FAQ: Nitrogen Fixation

Can all pasture legumes fix nitrogen?

Generally, yes. But it is actually the rhizobia bacteria which inhabit nodules on the legume roots which fix the nitrogen (N). The plant swaps carbohydrate (formed during photosynthesis) for fixed N from the bacteria in a mutually beneficial symbiotic relationship where both partners benefit from the relationship. However, some legumes require specific bacteria strains to fix N for them (e.g. lucerne, Caucasian clover, Lotus corniculatus) and this is why it is important to inoculate legume seeds before sowing especially if there is no recent history of that legume in that paddock. Other rhizobia present in the soil can also infect the nodules but if they aren’t the ones needed they form small ineffective nodules – that is they become a carbon drain on the plant. The plant continues to supply the bacteria in the nodule with carbohydrates but it doesn’t receive any fixed N in return. Effective nodules are pink inside if you cut them in half.

How much nitrogen can pasture legumes fix?

In general both annual and perennial clovers fix nitrogen (N) in direct proportion to the amount of dry matter (DM) they produce. You can expect to get at least 25 kg N to be fixed per ton of clover DM grown. So if a 10 t DM/ha/year pasture has an average of 20% clover content in the DM, the 2 t/ha of clover produced would contribute 50 kg N/ha of pasture annually. This is, of course, insufficient for maximum grass growth and dark green urine patches will be an obvious indication that most of the grass in the pasture is N deficient.

We have detected little difference between species in N fixing ability. A comparison between irrigated white and Caucasian perennial clovers growing in ryegrass pastures showed that both fixed about 26 kg N/t clover DM produced (Widdup et al. 2001). In dryland pastures we have shown that sub or white clover sown with cocksfoot fixed about 28 kg N/t of clover produced (Lucas et al. 2010). Other studies on volunteer clovers such as haresfoot and striated clovers have revealed similar rates of N fixation per ton of clover herbage produced. Australian work (Peoples & Baldock 2001) on N fixation by a wide range of herbage legume species, including clovers, has also conformed to this general rule.

The 25 kg N/t legume DM grown is a conservative estimate. It takes no account of the N content of roots and nodules (because basically it’s difficult and time consuming to routinely dig up all the roots and nodules). Work where roots and nodules have been measured and included in N fixation estimates indicate that total N fixed is over 40 kg N fixed/t of legume DM.

What factors limit N fixation?

Anything that reduces legume production will reduce the amount of N fixed!!

Nutrient availability, soil acidity, seasonal temperatures and the development of water stress can all compromise the ability of legumes to grow and thus the amount of N which can be fixed. BUT: the main limitation is caused by competition from companion grasses in pasture mixes.

Legume production, and with it, N fixation are limited by nutrient deficiencies such as phosphorus (P), sulphur (S), molybdenum (Mo) and boron (B). Low soil pH, especially soil pH (in H20) <5.5 , causes an increase in soluble aluminium levels which can be toxic to some legumes and inhibit the ability of the plant to explorer deeper soil layers for water and nutrients. Acid soils can also impact on the ability of some N fixing rhizobia to survive.

Cool winter temperatures restrict perennial legume production more than annual clovers which are adapted to grow rapidly in late winter early spring before the onset of summer drought.

Soil moisture deficits will also limit legume productivity because as the soil dries less water is available. This also restricts uptake of nutrients from the soil, which enter the plant in solution, via the roots.

However, the main limitation to legume production and the associated N fixation is competition from grasses. A pure stand of white clover in a summer rainfall area or under irrigation will produce at least 10 t DM/ha/yr and fix 250 kg N/ha/yr. Red clover and lucerne can produce much more DM and N than that. Annual clovers grown as a pure stand in a deep soil in a summer dry, 750 mm rainfall climate will also produce at least 10 t DM/ha/yr and inject 250 kg N/ha into farm system. Under ideal conditions some annual clovers will produce 14 to 16 t/ha DM. This can therefore be equivalent to about 400 kg N/ha.

Where legumes are grown alone and grazed in situ (e.g. lucerne) the N fixation rate per ton legume DM grown may decline over a period of several years. This is because soil N accumulates in the absence of a grass to exploit the high soil N. It may therefore, eventually be more efficient for the legume plant to take up soil N rather than use carbohydrates from its current photosynthesis to fix more N. Ultimately, legumes are lazy – there is a lower energy cost to the plant if it uses “free” soil N rather than paying for N fixation by supplying rhizobia bacteria in the root nodules with carbohydrate.

As soil N accumulates under legume dominant stands grass weeds tend to invade. Management options then include grass killing herbicides, hay or silage crops to deplete soil N, over drilling desirable grasses or exploiting the accumulated soil N by replacing the legume with a non-legume forage or cash crop.

What are some of the factors which can affect nodulation of legumes?

There are a few things that can affect nodulation

a) White clover is usually fine for nodulation on resident bacteria in the soil and people are no longer inoculating every time they sow white clover. However, some of the resident bacteria can be less effective (they fix less N for the plant but cost the plant the same amount of energy) than the introduced ones in commercial inoculants. Also some nodule invaders may actually not be rhizobia and are parasitic – they have the gene to tell the plant they are fixing N but actually aren’t fixing any – cunning bacteria! There is work in progress at Lincoln on this at the moment.

White nodules on white clover may be:

Ineffective and not rhizobia

Effective but not currently active.

b) Lucerne – usually needs to be inoculated unless it has been sown into a paddock that had lucerne within the previous 4-5 years and an ongoing high pH (>5.8 in H2O). However, even when inoculated not all of the bacteria that occupy the nodules will be from the inoculant – some will be resident bacteria from the soil and may be less efficient at N fixation – we actually don’t know what bacteria populates the nodules over time especially because lucerne periodically sloughs off the nodules during periods of low N demand (winter, water stress, high soil N conditions).
On lucerne the nodules are more like long sausages in clumps – and sometimes pink (see photo below) and sometimes white- they are difficult beasts to work on for that reason.

c) Longevity of rhizobia – at this stage we suspect the soil around white clover may only contain the inoculant for a few months. For lucerne it may be in proximity for several years. After that it is a jungle of soil bacteria and no-one really knows the dynamics hence we have some work happening regarding this issue at the moment.

d) Lack of pink nodules – rhizobia are not always active and fixing N for the plant- they will be pink and operating when the plant is actively growing but may be sloughed off when the plant is not actively growing. This occurs because the plant is in charge of the relationship and it is lazy. There is a cost to the plant of fixing N (N supplied in return for carbohydrate formed by the plant) so if they don’t need N because:

They are not growing much or

There is adequate soil N or

They won’t be fixing much N (although when they switch off we really don’t know – it differs between species.

e) At establishment lucerne may go all of the first year without fixing N because there is adequate soil N (from mineralisation after the soil is cultivated) to keep it going so it will form the bumps and take the rhizobia in but not actually start fixing until year two.

f) Clover root weevil (Sitona lepidus) – check for notching on the leaves – also attacks lucerne and can take 2-3 years to be controlled by its parasitoid. The weevil causes more damage on white and red clovers than on sub clover and lucerne.

Why are there no nodules on the roots of my lucerne plants in winter?

The relationship between rhizobia and lucerne is largely driven by the lucerne. In the middle of winter when the plant is not growing it may not have nodules on it. The plant has a low demand for nitrogen at this time so it may have sloughed off the nodules. This is not a problem and there is no need to panic.

We believe the lucerne preferentially uses soil available nitrogen before fixed nitrogen. When spring kicks into gear lucerne can become yellow and this may be due to any number of issues such as a sulphur deficiency or a lack of nodulation. The symbiosis (the forming of the relationship between the rhizobia and the lucerne plant) takes time to re-establish each spring and because the lucerne is responding to air temperatures and the root to soil temperatures there may be a mismatch in nitrogen demand. However, the lucerne then gets fixation going and usually fixes about 28 kg N/t DM grown.

The rhizobia in the soil that have sloughed off can multiply and repopulate nodules relatively quickly. In the absence of any lucerne for a five year period their can still be sufficient rhizobia in the soil to nodulate a stand- provided the soil pH has remained sufficiently high (around 5.8 in H2O in NZ or 5.2 in CaCl if you’re in Australia) for them to survive.

In our experience, knowing exactly when you will find nodules has been difficult – but their absence is not always a bad sign – it is quite common. The only time to panic is if you are going into a site that has low pH (<5.5 in H2O or about 4.4 in Australia), no history of lucerne and you forget to add the rhizobia. Even then it can be recovered by applying a slurry in the evening before rain to inoculate the soil. So in winter n absence of nodules is expected.

I have just sown my lucerne but there is no sign of nodulation. What should I do?

In an establishing/1st year crop the availability of soil N from mineralisation following cultivation often means lucerne will not form nodules for at least a year – until it runs out of “free” soil N. White pimples on the roots could then be the root hairs saying hello to the bacteria and capturing it but not actually switching on the N fixation because they don’t need to yet. Then when soil N is low they may “turn on” and then grow and become pink. The process is actually controlled by the lucerne – not the rhizobia bacteria.

The healthy paddock may also have small nodules if there is adequate soil N. So don’t judge a lucerne paddock by its nodules – it will always disappoint you! The thing is it can have a few clusters of nodules that are fixing lots of N really efficiently or one nodule doing exactly the same! What you don’t usually find is masses of nodules all over lucerne roots, which is common in clover species.

Why are there no nodules on my 2-year-old lucerne stand in mid-October?

At this time of year a lot of lucerne does not have nodules on it if the soil N levels are high – but in a second year crop it may not have nodulated at all last year depending on yield. At Lincoln we have had an inoculated crop grow as much as an inoculated one for the first year based on soil N levels but observed a yield loss in the inoculated stand in Year 2. Ideally, sow fresh peat slurry inoculated seed or commercially available coated seed. If the stand was not inoculated at sowing spray a slurry of peat inoculant over the paddock in the evening when there is about 15 cm of herbage – no more – The rhizobia bacteria will be killed by exposure to UV light (and a range of other factors like storing inoculated seed in areas with high temperature etc.) so you need ground cover to prevent the sunshine killing the bacteria. Applications just before irrigation or rain is best to wash it into the soil.

It’s autumn and we have a mid-age lucerne stand (4-5 years-old). It looks healthy on top but underneath the nodules are white and very, very small and another paddock that is extremely healthy but the clover is looking yellow around the edges. Is there anything wrong and what should we do?

Everything sounds normal for this time of year. Lucerne is a lazy plant and fixing nitrogen has a carbohydrate cost. This means if there is high soil N – which may come from the urine returns from direct grazing in a mid-age stand – lucerne will preferentially use the freely available N in the soil and N fixation is likely to be reduced. This means there will be fewer nodules.

The small white pimple-like nodules could be either

Ineffective nodules formed by bacteria other than rhizobia trying to claim carbohydrate from the lucerne but which don’t supply N to the plant in return OR

Rhizobia that are not currently fixing N because there is low plant demand for N or it is being met by soil N.

If the paddock is poor performing the lack of N could be due to

The soil pH is too low and Molybdenum is an issue OR

It is a paddock that didn’t nodulate well at establishment.