Fixed nitrogen benefit and
inoculation response of field pea
Ros s Ballard, Li z Drew, Nigel Charman and Mi ck lines1, Ma tthew Denton 2 a nd Ja s on Bra nd 3
1South Aus tra l i a n Res ea rch a nd Devel opment Ins ti tute (SARDI), 2Uni vers i ty of Adel a ide,
3Vi ctori a n Depa rtment of Envi ronment a nd Pri ma ry Indus tri es (DEPI).
Peer Revi ew: Ja ke Howi e (SARDI).
Why was the trial done?
The amount of symbiotically fixed nitrogen produced by different cultivars of field pea is being
determined and related to their compatibility with soil rhizobia, sensitivity of the symbiosis to soil
nitrogen and their production and partitioning of above ground dry matter. The potential to improve
nitrogen fixation through rhizobial inoculation is also being assessed.
The work will be used to develop cultivar specific symbiotic ratings to direct growers and peabreeding
programs towards cultivars that maximise the amount fixed nitrogen returned to the farming system.
It adds to previous and ongoing work that aims to understand the diversity and nitrogen fixation
capacity of pea nodulating rhizobia that persist in many soils and the environmental conditions that
favour their persistence.
How was the trial done?
A pea genotype and an inoculation trial are being established at two sites each year. In 2013, trials
were established at Curyo (Vic.) and Pinery (SA). The genotype trial comprised 13 field pea lines
(cultivars and advanced breeding lines) sown with or without the addition of mineral nitrogen (+30kg
N as Urea). The trial was not inoculated with rhizobia. A supplementary inoculation trial wasalsosown
at each site. The pea cultivar Kaspa was sown (+/- a high rate of inoculation) to better understand the
conditions where inoculation may be beneficial.
Nodulation, dry matter production, grain yield and grain N concentration were determined. Samples
are currently being analysed to determine the % of herbage nitrogen derived from fixation. Parallel
greenhouse tests were used to estimate number of soil rhizobia at the sites and their intrinsic
compatibility with the different pea genotypes.
Key Messages
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•
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All pea genotypes nodulated satisfactorily with the soil rhizobia at both sites, producingmore
than 40 nodules per plant. Although some genotypes had fewer nodules, it did not appear to
limit their dry matter production.
Total above ground biomass production of the pea genotypes varied by more than 60% at
both sites, and is likely to be the most significant driver of their fixed nitrogen contribution.
The predicted fixed N benefit of the different pea lines is estimated to range from 80 to 160
kg/ha at Curyo and from 78 to 123 kg/ha at Pinery. The cultivar Hayman performed best.
•
•
Addition of mineral nitrogen was less significant than plant genotype for all measured
parameters.
Inoculation failed to significantly increase nodulation, above ground biomass or grain yield
and N concentration. The lack of response to inoculation for field pea was not unexpected
given the slightly alkaline soil conditions and the presence of >300 pea rhizobia pergsoil.Onehundred rhizobia per g soil are considered adequate for prompt nodulation.
Acknowledgements
The work is funded by GRDC.
Background
Work is underway to develop symbiotic (nitrogen fixation) ratings for different genotypesoffieldpea.
It will ensure that new cultivars have strong symbiotic competence and identify cultivars with high
nitrogen fixation potential that can be used by growers where input of fixed N into the farmingsystem
is a priority, whether grown as a grain crop or used as green or brown manures.
The compatibility of pea cultivars and breeding lines with soil rhizobia, sensitivity of theirsymbiosisto
soil nitrogen, and above ground biomass production and partitioning are being measured and related
to the amount of fixed nitrogen they produce.
The field work reported here is being supplemented with greenhouse studies to characterise the
compatibility of different pea genotypes with rhizobial inoculant strains and with an expanded setof
soil rhizobia.
The potential to improve field pea nodulation and nitrogen fixation using high rates of rhizobial
inoculation is also being examined, at multiple field sites.
About the trials
In 2013, pea genotype and inoculation trials were sown at Curyo in Victoria and Pinery in SA (Table 1).
Soils were of neutral to slightly alkaline pH, contained a moderate number of pea rhizobia (100 per g
soil is usually enough for prompt nodulation) and had low levels of available N (0-10 cm).
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Table 1. Details of the trial sites
Year of last
Approx.
Trial
Soil pH
Mineral N
legume host
2013
location
(0.01M CaCl2)
(mg/kg)
nodulated by ‘pea
rainfall*
rhizobia’
(mm)
Curyo, Vic.
7.9
10
360
Lentil, 2010
255
Pinery, SA
7.8
4
>420
Lentil, 2010
336
*Amounts for April to Oct. inclusive. Source: BOM records for nearby towns of Beulah (Vic.) &
Balaklava (SA).
Number of
pea rhizobia
(per g soil)
Pea genotype trials: Thirteen pea genotypes (see Fig. 1) were sown +/- urea (equivalent of 30 units N
per ha). Urea was applied to test the sensitivity of the symbioses to soil N. No rhizobial inoculantwas
applied.
Inoculation trials: Kaspa field pea was sown (i) without rhizobial inoculation or (ii) with a high rate of
inoculum (strain WSM1455 applied at 39 and 98 times recommended rate at Curyo and Pinery,
respectively).
In all trials, treatments were arranged in a randomised block design with 4 replications at Curyo and3
at Pinery.
Plant density, early biomass production, nodule number (at about 8 weeks after sowing), peak
biomass production (at mid pod fill), grain yield and N content have been determined. Biomass
samples collected at mid pod fill have been subsampled to determine the proportion of nitrogen
derived from atmosphere (%Ndfa) using the 15N natural abundance method.
Results
Pea genotype trials
All pea genotypes nodulated satisfactorily with the soil rhizobia at both sites, producing more than40
nodules per plant (Fig. 1). Although the cultivars differed in nodule number e.g. Hayman had the least
number of nodules per plant (48 at Curyo and 54 at Pinery) and Percy the most (109 at Curyo and 127
at Pinery); the variation did not explain subsequent differences in biomass production. Nodulation
was therefore unlikely to be limiting to production.
Total above ground biomass varied by more than 60% between pea genotypes, at both sites (Fig. 1).
At Curyo, the cultivar Pearl produced least biomass (5.2 t DM/ha) and Hayman most (10.4 t DM/ha).
In contrast, Pearl produced most at Pinery (8.0 t DM/ha) and Twilight least (5.0 t DM/ha).
Grain yield also varied between pea genotype (about 0.9 t/ha for Hayman to 2.7 t/ha for OZP1101),
but for a given genotype was more stable than biomass across the two field sites.
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PBA Pearl
OZP0903
PBA Twilight
Morgan
PBA Oura
PBA Coogee
PBA Gunyah
PBA Percy
Kaspa
Parafield
OZP1104
OZP1101
PBA Hayman
Nodulation
Pinery
Curyo
0
20
40
60
80
100
120
140
Nodule number per plant
PBA Pearl
OZP0903
PBA Twilight
Morgan
PBA Oura
PBA Coogee
PBA Gunyah
PBA Percy
Kaspa
Parafield
OZP1104
OZP1101
PBA Hayman
Herbage
Pinery
Curyo
0
2
4
6
8
10
12
Total above ground biomass (t/ha)
PBA Pearl
OZP0903
PBA Twilight
Morgan
PBA Oura
PBA Coogee
PBA Gunyah
PBA Percy
Kaspa
Parafield
OZP1104
OZP1101
PBA Hayman
Grain
Pinery
Curyo
0
1
2
3
4
Grain yield (t/ha)
Figure 1. Nodule number per plant (top), total above ground biomass (middle) and grain yield
(bottom) of 13 field pea genotypes at Curyo (Vic.) and Pinery (S.A.). Treatments without nitrogen
addition shown.
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Grain N concentration differed between pea genotypes (data not shown). Breeder’s line OZP0903had
the lowest grain N at both sites (2.9% at Curyo, 3.1 % at Pinery), whereas the cultivar Hayman had the
highest concentration (3.6% at Curyo and 4.2 % at Pinery).
The proportion of N derived from the atmosphere (%Ndfa) is still being determined, so we have used
mean data (77% Ndfa and 1.6% N) from a similar trial in 2012 to calculate an estimate of the amount
of N fixed, adjusted (+25%) to take account of root contributions. On this basis, we predict that the
amount of total fixed N is likely to range from 80 to 160 kg/ha at Curyo and from 78 to 123 kg/ha at
Pinery. After grain removal, estimates of residual fixed N are 6 to 122 kg/ha at Curyo and -8 to +74
kg/ha at Pinery.
The addition of mineral nitrogen (urea) at sowing was less significant than plant genotype for all
measured parameters. At Curyo, nitrogen addition had no significant effect on nodulation, although
there was a trend for reduced nodulation in some cultivars and most evident for Percy(nodule number
reduced from 109 to 75 per plant). There was no effect on above ground biomass or grain yield and
N concentration. At Pinery, nitrogen addition resulted in small but consistent reductions in nodule
number across the different pea genotypes. Mean nodule number was reduced from 89 to 76 per
plant. Grain yield was also reduced from 6.2 to 5.8 t/ha. There was, however, no effect on above
ground biomass or grain N concentration.
Inoculation trials
Without inoculation, the cultivar Kaspa produced about 90 nodules per plant (similar to the number
in the genotype trials reported in Fig 1). Inoculation failed to significantly increase nodulation,above
ground biomass or grain yield and N concentration.
Key messages
Differences in the nodulation, production and nitrogen fixation of the different pea genotypes at
Curyo and Pinery will be combined with data from other field sites and greenhouse studiestodevelop
nitrogen fixation indices for field pea. This information will allow growers to considernitrogenfixation
potential when making cultivar selections. We estimate that differences in total fixed N will exceed
30 kg/ha, dependant on the pea cultivar used. Hayman performed best in this regard.
Slightly alkaline soils and recent pulse (host) histories are favourable to the survival of pea rhizobia,
which were present at >300 per g soil at Curyo and Pinery. It is a level considered adequate for prompt
nodulation. Even at the high rates of inoculation employed (>39 times recommended rate) there was
no increase nodulation or plant growth. The finding supports current recommendations for field pea
that where soil pH is neutral to alkaline and a well nodulated host crop had been grown in the previous
4 years, there is unlikely to be a response to inoculation.
Trials will be repeated at two field sites in 2014 and contribute to the development of symbioticindices
for field pea.
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