropics of Brazil. Agron. J. 113, 1882-1894.

Torabian S, Farhangi-Abriz S, Denton MD (2019). Do tillage systems influence nitrogen fixation in legumes? A review. Soil Till. Res. 185, 113-121.

Unkovich M, Herridge D, Peoples M, Boddey R, Cadisch G, Giller K, Alves B, Chalk P (2008). Measuring plant-associated nitrogen fixation in agricultural systems. ACIAR Monograph No. 136; Australian Centre for International Agricultural Research, Canberra. 1-258.

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

Vanlauwe B, Hungria M, Kanampiu F, Giller KE (2019). The role of legumes in the sustainable intensification of African smallholder agriculture: Lessons learnt and challenges for the future. Agric. Ecosyst. Environ. 284. 106583. https://doi.org/10.1016/j.agee.2019.106583.

Voisin AS, Salon C, Munier-Jolain NG, Ney B (2002). Quantitative effects of soil nitrate, growth potential and phenology on symbiotic nitrogen fixation of pea (Pisum sativum L.). Plant Soil 243, 31–42.

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