cropping
Nitrogen from legumes Part II
This article appeared in the July 2009 edition of the Kondinin Group’s monthly magazine Farming Ahead. The Kondinin Group holds
the copyright on the article. Reproduction of this text in whole or part by any other publication or for any other purpose is not
permitted without permission of the Farming Ahead editor. For more information contact Kondinin Group on (08) 9478 3343.
Photo: Kondinin Group
The legume story — what
happens to fixed nitrogen?
Mark
Peoples
Jill
Griffiths
For csiro plant industry
Legume benefits: Including legumes in crop
rotations replenishes soil nitrogen for the midto long-term.
At a glance
The nitrogen contributed below
ground by roots and nodules is
important in assessing how much
nitrogen legumes return to the soil.
The action of soil microbes is
required to convert the nitrogen
in the legume organic matter
to ammonium and nitrate, so
generally only a portion of
nitrogen is plant-available during
the short-term.
Cereal crops tend to recover
more than double the amount of
nitrogen from fertiliser than from
legume residues. But, nitrogen
losses from the two sources tend
not to vary greatly.
This article, the second in a series of articles on the contributions of
legumes to farm productivity, looks at what happens in the soil after
legumes fix nitrogen. The first of the series, published in the June edition
of Farming Ahead, looked at how legumes fix nitrogen.
It is often assumed because such large
amounts of nitrogen are removed from the
field as grain, legume pulse crops might not
return much fixed nitrogen to the soil for
the benefit of following crops. However, the
measurements of fixed nitrogen have often
been determined solely from measures
of legume shoot biomass and below
ground contributions of fixed nitrogen have
been ignored.
Below-ground contributions
Field research now suggests that nitrogen
associated with nodules and roots may
represent between 30–60 per cent of the
total nitrogen accumulated by legumes.
Therefore, total inputs of fixed nitrogen by
pulses are likely to be much greater than
had previously been believed from shootbased determinations.
It follows that, when below ground
contributions of fixed nitrogen are included
in nitrogen budgets compared to when
they are not, different conclusions will be
drawn about the potential for legume crops
to return fixed nitrogen to soil following
grain harvest.
Patterns of nitrogen release for crops
It is incorrect to assume all of the nitrogen
fixed by legumes will immediately be
available to crops following either a pulse or
a pasture phase. The nitrogen in legume
organic matter decomposes to produce
ammonium. A specialised group of microorganisms (nitrifiers) convert the nitrogen
in ammonium to nitrate.
Both ammonium and nitrate
can be used by plants for
growth, but many plants
prefer nitrate.
Because the conversion of organic
nitrogen into inorganic nitrogen (the
process of mineralisation) is mediated by
soil microbes, only a portion of the nitrogen
in legume root and shoot residues will
become available for plant uptake in the
short-term. This can be influenced by:
• Soil water content and temperature — Peak
rates of mineralisation during favourable
conditions, where soil water and
temperatures are optimal, are about
1–1.2 kilograms of nitrogen per hectare
per day. When temperatures fall to about
7°C the rate drops to 0.2 kgN/ha/day. The
rate drops to zero in a dry soil.
• Soil pH — Low soil pH inhibits the activity
of nitrifiers.
• The ‘quality’ of the residues — The release
of nitrogen from roots and shoots depends
on the carbon to nitrogen ratios (C:N).
Residues with a low C:N ratio decompose
fastest, and those with a high C:N ratio
can actually lead to a reduction in mineral
nitrogen
by
absorbing
nitrogen
mineralised from the soil organic matter
(this process is called immobilisation).
Not all legume residues are the same.
At the end of a growing season roots are
likely to represent the single largest pool
of legume nitrogen for mineralisation.
Farming Ahead July 2009 No. 210 www.farmingahead.com.au
33
cropping
Nitrogen from legumes Part II
For sub-clover the breakdown is usually rapid
because of the low C:N ratio (about 13:1).
The C:N ratio for lucerne roots, on the
other hand, is 25–30:1 and this results in an
initial transient immobilisation of nitrogen
followed by a slow mineralisation of lucerne
residues. This may partly explain why there
is sometimes nitrogen deficiency in crops
immediately after lucerne.
However, the slower initial release of
nitrogen from lucerne residues, combined
with a larger soil pool of organic legume
nitrogen after a long-term lucerne pasture,
means that following crops may benefit from
an improved supply of inorganic nitrogen for
a number of years after a lucerne pasture is
removed, as is illustrated in Table 1.
Nitrogen following pasture
More than 90% of the 4–5 million tonnes
of nitrogen estimated to be fixed by legumes
in Australian farming systems each year
comes from pastures.
The concentrations of inorganic nitrogen
after a pasture are affected by several factors,
as explained below.
• Studies have linked soil nitrate
concentrations to the cumulative amount
of legume biomass grown. On average,
an additional 15kg of nitrate-N/ha was
accumulated for every additional tonne of
legume dry matter (DM) (see Figure 1).
The data for this figure were derived from
experiments at two locations in southern
New South Wales. Mineral nitrogen was
measured in the top one metre of soil.
• Grazing intensity — High grazing pressure
results in greater concentrations of
mineral nitrogen. This is because a much
higher proportion of the nitrogen in the
foliage of pasture legumes is consumed
by livestock, excreted as urine, and rapidly
Table 1 Wheat yield without fertiliser*
Sequence of pastures or crops
2001 yield
1997
1998
1999
2000
2001
(t/ha)
Barley
Canola
Wheat
Canola
Wheat
2.8
Barley
Sub-clover
Sub-clover
Canola
Wheat
3.4
Barley
Sub-clover
Sub-clover
Sub-clover
Wheat
4.2
Lucerne
Canola
Wheat
Canola
Wheat
3.9
Lucerne
Lucerne
Wheat
Canola
Wheat
4.1
Lucerne
Lucerne
Lucerne
Canola
Wheat
4.1
Lucerne
Lucerne
Lucerne
Lucerne
Wheat
4.1
*This table shows the yield of wheat harvested during 2001 in an on-farm trial near Temora, southern NSW.
Plants were grown without fertiliser nitrogen for varying periods after either sub-clover or lucerne-based
pasture phases. The first line of the table shows the control site, which was continuously cropped with a
canola-cereals rotation. In the control, the yield was increased to 3.4t/ha by applying urea at a rate of 60kg
fertiliser N/ha. The lucerne pasture was four years old at the start of the experiment.
Source: Angus, McCallum, Peoples and Swan, unpublished data
Figure 1 Mineral nitrogen and legume dry matter*
400
350
Mineral N (kgN/ha)
300
250
200
150
100
Junee
Ardlethan
50
0
0
2
4
6
8
10
12
14
Legume DM (t/ha)
* Relationship between concentrations of mineral nitrogen in the top one metre of soil just before cropping and
the total above ground legume dry matter (DM) accumulated during the previous three-year pasture phase.
Source: Virgona, Dear, Sandral and Swan
34 Farming Ahead July 2009 No. 210 www.farmingahead.com.au
converted to ammonium and nitrate in
the soil.
• The timing of lucerne removal before cropping
— The length of time between removing
the lucerne pasture and sowing a wheat
crop affects crop nitrogen uptake and
grain yield (see Table 2). In the experiment
illustrated in Table 2, soil mineral nitrogen
was increased by about 0.75kgN/ha of
every additional day of fallowing, or by
0.5kgN/ha/millimetre of rainfall during
the fallow period.
Legume nitrogen versus fertiliser nitrogen
Under Australian field conditions wheat
has been reported to use 3–10% of the
residual below-ground nitrogen from a
previous lupin crop, or 8% and 16% of the
below-ground nitrogen of previous faba
bean and chickpea crops, respectively. In
the case of faba bean and chickpea, this
uptake of below-ground nitrogen contrasted
with an uptake by wheat of just 3% of the
residual shoot nitrogen. Other studies have
suggested that below-ground legume
nitrogen could be the source of between
30–75% of the total mineral nitrogen
accumulating after legumes.
As such, the below-ground
pool of legume nitrogen
appears to be an important
source of nitrogen for
following crops — and
one that has often been
ignored.
Studies that estimate uptake efficiencies
of nitrogen from legume residues also have
a tendency to underestimate the overall
nitrogen-supplying capacity of a legumebased system. This is likely to be the result
of nitrogen ‘pool substitution’ whereby the
newly applied legume nitrogen is
immobilised in the microbial biomass and
older, pre-existing nitrogen already present
in soil organic matter is mineralised. The
importance of pool substitution was
illustrated in a field experiment in Western
Australia. In the second year of a lupinwheat rotation, researchers found gross
nitrogen mineralisation in the top 10cm of
soil to be 120kgN/ha, and net nitrogen
mineralisation (gross mineralisation —
immobilisation) to be only 59kgN/ha, 69%
of which (41kgN) originated from the soil
microbial pool.
These data suggest that most of the
nitrogen initially released from lupin residues
was immobilised and thus inaccessible to the
wheat crop in the short-term.
This was compensated for by mineralisation
of older microbial nitrogen that subsequently
became available for crop uptake.
The net result of such processes is that
calculations based on the direct crop
recovery of legume nitrogen such as
presented in Table 3 will underestimate the
full ‘agronomic’ nitrogen benefits derived
from including a legume in a rotation.
cropping
Nitrogen from legumes Part II
nodules, and focus only on shoot material.
Some studies also underestimate the overall
nitrogen-supplying capacity of a legumebased system.
Table 2 Effect on soil nitrogen of elapsed time between lucerne removal and cropping*
Time of lucerne removal
Soil mineral nitrogen
(top 2m) at sowing
(kgN/ha)
Wheat above-ground
nitrogen at maturity
(kgN/ha)
Grain yield (wheat)
6
206
137
5.9
4
111
109
5.0
2
59
86
3.8
(Months before sowing)
(t/ha)
Further, all the consequences
of using fertiliser or legume
nitrogen may not become
apparent in the relatively
short-term of many
nitrogen studies.
* Results from an on-farm trial near Junee, southern NSW. Values represent the combined data from both
cultivation and herbicide removal treatments.
Source: Angus et al. (2000), Australian Journal Agriculture Research 51: 877–890
Table 3 Fate of nitrogen in cereal crops*
Source of nitrogen
applied
Crop uptake
(% applied N)
Recovered in soil
(% applied N)
Unrecovered [assumed
lost] (% applied N)
17–50
21–40
16–62
36
31
33
5–27
37–90
4–54
15
62
23
Fertiliser
Average
Legume
Average
It is not essential to grow legumes to
have nitrogen available to plants. However,
without legumes, the nitrogen resources
of the soil will be mined and the practice is
not sustainable. There is also more to
the legume story than nitrogen alone,
as will be covered in a later edition of
Farming Ahead.
* Figures are from experiments using either 15N enriched fertilisers or legume residues applied to cereal crops,
and show the range of estimates of crop nitrogen recovery and the extent of losses of the applied nitrogen.
Acknowledgements Much of the data
presented here resulted from past
GRDC-funded projects.
Source: Peoples et al. (2009), Symbiosis (in press)
Nitrogen use efficiency — the low-down
Studies in rain-fed farming systems
indicate that cereal crops, on average, tend
to recover more than twice the nitrogen
from fertiliser than from legume residues
(see Table 3).
However, the estimates of nitrogen losses
from legume sources (on average 23%) tend
not to be very different from fertilisers
(33%). This is despite a higher proportion
of the legume nitrogen generally remaining
in the soil at harvest as compared with
fertiliser nitrogen.
CSIRO research
Contact Mark Peoples,
CSIRO Plant Industry
(02) 6246 5447
(02) 6246 5062
[email protected]
www.csiro.au/pi
It is important to remember that many
studies on legume nitrogen ignore the
below-ground contribution of roots and
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