BIOLOGICAL NITROGEN FIXATION (BNF) IN LEGUMES: IMPORTANCE IN SUSTAINABLE AGRICULTURE AND FOOD SECURITY
AI Hassen (PhD)ARC-Plant Protection ResearchP. Bag X134, QueenswoodEmail: [email protected]
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