Grains Research and Development

Date: 10.02.2015

Legume effects on soil N dynamics - comparisons of crop response to legume and fertiliser N

Author: Mark Peoples, Tony Swan, Laura Goward, James Hunt (CSIRO) Guangdi Li (NSW DPI), Rob Harris (DEPI Vic), De-Anne Ferrier, Claire Browne, Simon Craig (Birchip Cropping Group), Harm van Rees (CropFacts), James Mwendwa (Central West Farming Systems), Tony Pratt (FarmLink), Felicity Turner (MacKillop Farm Management Group), Trent Potter (Yeruga Crop Research), Allison Glover (Riverine Plains Inc) and Jon Midwood (Southern Farming Systems)

GRDC project code: CSP000146

Take home messages

  • The choice of legume species and management was found to influence the amount of nitrogen (N) fixed by both crop and pasture legumes by affecting either legume reliance upon N2 fixation for growth or changes in dry matter (DM) accumulation. On average 19 kg of N is fixed per tonne of shoot dry matter produced by pulse crops.
  • The amount of N fixed by field peas tends to be lower than either lupin or faba bean.
  • Medium to high soil mineral N concentrations at sowing (>50 kg mineral N/ha) appeared to suppress N2 fixation by field pea more than lupins or faba beans.  
  • Pulse legumes grown for grain generally result in lower net inputs of fixed N than either brown manured or forage legumes because large amounts of N are removed at grain harvest.  
  • Available soil N following either legume crops or pastures can represent an additional 40 to 90 kg N/ha in the 1st year and 20 to 35 kg N/ha for the 2nd year relative to continuous cereal sequences. This additional mineral N represented 7-11 kg N per tonne of pulse residue DM, or 15 kg N per tonne legume DM grown in a pasture.
  • The increased N uptake by the 1st wheat crop grown after legumes was equivalent to 27-40% of N in the previous year’s legume residues. In comparison, 47-59% of fertiliser N was recovered by wheat when it was applied at stem elongation just prior to a period of high crop demand for N.

Introduction

This paper reports results from previous research and some recent findings from a GRDC-funded project (CSP00146) where inputs of fixed N2 by different legumes have been routinely measured during experimentation and the nitrogen (N) dynamics of following wheat crops relying upon legume and/or fertiliser N have been assessed. The project examines the effect of legumes or canola break crops on subsequent cereal productivity in cereal-dominated cropping systems through participatory research, in partnership with leading grower groups and agribusiness consultants from NSW, Victoria and SA.

Legume inputs of fixed N

The amounts of N2 fixed by legumes are regulated by two factors: (i) the amount of legume N accumulated over the growing season (as determined by shoot dry matter (DM) production and %N content), and (ii) the proportion of the legume N derived from atmospheric N2 (often abbreviated as %Ndfa).

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

Comparisons of different legumes

Several project studies have demonstrated the impact of crop species and management on inputs of fixed N. This was exemplified recently by the SFS ‘Pulse Challenge’ competition where 21 farmer, agribusiness and researcher teams grew either field pea, lupin or faba bean at Lake Bolac or Inverleigh, Vic in 2011. When the three pulses were compared side-by-side, field pea fixed less N per tonne shoot dry matter (DM) production than either lupin or faba bean (Fig. 1a). Field pea’s lower reliance upon N2 fixation for growth (%Ndfa) suggested that it was more sensitive to soil nitrate at sowing than the other crops (40 and 84 kg N/ha at Inverleigh and Lake Bolac at 0-100 cm, respectively; Fig. 1b). The N2 fixation data collated across all 21 crops indicated a strong relationship between the amounts of shoot N fixed (ranged from 1-163 kg N/ha) and shoot DM (0.04 t DM/ha where grass weeds were not controlled to 7.2 t DM/ha) and on average 19 kg shoot N was fixed per tonne shoot DM accumulated (Fig. 1c and d). Similar relationships were observed in trials undertaken in association with MFMG in SA and FarmLink in southern NSW, and have been reported previously across a range of environments and legume species (see also Peoples et al. 2009; Unkovich et al. 2010). 

Figure 1. Estimates of (a) the amounts of shoot N fixed per tonne of above-ground dry matter (DM) by faba bean, lupin or field pea grown at Inverleigh and Lake Bolac, Vic in 2011, and (b) the percentage of legume N derived from the atmosphere (%Ndfa) for each pulse species and location.

Figure 1. Estimates of (a) the amounts of shoot N fixed per tonne of above-ground dry matter (DM) by faba bean, lupin or field pea grown at Inverleigh and Lake Bolac, Vic in 2011, and (b) the percentage of legume N derived from the atmosphere (%Ndfa) for each pulse species and location. The relationship between the amount of shoot N fixed and pulse shoot DM depicted in (c) and (d) represented 19.4 kg N per tonne DM accumulated across all crops (R2 = 0.85). Bars indicate standard deviation.

N2 fixation by commercial pulse crops

A total of 35 commercial pulse crops have been sampled in famers’ paddocks for determinations of N2 fixation in southern and central NSW, the Victorian Mallee and Wimmera, and the high-rainfall zone of south-eastern SA between 2001 and 2013. Amounts of shoot N fixed ranged from 12-180 kg N/ha (median 61 kg N/ha and 16 kg N/t DM) and %Ndfa from 8-87% (median 68%, Table 1). On-farm measures of %Ndfa were <50% N in nine of 35 crops, with <10 kg shoot N being fixed per tonne of shoot DM in seven of those crops. In some instances low inputs of fixed N could be related to direct effects of drought on growth, or high concentrations of soil nitrate where a period of drought was followed by a wet summer-autumn. In other situations this seemed to be related to routinely sowing legumes without inoculation, or the early termination of legume crops in mid-spring either by cutting for hay or sprayed with knock-down herbicides for weed control and to provide N benefits for subsequent crops (ie brown manuring).

Table 1. The range of measures of shoot dry matter (DM) production and estimates of N2 fixation for 35 commercial pulse crops sampled in farmers’ fields between 2001 and 2013. Mean values for each parameter and crop species are shown in brackets.

Legume

Number
of crops

Shoot DM

Shoot N fixed

(t DM/ha)

(%Ndfa)

(kg N/ha)

(kg N/t DM)

Faba bean

5

7.2-8.4

68-89

117-152

16-18

[7.6]

[74]

[135]

[17]

Lupin

11

0.9-10.2

20-82

20-150

9-21

[5.5]

[59]

[74]

[15]

Vetch

3

4.2-6.3

54-84

53-135

13-22

[5.1]

[69]

[89]

[17]

Lentil

3

2.0-5.3

17-82

20-104

4-20

[4.0]

[50]

[51]

[13]

Field pea

7

2.3-5.9

8-85

12-87

2-20

[3.9]

[53]

[45]

[14]

Chickpea

6

0.8-5.2

24-87

13-66

7-17

[2.9]

[67]

[34]

[13]

Median all crops

 

 

68

61

16

Net inputs of fixed N2

Knowing the amounts of shoot N fixed by legumes is 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 60% of the total plant N, this below-ground contribution of fixed N could 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 N2 fixation to include an estimate of how much fixed N might also be associated with the nodulated roots using a ‘root factor’ (Peoples et al. 2012; Unkovich et al. 2010). For many pulse legumes around one-third of the plant N may be below-ground in roots and nodules; in this case a ‘root factor’ of 1.5 would be used.

Total N fixed = (shoot N fixed) x root factor

The net inputs of fixed N are derived by comparing the total amounts of N fixed to the amounts of N removed in harvested grain or animal products, or lost from the system via volatilisation of ammonia from urine patches where the legumes are grazed (Peoples et al. 2012).

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

Various studies undertaken in SA, Vic and southern NSW have compared inputs of fixed N by pulses grown for grain or brown manure (BM) and pure legume swards either cut for hay or used as grazed pasture. Data generated by experiments indicated that brown manured crops and forage legumes generally provided greater net returns of fixed N to soils than grain crops since large amounts of N were removed in the high-protein legume grain at harvest (Table 2). However, it is also clear from these data that different legume species have different potential for growth and N2 fixation regardless of their eventual end-use (Table 2).  

Table 2. Examples of net contributions of fixed N where the total amounts of N2 fixed by different legumes grown for forage, brown manure (BM) or grain have been compared to estimates of the amounts of N removed in either hay, wool, or grain, or lost by volatilization from urine patches from grazed pastures (Peoples et al. 2012 and unpublished data).

Location

Legume

Total amounts of  N2
fixeda

N removed
or lost

Net input of
fixed N

(kg N/ha)

(kg N/ha)

(kg N/ha)

Naracoorte, SA

Subclover pasture

102

8 in wool + 24 lost

+70

Field pea for grain

125

128 in grain

-3

Faba bean for grain

180

120 in grain

+60

Hopetoun, Vic

Vetch for BM

130

0

+130

Vetch for hay

130

89 in hay

+41

Field pea for grain

125

136 in grain

-11

Yarrawonga, Vic

Field pea for BM

86

0

+86

Field pea for hay

86

65 in hay

+21

Vetch for hay

141

82 in hay

+59

Arrowleaf clover hay

138

70 in hay

+68

Subclover for hay

118

68 in hay

+50

Faba bean for grain

129

105 in grain

+24

Chickpea for grain

50

60

-10

Wagga, NSW

Forage legume mixb

71

8 in wool + 17 lost

+63

Vetch for forage

83

8 in wool + 23 lost

+52

Field pea for grain

65

104 in grain

-39

Lupin for grain

75

105 in grain

-28

Junee, NSW

Lupin for BM

246

0

+246

Field pea for BM

114

0

+114

Lupin for grain

310

214 in grain

+96

Chickpea for grain

141

77 in grain

+65

Lentil for grain

137

139 in grain

-2

aThe 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).

bPasture mixture with a forage dry matter composition consisting of 41% balansa clover, 29% subclover,  17% berseem clover, and 13% arrowleaf clover.

Impact of legumes on available soil N

Crop legumes

Although legumes exert a significant effect on total soil N content through biological N2 fixation it is often difficult to quantify significant short-term changes because the inputs of fixed N are generally small relative to the large (and variable) background concentration of organic N in soil. By contrast measurements of elevated soil inorganic (mineral) N after legumes are very common across a wide range of different cropping systems. For example, a large data set of pre-season measures of soil mineral N collected from farmer paddocks in SA between 2002 and 2014 suggested that, on average, concentrations of soil mineral N after legumes can be expected to be 25-35 kg N/ha higher than following cereals (Table 3).

Table 3. Examples of autumn measures of concentrations of available soil N (0-0.6m) following cereals or break crops from commercial cropping paddocks located on the Yorke Peninsula, the mid-north and upper north of South Australia between 2002-2014a

Paddock use in the

Number of paddocks

Soil mineral N

previous year

sampled

Measured range
(kg N/ha)

Average
(kg N/ha)

Wheat

847

8 - 200

67

Barley

267

9 - 203

56

Faba bean

99

36 - 187

97

Field pea

110

43 - 158

90

Lentil

248

26 - 245

87

aClient data courtesy of Allan Mayfield Consulting (Clare), Holmes Farm Consulting (Maitland) and McC Ag Consulting (Laura, SA).

Results from an experimental trial undertaken near Junee in southern NSW in 2011 (Table 4) indicated that soil mineral N measured just prior to sowing wheat in 2012 was 42 or 92 kg N/ha greater following lupin than after wheat or canola where lupin crops had been grown for either grain or brown manure (BM), respectively (Table 5).  This represented the equivalent of 7-11 kg mineral N/ha per tonne of legume residue biomass. Concentrations of soil mineral N were still 18 or 34 kg N/ha higher under the lupin grain crop-wheat and lupin BM-wheat sequences, respectively than for wheat-wheat in 2013 when another wheat crop was grown (Table 5).

Table 4. Dry matter (DM) accumulation, grain yield and N remaining in crop residues where either lupin grown for grain or brown manure (BM), wheat, or canola were sown at Junee, NSW in 2011a.

Crop grown in 2011

Peak biomass
(t DM/ha)

Grain yield
(t/ha)

Grain N 
harvested
(kg N/ha)

N remaining
in residues
(kg N/ha)

Lupins BM

8.4

0

0

290

Lupins

9.9

3.5

210

188

Wheat +Nb

11.1

4.8

87

64

Canola +Nb

10.6

3.2

94

111

LSD (P<0.05)

1.3

0.5

11

22

aGrowing season rainfall = 216 mm compared to long-term average of 311 mm.

bUrea fertiliser was applied to wheat @ 49 kg N/ha and canola @ 66 kg N/ha.

Table 5. Concentrations of soil mineral N (0-1.6m) measured in autumn 2012 and 2013 following either wheat, canola and lupin grown for grain or brown manure (BM) at Junee, NSW in 2011, and calculations of the apparent net mineralisation of N from lupin residues. 

Crop grown in 2011

Soil mineral Nautumn 2012

Apparent mineralisation of legume N

Soil mineral Nautumn 2013

Apparent net mineralisation of legume N

(kg N/ha)

(% 2011 residue)

(kg N/ha)

(% 2011 residue)

Lupins BM

169

32%

167

12%

Lupins

119

22%

151

10%

Wheat

77

-

133

-

Canola

76

-

115

-

LSD (P<0.05)

35

20

It is possible to calculate the apparent net mineralisation of lupin N by dividing the differences in soil mineral N data following the 2011 lupin and wheat treatments (Table 5) by the amount of N present in the original lupin residues at the end of the 2011 growing season (Table 4) – note: this assumes a negligible net N release from the 2011 wheat stubble or roots and provides a conservative estimate of the apparent net mineralisation of the lupin N.

Apparent mineralisation = 100x [(mineral N after legume) – (mineral N after wheat)] /(legume residue N)

These calculations suggested that net mineralisation over the wet 2011/12 summer fallow (474 mm 25th Nov11 – March12) represented the equivalent of 22-32% of the 2011 lupin N, or 7-11 kg mineral N per tonne of residue DM. A further 10-12% of the residue N was subsequently released during the 2012/13 fallow period prior to sowing the 2013 wheat crop. 

Pasture legumes

Information on the release of mineral N after legume-based pastures can be gleaned from data generated following three years of different pasture treatments imposed at two locations in NSW that differed in total average annual rainfall (550mm at Junee, and 430mm at Ardlethan). These data (Fig. 2) indicated that concentrations of soil mineral N measured in the autumn immediately after a pasture were related to the cumulative amount of legume shoot biomass grown during the pasture phase (Fig. 2). In this study, an additional 15 kg mineral N/ha (on average) was accumulated over and above background mineralisation of soil organic N for every additional tonne of legume foliage DM grown (Fig.2).

In systems where alternating phases of lucerne-based pasture and grain crops are used, the lucerne needs to be terminated with herbicide or tillage prior to sowing a crop. On-farm experimentation also undertaken near Junee indicated that both the concentrations of soil mineral N measured when sowing the first wheat crop after the lucerne pasture, and the subsequent impact on crop N uptake and grain yield, were closely related to the timing of the removal of the lucerne prior to cropping (Table 6). In this particular experiment soil mineral N was increased by around 0.75 kg N/ha for every additional day of fallowing, or by 0.5 kg N/ha per mm of rainfall over the fallow period (Angus et al. 2000).

Figure 3. Relationship between concentrations of mineral N in the top 1m of soil just prior to cropping and the total shoot dry matter (DM) accumulated during the previous 3 years by pasture legumes.

Figure 3. Relationship between concentrations of mineral N in the top 1m of soil just prior to cropping and the total shoot dry matter (DM) accumulated during the previous 3 years by pasture legumes. Regression equation: Mineral N =  14.8 x (legume DM) + 130 (R2 = 0.66).

Table 6. Example of the effect of timing of removal of lucerne prior to cropping on concentrations of soil mineral N (0-2m) at the time of sowing wheat, the subsequent crop uptake of N and grain yielda.

Time of lucerne removal
(months prior to sowing)

Sowing
soil mineral N
(kg N/ha)

Wheat shoot N
at maturity
(kg N/ha)

Wheat
grain yield
(t/ha)

6

206

137

5.9

4

111

109

5.0

2

59

86

3.8

aData represent the combined results of cultivation and herbicide removal treatments (Angus et al. 2000).

Comparisons of crop use of legume or fertiliser N

Not all of the N in legume residues will be available immediately to crops following either a pulse crop or a pasture phase. The decomposition and mineralisation of residue N into inorganic forms are microbial-mediated processes with the breakdown of organic compounds providing the soil microbes with a C source for respiration and growth. Much of the simple organic N released is rapidly assimilated (immobilised) by the soil microbial population (Peoples et al. 2009). Mineral N for uptake by plants becomes available only when the amounts of N released from the organic residues exceed the microbial growth requirements (i.e. when gross mineralisation of N exceeds microbial immobilisation). This is more likely to occur with legume material than with cereals residues since legume organic matter has a higher N content and lower C:N ratio. Since the conversion of organic N into inorganic N is mediated by soil microbes, only a portion of the N originally present in the nodulated roots and legume shoot residues will become available for plant uptake in the short-term.

The large differences in soil mineral N observed following lupin grown for grain or BM in 2011 compared to wheat or canola top-dressed with fertiliser N applied at stem elongation, in the experiment described above in Tables 4 and 5 resulted in major increases in wheat N uptake and grain protein in 2012 after both lupin crops (Table 7). Unfortunately the impact of the additional N supply via either of the lupin treatments or the top-dressed fertiliser N was not reflected in grain production as yields were just 0.4-0.6 t/ha greater than that achieved by wheat grown only with basal fertiliser N after wheat or canola (Table 7). Essentially the dry growing season in 2012 (168mm cf 300mm long-term average) restricted the full benefits of the additional N supplied to the wheat being translated into grain yield. However, the design of the experiment was such that it was possible to derive estimates of the apparent recoveries of lupin N and top-dressed fertiliser N by wheat using the following equations and the lupin residue N data from Table 4:

Apparent recovery legume N = 100x [(wheat N after legume) – (wheat N after wheat)] /(legume residue N); and

Apparent recovery of fertiliser N = 100 x [(wheat N100N) - (wheat N49N)]/(51)

Table 7. Grain yield and crop N uptake by wheat in 2012 following either wheat, canola and lupin grown for grain or brown manure (BM) at Junee, NSW in 2011, and calculations of the apparent recoveries by wheat of either N from lupin residues, or top-dressed fertiliser N. 

Crop grown

in 2011

Sowing soil mineral N 2012

N fertiliser applied
2012

Grain
yield

Grain protein

Wheat N
uptake

Apparent N recovery

(kg N/ha)

(kg N/ha)

(t/ha)

(%)

(kg N/ha)

(%)

Lupins BM

169

49

4.0

13.6

184

27

Lupins

119

49

3.9

12.4

159

28

Wheat

77

49

3.4

9.9

106

-

Wheat

77

100

3.8

11.7

136

59

Canola

76

49

3.4

9.8

113

-

Canola

76

100

3.8

11.8

137

47

LSD (P<0.05)

35

0.3

 0.8

Note: All 2012 wheat plots received a total of either 49 or 100 kg N/ha comprising of either 2.5 and 46 kg N/ha, or 7.5 and 92 kg N/ha applied at sowing and stem elongation (GS31); respectively.

Subsequent calculations suggested that the 2012 wheat crop recovered the equivalent of 27-28% of the lupin residue N. This compared to apparent recoveries of 47-59% of the top-dressed fertiliser (Table 7). Data from an experiment undertaken at Breeza on the Liverpool Plains in northern NSW in the late 1990’s also provided another opportunity to undertake similar calculations to determine the apparent uptake of legume residue N by wheat. In this case, the equivalent of 40% of faba bean N was recovered by the next crop (Table 8). Comparisons of treatments with or without above-ground residues imposed in the Breeza study suggested that ~70% of the faba bean N assimilated by wheat came from the nodulated roots.

Table 8. Wheat N uptake in 1998 following either faba bean or barley grown at Breeza, NSW in 1997, and calculations of the apparent recoveries by wheat of the N from faba bean residuesa

Crop grown in 1997

Residue N
in 1997b

Wheat N uptake
in 1998

Apparent recovery legume N

(kg N/ha)

(kg N/ha)

(%)

Faba bean

96

97

40

Barley

73

59

-

a Source: Peoples et al (2009). Note: no fertiliser N treatments were included in this study.

b Includes an estimate of the contribution of below-ground N reported by Khan et al. (2003)

The relatively high recovery (47-59%, Table 7) of the top-dressed fertiliser by the Junee wheat crop is not totally unexpected since the N was applied just prior to the period of peak crop demand for N, which is consistent with the most appropriate timing for N applications to achieve the highest efficiencies of N use and lowest risks of N losses (Crews and Peoples 2005). Unfortunately the experimental design prevented a similar estimate for the recovery of the basal fertiliser N applied at sowing. However, a number of studies have monitored the fate of fertiliser N supplied at sowing using isotopic tracers in the past in various rainfed cereal systems around the world, and some of these data are summarised in Table 9. While there is a range of results, it might be concluded that on average roughly one-third of the fertiliser N tends to be assimilated by the crop. This value is comparable to the estimates obtained for the effects of lupin and faba bean on crop N uptake reported in Tables 7 and 8.

Table 9. Summary of the fate of fertiliser N applied at sowing collated from different rainfed cereal production systemsa.

Measures

Crop uptake

Recovered in soil

Unrecovered
[assumed lost]

(% applied N)

(% applied N)

(% applied N)

Range

17-50

21-40

16-62

Mean

36

31

33

a Source: Crews and Peoples (2005)

Conclusions

Legume inputs of fixed N

The choice of legume species and management were found to influence inputs of fixed N by legumes by affecting either %Ndfa or DM accumulation. Around 19 kg of legume shoot N is commonly fixed per tonne of shoot DM produced by pulse crops. On-farm measures of N2 fixation suggest constraints to N2 fixation in 20-25% of commercial pulse crops. Median estimates of %Ndfa across 35 farmers’ crops indicated that these crops were deriving ~70% of N requirements from atmospheric N2, and fixing ~16 kg shoot N/t DM produced. Residual fixed N from brown manured crops or pure pasture legume swards were generally greater than net inputs of fixed N remaining after pulses largely due to the export of large amounts of N in harvested grain.

Impact of legumes on available soil N

There is considerable evidence that the inclusion of legumes in cropping sequences results in higher available soil N for subsequent crops. Data collected from farmers’ paddocks in SA suggest that this might represent on average 25-35 kg N/ha more mineral N than after wheat. Information collected elsewhere in south-eastern Australia indicate that in the case of a pulse grown for grain or BM, concentrations of available soil N can be 42-92 kg N/ha greater than following wheat or canola in the 1st cropping season after the legume was grown representing apparent mineralisation of 20-30% of the N originally present in the legume residues, and 18-34 kg N/ha in the 2nd year, representing 10-12% of the residue legume N. The additional N mineralised prior to sowing the 1st subsequent crop can be equivalent to 7-11 kg N/ha per tonne of residue DM for pulses, and 15 kg N/ha per tonne of legume DM grown during a pasture phase.

Comparisons of legume and fertiliser N

As the release of inorganic forms of N from legume residues in soil is a microbial-mediated process, not all the legume N returned to soil becomes available in the short-term. Consequently, the apparent recovery of legume N by a following cereal crop (27-40% across two different studies) tends to be lower than top-dressed fertiliser (47-59%), but may not be too dissimilar from fertiliser applied at sowing. However, losses of N from the system are usually lower from legume sources than from fertiliser (Crews and Peoples 2005), and a major contribution of legumes is the maintenance of the long-term organic fertility of the soil.

Acknowledgements

GRDC is gratefully acknowledged for its support of on-going research on break crop effects on wheat (Project CSP000146), and Allan Mayfield, Matt McCallum and Sam Holmes are thanked for their generous provision of client soil nitrate data.

References

Angus et al. (2000) Australian Journal of Agricultural Research 51, 877-890.

Crews and Peoples (2005) Nutrient Cycling in Agroecosystems 72, 101-120.

Khan et al (2003) Australian Journal of Agricultural Research 54, 333-340.

Peoples et al.(2009) Symbiosis 48, 1-17.

Peoples et al.(2012) Crop & Pasture Science 63, 759-786.

Unkovich et al. (2010) Plant and Soil 329, 75-89.

Glossary of key terms used

  • Mineral Nnitrate (NO3) and ammonium (NH4) sources of N. They are considered to be the most readily plant-available forms of N in the soil and are sometimes also referred to as inorganic N.
  • Soil Organic Norganic forms of N in soil such as previous crop residues and humus that is not readily available for plant growth until it is converted into mineral N by soil microbes.
  • Fixed N the amount of atmospheric N2 biologically fixed by soil bacteria (rhizobia) via a symbiotic relationship with the legume in nodule structures on legume roots.
  • Residual fixed N the amount of fixed N calculated to remain in legume residues once N removed in agricultural produce (e.g. N exported in harvested grain, or N in animal products) is accounted for.
  • Shoot N – amount of N in all above-ground plant biomass.
  • Legume residue biomass – above-ground legume biomass that remains after grain harvest.
  • Apparent net mineralisation of legume N – the total increase in soil mineral N equivalent to the differences in concentrations of plant-available soil N following a legume and after a non-legume compared to the amount N estimated to have been present in the legume residues.
  • Apparent recovery of legume N – equivalent to the differences between wheat N uptake following a legume and wheat N after a non-legume compared to the amount N estimated to have been originally present in the legume residues the previous year.
  • Apparent recovery fertiliser N – equivalent to the differences between wheat N uptake at two rates of fertiliser N compared to the difference in the amount of fertiliser N applied.

Contact details

Mark Peoples and Tony Swan
CSIRO Agriculture Flagship, GPO Box 1600 Canberra ACT 2601
mark.peoples@csiro.au
tony.swan@csiro.au