BIOLOGICAL NITROGEN FIXATION (BNF) IN LEGUMES: IMPORTANCE IN SUSTAINABLE AGRICULTURE AND FOOD SECURITY

AI Hassen (PhD)ARC-Plant Protection ResearchP. Bag X134, QueenswoodEmail: hassena@arc.agric.za

NSTF- Discussion Forum on Pulses and Food Security, 2-3 June 2016, Emperors Palace Convention Centre, Kempton Park

Nitrogen fixation is an essential process for all organisms

Biological Nitrogen Fixation (BNF):

The Nitrogen Cycle in the tropics.

Protein contents of human and animal foods

Diets of resource poor communities

Yield increase and income generation

Synergistic biological interaction

Plants: carbon and energy source

Rhizobia/Bradyrhizobia: fixed nitrogen

Pulses: Pea (105kg/ha), chickpea (75kg/ha), Faba bean

(110kg/ha) (Various limiting factors)

Free living N2 fixation: 60Kg N ha-1y-1

SNF: 100 – 360 Kg N ha-1y-1 = 30 – 80 Kg Fertilizer N ha-1

Schultze and Kondorosi, 1998. Ann Rev Gen; www.cilr.uq.edu.au

Pulses (other legumes) prefer to use Nitrogen in mineral

forms (NO3, NH4) as this requires less energy than making

their N from BNF ( to use ‘N’ from the soil) .

If it is a question of adding fertilizer ‘N’, BNF is the best

alternative than adding inorganic fertilizer.

Pulses high nitrogen requirement for protein synthesis

Need to apply very large quantity of inorganic fertilizer

Example: to obtain soybean yield of 2000kg ha-1

Need to apply 600 – 900 Kg Urea ha-1

280 – 413 kg Nitrogen

The same yield ( 2000 kg ha-1 can be obtained by BNF, no

addition of N-fertilizer; BNF is cost effective

Necessary to inoculate pulses and other legumes with

Rhizobium, symbiotic nitrogen fixing bacteria

Compete with indigenous rhizobium

Effective in nodulating the target legume

Fix atmospheric N with a wide range of host genotypes

Increase yield to the desired level

Persist in the soil

Maintains genetic stability

• ARC-Plant Protection Research Institute

• South African Rhizobium Culture Collection (SARCC)

• Several hundreds of rhizobia (National Assets)

• Long term projects, field collections, client samples and

international institutions

• Rhizobium, Bradyrhizobium, Sinorhizobium, Mesorhizobium

• Routinely maintained (viability, purity, effectiveness)

About 15 commercial strains of N-fixing bacteria

Rhizobium sp. TJ14 (Peas)

B. japonicum strain XS21 (Cow pea, ground nut)

Rhizobium sp. Strain UD5 (Faba bean & broad beans)

Bradyrhizobium sp. Strain VK10 (Lupin)

Bradyrhizobium japonicum WB74 (Soybean)

Sinorhizobium meliloti RF14 (Lucern)

Currently Mesorhizobium sp (Rooibos)

Nodulation occurs during early flowering

Starts to emerge 14 days after crop emergence

Nodule numbers and N-fixation maximum during early

to mid flowering

Many, big size, mainly on the crown, red in color inside

Leghaemoglobin

A B C

No indigenous strains of the required Rhizobium

E.g. The case of soybean in South Africa

Bradyrhizobium japonicum strain WB74

(Australian strain under the name CB1805)

When the level of indigenous Rhizobium population is

extremely low (10 – 1000 cells/g soil).

Adding small amount of fertilizer ‘N’ stimulates

nodulation and growth.

Not always true for some legumes, jeopardizes the

process of nodulation and N- fixation

Unutilized fertilizer N carried over from previous cereal

crops negatively influences BNF.

Level of N-fixation commonly low in pulse crops grown

in a rotation with fertilized maize. (Crop rotation??)

Recommended level of ‘N’ranges 15kg -30 kg/ha

Biologically fixed ‘N’ bound in the soil OM

Less susceptible to soil chemical transformations

Volatilization of N2-oxide (greenhouse gas)

Leaching of (NO-3)

Climate change

Reduced contamination of water bodies

Minimizes depletion of non renewable resources

Pulses, major source of dietary proteins and important

component of subsistence and low input agriculture

Global climate change posing threat

Fluctuations in pulse yield and frequent crop failure

Most pulses susceptible to abiotic & biotic factors

Acidity, salinity, drought, Reactive Oxygen Species

ABIOTICSTRESS

More focus be given to pulse BNF research

Investment in Rhizobium inoculation technology

Participation by public & private partnership

Effective BNF dissemination strategy (farmers)

Capacity building along BNF value chain

Partnership with research and academic institutions

in the developed nations